Improve finish and tolerance on a long thin rod?

[zondar]

Hi All,

I have procured a 1/4" by 3' rod of Invar. Given the price, I had hopes of a clean, tight-tolerance finish. Alas, the surface finish is relatively poor, as is the uniformity of the diameter.

Aside from improving the finish, I had wished to turn parts that would slide nicely on the rod without wobble or binding, but it looks like the rod would need work first. Therefore, I'd like to polish it a little with emery cloth, preferably with mechanical assistance, and maybe even see if I can improve the tolerance on the diameter.

I have a 17" Sherline lathe which can fit the rod down its throat, but of course it can only hold half of the rod at a time (which needs to remain full-length). I don't see using centers or cutting tools on something that thin and long (I do have a steady-rest, but no follower-rest), but emery cloth might work if the rod can be held safely.

In addition, I'll have to cut, face and turn down the diameter on both ends slightly to allow for threading. But again, using the lathe with almost 3 feet of rod spinning out the back of the lathe, even if "slowly," sounds like trouble.

I could likely improve the surface finish manually if necessary, but what other ideas are there for finishing the surface and possibly improving the tolerance of the rod?

Thank you!

 

[Aukai]

If you can center drill, then you can use a live center. I don't know how to get around no follower.

 

[SLK001]

Not 3 feet, probably 15-18" at the most. Support the end with a spider. If you don't want to build a spider, taper a piece of wood with a 1/4" hole thru it so that it will fit into the end of your spindle. That should keep whipping at a minimum. If you have to cut into the diameter of your rod, use something like a shear tool (have a look at this post). I would definitely center drill the rods at both ends so that it can be supported in the lathe. Cut the center off later if you want a clean end. Also use your steady to break up long sections of the rod.

 

[cathead]

It's not difficult to hold the stick out on the back side of the lathe with a wooden jig with a hole about the size
of the part. As far as between centers, you need a follower rest to machine something that thin and long.

A two by six inch diamond hone (400 grit) is something I will use occasionally to try and get surface finished
more uniform. I generally hand finish small parts that way. You could make a project out of it and build
yourself a follower rest if so inclined. It all depends on how perfect and smooth you want the rod.
Another thing one could do would be to drill a hole in a piece of steel with a fairly tight tolerance and
run that down the rod until it hangs up. Mark the spot and hone the imperfection till it passes through.
Trying to machine something like that may not turn out well even with a follower rest so something to
consider. I don't think I would attempt to machine it myself but then I'm running some big and old machines.
If you think of your end use for the rod, you could possibly cut it up in shorter pieces that would fit your application
making it easier to handle in the lathe.

I had to look up Invar to see what it was, interesting stuff,

 

[stupoty]

Watch out for "whip" with thin stock , you may be limited on the rpm's you can run to avoid it going bannana shape very quickly in a slightly scary manner !

Stu

 

[zondar]

Thanks for the feedback.

I can't reduce the length, so yes, it's "whip" of the very long end protruding from the end of the lathe that I'm worried about. A spider (I can 3-D print something temporary) at the lathe's exit won't do much given the extention of the rod out the back, it would have to be positioned a foot or two away from the lathe.

I'm also worried that a live center won't hold something that thin, long and somewhat flexible well either due to it spinning out of round.

I could start by gingerly trying it at very low speed (the Sherline goes down to a little under 100 RPM) and see how it behaves.

But in the end, I probably should just try to do it manually. I do like the idea of a tight-tolerance hole (or maybe a bearing with a 1/4" hole) to use as a gauge to find the high spots in the rod. That could be a time saver vs. endless use of a micrometer.

My other option is to just clean it up some and abandon the sliding fixture if it can't fit well enough down its length (it has to remain precisely square to the rod without any wobble). I have a few other ideas.

My other-other option is to abandon the Invar rod and just use a proper tight-tolerance (ground?) rod of something like a stainless bearing shaft. But that's no fun after splurging on Invar, which I did need due to thermal stability concerns.

Thanks again.

 

[zondar]

Quick update: I just tried it using the 3-jaw and a steady rest positioned all the way down the bed (rest only, no center at that end for the trial). At the lowest RPM, whip of the unsupported end isn't bad, and I'd say it's safe enough for polishing using emery cloth. With an hour or two of that, maybe I could improve the uniformity of the diameter some too.

Cutting is a different story. Even with a follow-rest, I don't feel confident that I could get the entire rod turned to a uniform diameter. But I think facing and turning the ends down slightly as prep for the use of a die is doable.

Thanks again.

 

[homebrewed]

If your rod is only a few thousandths off you might want to consider lapping it. A description of the lapping process can be found here. A more expensive option might be to use a toolpost mounted grinder (assuming you don't already have one).

 

[compact8]

It will be a lot simpler to just buy a linear shaft / guide rod like this kind. The finish and tolerance should meet your requirements. Cheap too. I have a bigger lathe but still I won't try to make it. Just not worthwhile.

 

[RJSakowski]

Your requirement that the mating parts remain square to the rod and not wobble but are able to slide freely put some strict requirements on uniformity of diameter, roundness, cylindricity, and surface finish. These requirements would be calling for a centerless grinding operation rather than turning.

A description of your application would help in determining a suitable course of action.