Unintended asymetric turning

snipped-for-privacy@gmail.com wrote in news:94g3g65m52r78e46mvfiknjsrntsul4khv@

If it was me, I'd turn the OD, and then use a "Step collet" for the rest. They can machined in place to fit the part precisely, so are guaranteed concentric. They also apply very even force to the OD of the part, unlike the high point pressure of a bunch of chuck jaws.

Doug White

Assuming stress isn't the cause of the distortion, another approach would be to use a longer piece of material. Face, turn ID and OD, then turn around and machine off the excess. More work removing the excess, but -should- result in concentric ID/OD leaving only parallelism of the two faces as an issue for the last operation.

Jon

Could it have been as simple as a .014" thick bit of crud on one of the jaws?

O.K. So far, it sounds like a standard independent jaw 4-jaw.

Strange. This is not a "universal" (e.g. scroll-back 4-jaw is it?

Nearly doubling the eccentricity from before.

Possible.

That still should not result in eccentricity -- just greater difficulty achieving concentricity.

You indicate from the *jaws*? With no certainty that they are equal sized? This is likely the problem.

Good Luck, DoN.

Instead of reading runout on the indicator you could feed it in to zero and read the leadscrew dial. Then you could move the carriage in and out of position to indicate the work beside the jaws.

This job may justify a spacer plate with slots for the jaws to keep it in place and standoffs to position the dial plate out past the jaw ends.

jsw

=========== One way around this from the old machining books is to use what is called a bridge or pump staff.

You can make one out of a piece of stiff wire that you can clamp in the tail stock chuck or make a bushing. To use, you bend the free end so that the tip of the bridge rides on what you are trying to get concentric and you can use a drop indicator on the bridge to measure the runout. The actual measurements won't be correct, but generally you are trying for zero runout with no or minimal needle movement.

You can also fabricate a bridge out of a used hacksaw blade that you can clamp in either the tail stock or tool post. Works great for indicating square/hex stock or rough cast preforms or rough cast bores.

If it would be helpful I can post pictures of the ones I made on either the drop box or my web site.

Another possibility if you have several of these to do is to fabricate a holder/fixture that will clamp the donut that you can bolt to the faceplate.

Let the group know how you make out and what your solution was.

-- Unka George (George McDuffee) .............................. The past is a foreign country; they do things differently there. L. P. Hartley (1895-1972), British author. The Go-Between, Prologue (1953).

I have never seen one of those so I had a look around the Google to see how they work.

If I understand it correctly for a work piece like mine one would get a 5" step collet and machine it closely to fit the OD of the work piece. Presumably the clamping range of these collets is the same as ordinary 5C collets - rather small. The recess would have to be deep enough to accommodate the work piece and a spacer to allow boring of the central hole.

Even at $172 it would be a good investment if one were to make more of these pieces. Unfortunately I do not think I can use 5C collets on my lathe.

Michael Koblic, Campbell River, BC

It's a thought. I do try to be fairly anal about cleaning everything before clamping.

No, it's an independent 4-jaw. Looks like this:

You'd better believe it...

I measured the jaws when I got the chuck. I have also used the same procedure on similar work pieces with nothing like this happening before.

Thinking about it I should be able to tell by indicating the face and tightening the jaws.

Michael Koblic, Campbell River, BC

I am being slow. Cannot visualize this.

What I have is a piece that does both. It is smaller than the work piece and I center it and attach it to the work piece with superglue.

Michael Koblic, Campbell River, BC

I think a faster and easier way to make this part would be to start with .187 plate, use a hole saw or tram cutter in the drill press to cut the ID and then trim the corners with a shear or saw and then finish the OD by mounting the piece on a mandrel.

You could also make a fly cutter / tram that would cut the ID and the OD in one op. Here it would probably be best to have the ID cut first and then have the od cut as you might need to have a speed chang

If for some reason you need to use bar stock rather than plate, Then I would face one end, drill/boar the ID part or cut off the blank and then use the mandrel to face and size the OD.

As I see it the disadvantage top the way you are doing it now is that it requires a whole lot of set up changes and having to reestablish center each time.

Roger Shoaf

And I need to practice writing clear descriptions.

When centering work in a 4-jaw you normally turn the chuck while watching the indicator dial, right?

You can't do this when the jaws grab the OD, and moving the carriage out of the way to turn past a jaw means you have to slide the indicator point back onto the work afterwards without shifting it.

I suggest initially advancing the crossfeed until the indicator reads zero (close to a jaw) and then perhaps zeroing the crossfeed dial. Then you can back it out and slide the carriage to the right so the jaws clear the indicator. Rotate the work half a turn, return the carriage and advance the crossfeed to a zero indicator reading again. Adjust the jaws to remove half of the difference between the new and previous crossfeed readings. If you zeroed it and the new one is 30 then move the jaws to make it 15. At this point I reset the zero.

