6" D1-4, 3 jaw chuck

[canuck750]

Today I removed the jaws from my 6” scroll chuck and cleaned the scroll and jaws with brake cleaner, compressed air dried everything and lightly oiled the scroll and jaws. I wiped down the spindle nose and re-attached the chuck with the three D4 studs. I have a new piston pin, clamped in the 3 jaw and with a DTI measured the runout. I took several readings rotating the piston pin each time and read a consistent 5 thousandths on the nose each time.
I assume 5 thousandths is the limit one should accept in a cheap chuck?
Would it help much to set up a die grinder in the cross slide and skim the contact faces of the jaws with spacers made to load the jaws towards center closed position?
I have seen several YouTube videos of guys doing this to improve runout by a couple thou.
I may do this if by chance I can find a set of outside jaws to fit this chuck.

 

[Susquatch]

Would it help much to set up a die grinder in the cross slide and skim the contact faces of the jaws with spacers made to load the jaws towards center closed position?

Yes, it will help. But you don't load them inward, you load them outward as though they were tightening onto something.

Runout will vary with diameter setting so you should try to load and grind them at the approximate diameter you plan to use most and hope that's its close enough when you make small changes. That's just the nature of scrolling jaws. It's virtually impossible to get low runout at all diameters.

 

[jorogi]

I have a new piston pin, clamped in the 3 jaw and with a DTI measured the runout. I took several readings rotating the piston pin each time and read a consistent 5 thousandths on the nose each time.

Is your piston round ?

 

[Susquatch]

I have a new piston pin, clamped in the 3 jaw and with a DTI measured the runout. I took several readings rotating the piston pin each time and read a consistent 5 thousandths on the nose each time.

Why are you surprised by that. I'd expect it to be the same each time. It's only when you change the diameter that it might change.

Mark the runout high spot on your chuck. Test a 1/4" pin, a 1/2" pin, a 1" pin, a 1.5 " pin and so forth. The maximum runout will probably change in both magnitude and location. If it doesn't, then you have a fantastic scroll mechanism well worth grinding and truing the jaws. If it does, you will get an idea of what it can do at certain conditions but it's likely still worth truing.

Edit - If I had one that varied a lot, I might consider graphing the runout vs diameter to establish a mean. Then grind the jaws at that mean knowing it would then be true at that diameter and only vary plus or minus that by the deviation.

 

[Susquatch]

Is your piston round ?

I believe he said pin, not piston. I assume he is using the pin in his chuck as a gauge pin to evaluate runout.

See my other response.

 

[canuck750]

The ‘pin’ is the hardened pin that runs through the piston and the small end of the connecting rod, in this case 18mm. I don’t have a gauge pin so this is the best I have on hand plus I assume it’s a precision ground part.

I watched several chuck grinding videos, this one by Mr. Crispin, he goes into a lot of detail and measuring, interesting how runout changes after grinding depending upon which of the three scroll screws are tightened (obvious once he explains this) He also mentioned how wear of the scroll itself and the corresponding chuck engagement teeth lead to more runout.

When I rotated my piston pin to compare runout I did not change the scroll screw I was tightening, Mr. Crispin explained why I need to try different tightening screws while checking runout.

 

[140mower]

Some chucks have one pinion that is marked differently and from what I understand, is to be tightened last. I tighten up using all three pinions increasing the torque each time and finish with the marked one ....... Of course, I might just be superstitious.. 🙄

 

[canuck750]

Another video on making spacers to load the church jaws, this appears to be much simpler to do than the Mr. Crispin spacers

 

[Susquatch]

Before you plow ahead with your chosen task, I'd highly recommend watching a few relevant videos by Robin Renzetti. He does a good job of describing how all the heavy metal in a chuck is just like rubber. "Everything is rubber" is his way of saying it. My own version of that is "everything moves". It's only a matter of how much. If you want to improve the performance of your chuck, you will need to begin by gaining an understanding of how the chuck and jaws distort as the forces and dimensions change. Otherwise you might do more harm than good.

Anyway, a lathe chuck is one of those pieces of hardware that moves in many ways that we wouldn't normally guess without a strong background in machine design.

As I mentioned in my previous post, it's unlikely that you can get a 3 jaw chuck to have good runout across its entire clamping range. It's more likely that you can only do that at one diameter. It's also true that the forces driving jaw distortion will change as the jaws are moved in and out within and beyond the chuck body.

To be honest with you, I think I'd be buying a high quality chuck with reversable jaws before I put a lot of effort into improving the clamping and runout of yours. But that's a personal bias. You might also just want to have a go at it for the learning experience.

The nice thing about reversable jaws is that they normally also use machine screws that can be used to bias the closing forces or even as jaw surfaces. I often use mine that way.

Last but not least, I'd suggest that you consider buying a 5C collet chuck. They are inherently more concentric than a 3 jaw and can even be had with set tru type adjustments. I use my collet chuck more than all my other chucks combined!

 

[canuck750]

Very interesting video, hard to believe how much deflection occurs when the chuck is loaded!
I can now see how an unskilled attempt (me) at grinding the jaws can result in a worse condition.
Thanks for this.
I think it’s best I just purchase a new chuck of decent quality if I want to improve runout.

 

[Susquatch]

I think it’s best I just purchase a new chuck of decent quality if I want to improve runout.

Yes, that's essentially what I was suggesting. But maybe you just want to try for the experience and the learning. Nothing wrong with that as long as you can manage your expectations.

For most of us, low runout is what 4jaw chucks are for. In fact, I have made several 4 jaw like chucks that even provide a way to eliminate axial runout.

And I do LOVE my Bison 5c collet chuck...... It was worth every penny I paid for it!

 

[PeterT]

I think it’s best I just purchase a new chuck of decent quality if I want to improve runout.

The chuck jaws & scroll usually get the bulk of the blame, either a factory QC issue or have seen some use/abuse. Just so you are aware, other issues can enter the picture. If the spindle nose surface on the chuck backplate is (pick 1 or 2 you like) slightly too tight, slightly too loose, oval/misshaped, has seen a ding, had a bit of swarf clamped in there.... can affect how the chuck sits on the spindle which affects how the test bar measures. the D-pin requires a) full seat on the taper spindle nose AND b) contact to the vertical spindle face. Examine the seats & faces very carefully. Do the felt pen transfer trick & see what it says. Keep the same dowel clamped in the jaws & same D-pin orientation & repeat runout test. Do you get EXACTLY the same results? What about the outboard dowel position (indicating potential seating cant).

Now if you have another 3J & its awesome with 0.001" TIR then all indicators point to an inferior chuck & what to do with it. But if this is your one & only, maybe a new-to-me lathe (sorry I didn't follow that part of the story) then you may need to investigate a bit more.