Another silly question...

[Allen Smithee]

So I'm progressively accumulating the bits I need to do the coffee-cup stirling engine I've chosen as a practice project. This engine has a perspex displacer cylinder with aluminium disks above and below, and the disks have a 3mm groove cut in the face to seat the perspex against a soft gasket. It occurred to me that cutting face-grooves is on the list of things I've never done before*.

Thinking from scratch leads me to two tool designs, one made from a bit of round toolsteel and the other from a bit of square. Am I on the right track or is there a better way of doing this?

AS

*This list is rather a long one

[Jasonb]

Thats the right idea, plenty of clearance so the tool does not rub on the outer edge of the groove. I usually grind from square but no reason not to use round. Infact you could just grind right across a bit of small round, so long as its held rididly with minimal overhand it will do the job.

J

[Vixen]

Allen
Do you have the equipment necessary to mount the discs on a rotary table and mill the groove with a 3mm mill cutter? Alternatively you could mill the groove with a spindle on the lathe saddle and the disc on the faceplate. It all depends on what equipment you have or intend to get.

Mike

[mklotz]

I've built several of this style engine and never grooved the aluminum disks.  Assuming that the perspex has been faced flat and smooth and the sisks are pulled onto it with sufficient non-heat-conducting bolts, leakage is minimal.  I generally coat the joint with a thin film of Vaseline or similar for even better sealing.  Remember that the pressure in these engines only deviates slightly from atmospheric so sealing needn't match steam engine standards.

[Jasonb]

Held in the 4-jaw and turned should be OK this is 3mmish trepanning cut in a 6" steel disc 5mm thick



And I found a picyure of the tool I use. 8mm deep into brass

[Allen Smithee]

Thanks chaps. I thought about milling it in a rotary table, but I'm too lazy and impatient for that!

The mental image I had was a parting tool with a circular clearance on one side - ie a tool that's intended to cut across the whole width of the front edge rather than just one corner. But I thought grinding it from a bigger chunk of toolsteel would be more rigid.

Jason - yes, that's exactly what I had in mind when I sketched the circular one. A piece of round toolsteel with the front ground off twice (at <88degrees to eachother) and then some top-rake added as it's going to cut aluminium. Held in a boring bar holder.

AS

[Vixen]

Listen to Marv

[b.lindsey]

Allen, you might consider whether or not the grooves are even needed. Both of the LTD stirlings I have made did not have grooves, just a nice flat finish on the aluminum plates and an equally good flat finish on both sides of the displacer cylinder.

Bill

Edit: Sorry, got distracted while writing this and didn't notice Marv's post saying the same thing.

[Bertie_Bassett]

id agree with not worrying about the groove, unless you want some practice??

I only grooved mine as I made the cylinder from glass which didn't come out smooth, sealing was done with a  thin smear of clear silicone.

[Allen Smithee]

Interesting thoughts which I'll ponder while I finish the drawings in Solidworks. I'm basiung the design on Jan Ridders one, but adapting bits to suit the paterials and tooling I have available (and changing the con rods from brass strip to carbon rod woth ball-raced ends). I find transcribing the drawings is the best way of understanding a design, especially of I'm modifying it.

I originally intended to do the displacer cylinder from glass by cutting a slide outof a caffetiere jar - in a local shop I can but a replacement jar which is 100mm dia with 1.5mm walls for a couple of quid, and has enough length to have at least six attempts! But being pyrex rather than glass I lack confidence in cutting it. I don't think the thermal and physical (scribe & tap) shock methods would work on it, so the only way I could think of cutting it was to make a roller jig to accurately run a dremel cutting disk round and round until it broke through. I'm told you can then reflow the rim with a small blowtorch, but min is probably too big and would heat far too much of the glass. So I chickened out and bought some perspex tube!

AS