This works with a lever test indicator with a cosine error because you only read it at one point, its zero.

Rotate the chuck a quarter turn and repeat. The work should now be nearly centered.

HTH JSW

OK, now I get it, thanks. What you are doing is attaching the indicator in such a manner that it need not be touched by human hand while doing the series of measurements except to re-set the zero. You are also indicating next to the jaws.

If I thought about it I should have done that or something similar then. I shall be aware of it with the next piece.

I am more and more convinced that the issue is the flexing of the work piece: All the previous ones I had done I only faced off one side. Also, the recent batch I bought were definitely thinner than the ones I bought in the past. I said so much in the shop at the time.

Recession is hitting everywhere. Even baby bum-wipes are smaller and thinner these days...

Michael Koblic, Campbell River, BC

Yes please. I am having a hard time visualizing it: How do you get around the jaws with the bridge?

Right. That's where that 5C step collet would have been so nice, too. Up to now the pieces have been all different as I am experimenting. At some stage I would like to develop a method where the edge, one face and the hole in the middle can be all done in one clamping.

Next time I do a similar piece I shall take notes and do a few extra measurements.

It is not impossible that Santa will drop a new 4-jaw down the chimney, too.

Michael Koblic, Campbell River, BC

Do you standardize the size of these dials? If so, maybe you could find a chuck with removeable top jaws and make your own pie jaws, with short protrusions on both ends to clamp either the ID or OD and an undercut so you can cut across the unclamped edge.

/'''|__---------------__|'''\

jsw

On Sat, 11 Dec 2010 13:29:15 -0800 (PST), RS at work

Interesting. You think it would be faster doing the pieces from scratch rather than using prefabricated items as I am doing now? Also what is the advantage of a mandrel as opposed to a 3-jaw chuck?

Again, I am not sure of the advantage of doing it from scratch rather than finishing the pre-fabs. The only hassle part of the procedure is the 4-jaw chuck to get the ID concentric with the OD.

Michael Koblic, Campbell River, BC

O.K. This does not show the back side. Is this, by any chance, the chuck with studs sticking through the backplate, and a nut and washer holding it in, and tightened after you get it adjusted?

I have read descriptions of these with people who got them on metalworking lathes saying that they are only good for woodworking. :-)

I see a hint at the bottom of the page that if you are interested in this chuck -- you should also be interested in this chuck (again) at the bottom of the page. :-)

Sounds like the type with the nuts on the back holding the jaws, then.:-)

O.K. But *never* trust the jaws to be right -- measure the workpiece, even if it is more difficult.

O.K.

Good Luck, DoN.

I'm in late on this conversation, and have not read all the responses. However, a ready solution, if it hasn't been mentioned, is to run the part in soft jaws. Turn the OD to size, then fashion pie shaped jaws that will encompass the major portion of the diameter. By creating a step of the proper depth, you'll be able to face the item parallel and bore the hole concentric. Used properly, you shouldn't have any problem holding a half thou in concentricity and parallelism. Facing should be the last operation, and it should be done by taking equal amounts off each side, to rough, then equal amounts off each side to finish, flipping the part over for each pass. Leave a few thou only for the finish cuts

Harold

This type of chuck isn't ideally suited for most large lathes, but it's sufficiently adequate for use on a generic 9x20 model. I don't know if the chuck is a good choice for thin rings, though, as jaw pressure could easily distort the ring (much like thin tubing).. the flat faceplate may be a better choice (with some fittings added).

I've used the same type 4-jaw on my own 9x20, and it isn't substantially any more difficult to use than a heavy duty 4-jaw chuck. Like many pieces of tooling from China/elsewhere, it was kinda rough as supplied. After deburring all the parts and edges, and smoothing the sliding surfaces, I replaced the soft flat washers under the nuts with hardened, ground flat washers. That may not seem like a major issue, but those original soft washers don't give a good feel of when a fastener is tight.

Backing off the jaw nuts just enough to allow for minor adjustments may be the key to ease of use with these chucks.

Hardened/ground washers offer a significant improvement in feedback of tight/just-snug within a short wrench swing.. I generally install them where repeated use is common. When a fastener has 90+ degrees of swing from tight-to-snug (backing off), something is wrong.. the bolt isn't stretching, and the cast iron isn't compressing, so it's the soft flat washer. Another cause of wrench "overswing" could be that the threaded hole in the nut isn't perpendicular with the flat sides, which is a fairly frequent occurrence with Chinese hardware.

A HD 4-jaw will have other issues, such as the correct spindle thread and matching register seat for the 9x20 models with metric thread spindles. Another issue with a HD 4-jaw is the workpieces will always be supported further away from the spindle nose bearing, which will most likely result in more tool chatter when not using tailstock center support.