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10.17 Threading

Micky R. Jennings

A finished thread on a part in a 3 jaw chuck.
Figure 10.172. A finished thread on a part in a 3 jaw chuck. / Image Credit: Micky R. Jennings, courtesy of Wenatchee Valley College, CC BY 4.0

Threading is the process of creating a precision angular helical groove down the length of a shaft. The result of this cutting operation is known as a thread. There are many types of threads that could be discussed, but the most common are unified inch threads and metric threads. This text will discuss the basics of single point cutting unified threads. This process can easily be adapted for cutting metric threads. Before beginning to cut threads, it is important to define some thread terminology.

Terminology

A thumb screw is shown with the components of the thread labeled. Crest, root, flanks, thread relief, thread angle, pitch, major diameter, pitch diameter, and minor diameter are shown.
Figure 10.173. A thumb screw is shown with the components of the thread labeled. Crest, root, flanks, thread relief, thread angle, pitch, major diameter, pitch diameter, and minor diameter are shown. / Image Credit: Micky R. Jennings, courtesy of Wenatchee Valley College, CC BY 4.0

Nominal Diameter

The nominal diameter of the thread is a general reference term. For example, you might hear a machinist mention a 1/2″ bolt. The nominal diameter is 1/2″, but that isn’t what the outside of the thread really measures.

Major Diameter

The major diameter of an external thread is what the outside diameter of the thread actually measures, generally slightly smaller than nominal. For internal threads, the major diameter is the largest diameter of the thread being cut.

Minor Diameter

The minor diameter of an external thread refers to the smallest diameter of the thread being cut. For internal threads, the minor diameter is the same as the tap drill size when looking to maintain 75% thread engagement.

Pitch Diameter

The pitch diameter is the imaginary diameter that exists at the point where 1/2 of the thread’s pitch would lie horizontally on a single thread. This is one of the most important parts of a thread to measure because it is where internal and external threads meet.

Threads Per Inch

Also known as TPI, threads per inch is used in describing inch thread sizes. For example, a 1/2-13 thread has a nominal diameter of 1/2″ and a TPI of 13. Metric threads don’t use TPI to describe threads; they just use the thread pitch.

Pitch

The pitch of a thread is the distance from one point on a thread to the same point on another thread. Knowing the size of a thread, the pitch can be calculated with the formula:

For example, a 1/2-13 thread has a pitch of .077 because 1/13 = .077

Lead

The lead of a thread is the distance a threaded shaft would travel inside a nut when turned one full rotation. For a single start thread, the lead is equal to the thread pitch. For a double start thread, the lead would be twice the thread pitch, and so on.

Crest

The crest of a thread is the small flat at the major diameter of an external thread and the minor diameter of an internal thread.

Root

The rootof a thread is the bottom surface generated by the thread cutting tool. On an external thread, the root would be at the minor diameter, and on an internal thread, the root would be at the major diameter.

Flank

The flank of the thread is the angular portion between the crest and root. The vee shape cut on the thread, from one flank to the other flank, measures 60 degrees on both inch and metric threads.

Step by step process for external threading:

  1. Turn the outside major diameter of the area to be threaded. The major diameter and tolerance for any thread can be looked up in a manufacturing reference book.
  2. Chamfer the face end of the shaft to be threaded. The size of this chamfer should be about down to the minor diameter.
  3. Load a 60 degree threading tool into a tool holder and onto the tool post of the lathe and set the height.
  4. Adjust the angle of the compound rest so that it is pointing 29 degrees from being perpendicular to the work. It is important to note that the compound rest should not be set to 29 degrees. This is the angle at which the tool will be advancing for the depth of cut. The reason for the angle is so that the tool cuts a majority of the material from the leading edge of the tool and the chip has room to flow across the chip tool interface. If the depth of cut were to be made strictly perpendicular to the work, the chip would try to form equally from both sides of the cutting tool and bunch up near the fragile tip and potentially break it.
  5. Using a center gage, make sure the tool is pointed perpendicular to the work.
  6. Select a slow spindle speed for threading, generally about 100 RPMs for beginners. On a gear head lathe, once the threading process has been started, the spindle speed cannot be altered without ruining the alignment of the thread.
  7. Using the quick change gear box, select the TPI required.
  8. Turn the spindle on and make sure the machine is in gear and that the lead screw is turning.
  9. Apply layout dye to the outside of the portion to be threaded.
  10. Scribe a line with odd leg calipers at the point where the thread will end.
  11. Move the cross slide so that the tool is as close as it will get to the work with the handle of the hand wheel in the three o’clock position.
  12. Set the graduated collar on the cross slide to zero.
  13. Adjust the compound rest so that the tool just touches the outside of the work.
  14. Zero the graduated collar on the compound rest.
  15. Move the carriage to about 1/2″ in front of the part.
  16. Advance the compound rest a couple of thousandths to make a scratch pass for thread inspection.
  17. Select a timing mark or series of marks to engage the half nut lever.
  18. With a brush, spread cutting oil on the outside of the shaft.
  19. Place the left hand on the cross slide handle, being prepared to pull out of the thread.
  20. Engage the half nut and keep a hand on the lever. Make sure it is fully engaged and on the correct timing mark.
  21. Watch the tool cut along the outside of the shaft. Once the tool has reached the scribe mark, quickly pull out one full turn of the cross slide with the left hand and then disengage the half nut lever with the right hand. This motion needs to happen quickly and precisely in order to not overtravel the designated thread length or put grooves in the threaded section. Pulling out first is the safest option whenever needing to end the thread cutting operation.
  22. Move the carriage to the front of the thread again.
  23. Reset the cross slide to the zero and three o’clock position.
  24. Use a thread pitch gage or a pair of calipers to check that the threads are the correct TPI.
  25. Adjust the compound rest to about .010, add oil to the vee groove, and make another cut in the same fashion as the scratch cut. Generally, the first .040 or so can be made .010 at a time. After that, the chip created starts to become really large, and reducing the depth of cut is recommended.
  26. Continue the thread cutting process and stop when the width of the thread vee surpasses the width of the crest.
  27. Execute a dwell cut (also known as a zero cut or spring cut) of the threads. This is accomplished by making the cutting motion again, but without advancing the compound rest. Basically, following the same path as the last cut allows the tool to remove any material that may have been left due to the setup of the machine deflecting. This should be done before each measurement and after the last cut.
  28. Thoroughly clean the threads of chips and oil.
  29. Inspect the flank of the threads for tearing. Now is the opportunity to fix an issue like this.
  30. Measure the pitch diameter of the threads and compare the measurement to the correct pitch diameter range to determine the material left to be removed.
  31. Continue cutting threads deeper until reaching a point of within .005 of the pitch diameter range. It is important to note that the compound rest movement is measured as the actual distance, whereas the cross slide movement is measured by the diameter change. It is easy to over-cut threads by not understanding this fact. If the crest of the threads becomes sharp and without a flat, the threads have been cut too far.
  32. At this point in the thread cutting process, the remaining material is removed by advancing the cross slide in smaller finish cut increments. This allows both sides of the 60 degree cutting tool to cut evenly and create smooth, symmetrical flanks.
  33. Take the final measurement.
  34. Gently deburr the crest and the end of the thread.

