Holding a small part

[trlvn]

If I haven't said it enough, I'm a newbie. I've turned a boss that I need to part off and then finish the back:

The finished boss will be about 3/8" high. To finish the back, I believe I would have to hold it by the 0.090 flat that is on the near side in the picture. I don't want to mar it while I face it to the finished thickness. The part is 1.248" in diameter if that matters.

I have 3-jaw and 4-jaw chucks and a face plate. I don't have soft jaws.

Suggestions on how I can mount this securely, parallel to the currently faced end, and without marring the part?

Craig

 

[DPittman]

I don't know if you've tried it but parts can be held together with crazy glue for light turning cuts. You then use heat to break the bond to release. I use loctite also for the same purpose but it is more expensive.

 

[trlvn]

I don't know if you've tried it but parts can be held together with crazy glue for light turning cuts. You then use heat to break the bond to release. I use loctite also for the same purpose but it is more expensive.

Did not think of that! Might work. I guess in the worst case the part pops off but it is fairly small and I'll be running at pretty low RPMs to start with. I suppose both surfaces would have to be pretty smooth for this to work.

If I try to hold it in my 3-jaw, what kind of material should I use to protect the finished surface? Is the aluminum from a soft-drink can too thin? What do you guys use?

Craig

 

[DPittman]

Yes smooth clean surfaces are a must and more surface area the better. I use beer/pop can aluminum in jaws and find it amazingly consistent in thickness.

 

[Johnwa]

I made exactly the same piece for my southbend. Darned if I can remember how I did it.

Maybe I had the dovetail the other way. That way I could finish of the top without removing from the chuck. It could then be parted off. I don’t think the bottom needed anything other than burr removal.

 

[PeterT]

If you mean you are wanting to face the opposite side of the 'thick washer' to 0.090"? One issue is you don't have a lot to grip onto so light cuts are needed. A 4 jaw would be 1 better than a 3.
Another is aligning it to the original face. Some guys use spiders which fit between the jaws & the face of the part butts up against it for these instances.

Method 1
Use a scrap piece of the same diameter as washer (maybe off cut from the same operation). Turn a good end flat & true. CA glue the washer to the end so they are 100% flush = concentric. Chuck jaws grip both the scrap & washer with just a bit sticking out for finishing. Technically you are not putting full clamp on the washer but might be enough to turn safely. But this helps eliminate part wandering. Some guys will put a strip of tape on the joint, not to hold it or prevent marring but to put ever so slightly more clamping pressure on the washer part.

Method 2 pot chuck
Chuck a piece of scrap aluminum. Turn a relief hole maybe .050" deep & couple thou larger than your disk diameter. Use the CA glue method to encapsulate the steel disc. Now you have much more lateral support for cross feed cutting. I keep a short chunk of sacrificial aluminum for exactly these finicky thin washer jobs. One warning - it can stay attached 'too good' & be difficult to remove even with a torch because you cant get under the edge to pry off. So might be advisable to have an enlarged center hole so you can pop it from behind with a dowel once heated. I have not found that CA debonders or acetone work very well. It just cant penetrate a the glue joint. Heat is best. Aluminum expands more than steel which helps to part the two.

 

[Dabbler]

Craig, this may not help much, because it involves starting from scratch... but here goes. I would make the part with the flange being 1 1/2" long, turn it around, finish the part, then part it off.

This extra metal is referred to by some as a 'holding feature' Joe Pieczinski on youtube calls them 'disposable features'. It is common to machine a part with features you can hold on to until the very last operations. You can see his suggestions on this at:

 

[trlvn]

Thanks for all the suggestions, guys. Lots of ammunition for future work.

For this part, I decided that jaw marks were not going to be a big deal. (And they turned out to be negligible.) One of my concerns was parting off. I've had really bad chatter and crappy finishes previously. This time, I came up with a slightly different approach. When I got the lathe, it had a 4-way turret tool post (homemade) that I haven't been using. I found that one of my parting tool holders fit in it and I could get the cutting edge on centre with just a little grinding. It is solid steel and goes on in place of my compound. This eliminates the play in my compound which is a weak spot at the moment.

Long story short, this was a big step up in parting off for me. Slow, but a very nice finish. I could have gone straight for finished size. Any parting I've done previously had so much chatter that I had to leave material for a clean up cut.

Here is the part in question:

Thanks again,

Craig