Lathe chuck on rotary table, jaw alignment problem
I have a 4-jaw Vertex 5” chuck fixed to a Vertex 6” vertical rotab as a 4th axis on my mill. Problem is that when I center-up reference stock (i.e. 1” drill rod) with a dial indicator, as measured just off the face of the jaws, the far end (say 6” extended from that point) is way out of alignment. By way out, I mean hundredths coming on a tenth on an inch. I have checked the rotab body and face for alignment- they are both within .002” around/across. So, just seems like the jaws are not holding the stock parallel with the axis of rotation. And, I don’t know if there is a spec for this, but I do know that on my lathe, I see nothing like this error. I might see .005” at most on the 4-jaw in the same setup, zeroed at the jaw face.
Does anyone have insight as to what the cause here typically is? I can see several scenarios- one where the jaw slots in the rotab body are not parallel with the face, so the jaw interface is not aligned with the axis of rotation. Another could just be that the jaw's interface/teeth are ground incorrectly.
I have written to Vertex, but no response.
It is not a big deal to put the chuck on the lathe and setup a fixture to load the jaws- to allow them to be reground. But if this is more likely a problem with the slots in the body, then Im going to be chasing slop every time I tighten the jaws up- which is no fun on a 4-jaw.
Re: Lathe chuck on rotary table, jaw alignment problem
I had the same problem with cheap junk 4-jaw on my lathe. What I did was to put a bar in my collet chuck, then mount the 4-jaw on the bar, backwards, dialed it in to the body, then I could work on the back of the 4-jaw. Then I took a cleanup cut on the mounting surface. This squared the mounting surface with the jaws. Then put it back on the lathe and took a clean up cut on the front face. Like yours, mine was out about 0.100 at 8 inches, when I finished with it, it was < 0.001 out at 8 inches. And yes, that T-slot is canted off about 10°, like I said, cheap junk chuck, but it works for what I need.
Re: Lathe chuck on rotary table, jaw alignment problem
Yea, its strange. I know Vertex isn’t Bison etc, but they tend to keep far better tolerances than this. The 6” HV rotab is $200, and the thing is remarkable as regards basic tolerances and lash.
I had thought about your approach, but am concerned that if the issue is the jaw face and not the slot(s), then a jaw reversal for larger work holding could re-introduce the problem.
I just haven’t had much inspiration as to how to determine where this error is coming from. Im sure there is some obvious setup to measure the jaw face or slots that Im not considering. Maybe flipping the jaws isn’t a terrible start though- since the bias should remain the same magnitude and azimuth if the error is in the body slot(s)… But that assumes the jaws aren’t garbage…
I appreciate the insight on your fix- and that you were able to get something functional out of what was a disaster. I may end up doing that, and re-truing it later if I figure it out, using the outer body as the reference…
Re: Lathe chuck on rotary table, jaw alignment problem
Well, all I can add to this conversation is .....you only get what you pay for.......but that is not always the case.
Back in '82 I bought a relatively cheap 150mm 3 jaw chuck.....made in China...... and if anything is likely to be cheap and nasty it's an "inexpensive" 3 jaw chuck....they just can't be made with the quality that you need when a 3 jaw is doing it's thing.
Long story short.......after 35 years of not too constant use, but still being put to the test when it's needed, I can still hold a 25mm bar stock and have it run true to .02mm concentricity.........I have several hardened and ground steel test bars that confirm it.
The real secret is not to try and bend the chuck key when you hold a job.......if you need force to hold rough bar stock then I use a 4 jaw.....I consider the 3 jaw as an accurate work holder and treat it with extreme care........a 3 jaw is a poor mans collet chuck with infinitely variable holding capacity.
BTW.......anyone who mounts a 3 jaw rigidly on the back plate spigot is not going to have a chuck that will run true for all diams......expensive or a cheapie.
If you have the spigot on the back plate mount a clearance fit in the chuck of at least .2mm or 5 thou..... whatever, you can dial up any work piece and make the chuck run dead true by slacking the holding bolts and tapping the chuck true with a lead hammer.....this is a pseudo Griptrue method, but the results are the same.
Unless you're running at 10,000 rpm the .2mm run out of the chuck itself will not affect the balance at all.
This is also very important when a 3 jaw is mounted on a dividing head (or rotary table) .......any run out is impossible to correct otherwise.
In the case that this thread is about I'd use that 4 jaw as a door stop.....sometimes you just have to decide that a crap work holder will never get better.
Ian.
Re: Lathe chuck on rotary table, jaw alignment problem
I guess I lucked out with my Vertex 6 inch super spacer. It came with a 3-jaw, and is very accurate. < 0.002 at 6 inches out of the box. It became my 4th axis after I added a stepper. Works well.
Re: Lathe chuck on rotary table, jaw alignment problem
Hi.....not wanting to be argumentive.........anything that has a worm and worm wheel for a drive is not really suitable for a 4th axis.
Backlash being the bogey of such devices makes them worse than Acme threaded leadscrews on all axes........they will suffice if the work piece is simply a profiled pattern piece where surface pattern is more important than the surface incremental accuracy, but for work where radial pitching is of prime importance.........it's as reliable as a rubber band.
The other point is.....out of the box is not at the coal face where the work is done and will soon "wear it in".....any run out in a 3 jaw chuck is impossible to tolerate if you want to have concentricity on any component you wish to produce accurately.
This factor has been discussed in deep detail some time back on another thread and no solution was forthcoming then so the bogey still remains........a .002" run out would make any gear cutting exercise, using either a stub mandrel or between centres, a non event, but you wouldn't cut gears or radially pitch holes with an unlocked dividing head in any event.
This is one main reason I advocated the slack backplate mounting method for a 3 jaw chuck as it eliminates run out completely.
Ian.
Re: Lathe chuck on rotary table, jaw alignment problem
I guess I got lucky with my Vertex. It is adjustable to < 6 arcsec backlash, checked at 3 points in the rotation. As I said, out of the box the TIR was <0.002 at 6 inches, but I corrected that and is now <0.0005 TIR both near the jaws and out at 6 inches. Seems to be repeatable. As it wears in, that may change, but at my use rate that may take years.
If I needed extreme accuracy, I would just build another servo driven rotary and put Renishaw 0.5 µM ring encoder on it.
Lathe chuck on rotary table, jaw alignment problem
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