Dicing with Delrin - A Repair Job.

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Pete W.:
As I said to Bob, I've made a bit more progress even though Mr. Bozo got in on the act! :bang: :bang: :bang:

The first photo (see below) shows the drilling template and four of the five dominos after drilling. Those reading this who are experienced in making jigs & fixtures will probably have anticipated the problem I experienced. You will notice that three corners of the template are rounded while the fourth corner is left sharp - I did that to denote the reference corner and the associated reference long edge. Aligning the template with each of the dominos in turn seemed to make me all fingers & thumbs and I had to juggle a bit to get the domino & the template properly aligned without the tool-maker's clamp obstructing the hole positions. I should have designed and made a more complex jig with stops to press the dominos and the template against to ensure accurate alignment. It didn't seem worth the effort for only five-off dominos. The hole marking the eventual position of the bored hole for the optic is a nominal 0.100" while the other two are a snug 4 BA clearance.

After I'd drilled all the holes, I recognised Mr. Bozo's contribution - the hole pattern is 100 thou too far from the edge with the tongue. Oh, well, at least they are all the same and I think the mounting to the head-band will enable the optics to still be set to eye level.

The next operation was to make a boring fixture. I cut a square of 5/16" hard aluminium alloy plate and milled the edges to clean up after the hacksaw. I made a slight error with the last side as you might be able to see in the second photo - the diagonal scribed line doesn't quite end at the corner! Then I used the drilling template to position a set of holes astride the diagonal with the centre hole over the centre of the fixture. I opened two of the holes and tapped them 4 BA. I was concerned that the tongue end of the domino would need some support in case the domino tried to 'walk up' the drill or boring tool so I contrived the clamp shown in the photos - it's cut off the end of a computer hard drive slide, 'waste not, want not'! The 'back end' of the clamp rests on the head of another 4 BA screw while the clamp is held onto the domino with another 4 BA screw.

The next operation is to mount the boring fixture in the four jaw chuck and set the centre hole to run true, then to mount each domino in turn and bore the stepped hole to take the optic. I've stamped each domino so I know which face to bore. Before I can do that, I have to make a 'wobble bar', á la Doubleboost. Once that's all done, I have to profile the parts to (approximately) match the shape of the original mount, I think that's going to be a bit of a challenge!

Through all these stages, I've found my optical centre punch to be very helpful.

Right, photos:

tom osselton:
I found this the other day have you thought of casting it with plastic resin?
I'm going to have to try this! :beer:

SwarfnStuff:
Interesting Link Tom, duly bookmarked in the "try this perhaps" folder. Could be useful in getting impressions of fiddly bits. Or just to mess with? :scratch:
John B

tom osselton:
Thats what I thought I even bought some corn starch the other day and still have a half tube of silicone sitting in the kitchen.

SwarfnStuff:
Ha ha, Last tube of silicone I bought had set solid in the tube when I finally needed it. :bang: Thought I would have to bodge something up, fortunately a mate had something that did the job.
John B

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