Step 3: Load a 60 degree threading tool into a tool holder and onto the tool post of the lathe and set the height.

Checking the tool height of a threading tool with a tailstock center.
Figure 10.174. Checking the tool height of a threading tool with a tailstock center. / Image Credit: Micky R. Jennings, courtesy of Wenatchee Valley College, CC BY 4.0

Step 4: Adjust the angle of the compound rest so that it is pointing 29 degrees from being perpendicular to the work. It is important to note that the compound rest should not be set to 29 degrees. This is the angle at which the tool will be advancing for the depth of cut. The reason for the angle is so that the tool cuts a majority of the material from the leading edge of the tool and the chip has room to flow across the chip tool interface. If the depth of cut were to be made strictly perpendicular to the work, the chip would try to form equally from both sides of the cutting tool and bunch up near the fragile tip and potentially break it.

Setting the compound rest to the angle required for threading.
Figure 10.175. Setting the compound rest to the angle required for threading. / Image Credit: Micky R. Jennings, courtesy of Wenatchee Valley College, CC BY 4.0

Step 5: Using a center gage, make sure the tool is pointed perpendicular to the work.

A hand uses a center gage to test check the angle of the toolpost and threading tool.
Figure 10.176. A hand uses a center gage to check the angle of the tool post and threading tool. / Image Credit: Micky R. Jennings, courtesy of Wenatchee Valley College, CC BY 4.0

Step 7: Using the quick change gear box, select the TPI required.

A threading chart on the front of a lathe.
Figure 10.177. A threading chart on the front of a lathe. / Image Credit: Micky R. Jennings, courtesy of Wenatchee Valley College, CC BY 4.0

Step 7: Using the quick change gear box, select the TPI required.

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Figure 10.178. Quick change gearbox selection knobs set to “C”, “T”, “Y”, and 2. / Image Credit: Micky R. Jennings, courtesy of Wenatchee Valley College, CC BY 4.0

Step 9: Apply layout dye to the outside of the portion to be threaded.

A hand applies layout dye to the outside of a part.
Figure 10.179. A hand applies layout dye to the outside of a part. / Image Credit: Micky R. Jennings, courtesy of Wenatchee Valley College, CC BY 4.0

Step 11: Move the cross slide so that the tool is as close as it will get to the work with the handle of the handwheel in the three o’clock position.

The cross slide handle set to the 3 o'clock position.
Figure 10.180. The cross slide handle set to the 3 o’clock position. / Image Credit: Micky R. Jennings, courtesy of Wenatchee Valley College, CC BY 4.0

Step 12: Set the graduated collar on the cross slide to zero.

Step 13: Adjust the compound rest so that the tool just touches the outside of the work.

A hand uses the compound rest to touch off the tool to the OD of the work.
Figure 10.181. A hand uses the compound rest to touch off the tool to the OD of the work. / Image Credit: Micky R. Jennings, courtesy of Wenatchee Valley College, CC BY 4.0

Step 13: Adjust the compound rest so that the tool just touches the outside of the work.

Step 14: Zero the graduated collar on the compound rest.

Two hands work in conjunction to zero the graduated collar of the compound rest.
Figure 10.182. Two hands work in conjunction to zero the graduated collar of the compound rest. / Image Credit: Micky R. Jennings, courtesy of Wenatchee Valley College, CC BY 4.0

Step 14: Zero the graduated collar on the compound rest.

Step 17: Select a timing mark or series of marks to engage the half nut lever.

Examining a threading chart near the threading timing wheel.
Figure 10.183. Examining a threading chart near the threading timing wheel. / Image Credit: Micky R. Jennings, courtesy of Wenatchee Valley College, CC BY 4.0

Step 20: Engage the half nut and keep a hand on the lever. Make sure it is fully engaged and on the correct timing mark.

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Figure 10.184. A hand engages the half nut lever on the number “1” on the timing wheel. / Image Credit: Micky R. Jennings, courtesy of Wenatchee Valley College, CC BY 4.0

Step 20-21: Engage the half nut and keep a hand on the lever. Make sure it is fully engaged and on the correct timing mark. Watch the tool cut along the outside of the shaft. Once the tool has reached the scribe mark, quickly pull out one full turn of the cross slide with the left hand and then disengage the half nut lever with the right hand. This motion needs to happen quickly and precisely in order to not overtravel the designated thread length or put grooves in the threaded section. Pulling out first is the safest option whenever needing to end the thread cutting operation.

Step 21: Watch the tool cut along the outside of the shaft. Once the tool has reached the scribe mark, quickly pull out one full turn of the cross slide with the left hand and then disengage the half nut lever with the right hand. This motion needs to happen quickly and precisely in order to not overtravel the designated thread length or put grooves in the threaded section. Pulling out first is the safest option whenever needing to end the thread cutting operation.

Step 24: Use a thread pitch gage or a pair of calipers to check that the threads are the correct TPI.

A hand uses a pair of calipers to check the pitch of the scratch pass on the work.
Figure 10.185. A hand uses a pair of calipers to check the pitch of the scratch pass on the work. / Image Credit: Micky R. Jennings, courtesy of Wenatchee Valley College, CC BY 4.0

Step 25: Adjust the compound rest to about .010, add oil to the vee groove, and make another cut in the same fashion as the scratch cut. Generally, the first .040 or so can be made .010 at a time. After that, the chip created starts to become too large, and reducing the depth of cut is recommended.

