Grinding the Jaws on 10" Pratt Burnerd 6-Jaw Chuck

[jbacc]

Good Morning,

I think I know the answer but I trust the collective wisdom of the forum members and their/your vast knowledge and hope I may tap into it once again.

When I purchased my used Whacheon WL-435 lathe about 6 years ago, it came with 6-jaw PB Setrite Chuck. A couple of the jaws were cracked due to abuse and before checking the rest of the chuck out, I naively purchased a new set of jaws.

The salesman from the company I purchased the jaws from advised that ideally, the new jaws should be ground (just kissed in his words) with a grinder.

I was not set up for that and I was not confident enough to do it at the time so I held off. The chuck would not repeat. I would dial in a piece of ground drill rod and if I loosened and retightened the same part, I would be off by many thousandths. I wound up buying a 3-jaw chuck which is used most often and if necessary, I have a 4-jaw chuck.

Fast forward 5-years later and I get the bug to try and grind the PB 6-jaw chuck. I made a mount for my air die grinder which made for a very rigid attachment to my tool post. I did make an extended shaft which is probably the weak point of my set up but it seemed to work fine and I took very light cuts.

I should also note that the chuck is for my A1-6 spindle nose and as such, the chuck has two pieces that mount when installed.

After all set up and grinding was completed, I anxiously set up varying sizes of drill rod to see if the condition was corrected or at least improved... Much to my dismay, I am in the same boat.

I have disassembled the chuck prior to grinding to clean it and inspect it and to the naked eye, the scroll, pinion and everything else looked fine and the jaws fit snug and were not sloppy or loose. After grinding, I repeated the procedure.

As I said, this is a setrite chuck with a single pinion so I dialed in the ground rod, loosened the chuck, rotated the same piece of stock, tightened the chuck and it was off by more than 10 thousandths.

I could not get it to repeat with the same stock without fiddling around and moving the rod around. It would behave the same way with any piece of stock I tried, in fact, sometimes it would be off by as much as 20 thousandths from one piece of rod to the next.

Based on all I have read I conclude the scroll is worn and unless I want to dial in each part (might as well just use the 4-jaw chuck), it seems the chuck is unfixable.

Before I relegate it to a spot in the corner of my shop never to be used again, I am hoping to get some insight from the experts on PM.

I attached a short video of the grinding operation.

As always, my sincerest thanks and appreciation for any and all advice, comments, insight and assistance.

Joe

 

[Earl Sigurd]

Have you checked the clearance of the scroll to the part on which it rotates? This messed up my attempts to regrind jaws. 0.2mm clearance and 0.2mm runout randomly.

 

[jbacc]

Have you checked the clearance of the scroll to the part on which it rotates? This messed up my attempts to regrind jaws. 0.2mm clearance and 0.2mm runout randomly.

I have not.... May I ask how you took that measurement? Are we talking where the pinion meets the the scroll plate? I apologize if I am using the wrong terminology.

Thank you.

Joe

 

[eKretz]

I have not.... May I ask how you took that measurement? Are we talking where the pinion meets the the scroll plate? I apologize if I am using the wrong terminology.

Thank you.

Joe

You'll need to disassemble the chuck. The scroll plate rides on a central boss or pivot with its ID. That fit needs to be a close running fit for the scroll to stay centered well.

 

[jbacc]

You'll need to disassemble the chuck. The scroll plate rides on a central boss or pivot with it's ID. That fit needs to be a close running fit for the scroll to stay centered well.

Thank you. If I may, what am I actually measuring? When I took the chuck apart to clean it, the scroll fit very precisely to the point I had to tap it slightly to get it to seat properly.

Thanks again.

Joe

 

[jcarey88]

I should also note that the chuck is for my A1-6 spindle nose and as such, the chuck has two pieces that mount when installed.

Have you indicated the chuck on the machine to see its runout compared to the spindle?

 

[L Vanice]

Pratt Burnerd America in Kalamazoo, MI will regrind the jaws on the chuck and presumably do it correctly. They also inspect the chuck and give a quote on repairs/replacing parts before doing the work. That is an expensive chuck, so it is probably worth fixing.

That said, a six-jaw is not the best chuck for everyday use. Likely, only three jaws will actually be doing the gripping on round or hex stock, with the other three more or less close, but could be actually loose. A six-jaw will grip tubing with all six jaws if the tubing is thin enough to deform enough when clamped. That deformation will be less noticeable than if you clamp the same tube in a three-jaw. You have probably seen comments about why a six-jaw is better for holding tubing.

