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5.5 Layout: Arcs

D.M. Donner

The other type of layout line is the arc. An arc is any portion of a circle. When working with arcs, we measure radius. There are specialty tools for creating arcs, and there are simple tools we can use to create arcs for layout. Let us begin with purpose-built tools first.

Divider

A layout caliper has two legs with points connected at one end and held apart with an adjustment screw.
Figure 5.31. A layout caliper has two legs connected at one end and held apart with an adjustment screw. / Image Credit: Damon Donner, CC BY 4.0

A divider is a tool with two equal length legs with pointed ends.

A close up view of the two points on a divider.
Figure 5.32. Two pointed legs on a divider / Image Credit: Damon Donner, CC BY 4.0

The tips of a divider need to be sharp and pointy. In the figure above, maintenance has been performed over the years to keep the points sharp.

Opposite the sharp tips, the two legs come together at a pivot point. The leg distance is adjusted by tightening or loosening the adjustment nut to bring the legs together or farther apart.

The two points of a divider are placed on a ruler with one point on zero and the other on the one inch line.
Figure 5.33. The two points of a divider are placed on a ruler with one point on zero and the other on the one inch line. / Image Credit: Damon Donner, CC BY 4.0

A ruler is used to set the distance between points. This distance will equal the radius of the arc. Once again, sharp points will aid in accurate settings.

A close up view of material displaced from the point of a punch when struck with a hammer.
Figure 5.34. A close-up view of material displaced from the point of a punch when struck with a hammer. / Image Credit: Damon Donner, CC BY 4.0

A punch is used to create a conical divot at the center mark of the layout lines. A punch is a tool that is designed to be struck with a hammer. The term conicalmeans cone-shaped, such as an ice cream cone. One point of the divider will reference this punch mark as a center for the arc scribe. This punch mark is rather heavy because it was made with a center punch. A prick punch leaves a smaller divot and is easier to align with the layout lines.

A divider point placed in a punch indent scribing an arc with the other point.
Figure 5.35. A divider point placed in a punch indent scribing an arc with the other point. / Image Credit: Damon Donner, CC BY 4.0

Gently turn the divider around the pivot point of the punch mark to create a clean radius. Dragging the points a couple of times will create more contrast, but be careful not to damage the lines from multiple attempts.

Trammel

Two trammel points clamped to a straight piece of stock.
Figure 5.36. Trammel points being set with a ruler / Image Credit: Damon Donner, CC BY 4.0

A trammel shares some similarities with the divider in that it uses two points to create an arc; however, a trammel is used for arcs that are too large for smaller tools such as a divider. The trammel’s points are mounted on a beam. In the figure above, the points are set larger than 5 inches and are only limited by the size of the beam.

Gage Pins

What type of circular equipment do we have in abundance in a machine shop? Gage pins. Gage pins are accurately sized and almost infinite in their selection, usually up to one inch.

A set of gage pins in a plastic case.
Figure 5.37. A set of gage pins in a plastic case. / Image Credit: Damon Donner, CC BY 4.0

When using a gage pin to lay out an arc, remember to select the pin for the radius of the arc.

A scribe creating a layout arc using the surface of a gage pin as a guide.
Figure 5.38. A scribe creating a layout arc using the surface of a gage pin as a guide. / Image Credit: Damon Donner, CC BY 4.0

A gage pin is not easy to hold still while tracing the circumference, and the ability of the operator to align the pin with the layout lines also takes some care. Use a gentle touch when scribing the lines to prevent moving the pin.

Radius Gages

Radius gages are another tool at our disposal, but there is a concern we need to address. We are using measuring equipment to perform layout. This is not an accepted practice unless the equipment has been set aside for this purpose. Check with your supervisor before using any inspection equipment to perform layout.

A radius gage blade placed on a part that has layout dye applied and a scribe applying an arc along the edge of the radius gage.
Figure 5.39. A radius gage blade is placed on a part that has layout dye applied. A scribe is drafting an arc along the edge of the radius gage. / Image Credit: Damon Donner, CC BY 4.0

Select the correct radius and place it on the reference marks. In the figure below, two straight lines are laid out with a radius where they meet. The radius gage is much easier to control than a gage pin during the scribe process.

A close up view of two different layout line arcs.
Figure 5.40. A close-up view of two different layout line arcs. / Image Credit: Damon Donner, CC BY 4.0

The arc to the left in the figure above was created with a gage pin, and the arc to the right was created using a radius gage. More pressure could be applied with the radius gage, which permitted a deeper scribe.

Attributions

  1. Figure 5.31: Divider by Damon Donner, for WA Open ProfTech, © SBCTC, CC BY 4.0
  2. Figure 5.32: Two points on a divider by Damon Donner, for WA Open ProfTech, © SBCTC, CC BY 4.0
  3. Figure 5.33: Divider legs set to 1″ using a ruler by Damon Donner, for WA Open ProfTech, © SBCTC, CC BY 4.0
  4. Figure 5.34: A punch mark by Damon Donner, for WA Open ProfTech, © SBCTC, CC BY 4.0
  5. Figure 5.35: Arc scribe with divider by Damon Donner, for WA Open ProfTech, © SBCTC, CC BY 4.0
  6. Figure 5.36: Trammel points being set with a ruler by Damon Donner, for WA Open ProfTech, © SBCTC, CC BY 4.0
  7. Figure 5.37: Gage pins used for laying out arcs by Damon Donner, for WA Open ProfTech, © SBCTC, CC BY 4.0
  8. Figure 5.38: Using a gage pin to create an arc by Damon Donner, for WA Open ProfTech, © SBCTC, CC BY 4.0
  9. Figure 5.39: Radius gage to layout an arc by Damon Donner, for WA Open ProfTech, © SBCTC, CC BY 4.0
  10. Figure 5.40: Arc payout lines by Damon Donner, for WA Open ProfTech, © SBCTC, CC BY 4.0
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Introduction to Machining Copyright © by SBCTC is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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