Step 26: Continue the thread cutting process and stop when the width of the thread vee surpasses the width of the crest.

Step 27: Execute a dwell cut (also known as a zero cut or spring cut) of the threads. This is accomplished by making the cutting motion again, but without advancing the compound rest. Basically, following the same path as the last cut allows the tool to remove any material that may have been left due to the setup of the machine deflecting. This should be done before each measurement and after the last cut.

Step 28: Thoroughly clean the threads of chips and oil.

An air nozzle is used to clean the threads with compressed air. A towel is used to catch the oil being blown from the threads.
Figure 10.186. An air nozzle is used to clean the threads with compressed air. A towel is used to catch the oil being blown from the threads. / Image Credit: Micky R. Jennings, courtesy of Wenatchee Valley College, CC BY 4.0

Step 30: Measure the pitch diameter of the threads and compare the measurement to the correct pitch diameter range to determine the material left to be removed.

A hand uses a micrometer to measure a thread using thread wires.
Figure 10.187. A hand uses a micrometer to measure a thread using thread wires. / Image Credit: Micky R. Jennings, courtesy of Wenatchee Valley College, CC BY 4.0

Step 31: Continue cutting threads deeper until reaching a point within .005 of the pitch diameter range. It is important to note that the compound rest movement is measured as the actual distance, whereas the cross slide movement is measured by the diameter change. It is easy to over-cut threads by not understanding this fact. If the crest of the threads becomes sharp and without a flat, the threads have been cut too far.

A hand uses a micrometer to measure a thread using thread wires.
Figure 10.188. A hand uses a micrometer to measure a thread using thread wires. / Image Credit: Micky R. Jennings, courtesy of Wenatchee Valley College, CC BY 4.0

Step 32: At this point in the thread cutting process, the remaining material is removed by advancing the cross slide in smaller finish cut increments. This allows both sides of the 60 degree cutting tool to cut evenly and create smooth, symmetrical flanks.

Adjusting the cross slide to take the finishing cuts of a thread.
Figure 10.189. Adjusting the cross slide to take the finishing cuts of a thread. / Image Credit: Micky R. Jennings, courtesy of Wenatchee Valley College, CC BY 4.0

Step 32: At this point in the thread cutting process, the remaining material is removed by advancing the cross slide in smaller finish cut increments. This allows both sides of the 60 degree cutting tool to cut evenly and create smooth, symmetrical flanks. ”

Step 33: Take the final measurement.

A micrometer measures a thread using thread wires.
Figure 10.190. A micrometer measures a thread using thread wires. / Image Credit: Micky R. Jennings, courtesy of Wenatchee Valley College, CC BY 4.0

Step 34: Gently deburr the crest and the end of the thread.

A file is used to deburr the crests of the thread.
Figure 10.191. A file is used to deburr the crests of the thread. / Image Credit: Micky R. Jennings, courtesy of Wenatchee Valley College, CC BY 4.0

Step by step process for internal threading:

  1. Drill, and or, bore a hole to the minor diameter in the area to be threaded. The minor diameter and tolerance for any thread can be looked up in a manufacturing reference book.
  2. Chamfer the face end of the shaft to be threaded. The size of this chamfer should be out to the major diameter.
  3. Adjust the angle of the compound rest so that it is pointing 29 degrees from the operator. It is important to note that the compound rest should not be set to 29 degrees. This is the angle at which the tool will be advancing for the depth of cut. The reason for the angle is so that the tool cuts a majority of the material from the leading edge of the tool and the chip has room to flow across the chip tool interface. If the depth of cut were to be made strictly perpendicular to the work, the chip would try to form equally from both sides of the cutting tool and bunch up near the fragile tip and potentially break it.
  4. Square the tool post.
  5. Load a 60 degree threading bar into a tool holder and onto the tool post of the lathe. Only have the bar out as far as is needed, with 1/8″ of clearance, to combat chatter.
  6. Using a center gage, make sure the tool is pointed perpendicular to the work.
  7. Select a slow spindle speed for threading, generally about 100 RPMs for beginners. On a gear head lathe, once the threading process has been started, the spindle speed can not be altered without ruining the alignment of the thread.
  8. Using the quick change gear box, select the TPI required.
  9. Turn the spindle on and make sure the machine is in gear and that the lead screw is turning.
  10. Apply layout dye to the threading bar at approximately the depth of the thread. (Blind thread only.)
  11. With odd leg calipers, scribe a line on the threading bar at the point where the thread will end. (Blind thread only.)
  12. Move the cross slide so that the tool is as close as it will get to the inside diameter with the handle of the hand wheel in the nine o’clock position.
  13. Set the graduated collar on the cross slide to zero.
  14. Adjust the compound rest so that the tool just touches the inside of the work.
  15. Zero the graduated collar on the compound rest.
  16. Move the carriage to about 1/2″ in front of the part.
  17. Advance the compound rest a couple of thousandths to make a scratch pass for thread inspection.
  18. Select a timing mark or series of marks to engage the half nut lever.
  19. With a brush, spread cutting oil on the inside of the hole.
  20. Place the left hand on the cross slide handle, being prepared to push out of the thread.
  21. Engage the half nut and keep a hand on the lever. Make sure it is fully down and on the correct timing mark.
  22. Watch the tool cut along the outside of the shaft. Once the cutting edge is through the part (for blind holes, once the face of the work has reached the scribe mark), quickly push out one full turn of the cross slide with the left hand, and then disengage the half nut lever with the right hand. This motion needs to happen quickly and precisely in order to not overtravel the designated thread length, or put grooves in the threaded section. Pulling out first is the safest option whenever needing to end the thread cutting operation.
  23. Move the carriage to the front of the thread again.
  24. Reset the cross slide to the zero and nine o’clock position.
  25. Use a thread pitch gage or a pair of calipers to check if the threads are the correct TPI.
  26. Adjust the compound rest to about .010, add oil to the vee groove, and make another cut in the same fashion as the scratch cut. Generally, the first .040 or so can be made .010 at a time. After that, the chip created starts to become really large, and reducing the depth of cut is recommended.
  27. Continue the thread cutting process and stop when the width of the thread vee is equal to that of the crest.
  28. Execute a dwell cut (also known as a zero cut or spring cut) of the threads. This is accomplished by making the cutting motion again, but without advancing the compound rest. Basically, following the same path as the last cut allows the tool to remove any material that may have been left due machine setup deflection. This should be done before each measurement and after the last cut.
  29. Thoroughly clean the threads of chips and oil.
  30. Inspect the flank of the threads for tearing. Now is the opportunity to fix tears and similar problems.
  31. Check the thread by attempting to insert a bolt into the hole. The bolt will be smaller than the threaded plug gage and will start into the hole sooner. It is also nice to test with a bolt so the expensive threaded plug gage doesn’t get damaged from overuse or accidents.
  32. Continue cutting threads deeper until the bolt starts to thread into the hole. It is important to note that the compound rest movement is measured as the actual distance, whereas the cross slide movement is measured by the diameter change. It is easy to over-cut threads by not understanding this fact. If the crest of the threads becomes sharp and without a flat, the threads have been cut too far.
  33. Check with a threaded plug gage from this step on.
  34. At this point in the thread cutting process, the remaining material is removed by advancing the cross slide in smaller finish cut increments. This allows both sides of the 60 degree cutting tool to cut evenly and create smooth, symmetrical flanks.
  35. Gently deburr the crest and the end of the thread.

