Wednesday, June 25, 2014

Test Tube Stirling - 2 Cutting The Glass

I remade the one I cracked and I shortened the displacer by about a half inch. I'm getting pretty damn good at cutting this glass! Ill take this post to briefly explain how I cut them on the lathe with a diamond file and to show the reworked and new parts.

Masking tape is the biggest thing you can do to help yourself since until a groove is established the file wants to slide all around. The gap I leave is not much wider than the width of the edge of the file I use.

Chucked up in the lathe and ready to go, I turn at about 1200 rpm and use no lubricant/coolant.These are the two files I usually use, though I do use a flat one sometimes if the edge is jagged from chipping or breaking catiwampus, the flat file lets me bring it all flat again relatively easy. The one I cut with is a knife shaped one with a fairly fine "cutting edge" of about .03"(I put that in quotes because technically the whole file is a cutting edge since its coated all over). I use the round file to clean up the inside edge after the cut is complete. You can get a cheap set of diamond files for $10 online or at Harbor Freight. It cuts this borosilicate glass great and they're also great to have around for doing detail when shaping a HSS tool bit or just putting a nice radius on a newly ground tool tip. They're nice to have around, get some!

You never want to put any pressure on the file besides what is necessary to keep the file flat and in contact with the workpiece. If you're impatient this may be a bit of a challenge to not put any pressure at all on it, but trust me, its cutting and after a couple minutes itll just fall off nice and easy and there'll be no chips, cracks, jagged or sharp edges, just a beautiful smooth edge. I polish the edge with some 220 grit emery cloth to finish up, seems to work pretty good.

Here's some more pr0n-

This will be the power cylinder. I did put a scratch around this one on the one end. Taping it up good is necessary unless you can manage to not deviate at all from where you first put the file, and before a groove is established that's pretty much impossible.

Here's the reworked displacer. I shortened it by ~.5". On the left is the cut end of the new main cylinder, the one I cracked in the previous post. Look at that beautiful edge! 

Here they are they way they will be assembled in the end. The fit is so close that dropping the displacer, even with the heavy shaft attached, it comes to a fairly soft stop. It's not so much that friction or drag will be an issue, but just the perfect amount, .006" per side, .012" total, to be exact.

Here's a sketch that is still evolving, but it does show how I intend the main body to be laid out. The flywheel will be situated in between the shaft from the displacer and the power piston on the left. I will provide actual cad files for things soon as I need to stop using my notebook for EVERYTHING. I know how to use the software, I just need to install it and do it. But, for now, this will give you some idea of the direction Im headed with this thing.

This is gonna be a pretty fun one to get going. Stay tuned!


  1. You have a way of making a benchtop lathe look like something no one should be without.

  2. That's because they shouldn't. There really is no excuse :^D