Taking Time

I'm back to making parts at work. The past two weeks I've been designing and making parts for a temporary instrument. It's only likely to be used a handful of times, but its purpose is to demonstrate whether a particular idea works. So the results from the experiment could steer the course for future instrument development. It's been a fun project.

Today I made a part for a completely different instrument, one of our permanent ones. Every time it's taken down and set back up, there's an alignment that needs to be done to better than 0.7 microns. (Yeah, better than the wavelength of red light...) Oddly enough, the instrument had no facility for making this adjustment! The "proper" method was to knock a mirror into position, then fine tune it with the tension on the clamping screws. Ergh...

We've been kicking ideas around for a while, and a couple of attempts have been made, none all that successful. But today I took the drawings for the latest design and parked myself out in the shop. It was a good day.

The part's basically a ring with three clear holes and three threaded holes in the front face. So far so good. Some time on the lathe making all the different profiles on the part and some drill work on the mill, and the basics were done. For the next trick, it required four holes spotted, drilled, and tapped, at a 38 degree angle from horizontal, all around the periphery of the part.

It's really tempting to take shortcuts. Really tempting. But this time I resisted. Rather than file the bevels on the edges of the part, I used the cross-slide and did it right. Rather than try to C-clamp the thing down for drilling, I made a fixture and held it down with a screw. Rather than try to fudge that 38 degree angle, I made a gauge block and used that to set the part up in all four orientations. And rather than trust to fate with a center drill, I actually stuck to the drawings I'd made and spotted a 5mm flat at the location for each hole.

While I was at it I added some witness lines on top of the part so you could tell what axes the push-push screws were going to move the mirror. In the end it wound up actually looking nice!

But the proof is in the pudding. That instrument is coming off tomorrow, so we should have some opportunities to test it in the not-so-distant future. I hope it works. This has been a major thorn in my side. I have one of the other failed attempts sitting on my desk at the moment. It's a removable kinematic mount design that should've worked, but didn't because I took a shortcut with the geometry and screwed it up. No more short cuts! Take the time and get it right!

Tom