Step 1: Drill, and or, bore a hole to the minor diameter in the area to be threaded. The minor diameter and tolerance for any thread can be looked up in a manufacturing reference book.

Step 1: Drill, and or, bore a hole to the minor diameter in the area to be threaded. The minor diameter and tolerance for any thread can be looked up in a manufacturing reference book.

A hand prepares to install a taper shank drill into a tailstock.
Figure 10.192. A hand prepares to install a taper shank drill into a tailstock. / Image Credit: Micky R. Jennings, courtesy of Wenatchee Valley College, CC BY 4.0

Step 1: Drill, and or, bore a hole to the minor diameter in the area to be threaded. The minor diameter and tolerance for any thread can be looked up in a manufacturing reference book.

Step 1: Drill, and or, bore a hole to the minor diameter in the area to be threaded. The minor diameter and tolerance for any thread can be looked up in a manufacturing reference book.

Step 4: Square the tool post.

Aligning the toolpost using the tool holder against the face of the work. The toolpost is out of square with the face of the work.
Figure 10.193. Aligning the tool post using the tool holder against the face of the work. The tool post is out of square with the face of the work. / Image Credit: Micky R. Jennings, courtesy of Wenatchee Valley College, CC BY 4.0

Step 4: Square the tool post.

Aligning the toolpost using the tool holder against the face of the work. The toolpost is square with the face of the work.
Figure 10.194. Aligning the tool post using the tool holder against the face of the work. The tool post is square with the face of the work. / Image Credit: Micky R. Jennings, courtesy of Wenatchee Valley College, CC BY 4.0

Step 5: Load a 60 degree threading bar into a tool holder and onto the tool post of the lathe. Only have the bar out as far as is needed, with 1/8″ of clearance, to combat chatter.

A hand holds a threading bar inside the part to check for clearance and determine the bar installation length.
Figure 10.195. A hand holds a threading bar inside the part to check for clearance and determine the bar installation length. / Image Credit: Micky R. Jennings, courtesy of Wenatchee Valley College, CC BY 4.0

Step 5: Load a 60 degree threading bar into a tool holder and onto the tool post of the lathe. Only have the bar out as far as is needed, with 1/8″ of clearance, to combat chatter.

Checking the threading bar height with the tailstock center.
Figure 10.196. Checking the threading bar height with the tailstock center. / Image Credit: Micky R. Jennings, courtesy of Wenatchee Valley College, CC BY 4.0

Step 5: Load a 60 degree threading bar into a tool holder and onto the tool post of the lathe. Only have the bar out as far as is needed, with 1/8″ of clearance, to combat chatter.

Checking for clearance and the threading bar installation length.
Figure 10.197. Checking for clearance and the threading bar installation length. / Image Credit: Micky R. Jennings, courtesy of Wenatchee Valley College, CC BY 4.0

Step 6: Using a center gage, make sure the tool is pointed perpendicular to the work.

Checking the angle of the toolpost using a center gage.
Figure 10.198. Checking the angle of the tool post using a center gage. / Image Credit: Micky R. Jennings, courtesy of Wenatchee Valley College, CC BY 4.0

Step 13: Set the graduated collar on the cross slide to zero.

Zeroing the cross slide handwheel.
Figure 10.199. Zeroing the cross slide handwheel. / Image Credit: Micky R. Jennings, courtesy of Wenatchee Valley College, CC BY 4.0

Step 14: Adjust the compound rest so that the tool just touches the inside of the work.

Touching off the tool on the inside of the part using the compound rest.
Figure 10.200. Touching off the tool on the inside of the part using the compound rest. / Image Credit: Micky R. Jennings, courtesy of Wenatchee Valley College, CC BY 4.0

Step 15: Zero the graduated collar on the compound rest.

The compound rest has been zeroed after touching the tool off.
Figure 10.201. The compound rest has been zeroed after touching the tool off. / Image Credit: Micky R. Jennings, courtesy of Wenatchee Valley College, CC BY 4.0

Step 21-22: Engage the half nut and keep a hand on the lever. Make sure it is fully down and on the correct timing mark. Watch the tool cut along the outside of the shaft. Once the cutting edge is through the part (for blind holes, once the face of the work has reached the scribe mark), quickly push out one full turn of the cross slide with the left hand and then disengage the half nut lever with the right hand. This motion needs to happen quickly and precisely in order to not overtravel the designated thread lengt, or put grooves in the threaded section. Pulling out first is the safest option whenever needing to end the thread cutting operation.

Step 31: Check the thread by attempting to insert a bolt into the hole. The bolt will be smaller than the threaded plug gage and will start into the hole sooner. It is also nice to test with a bolt, so the expensive threaded plug gage doesn’t get damaged from overuse or accidents.

