Chapter 10 Part 4: Manual Lathes
SPINDLE SPEED, FEED RATE, DEPTH of CUT and FINISH ALLOWANCES
Spindle speed
Lathe spindle speeds are calculated very similarly to other machining processes. The machinist will use the same formula in its simplified form to calculate RPM:
RPM = SFPM x 4 / Diameter
In the case of the lathe, the diameter refers to the point at which the tool touches the workpiece. For example, if there was a HSS turning tool cutting a 2” piece of mild steel, the calculation would look as follows.
RPM = SFPM x 4 / Diameter
RPM = 100 x 4 / 2
RPM = 400 / 2
RPM = 200
This calculation is just to get a foot in the door and start creating chips. Many factors could alter this spindle speed and the operator may need to adjust accordingly.
Carbide tooling is much stronger and more temperature and abrasion resistant than HSS. For these reasons, it can be used at a much higher SFPM than HSS. With the case of carbide inserts, the manufacturer will give a range of SFPM that is most optimal for the tool to operate. Whenever possible, use the numbers provided by the manufacturer as a guide. Sometimes, with some general use carbide, that data isn’t available, and so going with a conservative estimate usually works well to get out of the gate. 4 x the SFPM of HSS is a good starting number.
An example of this calculation cutting 2” mild steel with a carbide insert would be.
RPM = SFPM x 4 / Diameter
RPM = 400 x 4 / 2
RPM = 1600 / 2
RPM = 800
Author’s Tip
I will generally make a conservative calculation based on generalized numbers and then adjust as I am running the parts. I mainly look at chip color when working with a rigid setup. A majority of the heat of a cutting operation will be in the chip produced, so the chips are a good indicator of the temperature at the point of contact. HSS has a lower working temperature, and so I will just increase my spindle speed to where I can achieve a straw to golden yellow color (400 degrees). With a general carbide insert that can withstand more heat, I will adjust until I get a purple to blue-colored (550 degree) chip.
Feed rate
Feed rates used on a lathe are determined by multiple factors. Operation, surface finish, tool nose, and tool type are all aspects to be considered. When using carbide inserted tooling, the manufacturer is a good place to look for this information. They will give the operator feed rates in Inches Per Revolution (IPR) that will give the best tool life. Using that information, coupled with the part surface finish requirements, will generally give the best results.
Depth of cut
The insert manufacturer is also an excellent place to look for the recommended depth of cut for an insert. Depth of cut is the one cutting condition variable that can be altered with the least effects on tool life. Spindle speed and cutting feed rate will shorten tool life much quicker than depth of cut.
Finishing allowance
The finishing allowance is the intentional amount of material left on surfaces by the roughing tool. This remaining material is removed by the finishing tool when the part is brought to final size.
The size of the finishing allowance depends on the type of tool used for the cuts. Because of the way carbide inserts remove material it is often not a good plan to attempt small depths of cut. For this reason a diameter increase of 1/32”, and a length of 1/64” is recommended. The remaining material can be removed in two two finish cuts of equal size, measuring between the two. This practice will give the machinist a more accurate final size because the pressures between the two finish passes will behave more similarly to one another than the heavy roughing cuts.
High speed steel and cement often have shapely ground edges and cut more freely than carbide. Because of this, they are capable of much lighter cuts, sometimes .001 or less. Many carbide inserts will ride up on small depts, burnishing the surface instead of cutting it.
Author’s Tip
Speaking in general terms, our roughing operations are going to be fed faster and deeper than the finishing cuts. For that reason, when roughing a part, I generally choose somewhere in the .010 IPR range for feed rate, and at a depth of cut the machine can handle. I like to start off small and increase the depth until I can hear the machine working. I keep my depths at an increment that is easy to count by. Don’t use .037 as an incremental depth because you will foul yourself up in the addition at some point. My favorite increment depths are multiples of .010 or .025.
For finishing operations, I am really looking at the accuracy of the feature and the surface finish callout. If the part I am turning has a +- .020 tolerance, I may not even spend the time to switch to a finishing tool or slow my feed rate down. Just rough it right to size. However, if I see ± .005 or less, I am going to use a separate tool and set my feed about half as fast at about .005 IPR. It is important for me to note that the tool nose radius also plays an important role in surface finish. The smaller the nose radius is, the slower the feed rate has to be to achieve acceptable surface finishes.
FACING
What is it?
Facing on a lathe is the process of making a radial cut along the end of the workpiece. It is generally the first cut made and serves as a starting point on which many of the other processes are based. Before making the facing cut, the material is often a rough saw cut that isn’t square and is out of general surface finish tolerance. Facing is generally performed in multiple roughing cuts and at least one finishing cut.
Step by step process for facing:
- Chuck up on material in either a three or four jaw chuck and dial in to suit requirements.
- Load a right handed turning tool onto the squared tool post.
- Calculate and set the proper spindle speed for the combination of cutting tool and material to be used.
- Set the quick change gear box for the feed rate desired to travel across the face of the work.
- Turn the spindle on.
- Manipulate the carriage and cross feed hand wheels to bring the cutting tool close to the face of the part and about 1/8” inside of the outer diameter.
- Gently turn the carriage handwheel until the tip of the cutting tool starts touching the end of the work.
- Back the tool out past the outer diameter using the cross slide handwheel.
- Adjust the graduated collar on the carriage handwheel to zero.
