Adjusting a 3 jaw in a rotary positioner?

[Taylorwso]

Okay I have a rotary position/index table that I added a 3 jaw to. Back plate was cut on a lathe with a threaded rod ( I cut 1.125" -8" IIRC) that fit the back plate. I faced/plug cut then turned diameter to fit 3 jaw.

I mounted it to index table after mounting the back plate. Back plate was tight to the internal rim of the 3 jaw, Run out on the 3 jaw chuck was .010-.015" , run out on the ground rod in the 3 jay was .040-.060" -ouch

So to true the index table I put a .621" plug in the 3 jaw to hold them tight, and used a 625 bit on the mill to cut the jaws in the verticle position, and I also rotated the positioner while doing it to hopefully get a near zero. This only got it to 15-10 thou.

Im not a machinest, I do gun work. I looked at all the videos on 3 jaw tuning but its always mounted in a lathe. So how would you true this to at least .005
1. Why did my setup not get it closer than .010"?
2. I cant mount the 3 jar to my lathe- maybe with the treaded rod I could, but its a lot of weight for my lathe.
3. Do i just cut the back plate again, to give wiggle room and adjust with the set screws holding the plate.
4 Its grizzly taiwan stuff and I know its not the best but I can get barrel less than .0005 i would at least lik to get this to a few thou.
5. Thanks

 

[atex57]

Okay I have a rotary position/index table that I added a 3 jaw to. Back plate was cut on a lathe with a threaded rod ( I cut 1.125" -8" IIRC) that fit the back plate. I faced/plug cut then turned diameter to fit 3 jaw.

I mounted it to index table after mounting the back plate. Back plate was tight to the internal rim of the 3 jaw, Run out on the 3 jaw chuck was .010-.015" , run out on the ground rod in the 3 jay was .040-.060" -ouch

So to true the index table I put a .621" plug in the 3 jaw to hold them tight, and used a 625 bit on the mill to cut the jaws in the verticle position, and I also rotated the positioner while doing it to hopefully get a near zero. This only got it to 15-10 thou.

Im not a machinest, I do gun work. I looked at all the videos on 3 jaw tuning but its always mounted in a lathe. So how would you true this to at least .005
1. Why did my setup not get it closer than .010"?
2. I cant mount the 3 jar to my lathe- maybe with the treaded rod I could, but its a lot of weight for my lathe.
3. Do i just cut the back plate again, to give wiggle room and adjust with the set screws holding the plate.
4 Its grizzly taiwan stuff and I know its not the best but I can get barrel less than .0005 i would at least lik to get this to a few thou.
5. Thanks

Number three to get around your problem. Number four is most likely the reason.

Ed.

 

[michiganbuck]

If you chuck something and then use a boring bar, the something should run dead true until you take it out of the holding.
Putting a line-up line on everything all the way to the spindle nose can help keep it close to true, perhaps.005..

Q (Do I just cut the back plate again, to give wiggle room and adjust with the set screws holding the plate.)
Sounds like you aim to make it adjust-true..likely that should work.

.005 is a lot of error for any gun work...Even a G machine should get .001 on most jobs. I would not like a muzzle break or silencer off .005.

 

[Taylorwso]

Number three to get around your problem. Number four is most likely the reason.

Ed.

I know its taiwan but .060" run out? I have a lot of grizz stuff but its never this bad.

 

[Taylorwso]

If you chuck something and then use a boring bar, the something should run dead true until you take it out of the holding.
Putting a line-up line on everything all the way to the spindle nose can help keep it close to true, perhaps.005..

Q (Do I just cut the back plate again, to give wiggle room and adjust with the set screws holding the plate.)
Sounds like you aim to make it adjust-true..likely that should work.

.005 is a lot of error for any gun work...Even a G machine should get .001 on most jobs. I would not like a muzzle break or silencer off .005.

that was .0005" on barrels. the rotary just nees to be w/n .005" to do muzzle brake holes

thanks

 

[AlfaGTA]

Take that 3 jaw and tape a quarter to the side of the body to make it worth throwing away....
Then get yourself a good quality 4 jaw independent, a good indicator and magnetic base.
Mount the 4-jaw best you can, chuck the work and indicate it to run true.....Accuracy to tenths , more rigid ,handles more types of jobs....
Cheers Ross

 

[Mtndew]

Make an adapter plate and turn it into a set-tru style chuck.
You're working with garbage, so you're not going to get good results without an adjustable plate.

 

[Taylorwso]

Take that 3 jaw and tape a quarter to the side of the body to make it worth throwing away....
Then get yourself a good quality 4 jaw independent, a good indicator and magnetic base.
Mount the 4-jaw best you can, chuck the work and indicate it to run true.....Accuracy to tenths , more rigid ,handles more types of jobs....
Cheers Ross

While I appreciate that, that's not what I need. My lathe can do that but I need a positioner for simple muzzle breaks, And I can't drop more money for accuracy I will never use.

 

[Taylorwso]

Make an adapter plate and turn it into a set-tru style chuck.
You're working with garbage, so you're not going to get good results without an adjustable plate.

Thats about where I'm at since No one can give me info on why my truing setup dint work.

 

[DDoug]

While I appreciate that, that's not what I need. My lathe can do that but I need a positioner for simple muzzle breaks, And I can't drop more money for accuracy I will never use.

"And I can't drop more money for accuracy I will never use."
You have un-acceptable accuracy now.

And so you'll forever be fighting it, pissing away with the 3-jaw.
Learn how to use/align a 4 jaw. You should be able
to dial a part in quickly.

 

[kenton]

Thats about where I'm at since No one can give me info on why my truing setup dint work.