Thread Measurement

Measuring threads can be accomplished by a few different methods. Fixed gages, known as threaded ring and threaded plug gages, are an easy way to check both thread size and form. Thread micrometers are very accurate, but they are often costly and require measuring skills to read. Lastly, there are thread wires. Thread wires can only measure external threads; however, they are the cheapest option because they utilize existing measuring equipment. Thread wires are also the most complicated to use because there are multiple pieces to manipulate, and there is math involved in taking a measurement.

Threaded ring and plug gages

image
Figure 10.202. A pair of threaded ring gages, one “go” and one “no-go”. / Image Credit: Micky R. Jennings, courtesy of Wenatchee Valley College, CC BY 4.0
image
Figure 10.203. A double ended threaded plug gage, one end “go” and one end “no-go”. / Image Credit: Micky R. Jennings, courtesy of Wenatchee Valley College, CC BY 4.0

Threaded ring gages and threaded plug gages will not measure the size of the threads, they will only indicate if the part is bigger or smaller than the gage. For this reason, they are good for checking both ends of the pitch diameter tolerance zone and distinguishing conforming from non-conforming parts for inexperienced operators. One drawback is that different gages are needed to check each size of thread. That can become expensive if many sizes are required.

  1. Clean the threads thoroughly.
  2. Apply a small amount of gage oil to the “go” fixed gage. This gage would be the maximum allowable pitch diameter for an external thread, and the minimum allowable pitch diameter for an internal thread.
  3. Delicately screw the gage onto or into the part. Stop if there is any resistance. This gage is not designed to cut, clean, or debur threads; it only measures them. If the gage goes on or in fully, then enough material has been removed to be within the pitch diameter tolerance. This result means the part may be good. If the gage doesn’t go onto or into the part, then it should be reworked to be within pitch diameter tolerance.
  4. Remove and clean gage.
  5. Apply a small amount of gage oil to the “no-go” fixed gage. This gage would be the minimum allowable pitch diameter for an external thread and the maximum allowable pitch diameter for an internal thread.
  6. Delicately screw the gage onto or into the part. Stop if there is any resistance. This gage is designed to go on by no more than one thread and stop. If the gage doesn’t thread onto or into the part, then enough material is remaining to be within the pitch diameter tolerance. This result indicates that the part may be good. If the gage goes onto or into the part fully, then the part has had too much material removed, and the part is scrap.
  7. Remove and clean gage.

Step 3: Delicately screw the gage onto or into the part. Stop if there is any resistance. This gage is not designed to cut, clean, or debur threads; it only measures them. If the gage goes on or in fully, then enough material has been removed to be within the pitch diameter tolerance. This result means the part may be good. If the gage doesn’t go onto or into the part, then it should be reworked to be within pitch diameter tolerance.”

Step 3: Delicately screw the gage onto or into the part. Stop if there is any resistance. This gage is not designed to cut, clean, or debur threads; it only measures them. If the gage goes on or in fully, then enough material has been removed to be within the pitch diameter tolerance. This result means the part may be good. If the gage doesn’t go onto or into the part, then it should be reworked to be within pitch diameter tolerance.

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Figure 10.204. A hand uses a threaded plug “go” gage to check a thread. / Image Credit: Micky R. Jennings, courtesy of Wenatchee Valley College, CC BY 4.0

Step 6: Delicately screw the gage onto or into the part. Stop if there is any resistance. This gage is designed to go on by no more than one thread and stop. If the gage doesn’t thread onto or into the part, then enough material is remaining to be within the pitch diameter tolerance. This result indicates that the part may be good. If the gage goes onto or into the part fully, then the part has had too much material removed, and the part is scrap.

Step 6: Delicately screw the gage onto or into the part. Stop if there is any resistance. This gage is designed to go on by no more than one thread and stop. If the gage doesn’t thread onto or into the part, then enough material is remaining to be within the pitch diameter tolerance. This result indicates that the part may be good. If the gage goes onto or into the part fully, then the part has had too much material removed, and the part is scrap.

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Figure 10.205. A threaded plug “no-go” gage checks a thread. / Image Credit: Micky R. Jennings, courtesy of Wenatchee Valley College, CC BY 4.0

Thread micrometer

A thread micrometer, adjustment wrenches, and interchangeable anvils, in a case lined with foam.
Figure 10.206. A thread micrometer, adjustment wrenches, and interchangeable anvils in a case lined with foam. / Image Credit: Micky R. Jennings, courtesy of Wenatchee Valley College, CC BY 4.0

Thread micrometers are an excellent way to check parts. Although fairly expensive, thread micrometers come in inch increments, some with interchangeable anvils, and can measure many threads within their range. Thread micrometers also offer direct readings for ease of use. Anyone who can read a regular micrometer can read a thread micrometer.

  1. Clean the threads thoroughly.
  2. Remove the thread micrometer from the case, clean, install the correct anvils, and calibrate.
  3. Measure the thread by inserting the concave vee anvil over a single thread and the convex fee anvil in between two threads. Adjust the thimble of the thread micrometer onto the thread and take the reading.
  4. Compare the reading on the micrometer to the pitch diameter tolerance in a manufacturing reference book.

Step 2: Remove the thread micrometer from the case, clean, install the correct anvils, and calibrate.

A thread micrometer zeroed out to check calibration.
Figure 10.207. A thread micrometer zeroed out to check calibration. / Image Credit: Micky R. Jennings, courtesy of Wenatchee Valley College, CC BY 4.0

Step 3: Measure the thread by inserting the concave vee anvil over a single thread and the convex fee anvil in between two threads. Adjust the thimble of the thread micrometer onto the thread and take the reading.

A thread micrometer being used to measure a thread on a lathe.
Figure 10.208. A thread micrometer being used to measure a thread on a lathe. / Image Credit: Micky R. Jennings, courtesy of Wenatchee Valley College, CC BY 4.0

Thread wires

Thread wires organized by size in a vinyl pack.
Figure 10.209. Thread wires organized by size in a vinyl pack. / Image Credit: Micky R. Jennings, courtesy of Wenatchee Valley College, CC BY 4.0

The three wire method of reading threads is a convenient and cost-effective way for anyone to measure threads. The reason it is so economical is because the measuring kit only consists of multiple 3 wire sets and utilizes existing micrometers for measuring. It can be a little tricky to manipulate all the wires and a micrometer with only two hands, but with a little practice it becomes easier.

