Machined Brass and Pearl Knob

Not long ago, I built a custom DC power supply. The knob that came with it was kind of boring, so I set out to make a unique, machined knob for it.

I wanted to incorporate some type of decorative front face and went with black mother of pearl.

Here's what you'll need:

Materials:

-Brass bar stock - https://amzn.to/49AbpiT

-Mother of pearl - https://amzn.to/3NoCsWU

-Delrin - https://amzn.to/4qOz1Y3

Tools:

-Two-part epoxy - https://amzn.to/4sDIsv2

-E6000 - https://amzn.to/4jIwFrw

-Hot glue - https://amzn.to/4552jcG

-Sand paper - https://amzn.to/3NoH6UQ

-Cotton buffing wheel - https://amzn.to/3LCsl02

-Polishing compound - https://amzn.to/3ZakfyY

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Here are the steps I took:

1. Machine the outer diameter to size
2. Bore the cutout for the pearl
3. Cut a 5-degree taper on the outside (drawing shows 2 degrees, but that wasn't enough)
4. Machine "knurl" on OD.
5. While not a true knurl, it gives grip and looks cool.
6. I used auto-feed and 20 TPI to achieve this look.
7. Part off
8. Machine a new part to hold the knob so the back side can be machined
9. I machined an aluminum shaft to glue inside the pearl bore
10. This should be machined to be a nice slip fit to locate inside the recess for the pearl
11. Clean both pieces thoroughly and glue to the front (I used E6000 glue)
12. Drill and bore the opening on the back of the knob for the spline adapter
13. To remove the knob from the knob holder, I had to:
14. Part off the holder
15. Make a "cup" to hold the knob from the front, but have an opening to let the holder be pressed through
16. You'll then need to press the press the holder out, through the spline hole
17. The CAD model shows this as a blind hole, but I accidentally drilled all the way through, so I was able to press the holder out this way

1. Spline adapter is #2 in the image
2. Use Delrin plastic for this piece
3. Machine the OD to give a light press fit to the knob bore (0.001" over)
4. Measure your spline OD on the device your knob will attach to
5. Bore your Delrin ID to slightly under the spline OD (0.002-0.005" under) - you may need to experiment
6. This will be a press fit, and the spline will "form" into the Delrin
7. My spline adapter was 0.25" long

CAUTION - Pearl dust is hazardous to lungs. Please use a respirator when doing any cutting, grinding or sanding with this material!

I tried a few methods before arriving at one that worked well for me. Pearl is brittle and mainly sensitive to two things: heat and shock.

1. I ended up using a "live" tool in my lathe, with a 1/8" 4-flute end mill to cut it.
2. I ran this at about 20k rpm and used water to help cool and keep dust down.
3. The end mill was perpendicular to the face of the pearl.
4. The pearl I have was an odd shape, so my work holding method was unique
5. I used a brass rod, and hot-glued the pearl to the face of the rod
6. The rod was heated with a torch and small piece of hot glue was added to the end
7. While still hot, the pearl was added and then push flat against the rod by the tailstock
8. Light pressure was held against the pearl while the glue was cooling
9. This method worked surprisingly well - I thought it would have come loose during machining
10. I think the KEY here is heating the brass rod with a flame when the glue is applied and not just applying hot glue with a hot glue gun.

1. Rough up the inside surface where your pearl will glue in - I used a dental pick
2. Clean with isopropyl alcohol and allow it to dry
3. Rough up the back of your pearl, and then clean as well
4. Mix 2-part epoxy and apply a small amount to the inside of knob
5. You will want your pearl to stick slightly above the knob face - this will allow you to sand it flush
6. I 3d printed a small spacer to put behind the pearl while gluing it
7. Clear epoxy will work best if you do not want to see a different color between the knob/pearl gap
8. Once dry, sand with increasing grits of sandpaper, starting around 400 grit. I went up to 3000 grit.
9. Wet sanding will help remove dust and make sanding a little easier
10. After sanding, I buffed using:
11. Cotton wheel on a flex shaft (low rpm)
12. White compound