[Kingjamez]

I have been using my CNC mill as a lathe since my mini-lathe is not CNC. I bought an R8 mounted 3 jaw chuck and recently tested it for runout. The outside body of the chuck has less than 0.001" TIR when installed on the R8 arbor. However, when I put any size material in the jaws I get right at 0.009" TIR measured on the material. I have been very careful to position the material carefully, and after many loosen / re-tighten cycles, it is almost ALWAYS 0.009" TIR. For most tests I'm using an edge finder as my test surface.

I then take the chuck off of the R8 arbor, and mount it on my mini-lathe. Chuck up the same test piece of material, and I get~0.001" TIR.

Any bright ideas on what I'm doing wrong?


-Jim

Edit to mention that I have 2 3" chucks, one that came with the mini-mill, one that came with the R8 arbor. Both exhibit the same issue.

[pete from TN]

IF both chucks show good runout on the lathe and they are bad on the arbor it sounds like the arbor is bad or your spindle is out.... If the jaws are removable could it be possible you have the jaws installed in the wrong order. I know my lathe chucks jaws need to not only be installed in the right order but they need to be started in the right order so I played with different positions to get the best runout and then I used an automatic center punch to mark the start position for installing the jaws. My three jaw has reversible jaws so it is different. Barring all of that I have machined the jaws on my cheapest lathe chuck to improve runout and it worked okay. I just clamped something in the jaws as far back as I could and then machined them as far back as possible. Then I removed the jaws and carefully ground the part I could not reach manually. How do you find that r8 mounted chuck as far as actually using it. I have looked at them to try the mill to lathe setup for some parts but never pulled the trigger yet..... Peace

Pete

[Kingjamez]

Any idea what could be wrong with the arbor if the body of the chuck has so little runout?

As to the CNC mill as a lathe. It works wonderfully. This runout has not been a problem so far as my parts were all one operation. I just put my indexable tool in the vise and start turning. Super easy.

I'm using LazyTurn from Art of Mach 3 fame. It's free and the code doesn't require any modification to work with the mill. I don't have to swap any axis, or directions. It just makes my parts. I love it.

-Jim

[TXFred]

You didn't mention what the runout of the chuck body was when it is mounted on the lathe. I have a feeling that your chuck jaws are not concentric to the chuck body, but that there's also runout when mounting to the lathe that cancels it out.

On the mill, the chuck jaws have 0.009" and the body has 0.001" of runout.

When on the lathe, the chuck jaws have 0.001" runout.

I suspect that on the lathe, the chuck body will have 0.009" of runout.

The solution? I would buy another set of chuck jaws, or a complete chuck, for use on the mill. I would then grind both sets of chuck jaws. That should give you two very accurate 3 jaw chucks.

Frederic

[Kingjamez]

Thanks for the suggestions Frederic. I took them and went and measured the runout of the chuck body while mounted on the lathe: 0.0015". At that point I was stumped.

Then I thought about what Pete said, and investigated the R8 arbor. It occurred to me that if the chuck body had no runout at the top where I was measuring, and I knew that the jaws were good from the lathe test, then the only answer is that the chuck was mounted at an angle onto the arbor.

I cleaned the mating surfaces, then started differentially tightening the 3 mounting screws using a 4 jaw chuck centering technique and viola... I got all the bolts very tight and I got ~0.0016" TIR. I called it quits at that point as I think I can live with that.

Problem solved. Thanks!

-Jim

[Dano523]

Ever though of just chucking up a piece of stock in the chuck and truing the jaws instead of the body???

[Kingjamez]

Ever though of just chucking up a piece of stock in the chuck and truing the jaws instead of the body???

I'm not following you here. That seems to me like a bad idea.

-Jim

[Dano523]

MIni-lathe mods - Lathe chucks repair

Then, instead of dialing off the body when the chuck is set, dial off a true work piece in the chuck. I have a 3/8" end mill that I point converted when it gave up the ghost as a cutter, and just chuck it up backwards to dial off of when I set the chuck to the spindle. Right now, my three jaw chuck is running .0003 out of round true full range (good enough for that I use it for).

Or if you want to true the body, then go back and high speed grind the jaws to run true with the body.