Centering Parts On A Rotary Table

[Jeremy_BP]

Hi all,
I need to cut some radii on the ends of some stainless pieces, which means I get to use my new rotary table! Yay!
The piece is 5" x 1" x 0.075". I need to cut a 0.5" radius on the ends, and there's no hole under the center. Any tips for getting the piece centered? I recall reading about indicating the part in, moving the mill table, and then moving the part back to the indicator, but this doesn't seem like a very good way to go about it. Thoughts?

 

[gbritnell]

If you can't stand a center punch mark in the piece then it gets a little more complicated. If you can then just put a small punch mark on the piece, center your spindle with the rotary table and then pick up the punch mark. The other way is to lay out the piece and scratch a couple of lines on it. Then use a wiggler to pick up the lines.
gbritnell

 

[Jeremy_BP]

Thanks for that. I'm making some panels that need to have a perfect finish, so I think I'll go for the scribed lines.

 

[tel]

even pencil lines might do the needful and won't leave marks

 

[ninefinger]

I am no expert on such things but seems to me that as your part is very thin your going to need something under it. Couldn't you attach a sacrificial piece of aluminum to the rotab after you've centered it? Then cut the aluminum to the shape you want then attach your work piece piece to it and re-cut again? Not explaining it very well here but the sacrificial piece will be to final shape and located and you could use it to locate the work piece.

 

[mklotz]

Another option is to not use the RT at all. Using trig, make incremental plunge cuts positioned relative to the edges of the workpiece to establish the radius, then smooth to final shape with a fine file.

 

[Jeremy_BP]

Mike: I have a sacrificial backing, maybe I'll just make a jig. Since I need to make a few, that would make sense.

Marv: That would make sense, but I need to hold fairly tight tolerance and don't feel that confident in doing that kind of positioning (being without a DRO and all that). Also, I get to use my new rotary table.

 

[bob ward]

Whichever method of centring you use, you will need to fine tune a little even after you have it set up super dooper dead nuts. Making your straights and radii blend seamlessly using a rotab can be quite difficult, especially if you're making a full 180° sweep on the end of a piece of flat, you can end up with an undercut on one merge and an overcut on the other.

Its sometimes easier to accept that this will happen and cut a slightly larger radius, then blend the straight and radius by hand, assuming of course that appearance is more important then dimensional accuracy.