Truing 4-jaw chuck jaws

[cwins]

I've got a well-used Skinner/South Bend 6" 4-jaw chuck that came with my SB9 that works perfect for my hobby uses, but as I was aligning the lathe with a ground/polished bar the jaw wear/bell-mouthing was very evident. I've got about two thousandths wear in the first 1/4" of each jaw, leading to an odd pivot point when centering work. Here's an example with some .001 shim stock, one jaw removed for clarity.



I removed the jaws and on the face, it looks like there's a factory taper on the jaw face to bias the grip towards the front. Each jaw had about .002 rise from back to front, except mine drop back to 0 in the last 1/4 inch. As it's a 4-jaw independent chuck, and I don't have a tool post grinder, I clamped one jaw in the mill vise and indicated the jaw face flat. The wear on the jaws is very evident here:


As each jaw is independent, my thinking is I'll be able to take a light cut across the top of the jaw to true it up past the worn portion while staying within the case-hardening range, but I'll be ending up with a flat face instead of concave. To leave some concavity, I could cant the mill head and use a larger-diameter carbide end mill to duplicate the 1/2" radius in the jaw.

Has anyone tried this approach, and if so - how much reduction in grip is there with a flat jaw face?

 

[cwins]

Looks like the second picture dropped out of the first post. Here's the jaw with wear visible, indicated flat in the vise:

 

[markba633csi]

You might want to use a small Dremel-type grinding stone instead of an endmill - just protect the machine from grinding dust
A 3-jaw needs to be trued on the lathe itself but with a 4-jaw you can true the jaws separately like that
as long as the main body of the chuck isn't sprung- which usually means it's time for a new chuck

 

[Chipper5783]

Are you sure the jaws are hardened? Granted, they are not made of butter - but perhaps not hard like most 3J chuck jaws are. I cut the hardened jaws on my 3J (a cheap TDC insert off Ali), worked great.

You asked about going to flat face jaws, they would still work, but boring the jaws in place is not difficult and why not keep the features that are present from when the chuck was first made?

It is a bit of a trick to load the jaws, so that they cone out (normal deflection), then when you recut the jaw it biases to slightly smaller out near the outer part of the jaw (that factory taper you mentioned). The three methods I am aware of are:
- to drill holes in your jaws for a pin, then with the 4 little pins sticking out, squeeze a ring

- make up a “clover leaf” plate, 4 holes for the jaw tips and a bigger hole in the middle (made carefully so you can index on the OD of said ring).

- cut a groove in back of the first jaw step, slip a loading ring into that slot (as per Robin Renzetti - “Lathe Chuck Magic”).

Another method that comes to mind is if you flipped the jaw 90 in your mill vise (from your picture above), but hanging out the side a touch, then plunge mill with a good sized EM (or a boring head), targeting a ~1/2” radius curvature, set a stop so each jaw is in the same position, shim the jaw slightly off level - you would get the curved jaw face and the cone grip.

My preference is always to cut the jaws on the lathe it will be used on (marking the chuck mounting position and always mount it the same). Then the chuck axis will be dead nuts on to the lathe axis.

Let us know how it works out.

 

[cwins]

Are you sure the jaws are hardened? Granted, they are not made of butter - but perhaps not hard like most 3J chuck jaws are. I cut the hardened jaws on my 3J (a cheap TDC insert off Ali), worked great.

I found some old Skinner literature claiming the jaws were case-hardened, and a check with the Tsubosan files showed they were between HRC 60-65.

You asked about going to flat face jaws, they would still work, but boring the jaws in place is not difficult and why not keep the features that are present from when the chuck was first made?

It is a bit of a trick to load the jaws, so that they cone out (normal deflection), then when you recut the jaw it biases to slightly smaller out near the outer part of the jaw (that factory taper you mentioned). The three methods I am aware of are:
- to drill holes in your jaws for a pin, then with the 4 little pins sticking out, squeeze a ring

- make up a “clover leaf” plate, 4 holes for the jaw tips and a bigger hole in the middle (made carefully so you can index on the OD of said ring).

- cut a groove in back of the first jaw step, slip a loading ring into that slot (as per Robin Renzetti - “Lathe Chuck Magic”).

I was originally going to cut these on the lathe once I figured out how to preload the jaws, leaning towards drilling small holes for pins to squeeze a ring and possibly thread them later to secure shim stock tabs, similar to what Kurtis from Cutting Edge Engineering did on his larger chucks. This would also account for any wear in the slots, although I only saw minimal wear when indicating those.

