.012 runout in my 4 jaw

[fastfire]

I recently purchased a used 6 1/2"PRATT-BERNERD IRON BODY 4 JAW INDEPENDENT LATHE CHUCK.
I am hesant about grinding the jaws because I tried on a cheap 3 jaw and couldn't get it rite.
The outer gripping surface has more taken off than intended, it didn't fix the runout. I made a plate so the jaws were under the same pressure as gripping.
I used a high end flex shaft for the grinding mounted on the tool post.
What'd I do wrong?

[Blazemaster83]

How can you have runout on a 4 jaw? Can't you just dial out the runout with the 4 jaws? Unless it's a 4 jaw scroll...

[lakeside53]

Where is the run out? if the body.. then that's an issue with the backing plate. Not sure I understand the 4 jaw issue - each is adjustable, or do you have a 4 jaw scroll chuck?

[TN Pat]

Could be the chuck won't hold the parts straight axially. At an angle, even when concentric at the jaws. Not much you can do. Thing with four-jaw chucks is, they hold so much better. BUT - if they cock the part at an angle, you're forced to only grip by one or two teeth and tap the part at the far end to correct axial TIR, losing that advantage...

[Toolguy]

Sometimes the jaws are not parallel to the spindle axis. I reworked a 4 jaw like that and it came out almost perfect. I set the jaws, one at a time, in a screwless vise on the surface grinder, using the bottom of the jaws as a reference plane. I ground the gripping part of the jaws flat (they were originally radiused), put them back in and had a very true running chuck. It was out about .001 inch runout 6" from the face of the chuck.

I like the flat surfaces better for a 4 jaw (independent) because when you are dialing in you are moving over in a straight line, not going up or down a curve. It makes dialing in quicker and more positive.

[Rich Carlstedt]

Your problem is using the plate .
While it loads the jaws, it does not load the ends of the jaws where most work is held.
Drill holes ( .125 ) into the end face of the jaws . then place pins in the holes and then clamp onto a ring.
Now you can Grind the jaws as the out board ring forces the jaws to full bellmouth .
That will give you a true position under load.
I have PB chucks also and have drilled them OK.

Rich

[fastfire]

Chucked a 1" precision ground rod with 10" sticking out, I can indicate next to the jaws to -0- and the end reads .012 or visa-versa.

[fastfire]

Where is the run out? if the body.. then that's an issue with the backing plate. Not sure I understand the 4 jaw issue - each is adjustable, or do you have a 4 jaw scroll chuck?

4 JAW INDEPENDENT chuck
I took a skimmed pass on the backing plate before mounting.

[fastfire]

Your problem is using the plate .
While it loads the jaws, it does not load the ends of the jaws where most work is held.
Drill holes ( .125 ) into the end face of the jaws . then place pins in the holes and then clamp onto a ring.
Now you can Grind the jaws as the out board ring forces the jaws to full bellmouth .
That will give you a true position under load.
I have PB chucks also and have drilled them OK.

Rich

Such as this method? https://www.youtube.com/watch?v=-yWsjeY6nJU

[Rich Carlstedt]

No, thats not the way, as he uses a chuck with replace-able jaws.

Look at the picture under post # 8 by Jerry Tiers

http://bbs.homeshopmachinist.net/thr...-with-a-reamer

Get a carbide masonary drill bit and make the holes
Then place pins ( dowel or roll or drill rod) in the hole and clamp on a thin ring
VERY important. Mark the jaw socket you use to tighten the jaws AT THIS MOMENT .
That socket will be the most accurate as it shifts the scroll plate during the grind operation.
Paint the socket, or better yet, use the one nearest the chuck maker plate

Rich

[fastfire]

No, thats not the way, as he uses a chuck with replace-able jaws.

Look at the picture under post # 8 by Jerry Tiers

http://bbs.homeshopmachinist.net/thr...-with-a-reamer

Get a carbide masonary drill bit and make the holes
Then place pins ( dowel or roll or drill rod) in the hole and clamp on a thin ring
VERY important. Mark the jaw socket you use to tighten the jaws AT THIS MOMENT .
That socket will be the most accurate as it shifts the scroll plate during the grind operation.
Paint the socket, or better yet, use the one nearest the chuck maker plate

Rich

Mine is an independent 4 jaw.

[Rich Carlstedt]

The beauty of the above method is that the holes do not need to be matched or centered in anyway on the jaw face .
Just pick a spot on each jaw and go for it .
As long as they are on the face and enough room is left for the loading ring and grinding clearance , it will work.
Does not matter how many jaws you have , just load them .
With a 4 jaw, you will need to adjust the jaws somewhat to make them concentric to the spindle axis
Rich

[Richard P Wilson]

Chucked a 1" precision ground rod with 10" sticking out, I can indicate next to the jaws to -0- and the end reads .012 or visa-versa.

Is the runout always in the same place? In other words, is it just one jaw thats causing this? Really, for 12 thou runout 10" out, you are only looking at 1 1/2 thou in the length of a jaw, which isn't much. Examine the jaws really carefully, stone off any little raised areas before you start grinding. To be honest, for the runout you are talking about, I'd live with it.

[projectnut]

Chucked a 1" precision ground rod with 10" sticking out, I can indicate next to the jaws to -0- and the end reads .012 or visa-versa.

You have way to much material sticking out of the chuck without support. The rule of thumb is 5 times the diameter is the maximum length that should extend beyond the jaws. In this case 5" of material is the maximum amount that can extend beyond the chuck without deflection.

[Forestgnome]

Is the runout always in the same place? In other words, is it just one jaw thats causing this? Really, for 12 thou runout 10" out, you are only looking at 1 1/2 thou in the length of a jaw, which isn't much. Examine the jaws really carefully, stone off any little raised areas before you start grinding. To be honest, for the runout you are talking about, I'd live with it.

That's what I would check. I would want to determine that before blaming the jaws. You could also have a chuck body problem. If the runout occurs in the same place every time, I would try swapping jaws around and see if it follows the jaws. If it doesn't, then dismount the body from the backing plate and rotate it before remounting. Make lots of marks while you check things. It's always better to analyze the heck out of a problem before deciding the cure.