Intro: Optical Center Punch
The following instructable shows you how to make a non-slip optical center punch. This is a tool that lets you make pilot indentations in materials prior to drilling, so that the drill hole comes out centered. You will need:
- 1/16 inch Center Punch
- Acrylic or Polycarbonate Sheet
- Right Angle Reference
- Silicone Caulk for Plastic
- Ultra Fine Sharpie Compass
- Brayer (Available at Craft Stores)
- Wax Paper
Step 1: Transparent Square
Choose an optically clear transparent plastic for the body of the center punch. I happened to use a:
- 3 inch by 3 inch by 1/8 inch piece of Acrylic
The piece should be as wide as the largest center you would expect to center punch. I would recommend choosing something thicker than 1/8 inch because it is rather flimsy. A 3/8 or 1/4 inch thickness is probably much sturdier. I would also recommend using polycarbonate because it is a more forgiving plastic.
More important than the material is making sure that it is square:
- Use a right-angle triangle or straight edge to check for square.
- Remove excess material using a hand plane.
When this is done, draw an X from corner to corner on the non-protected (clear) side of the plastic.
Step 2: Mark and Drill
In this step, form the grid and center hole:
- Indent the center of the plastic using sharp point.
- Score from the center to the edge in 1/4 inch increments.
- Draw concentric circles at the center marks.
- Drill a 1/16 inch center hole for the punch tool.
Step 3: Glue Base
In this step, secure the plastic piece to a base for further gluing with Silicone.
- Apply contact cement to the glue base, a piece of scrap wood, MDF, etc.
- Apply contact cement to the paper / protected side of the plastic sheet which can be removed later.
- Wait 5 minutes.
- Join the two glue surfaces.
Step 4: First Layer
The reason why most plastic glue joints fail is because the plastic contains low surface energy. This property can be seen by running water over the plastic. If the water beads up and rolls off, it has low surface energy. If the water sticks, then the plastic is a suitable target for gluing.
In this step, the silicone caulk is added. It's essential to follow these steps or the low surface energy of the plastic could cause the caulk to peel off.
- Use silicone caulk specifically designed for plastic. Unlike conventional silicone caulk, it has additives which increase the surface energy of the plastic, hopefully resulting in a better joint.
- Flame activate the surface of the plastic to increase the surface energy. If you run a flame over the plastic, it can increase the surface energy. This activation dissipates over time, so as soon as the plastic cools, apply the caulk.
- Apply a glob of the caulk to the plastic.
- Place wax paper over the glob.
- Use a brayer (rubber roller) to flatten the glob.
- Let it dry for 1-3 days.
Step 5: Additional Layers
The first layer must be special silicone. Additional layers can use conventional silicone caulk. To add them:
- Remove the old wax paper.
- Use a razor to cut away excess caulk at the edges.
- Add glob of regular silicone to the center.
- Place wax paper over glob.
- Flatten with glob with a brayer.
- Allow 1 day to dry.
- Remove the wax paper.
I would recommend 2-5 additional layers. The silicone layers provide the following benefits:
- Protects the grid from wear.
- Grips the work piece prior to center punching.
The drawback however is that silicone is opaque, so the more layers, the harder it becomes to see what's below the grid.
Once you are satisfied with the silicone thickness, remove protective paper backing and use the 1/16 inch center punch to poke through the silicone in the center hole. Do this from the silicone side, so that you don't accidentally peel it off.
Step 6: Use
To use the center punch:
- Use the concentric circles for alignment on circular objects.
- Use the X for alignment on rectangular objects.
- Use the 1/16 inch center punch to create a center indentation.