The following formula converts the measurement taken with a micrometer over the wires into the pitch diameter. The pitch diameter can then be compared to the pitch diameter tolerance in a manufacturing reference book.

The constant in this formula refers to a known value in the thread wire instructions. Each size of thread wires has a different constant. (Insert picture of thread wire chart.)

For example, if attempting to measure 9/16 – 12 2A threads, the pitch diameter tolerance would be .5016 to .5068 (from reference book) and the constant would be .09283 (from thread wire instructions) because of the .055 thread wire required to measure a 12 TPI thread. If a measurement was taken over the wires at .5971, the math would be as follows:

When compared to the pitch diameter tolerance, it can be seen that the thread is within tolerance.

.5016 < .50427 < .5068

  1. Refer to the instructions that come in the thread wire kit and select the wire size for the TPI to be measured.
  2. Select a micrometer that is capable of measuring the nominal diameter of the thread.
  3. Use a rubber band to hold two thread wires in grooves underneath the thread, and balance one thread wire in a groove on top of the thread, centered between the bottom two.
  4. Carefully manipulate the micrometer to take an accurate measurement over the maximum point between the three wires.
  5. Lock the micrometer and read the measurement.
  6. Insert this measurement into the formula, along with the constant, to calculate the thread’s pitch diameter.
  7. Compare the thread cut to the pitch diameter tolerance.

Step 1: Refer to the instructions that come in the thread wire kit and select the wire size for the TPI to be measured.

Table 10.1 Example Thread Chart for All US 60 Degree Threads
THREADS PER INCH WIRE SIZE CONST THREADS PER IN WIRE SIZE CONST
48 .018 .03596 11 1/2 .055 .09869
44 .018 .03432 11 .055 .08627
40 .018 .03235 10 .055 .07840
36 .018 .02994 9 .063 .09277
32 .024 .04494 8 .072 .10775
28 .024 .04107 7 1/2 .081 .12753
27 .024 .03993 7 .081 .11928
24 .029 .05092 6 .092 .13166
20 .029 .04370 5 1/2 .108 .16654
18 .032 .04789 5 .120 .18679
16 .040 .06587 4 1/2 .127 .18855
14 .040 .05814 4 .143 .21249
13 .045 .06838 3 1/2 .185 .30756
12 .055 .09283 3 .185 .26632

Step 1: Refer to the instructions that come in the thread wire kit and select the wire size for the TPI to be measured.

Three thread wires being held by three fingers in front of a thread to be measured.
Figure 10.210. Three thread wires being held by three fingers in front of a thread to be measured. / Image Credit: Micky R. Jennings, courtesy of Wenatchee Valley College, CC BY 4.0

Step 3: Use a rubber band to hold two thread wires in grooves underneath the thread, and balance one thread wire in a groove on top of the thread, centered between the bottom two.

Two thread wires being held to the bottom of a threaded shaft by a rubber band.
Figure 10.211. Two thread wires being held to the bottom of a threaded shaft by a rubber band. / Image Credit: Micky R. Jennings, courtesy of Wenatchee Valley College, CC BY 4.0

Step 3: Use a rubber band to hold two thread wires in grooves underneath the thread, and balance one thread wire in a groove on top of the thread, centered between the bottom two.

Top thread wire balancing on top of a threaded shaft, in between the two bottom wires.
Figure 10.212. Top thread wire balancing on top of a threaded shaft, in between the two bottom wires. / Image Credit: Micky R. Jennings, courtesy of Wenatchee Valley College, CC BY 4.0

Step 4: Carefully manipulate the micrometer to take an accurate measurement over the maximum point between the three wires.

Measuring over thread wires with a micrometer.
Figure 10.213. Measuring over thread wires with a micrometer. / Image Credit: Micky R. Jennings, courtesy of Wenatchee Valley College, CC BY 4.0

Author’s Tip

  1. Timing marks and threading memory challenges: There are many charts that will give information about the correct timing marks that can be engaged for a given thread. Your lathe probably even came with one. They are all correct; however, they are difficult to memorize unless you do a lot of single point threading. Often, in today’s manufacturing environment, repetitive threading jobs are handled on CNC equipment. Manual lathe single point threading is generally left for odd jobs of low quantity. For that reason, I have never been able to remember exactly what threads you can engage on which number.
  2. A simple whole number approach: When further inspecting one of the half nut timing charts, you will notice that almost every thread can be cut by selecting the same whole number for each cut. So, pick one whole number as a timing mark and stick with it for the whole thread. This method may take a little longer than engaging on multiple points, but it has kept me from scraping threads for many years.
  3. Internal threads and compound rest depth: For internal threads, it can be difficult to know how much to advance the compound rest without going too deep. When I cut internal threads, I never cut more than the diameter deviation of the pitch diameter, and then I check depth after each cut. That means that if I am not within the pitch diameter range and I only move in the amount of the pitch diameter range difference, it is impossible to overcut the thread.
  4. Mastering timing and synchronous hand movements: Complex timing and synchronous hand movements are difficult to master and always a challenge no matter how much experience you have. For that reason, I always practice off the part. Have spots in mind where you will be engaging the half nut, pulling out, and disengaging the half nut. Practice it over and over until you can make the motion reliably every time. For beginners, I suggest practicing until you can do it at least ten times in a row.
A hand holding a center gage roughly checks the compound rest for the correct angle for external thread cutting.
Figure 10.214. A hand holding a center gage while a machinist roughly checks the compound rest for the correct angle (external thread cutting). / Image Credit: Micky R. Jennings, courtesy of Wenatchee Valley College, CC BY 4.0
A hand holding a center gage roughly checks the compound rest for the correct angle for internal thread cutting.
Figure 10.215. A hand holding a center gage while a machinist roughly checks the compound rest for the correct angle (internal thread cutting). / Image Credit: Micky R. Jennings, courtesy of Wenatchee Valley College, CC BY 4.0