You can remove the odd or even numbered top jaws on a six-jaw and it will then act just like a three-jaw. Your jaw gripping surfaces may be too irregular to properly grip work after your grinding attempt, but you could try the chuck with three jaws installed and see if it repeats. Try bluing a piece of hardened round stock like an endmill shank and see how well the jaws contact the shank. If the gripping surfaces of the jaws are tapered, runout will always be random because the part will wobble on its axis after tightening the jaws.

As for jaw grinding, it is essential that the jaws be preloaded before grinding, just as you need to do when boring soft jaws. Preloading a three-jaw is not too difficult and there are commercially made devices for the purpose. I have never seen a commercial device for preloading a six-jaw., but I suppose the chuck makers have a way. Grinding the jaws while they are free to move around in the chuck body will give very random and inaccurate results. And a die grinder is not equal to a Dumore or Themac tool post grinder in regard to bearing precision and rigidity.

Larry

 

[jbacc]

Have you indicated the chuck on the machine to see its runout compared to the spindle?

I have, in fact, prior to the grinding operation, I used the setrite feature to indicate the chuck to ensure that the chuck was centered on the backplate. The amount of runout was imperceivable.

Thank you.

Joe

 

[eKretz]

I would suggest using a thinner wall thickness on your preload ring. That will have a little more give so that the jaws can be individually held back while grinding.

To check your scroll fit you can either measure the ID of the scroll plate and the OD of the boss it fits on or just mount a magnetic base indicator to the scroll plate while you try to push it back and forth on the pivot/boss and check for slop.

How is your fit from chuck to the spindle also? Does the taper snug up before the flat faces meet? That is an awful lot of runout. Almost seems like something isn't secure.

And I'll echo Larry's comment on that die grinder setup being suboptimal. That long narrow shank is not doing you any favors, to say the least. A good toolpost grinder would be probably at least an order of magnitude stiffer.

 

[jbacc]

I would suggest using a thinner wall thickness on your preload ring. That will have a little more give so that the jaws can be individually held back while grinding.

To check your scroll fit you can either measure the ID of the scroll plate and the OD of the boss it fits on or just mount a magnetic base indicator to the scroll plate while you try to push it back and forth on the pivot/boss and check for slop.

How is your fit from chuck to the spindle also? Does the taper snug up before the flat faces meet? That is an awful lot of runout. Almost seems like something isn't secure.

And I'll echo Larry's comment on that die grinder setup being suboptimal. That long narrow shank is not doing you any favors, to say the least. A good toolpost grinder would be probably at least an order of magnitude stiffer.

I thank you for taking the time to enlighten me, I do appreciate it and will give you're advice a try. I will also consider trying some other method of grinding as I too thought the long shaft on the grinder might be an issue. And, you guys confirmed that.

Thanks again.

Joe

 

[jbacc]

Pratt Burnerd America in Kalamazoo, MI will regrind the jaws on the chuck and presumably do it correctly. They also inspect the chuck and give a quote on repairs/replacing parts before doing the work. That is an expensive chuck, so it is probably worth fixing.

That said, a six-jaw is not the best chuck for everyday use. Likely, only three jaws will actually be doing the gripping on round or hex stock, with the other three more or less close, but could be actually loose. A six-jaw will grip tubing with all six jaws if the tubing is thin enough to deform enough when clamped. That deformation will be less noticeable than if you clamp the same tube in a three-jaw. You have probably seen comments about why a six-jaw is better for holding tubing.

You can remove the odd or even numbered top jaws on a six-jaw and it will then act just like a three-jaw. Your jaw gripping surfaces may be too irregular to properly grip work after your grinding attempt, but you could try the chuck with three jaws installed and see if it repeats. Try bluing a piece of hardened round stock like an endmill shank and see how well the jaws contact the shank. If the gripping surfaces of the jaws are tapered, runout will always be random because the part will wobble on its axis after tightening the jaws.

As for jaw grinding, it is essential that the jaws be preloaded before grinding, just as you need to do when boring soft jaws. Preloading a three-jaw is not too difficult and there are commercially made devices for the purpose. I have never seen a commercial device for preloading a six-jaw., but I suppose the chuck makers have a way. Grinding the jaws while they are free to move around in the chuck body will give very random and inaccurate results. And a die grinder is not equal to a Dumore or Themac tool post grinder in regard to bearing precision and rigidity.

Larry

Larry,

Thanks so much for your guidance and advice, I do appreciate it so very much.