We can guess at why your truing setup didn't work but, without seeing your set up or at least looking at pictures, it is just a guess.

But with guesses being the best option, I would guess that the combination of an assumed lightweight mill and hardened lathe jaws your setup moved a lot more than you realized as you indexed it around. I would also guess the that there is a possibility that starting with .040 to .060 runout you didn't remove enough material to get your concentricity down to an acceptable level even if your setup was sufficiently rigid.

 

[Taylorwso]

"And I can't drop more money for accuracy I will never use."
You have un-acceptable accuracy now.

And so you'll forever be fighting it, pissing away with the 3-jaw.
Learn how to use/align a 4 jaw. You should be able
to dial a part in quickly.

It's not unacceptable if its fixed, apparently you have no idea how to fix it, so no need to reply anymore

Its "practical" machining, not spend money on tools machining. A true craftsman will make his tools work, and a business won't spend money when that's not required.
I have a grizz lathe and a good dial indicator and a pair of mics. I can get barrels trued and turned to .0005 for suppressor. If I want accuracy I can make it happen with just okay stuff. I just needed help with a mill project since I dont do that very often

 

[Taylorwso]

We can guess at why your truing setup didn't work but, without seeing your set up or at least looking at pictures, it is just a guess.

But with guesses being the best option, I would guess that the combination of an assumed lightweight mill and hardened lathe jaws your setup moved a lot more than you realized as you indexed it around. I would also guess the that there is a possibility that starting with .040 to .060 runout you didn't remove enough material to get your concentricity down to an acceptable level even if your setup was sufficiently rigid.

Thanks, I did not take that much off, but I had very smooth cuts on the jaws. I might have to try another cut.

 

[reggie_obe]

Whats the runout of the rotary table alone?

 

[Taylorwso]

Whats the runout of the rotary table alone?

.010" I assumed by turning the postilion with a light cut it would center the jaws even with the run out.

 

[kenton]

Thanks, I did not take that much off, but I had very smooth cuts on the jaws. I might have to try another cut.

If that is the approach you want to take I would make your finish diameter the same as your most common part diameter, or possibly at the diameter of your part with the highest concentricity requirements.

 

[DDoug]

It's not unacceptable if its fixed, apparently you have no idea how to fix it, so no need to reply anymore

Its "practical" machining, not spend money on tools machining. A true craftsman will make his tools work, and a business won't spend money when that's not required.
I have a grizz lathe and a good dial indicator and a pair of mics. I can get barrels trued and turned to .0005 for suppressor. If I want accuracy I can make it happen with just okay stuff. I just needed help with a mill project since I dont do that very often

You said you could NOT get it trued up in the rotary table....Now you say you can....
Practical does entail fixing things right, not continued cobbling for 30+ years.
Get rid of that habit.

 

[EPAIII]

As to the comment about the CoO being the reason for such a large error, total BS. Chinese chucks and RTs may not be as good as some name brands. but they easily can be used where 0.005" is the goal.

You do not say if the RT and chuck are new or repurposed, older items. If the three jaw chuck is used, then there is a very large chance that the scroll is damaged. A damaged scroll can show widely different errors when gripping different diameters. This is because the jaws only contact a small area of the scroll so excessive force while tightening the chuck or an accident while turning something will have an effect on only three or sometimes just one area on the scroll.

And if the scroll is damaged, NO amount of re-machining or grinding will cure the problem. When the scroll is damaged it is quite difficult to repair it short of completely replacing the bad scroll plate.

When I purchased my RT, I wanted to have a chuck for holding many parts easily. But instead of using a three jaw, scroll chuck, I purchased an imported, four jaw chuck. To mount that chuck on the RT I drilled three holes completely through the main body near the outer edge and at 120 degree separations for three mounting bolts. These three, long bolts go to three tee nuts in the slots of the table. I only roughly center the chuck on the RT, using a digital caliper set to a predetermined distance from the OD of the RT to the OD of the chuck. This is very fast. At that point I am under +/-0.010" and probably under 0.005". Then I use a DI and the four independent jaws to center the work to the accuracy needed. That can be 0.005" or 0.0005". Whatever is needed for the job at hand.

A big part of the beauty of using a four jaw chuck is that it does not matter if it is new or used or if it is an expensive, name brand or a Chinese import. The accuracy is achieved by dialing the part in, not relying on a scroll. Scrolls are convenient, but not known for their accuracy.

Number three to get around your problem. Number four is most likely the reason.

Ed.

 

[Mtndew]

Thats about where I'm at since No one can give me info on why my truing setup dint work.

It's not that simple. We can't just shit out a magical answer that solves your issue.
But if I had to guess, it's a combination of having sub-par machines with a sub-par chuck and being a beginner.
Without being there when you did this, we can't really tell you why.

 

[Taylorwso]

If that is the approach you want to take I would make your finish diameter the same as your most common part diameter, or possibly at the diameter of your part with the highest concentricity requirements.

YES! success.

So I recheck the runouts on my mill in all axis, was less than a .001.

This time I used a left over ground rod that was 1.00". Put that in the positioner, then aligned it to 1 jaw that was perpendicular to the bit. I was trying to cut one jaw at a time as a offset to the center.

I rotated the positioner till I knew which one was out the most, set that as my cut depth, turned the positioner very slowly and cut each jaw. Once I had the last big cut done, reset to take less than .001. I just kept rotating the positioner until I didn't hear any cutting.
Popped in a gauge block, less than .003" I was shocked. Checked it a few times, still at .003.

I dont think I could get much better unless I ground it in.

Thanks for all the help, and now I know who the naysayers are.