Another method that comes to mind is if you flipped the jaw 90 in your mill vise (from your picture above), but hanging out the side a touch, then plunge mill with a good sized EM (or a boring head), targeting a ~1/2” radius curvature, set a stop so each jaw is in the same position, shim the jaw slightly off level - you would get the curved jaw face and the cone grip.

That was another possibility I considered, balancing the vise with a jaw in each side taking a plunge cut to keep the radius. Hadn't considered the boring head, but I'll have to look into that as I imagine single-pointing it would leave a better finish than an end mill.

My preference is always to cut the jaws on the lathe it will be used on (marking the chuck mounting position and always mount it the same). Then the chuck axis will be dead nuts on to the lathe axis.

Let us know how it works out.

Am I correct in this assumption - as this is an independent 4-jaw, any deviation indicating in (within reason) during preloading and boring in-situ will be able to be adjusted out during subsequent use?

 

[pavyan]

- to drill holes in your jaws for a pin, then with the 4 little pins sticking out, squeeze a ring

- make up a “clover leaf” plate, 4 holes for the jaw tips and a bigger hole in the middle (made carefully so you can index on the OD of said ring).

Illustration of two methods described by @Chipper5783 - one picture is worth a thousand words

PS:
As for drilling the jaw faces - I drilled 5mm holes in them with carbide insert drill bits.

And to tell you the truth, they were cheap 5mm concrete/stone drill bits - of course, I sharpened/ground them appropriately for steel drilling (on a diamond grinding wheel; with such a tool you can sharpen carbide like a razor blade, without any nicks).

 

[Chipper5783]

Am I correct in this assumption - as this is an independent 4-jaw, any deviation indicating in (within reason) during preloading and boring in-situ will be able to be adjusted out during subsequent use?

Sort of yes, sort of no.

Yes, you can dial out any deviation at any given position along your part (one inch from the chuck jaw, 10” from the chuck jaws - whatever, it will be perfect). However, if you dial it in at the chuck to 0-0, then go 10” further down the bed, then there probably will be a little run out, ideally it would still be 0-0. I’m pretty happy if I can get it to better than 0.005” TIR 6” along the bed (when you’re going to cut the piece anyway, then the runout does not really matter, just don’t want it to be horrible). If you cut the jaws in the chuck on the lathe (assuming you have tuned up the chuck mounting so it is repeatable), then it will meet the machine accuracies.

 

[Chipper5783]

I like the pictures provided by pavyan - the last one is of a 4J scroll chuck. You are working on a 4J independent chuck so if you would need to think about how to locate the jaws so you are taking about the same amount off each jaw (provide a register ring, reach in with a finger dial (DTI)) - there would be a couple solutions depending on how you load the jaws.

Or go for quick and dirty - grab a ring with the OD part of the jaw (so the jaws are even), then come in with a boring bar and step bore the jaws larger as you go in. How much is up to you, say 0.001” bigger for every half inch further in - then try it, if you don’t like it cut again. Yeah, quick and dirty (real dirty). However, it will accomplish much of what you are trying to accomplish.

 

[RJSakowski]

Here was my approach to grinding the jaws on my 4 jaw chuck.

Thread 'Grinding My Jaws'

Oct 1, 2016

I recently took a critical look at my Grizzly 6 -1/2" 4 jaw chuck and found the jaw grind to be seriously deficient and I decided to try to regrind my jaws.

I have a Craftsman 1/4" die grinder that I would press into service but I needed to come up with a stable mount for the grinder. My design utilized the AXA style QCTP and a Shars 5/8" tool holder. This design permitted me to adjust the angle of the die grinder spindle axis to the lathe spindle axis as well as the height of the die grinder relative to the lathe spindle axis

In order to grind the jaws of a lathe chuck in situ, some...

• RJSakowski
• Replies: 6
• Forum: ALL ABOUT MACHINE RESTORATION & WAY SCRAPING

• 

 

[woodchucker]

since it's a 4 jaw independent, not a 4 jaw scroll...
I would just put them in a vise on my surface grinder and take off the material.
I realize you probably don't have a surface grinder.

So rather than do all jaws at once, I would probably just do one at a time. You are only looking to kiss that one jaw. You don't even have to spin it to speed, just rock it back and forth and advance the compound ... (assuming you have a tool post grinder).
Then do the next. Why am I recommending this over all 4 at once. Much easier to just true them up, vs zeroing them out and then truing them up.
This is not zeroing them like putting a piece of stock in. That averages the jaw, this will drive you nuts since it is a point ..