Author’s Tip

  1. Checking the compound rest angle with a center gage: After adjusting the compound rest to the correct angle, I like to use the 60 degree pointed end of the center gage to make sure I have adjusted correctly. To check using this method, put the right-side angle of the center gage against the left side of the compound rest and inspect the direction the center gage is pointing. If you have the compound rest set correctly, the center gage will be pointing within 1 degree of perpendicular to the axis of the work. The most common error when setting the compound is setting it to the 29 degree graduation. This method of checking will quickly expose the most common compound setup errors.
  2. Verifying TPI before threading: After selecting the TPI needed for threading, I like to keep the lathe off and check that I have it in the correct gear. To do this, I simply engage the half nut lever, rotate the chuck so all the backlash is out of the system, set a zero position for the carriage, and turn the chuck one full rotation to see how far the carriage has moved. If the machine is set for the correct TPI, the movement of the lathe should be equal to the pitch of the desired thread.
  3. Using calipers to check TPI: When I am checking that my machined threads are the correct TPI, I prefer using my calipers instead of thread pitch gages. You can do this in a couple different ways. The first is to set your calipers to 1″, put the jaws in one of the threads, and count the number of additional threads until you reach the next jaw. A second method that I use is to calculate the pitch of the thread and then set my calipers to that, hold it up to the threads, and see if it matches with one complete thread rotation. This method works well if you have less than 1″ of thread. A combination of measuring TPI and the major diameter with calipers can be used to distinguish metric threads from inch unified threads.
  4. Tricks for measuring with thread wires: I don’t have three hands, and neither do you. When I am measuring using thread wires, I have a couple little tricks that help me. First, get a couple quality rubber bands and keep them in the thread wire pack. You will want to wrap one of the rubber bands around the two bottom wires so they are held in place underneath your thread. Second, balance the last wire in the top groove. Now you have two hands to carefully bring the movable anvil of the micrometer onto the top wire, keeping it from falling, and take the measurement.