- Turn the carriage hand wheel in .005 or .010. to make an initial cut.
- Lock the carriage movement.
- Engage the cross slide power feed.
- Allow the cutting tool to travel across the face of the work and disengage the power feed once the tool reaches the center.
- Inspect the face of the work and continue making successive cuts until the face has been cut all the way across.
- Once the last pass is finished, and before moving the carriage, zero the graduated collar again. This is to prepare for turning the OD of the work a specific distance.
- Deburr the edge of the face with a file as the spindle is rotating. Hold the file handle in the left hand and the end of the file in the right hand to keep from reaching over the chuck.
“Step 1: Chuck up on material in either a three or four jaw chuck and dial in to suit requirements.“
“Step 2: Load a right handed turning tool onto the squared tool post.“
“Step 3: Calculate and set the proper spindle speed for the combination of cutting tool and material to be used.“
“Step 4: Set the quick change gear box for the feed rate desired to travel across the face of the work.“
“Step 5: Turn the spindle on.“
“Step 12: Engage the cross slide power feed.“
“Step 14: Inspect the face of the work and continue making successive cuts until the face has been cut all the way across.“
TURNING
What is it?
Turning is the process of removing material from the outside diameter of the work. It usually comes after the facing operation establishes the end of the part. Turning can look like steps, tapers, radiuses, and more. Turning is generally performed with multiple roughing cuts and at least one finishing cut.
Step by step process for turning:
- Chuck on a piece of material, dial it in if needed, and face the end.
- The spindle speed for turning is the same as for facing.
- Select the feed rate on the quick change gear box desired for the turning operation.
- Apply some layout dye to the outside of the part in the area to stop the turning operation.
- Turn on the spindle.
- Using an odd leg caliper, scribe a line from the face of the part the distance of the turned diameter.
- Bring the carriage up about 1/8” past the face of the part and close to the outside diameter.
- Apply cutting oil if required.
- Gently touch off on the outside of the work by moving in with the cross slide.
- Set the cross slide graduated collar to zero.
- Move the tool out in front of the face of the part and set an initial depth of cut.
- Engage the carriage power feed.
- Allow the tool to cut along the outside diameter of the part, stopping it about 1/8” short of the scribed line.
- Move the rest of the distance by hand, reading the graduated collar carefully.
- Continue making successive cuts until reaching the desired diameter.
- Deburr edges with a file.
“Step 4: Apply some layout dye to the outside of the part in the area to stop the turning operation.“
“Step 6: Using an odd leg caliper, scribe a line from the face of the part the distance of the turned diameter.“
“Step 7: Bring the carriage up about 1/8” past the face of the part and close to the outside diameter.“
“Step 8: Apply cutting oil if required.“
BORING
What is it?
Boring is the process of enlarging a drilled hole with a process similar to turning, but on the inside of the part. Boring is performed with a tool known as a boring bar. Facing can also be performed with a boring bare in a boring operation. The length and diameter of the tool are critical considerations when setting up for a boring operation. Boring is generally more unstable than other turning operations and the operator must make a conscious effort to optimize the setup to help reduce the chance of chatter. Boring is generally performed with multiple roughing cuts and at least one finishing cut.
Step by step process for boring:
- Insert material into a chuck, dial in if necessary, and face.
- Spot or center drill the part with the tailstock.
- Drill a hole large enough for the boring bar to fit inside. The insert offset can be measured and then added to the diameter of the to calculate the diameter required before boring. Sometimes this information is printed on the side of the boring bar.
- Load a boring bar into a tool holder and set the tool height. Make sure to only stick the bar out slightly longer than the depth required for the bored hole. Approximately 1/8”
- Select a spindle speed and feed rate for the boring operation. Calculate the spindle speed based on the diameter of the finished bore.
- Put layout dye on the boring bar at the approximate depth of the bored hole.
- Using odd legged calipers, scribe a precision depth gage line from the tip of the boring bar on the shank.
- Oil part if necessary.
- Turn the spindle on.
- Touch the boring bar off on the face of the part and set the graduated collar on the carriage hand wheel to zero.
- Touch the boring bar off on the inside of the bore and set the graduated collar on the cross slide to zero.
- Pull the bar out of the hole and set an initial depth for the first cut.
- Engage the carriage feed lever and watch as the bar continues into the work, disengaging when the face of the work reaches 1/8” from the scribed line on the bar.
- Take the remaining material out by hand while reading the graduated collar for accurate depth.
- Continue to make successive cuts until bore diameter is reached.
- Deburr opening of bored hole.
“Step 1: Insert material into a chuck, dial in if necessary, and face.“
“Step 1: Insert material into a chuck, dial in if necessary, and face.“
“Step 1: Insert material into a chuck, dial in if necessary, and face.“
“Step 2: Spot or center drill the part with the tailstock.“
“Step 2: Spot or center drill the part with the tailstock.“
“Step 3: Drill a hole large enough for the boring bar to fit inside. The insert offset can be measured and then added to the diameter of the to calculate the diameter required before boring. Sometimes this information is printed on the side of the boring bar.“
“Step 4: Load a boring bar into a tool holder and set the tool height. Make sure to only stick the bar out slightly longer than the depth required for the bored hole. Approximately 1/8”“
“Step 4: Load a boring bar into a tool holder and set the tool height. Make sure to only stick the bar out slightly longer than the depth required for the bored hole. Approximately 1/8”“
