Part length on a lathe (again)

[schwabw]

Hello all,

One of these days I'll figure this out - maybe I am designing my next strange gizmo (this one is medical disposables cast in jello[*]). Actually, I am making a frame that holds the parts while the gel sets, and we need various spacings. One way to do that would be to part off some "washers." I can face, turn and drill (boring would be overkill) with no problems.

Things get fuzzy when I think about parting off, and about the order of operations. Because of the parting, I don't see a way to get a bunch of parts ready for the last step. It seems that I would drill and turn far enough back to make a few of them[**] and then part off one. I might then need to face the next one, then part it. Every so often the stock moves, or should I advance it each time to keep things in about the same place?

Am I making any sense? What is the correct way to tackle this?

Bill


[*] Really. It's a ultrasound phantom.
[**] How thick will they be? I have not yet decided. We need to cover a range of 3-8 mm, but the actual parts don't have to be those thicknesses; they simply to span from their supports to the correct position, if the spacers get longer, the supports get shorter. CAD to the rescue.

[Harold_V]

Bill,
From practical experience.
Drill deep enough to make more than one, but not so deep that you have difficulty drilling (extracting the drill to clear chips). Do not drill deeper than an inch, for you'll use a long travel indicator to keep them the same length. There is an ultimate desired depth, which would yield the last part in the series with a full diameter hole, but the drill point would be removed from the remaining stock in parting, leaving the material without a center hole*.

Using a parting tool that is narrow, but wide enough to be rigid, pick up the face, taking a facing cut. Set your long travel to "0". Add the thickness of the parting tool to the desired thickness of the spacer, (plus anything you may wish to face off), and part the first one. Add that dimension to the first one, and move over to part the next one. Repeat, if the setup allows for more than two.

When you drill your hole, drill such that when you part the last piece, you have to re-establish center. That will address any drill drift, or concentricity issues that may rear their heads. You will move the stock out, picking up the parting tool set a few thou shy of the first facing cut. Center drill (*the old hole may not be on center at this point, so you want to eliminate traces with the last parting operation), then drill to size. Face the material to the (original) zero setting, then start parting again. This method uses the parting tool to re-establish the length of material sticking out of the chuck. Keep it as short as you can, for rigidity. You use only the parting tool, no need to keep changing tools (assuming you don't have to remove stock from the material diameter).

Debur each part (while spinning, using a fine file) before it comes off the stock, and break the one edge of the hole using a c'sink, or a three cornered scraper, before the spacer parts off.

I always use hand ground parting tools for operations like these----you can make them the desired width and length to make the tool function as desired.

Make sense?

Harold