Attributions

  1. Figure 10.172: Finished thread by Micky R. Jennings, courtesy of Wenatchee Valley College, for WA Open ProfTech, © SBCTC, CC BY 4.0
  2. Figure 10.173: Thumb screw by Micky R. Jennings, courtesy of Wenatchee Valley College, for WA Open ProfTech, © SBCTC, CC BY 4.0
  3. Figure 10.174: Tool height by Micky R. Jennings, courtesy of Wenatchee Valley College, for WA Open ProfTech, © SBCTC, CC BY 4.0
  4. Figure 10.175: Setting the compound rest angle by Micky R. Jennings, courtesy of Wenatchee Valley College, for WA Open ProfTech, © SBCTC, CC BY 4.0
  5. Figure 10.176: Checking the tool post angle by Micky R. Jennings, courtesy of Wenatchee Valley College, for WA Open ProfTech, © SBCTC, CC BY 4.0
  6. Figure 10.177: Threading chart by Micky R. Jennings, courtesy of Wenatchee Valley College, for WA Open ProfTech, © SBCTC, CC BY 4.0
  7. Figure 10.178: Quick change gearbox selection knobs by Micky R. Jennings, courtesy of Wenatchee Valley College, for WA Open ProfTech, © SBCTC, CC BY 4.0
  8. Figure 10.179: Applying layout dye by Micky R. Jennings, courtesy of Wenatchee Valley College, for WA Open ProfTech, © SBCTC, CC BY 4.0
  9. Figure 10.180: Cross slide handwheel in set position by Micky R. Jennings, courtesy of Wenatchee Valley College, for WA Open ProfTech, © SBCTC, CC BY 4.0
  10. Video 10.87: Micky R. Jennings, courtesy of Wenatchee Valley College, for WA Open ProfTech, © SBCTC, CC BY 4.0
  11. Figure 10.181: Touching off the tool by Micky R. Jennings, courtesy of Wenatchee Valley College, for WA Open ProfTech, © SBCTC, CC BY 4.0
  12. Video 10.88: Micky R. Jennings, courtesy of Wenatchee Valley College, for WA Open ProfTech, © SBCTC, CC BY 4.0
  13. Figure 10.182: Zeroing the compound rest by Micky R. Jennings, courtesy of Wenatchee Valley College, for WA Open ProfTech, © SBCTC, CC BY 4.0
  14. Video 10.89: Micky R. Jennings, courtesy of Wenatchee Valley College, for WA Open ProfTech, © SBCTC, CC BY 4.0
  15. Figure 10.183: Threading chart by Micky R. Jennings, courtesy of Wenatchee Valley College, for WA Open ProfTech, © SBCTC, CC BY 4.0
  16. Figure 10.184: Engaging the half nut lever by Micky R. Jennings, courtesy of Wenatchee Valley College, for WA Open ProfTech, © SBCTC, CC BY 4.0
  17. Video 10.90: Micky R. Jennings, courtesy of Wenatchee Valley College, for WA Open ProfTech, © SBCTC, CC BY 4.0
  18. Video 10.91: Micky R. Jennings, courtesy of Wenatchee Valley College, for WA Open ProfTech, © SBCTC, CC BY 4.0
  19. Figure 10.185: Checking pitch by Micky R. Jennings, courtesy of Wenatchee Valley College, for WA Open ProfTech, © SBCTC, CC BY 4.0
  20. Video 10.92: Micky R. Jennings, courtesy of Wenatchee Valley College, for WA Open ProfTech, © SBCTC, CC BY 4.0
  21. Video 10.93: Micky R. Jennings, courtesy of Wenatchee Valley College, for WA Open ProfTech, © SBCTC, CC BY 4.0
  22. Video 10.94: Micky R. Jennings, courtesy of Wenatchee Valley College, for WA Open ProfTech, © SBCTC, CC BY 4.0
  23. Figure 10.186: Cleaning the threads by Micky R. Jennings, courtesy of Wenatchee Valley College, for WA Open ProfTech, © SBCTC, CC BY 4.0
  24. Figure 10.187: Measuring a thread by Micky R. Jennings, courtesy of Wenatchee Valley College, for WA Open ProfTech, © SBCTC, CC BY 4.0
  25. Figure 10.188: Measuring a thread 2 by Micky R. Jennings, courtesy of Wenatchee Valley College, for WA Open ProfTech, © SBCTC, CC BY 4.0
  26. Figure 10.189: Adjusting the cross slide by Micky R. Jennings, courtesy of Wenatchee Valley College, for WA Open ProfTech, © SBCTC, CC BY 4.0
  27. Video 10.95: Micky R. Jennings, courtesy of Wenatchee Valley College, for WA Open ProfTech, © SBCTC, CC BY 4.0
  28. Figure 10.190: Measuring a thread 3 by Micky R. Jennings, courtesy of Wenatchee Valley College, for WA Open ProfTech, © SBCTC, CC BY 4.0
  29. Figure 10.191: Deburring a thread by Micky R. Jennings, courtesy of Wenatchee Valley College, for WA Open ProfTech, © SBCTC, CC BY 4.0
  30. Video 10.96: Micky R. Jennings, courtesy of Wenatchee Valley College, for WA Open ProfTech, © SBCTC, CC BY 4.0
  31. Figure 10.192: Installing a drill by Micky R. Jennings, courtesy of Wenatchee Valley College, for WA Open ProfTech, © SBCTC, CC BY 4.0
  32. Video 10.97: Micky R. Jennings, courtesy of Wenatchee Valley College, for WA Open ProfTech, © SBCTC, CC BY 4.0
  33. Video 10.98: Micky R. Jennings, courtesy of Wenatchee Valley College, for WA Open ProfTech, © SBCTC, CC BY 4.0
  34. Figure 10.193: Checking the tool post for alignment by Micky R. Jennings, courtesy of Wenatchee Valley College, for WA Open ProfTech, © SBCTC, CC BY 4.0
  35. Figure 10.194: Squared tool post by Micky R. Jennings, courtesy of Wenatchee Valley College, for WA Open ProfTech, © SBCTC, CC BY 4.0
  36. Figure 10.195: Checking the threading bar by Micky R. Jennings, courtesy of Wenatchee Valley College, for WA Open ProfTech, © SBCTC, CC BY 4.0
  37. Figure 10.196: Tool height by Micky R. Jennings, courtesy of Wenatchee Valley College, for WA Open ProfTech, © SBCTC, CC BY 4.0
  38. Figure 10.197: Checking the threading bar 2 by Micky R. Jennings, courtesy of Wenatchee Valley College, for WA Open ProfTech, © SBCTC, CC BY 4.0
  39. Figure 10.198: Checking the tool post angle by Micky R. Jennings, courtesy of Wenatchee Valley College, for WA Open ProfTech, © SBCTC, CC BY 4.0
  40. Figure 10.199: Zeroing the cross slide 2 by Micky R. Jennings, courtesy of Wenatchee Valley College, for WA Open ProfTech, © SBCTC, CC BY 4.0
  41. Figure 10.200: Touch off the tool by Micky R. Jennings, courtesy of Wenatchee Valley College, for WA Open ProfTech, © SBCTC, CC BY 4.0
  42. Figure 10.201: Zeroing the compound rest 2 by Micky R. Jennings, courtesy of Wenatchee Valley College, for WA Open ProfTech, © SBCTC, CC BY 4.0
  43. Video 10.99: Micky R. Jennings, courtesy of Wenatchee Valley College, for WA Open ProfTech, © SBCTC, CC BY 4.0
  44. Video 10.100: Micky R. Jennings, courtesy of Wenatchee Valley College, for WA Open ProfTech, © SBCTC, CC BY 4.0
  45. Figure 10.202: Threaded ring gages by Micky R. Jennings, courtesy of Wenatchee Valley College, for WA Open ProfTech, © SBCTC, CC BY 4.0
  46. Figure 10.203: Threaded plug gage by Micky R. Jennings, courtesy of Wenatchee Valley College, for WA Open ProfTech, © SBCTC, CC BY 4.0
  47. Video 10.101: Micky R. Jennings, courtesy of Wenatchee Valley College, for WA Open ProfTech, © SBCTC, CC BY 4.0
  48. Figure 10.204: Go gage checking by Micky R. Jennings, courtesy of Wenatchee Valley College, for WA Open ProfTech, © SBCTC, CC BY 4.0
  49. Video 10.102: Micky R. Jennings, courtesy of Wenatchee Valley College, for WA Open ProfTech, © SBCTC, CC BY 4.0
  50. Figure 10.205: No-go gage checking by Micky R. Jennings, courtesy of Wenatchee Valley College, for WA Open ProfTech, © SBCTC, CC BY 4.0
  51. Figure 10.206: Thread micrometer by Micky R. Jennings, courtesy of Wenatchee Valley College, for WA Open ProfTech, © SBCTC, CC BY 4.0
  52. Figure 10.207: Thread micrometer calibration check by Micky R. Jennings, courtesy of Wenatchee Valley College, for WA Open ProfTech, © SBCTC, CC BY 4.0
  53. Figure 10.208: Measuring with a thread micrometer. by Micky R. Jennings, courtesy of Wenatchee Valley College, for WA Open ProfTech, © SBCTC, CC BY 4.0
  54. Figure 10.209: Thread wires by Micky R. Jennings, courtesy of Wenatchee Valley College, for WA Open ProfTech, © SBCTC, CC BY 4.0
  55. Figure 10.210: Three thread wires by Micky R. Jennings, courtesy of Wenatchee Valley College, for WA Open ProfTech, © SBCTC, CC BY 4.0
  56. Figure 10.211: Bottom two thread wires by Micky R. Jennings, courtesy of Wenatchee Valley College, for WA Open ProfTech, © SBCTC, CC BY 4.0
  57. Figure 10.212: Top thread wire by Micky R. Jennings, courtesy of Wenatchee Valley College, for WA Open ProfTech, © SBCTC, CC BY 4.0
  58. Figure 10.213: Measuring over thread wires by Micky R. Jennings, courtesy of Wenatchee Valley College, for WA Open ProfTech, © SBCTC, CC BY 4.0
  59. Figure 10.214: Checking the compound rest angle for threading by Micky R. Jennings, courtesy of Wenatchee Valley College, for WA Open ProfTech, © SBCTC, CC BY 4.0
  60. Figure 10.215: Checking the compound rest angle for threading by Micky R. Jennings, courtesy of Wenatchee Valley College, for WA Open ProfTech, © SBCTC, CC BY 4.0
  61. Video 10.103: Micky R. Jennings, courtesy of Wenatchee Valley College, for WA Open ProfTech, © SBCTC, CC BY 4.0
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