How do I turn very thin cylinder

[Deferr]

I am starting to build Senft's "Tapper" but I need help on how to turn an unusual part. The part is stainless steel. I have attached a sketch of it. How would one go about this? I'm worried about the tube supporting itself during turning due to the very thin wall. Let me know what you guys think.

View attachment HotCap.bmp

 

[mu38&Bg#]

Turn the ID, put it on a mandrel, turn the OD.

 

[b.lindsey]

If you have enough extra stock length you may consider boring the id first as noted already, then turning the od in small increments while the stock is still chucked in the lathe. That will assure concentricity between the id and od. In addition, keeping the larger diameter end of the cap towards the chuck, turning from right to left down to that shoulder will be eaiseer as well. Once you are happy with the results you can part the cap off from the remaining stock and face the closed end to the final length required. This worked well when I was building Jerry Howell's mini sterling fan. The stiffness of stainless is an advantage too.

Bill

 

[rake60]

I would try to do it in one set up and part it off as Bill describes.
I wouldn't part it completely off. If it takes flight there is little
chance of it landing softly enough to stay round. Leave center
nub of about .100" and it will break off easily by hand.
A better option be sawing it off then chucking it up in a set
of soft jaws or a collet to finish the closed end face.

My greatest concern would be heat. With a wall thickness of .015
it is going to warm up quick and could easily warp out of round.
Flood coolant might help.

Chatter may also become a problem.
I have found that packing the bore of a thin walled part with
a piece of wet rag will help to dampen the chatter when turning
the OD. Just be sure to pack the rag so there is no chance of
the turning tool grabbing it.

Rubber banding wrapped around the OD will usually help control
chatter when cutting the bore in a thin wall piece.

Just thinking out loud (Ok, with the keyboard) here.

Rick

 

[nemt]

Hello Rake,

In the past I have turned very cylinders by making the bore first, still having a rather thick wall.
Then I turned up a mandrel rather precise and locktited the cylinder on it.
Turning the thin parts is no problem at all.
After finishing the cylinder, heat is applied and the locktite loosens. Get the cylinder off, and there you are!
Wall thicknesses of (sorry, I'm metric) of 0.2 mm is no problem. Stainless steel works well in this manner.

Greetings, .
Nemt.

 

[Kermit]

I'm very interested in this process, as I think it is what I will have to do with my old genny build. Forgoing the segmented plates by using iron powder(blah blah blah) ;D
anyway, I will need a non magnetic shell of sometype, stainless, brass, and these suggestions are very timely indeed.

Thanks everyone, and especially you Deferr, for starting this one. I'm taking notes.
Kermit

 

[rake60]

0.2MM is roughly .008"
That is impressive! I have never used a solid turned to fit mandrel.
My experience with mandrels has been with the expanding type.
Almost every time the material heats up, grows a little and the grip is lost.
Then the work piece spins loose resulting in me throwing things across
the shop. :-\

I'm still learning everyday.

Rick

 

[Shopguy]

Deferr
I've had success using seamless stainless steel tube, filling it with solder, turning to outside dimension then melting the solder out. 50/50 solder doesn't stick to stainless. I discovered/used this when building a stirling engine.
Regards
Ernie J

 

[DavesWimshurst]

Deferr,
I used wlindiii's method when I built this engine, I used free machining stainless (303) for the part and had little trouble except for one detail. When I parted it off from the bar it left a fairly large pip. I tried to machine it off in one pass and the thin wall top of the cylinder wouldn't stand the strain and popped a hole in the top. A silver soldered patch lasted one run. The next more carefully made cylinder has been demoed once a week for several years now. If you want to, or need to, use a mandrel an expanding mandrel like this has worked well for me. Probably not dead on for accuracy but not too bad:

The tapered headed bolt is tightened after slipping the closed ended cylinder on and then the mandrel is chucked up.
The mandrel is made by boring taper for the bolt head, the outside turned to the nominal size, and the saw cuts made. You can use a slitting saw in a mill if you're overly fussy.
I use a band saw and just eyeball the cut down the center. :
I hope this helps.
Dave

 

[shred]

Jerry Howell has a bit on his website that I used for my small sterling fan-- Bore the hole, turn up a rod to fit inside it with a center hole in one end, then bring up the tailstock and finish the outside thin part.

http://www.jerry-howell.com/HotCap.html