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I made the rods for the displacer pistons and pressed them all together a couple of days ago. As far as I can tell, that all worked okay. I did put a bit of JB-Weld on the parts before pressing them together, just for insurance against leaks. With the rods held in a collet in the lathe, I measure maybe 3-4 thousandths of runout at the far end of the pistons. So maybe there is some hope that they won't rub on the inside of the cylinder.
Then I spent way too much time cutting out a bunch of gaskets. The white ones are the thick parts that thermally insulate the hot end of the displacer cylinder from the base. I wound up using Teflon for these. We'll see what happens with that.
-- Russell Mac
gaskets.jpg
In theory, there should be no problem with Teflon. It is a durable and high-quality material. I like Teflon gaskets more than rubber or silicone gaskets.
Yesterday I somehow got the idea that the rod on one of the displacer pistons wasn't screwed in all the way. So I chucked the rod in a collet, gave the piston an overly zealous twist, and broke the rod off right at the point where the threads end!
I'll be back in a couple of days after I've made a new piston.
Russ Hanscom wrote: ↑Wed Mar 24, 2021 9:02 am
Why not remove the broken stub and save the piston? Challenges like these help build skills and character.
Well, I suppose nothing's impossible, but I'd say that saving the piston comes pretty close.
To start with, the 1/8" diameter rod broke off where the green arrow is pointing, about 1/4" from the bottom of a 5/8" deep hole. The drawing doesn't show it, but the part that remains is threaded. I don't know any way to even try to unscrew that little stub.
The next best thing might be to try to disassemble the piston and at least save the tube and the (blue) end cap. But, both ends are press fit and JB-Welded into the thin walled aluminum tube. Again, I don't know how to try to take that all apart without destroying it.
So I hope to build skill and character by just starting over on this assembly. I've got the time and aluminum is cheap!
Russ Hanscom wrote: ↑Wed Mar 24, 2021 5:59 pm
Make a guide bushing and drill a small hole into the stub. Try an easy out, or left hand drill or tap to remove the stub.
Oh, wow. That sounds so easy! I wouldn't have thought to "make a guide bushing" in a million years. Thanks for the hint.
I'm not finding left hand taps or easy outs that small online, but the left hand drill sounds like it might do the trick. Or maybe even something homemade with a tapered, square-cornered end that would jam down inside the hole enough to get a bite and unscrew the stub.
Again, thanks for the idea.
-- Russell Mac
Last edited by rmac on Wed Mar 24, 2021 11:14 pm, edited 1 time in total.
NP317 wrote: ↑Wed Mar 24, 2021 11:09 pm
For that same issue we bored out the entire top cap and installed a new one. That saved the rest of the displacer piston.
So I'm not the only one! That's comforting. Sort of.
New question: The plans for the Moriya engine contain this note in the section discussing the gland:
Dr. Senft wrote:
The central hole must be concentric with the portion to be turned. Since twist drills tend to wander--especially when they mustn't--it is worthwhile to make up a .120" diameter D-bit and follow with the reamer.
I tried to make a gland yesterday, ignored this warning, and just drilled the hole with a short, stiff screw machine drill. Sure enough, the drill wandered and I wound up with a crooked hole. It's bad enough that I don't think it's usable.
Did your students make D-bits? Are there other ways to solve this problem? I thought about drilling the hole first, then turning the outside on a tiny mandrel held in a collet and supported by a center in the tailstock. Does that sound reasonable at all?
Simple. Drill undersized, then bore to establish concentricity. You can then either bore to size, or open the bored hole to the desired size using a drill or reamer. Unless you have a badly damaged drill, you shouldn't lose concentricity establish by boring.
A tip when drilling. If you hope to have a drill cut size, open the hole so it's just a few thou smaller than the target diameter. When a drill removes a small amount of metal, it will pilot on the margins, almost a guarantee of size.
H
Wise people talk because they have something to say. Fools talk because they have to say something.
RMac:
We simply drilled them undersized and reamed to finish. I don't remember having concentricity issues.
I DID carefully prepare the drill for working with the brass, by flattening the cutting edges with a diamond flat file.
This might be a helpful solution.
And our most common problem with the top caps for the displacer piston was students stripping those internal threads in the aluminum from over tightening!
I was surprised you broke the rod! Hmmm...
RussN