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Don’t Trust Nuthin’

Posted by Tom Benedict on 03/09/2010

I’m getting seat time on some new tools at work, so I’ve had a chance to try to catch up on some long-standing projects.  One that I’m eager to get back on involves mounting a cryocooler on one of our smaller test cryostats.  It started life as an infrared camera, but several years ago it was recycled into a test cryostat for a new infrared detector.  This involved gutting it of all mechanics and optics, and basically turning it into a cold optics bench, complete with a 1/4″-20 screw hole pattern on 1″ centers.  We added a cold shield, installed baffles for the two rotary feedthroughs, the whole nine yards.  We really just wanted to characterize a new detector, but by the time we were done we had a general purpose cold optics bench and enough bits and pieces to build a complete infrared camera.  We could’ve bolted it to the telescope, but that was never its purpose in life.

Thank goodness we made it a general purpose tool.  Once again we need a cold bench, and once again we’re pulling it off the shelf.  Only this time we’re trying to move away from liquid nitrogen cooling and use closed-cycle cooling instead.  Some months ago I did a poster presentation at SPIE covering our conversion of one of our science cameras to closed-cycle cooling using a Polycold Compact Cooler (PCC).  When we did that conversion we picked up a full set of spares, and a second full set for lab work that we got off of Ebay.  (Yep, even science nerds buy hardware off of Ebay.)  The idea is to use our Ebay set to convert the cold optics bench to closed-cycle cooling.

Luckily this cryostat has all sorts of nice service hatches you can pull off and work with.  I popped off one that had enough surface area to mount the PCC cold head, and used that to get some seat time on the new tool.  It needed a clean face for mounting the cold head (0.020″ deep circular pocket clear with final outside clean), a clear bore (0.550″ deep circular pocket clear with final outside clean), and a bolt hole circle (drill cycle with a spot drill followed by a peck drill cycle with the tap drill).  I can’t say the machining was relaxing, but it went pretty quickly.  The seat time is paying off.

When it came time to tapping the holes, I went back to the cold head we had sitting on the shelf.  It had a #10-24 screw through it, so that’s what I used: #10-24.  The body tube bolted straight on, but when I tried to put the cold head in, the #10-24 screw jammed.  !!!!!  Turns out it’s threaded #10-32, but someone had driven a #10-24 screw through it.  This did a number on the thread, but nothing some cleanup work with the #10-32 tap wouldn’t fix.  The cold head and body tube cleaned up nicely, but the mating holes on that access panel were tapped to the wrong size.  Grrrrr!

Can’t trust nuthin’…

In the end we opted to keep things the way they are and just mark the two sets of bolt holes.  One is marked #10-24, the other is marked #10-32.  Same tool, totally different screws.  It’s aggravating, but that’s life.  Next time, I’ll check.

I hope to have the test cryostat bolted together some time next week.  So there should be some pictures to share.  I probably won’t be able to show the machining job since it’ll be buried, but a picture of the cryostat itself is worth a thousand hand-waving descriptions.

The part that really makes this project fun is that the PCC can only get down to about -150C the way we have it set up.  We need 77K, or -196C.  That’s 46C more than the PCC can give us.  We’ve got a test plan in place, and should know some time in the next two weeks whether it’s going to work, or if we’re just plain stuck.

– Tom

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