Joe

 

[jbacc]

I understand it would be best to use a toolpost grinder to perform the operation, I do not own one and do not know anyone I can borrow one from. Not particularly looking to purchase one so is there anything that would suit my needs that is better than my air die grinder without breaking the bank?

 

[michiganbuck]

RE: the scroll fit very precisely to the point I had to tap it slightly to get it to seat properly.

You might run an indicator on the chuck OD and face and back if some is showing to get an idea if the body is running near true..This to consider that the back may have been altered and the chuck Inners are at wobble.

A pate check, or a check of the inners when it was torn down and set on the spindle would have also been a check for this.

Good to mark a chuck to remount the radial place is good/same. /But won't help a wobble chuck very much

I ran a lathe that the part offs were good, but most everything out of the chuck had to go between centers. Getting used to doing that can be pretty quick with practice.

4 jaws can be good/tough..but a wobble 4Jaw can be a real headache.

For grinding a stout wheel arbor, perhaps a large enough wheel that your grinder nose/chuck can go in the bore to make the wheel arbor short.

If your wheel can push off the indicator with one finger then it will be tough to correct an ID or jaws.

Often good to spark out with coming out of jaws/chuck due to wheel break down and jaw taper.

 

[jbacc]

RE: the scroll fit very precisely to the point I had to tap it slightly to get it to seat properly.

You might run an indicator on the chuck OD and face and back if some is showing to get an idea if the body is running near true..This to consider that the back may have been altered and the chuck Inners are at wobble.

A pate check, or a check of the inners when it was torn down and set on the spindle would have also been a check for this.

Good to mark a chuck to remount the radial place is good./But won't help a wobble chuck very much

I ran a lathe that the part offs were good, but most everything out of the chuck had to go between centers. Getting used to doing that can be pretty quick with practice.

4 jaws can be good/tough..but a wobble 4Jaw can be a real headache.

For grinding a stout wheel arbor, perhaps a large enough wheel that your grinder nose/chuck can go in the bore to make the wheel arbor short.

Thank you, I am grateful to you and all the members who take the time to guide me, truly.

Joe

 

[michiganbuck]

Re:[I could not get it to repeat with the same stock without fiddling around and moving the rod around. ]

Be darn sure your test rod od very good/straight. Plate check to be sure.

 

[eKretz]

If you can't get a stiffer grinder setup worked out you could always try a good stout boring bar with a CBN insert. I think that would be better than the die grinder setup with the very long shank you're using now.

 

[jbacc]

If you can't get a stiffer grinder setup worked out you could always try a good stout boring bar with a CBN insert. I think that would be better than the die grinder setup with the very long shank you're using now.

Thank you.

 

[richard newman]

I have the same chuck, bought it used 25 yrs ago for my Leblond 17" Regal. It didn't grip as firmly as my 3 jaw, I called Pratt Bernerd, and they suggested sending to them to be re-ground. That's what i did, work ran very true, but it still didn't grip as tightly as I would have liked. Now I mostly use an 8" Bison, works great. I don't think I need 6 jaw, should probably sell it to someone who does.

As to your chuck, I'd be wondering kind of violence it suffered to crack the jaws. And that lack of repeatability suggest other problems. If you really want it to be right, it's probably a good idea to send it to Pratt Burnerd for an inspection. Won't be inexpensive tho.

 

[jbacc]

I have the same chuck, bought it used 25 yrs ago for my Leblond 17" Regal. It didn't grip as firmly as my 3 jaw, I called Pratt Bernerd, and they suggested sending to them to be re-ground. That's what i did, work ran very true, but it still didn't grip as tightly as I would have liked. Now I mostly use an 8" Bison, works great. I don't think I need 6 jaw, should probably sell it to someone who does.

As to your chuck, I'd be wondering kind of violence it suffered to crack the jaws. And that lack of repeatability suggest other problems. If you really want it to be right, it's probably a good idea to send it to Pratt Burnerd for an inspection. Won't be inexpensive tho.

Thank you, Richard.... I share your observations regarding it's past life. It was pretty stupid of me to just go out and purchase a new set of jaws without thoroughly checking out the rest of the chuck. That was around 5 years ago, I learned a lot since then but you gents probably forgot more than I will ever learn.

I will consider what you and others have said about sending the chuck to PB, I just don't want to keep spending good money after bad.

Thanks again.

Joe

 

[richard newman]

Well not me, I'm just an ignorant woodworker, but I wanted to share my experience