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Grinding a four-jaw chuck

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Roger Head23/11/2015 07:33:39
209 forum posts
7 photos

I have a Chinese 8" 4-jaw independant chuck that needs the jaws grinding - a lot! I won't bore you with the details, sufice to say that the manual/QC booklet is a true work of fiction.
My grinding experience is completely covered by off-hand preparation of HSS tool bits, so I have a few questions and would be gratefull for any advice offered.
My preferred method is to grind in-situ on the lathe, with the jaws loaded to accentuate bellmouthing via one of the 'spider' arrangements that I have seen used (on the web) by others. Obviously the radial position of the jaws will be set as close to equal as practical. I am contemplating using a Makita laminate-trimmer / router which appears to have decent bearings, a really chunky shaft, and an inbuilt speed control (10000 - 30000 rpm).

**LINK**

This unit has a cylindrical metal body (unlike many of the other brands which are mainly plastic), and what appears to be a fairly good collet chuck (I have examined one closely ay the store). I would need to make a short extension so that standard-length mounted points will reach to the full depth of the jaws, approx 60mm, but I feel comfortable doing that if I keep the revs reasonable and the feed light.
My main question relates to the dimensions of the 'W' mounted point and its engagement with the jaw during grinding. Once I get away from the very small diameters, the height of the wheel increases significantly e.g. 25mm dia. are available with heights of 3, 6, 10... mm, whereas the 40mm dia. start with a height of 13mm, and the next is 25mm. See the last page of this link:

**LINK**

Is it acceptable to use a wheel slightly angled, so that it is cutting just on the leading edge, or should the full height be in engagement with the work? The reason for asking is that if it can be angled, the router collet nut could enter the open chuck jaws (~53mm dia.) and I could use a shorter extension shaft. It wouldn't be much of a saving, but every mm shorter would be good.

I don't mind having a go at something, but I prefer to have a bit of knowledge to back it up.

Thanks, Roger

Chris Evans 623/11/2015 08:55:21
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2067 forum posts

I have heard about people doing similar things with a dremel lashed on to the lathe. All seems a little to dodgy for me to attempt even with my tool post grinder. Question is it the jaw that is tapered or are they belling out when tightened due to poor groove location. If it is poor location grinding in situ may not help and best to try and improve. Chris.

norm norton23/11/2015 09:23:19
188 forum posts
9 photos

Usually, you only need to remove a few thou from the tips of the jaws to get them aligned. I have successfully ground a couple of three jaw chucks by the established method of: place a centre in the headstock spindle, remount the chuck, support a MS bar of around 3/4" between centres, apply grinding paste to the bar and gently apply jaw pressure between rotating the chuck under slow rotation, hold the bar in a leather glove. This way you are closing the jaws on the scroll and will hopefully get a TIR of 0.0010"-0.0015". All the usual warnings of grinding paste and thinking how you CAREFULLY go about this apply.

If your jaw tips are very bad, then perhaps they could be reprofiled out of the chuck? I don't think you you want to end up with jaws that have a large convex contact face.

Paul Lousick23/11/2015 10:11:50
2075 forum posts
727 photos

Hi Roger,

A small wood router makes a good grinder and much more powerful than a Dremel and has a boss on the end for mounting in a suitable holder. Used one with a rotary burr to shape 10mm steel plate.

See my post for a 6" Ruston Proctor Traction Engine.

Paul

Edited By Paul Lousick on 23/11/2015 10:12:32

John Hinkley23/11/2015 10:15:17
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1354 forum posts
430 photos

I've never done this myself, but, surely, as it is an independent four-jaw, it doesn't matter where the jaws are set - all that matters is that you are grinding the gripping surfaces parallel to the lathe's longitudinal axis. As Norm says, above, this could equally be achieved off the lathe, assuming the location grooves are in good condition.

John

Ian S C23/11/2015 10:46:36
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7468 forum posts
230 photos

With the independent 4 jaw chuck the jaws can be removed from the chuck and set up at right angles on the mill and ground that way.

Ian S C

Roger Head23/11/2015 11:38:32
209 forum posts
7 photos

Just to give you an idea of what I am dealing with here: I have run the DTI over each jaw from the innermost 'tooth' to the outermost (at the nose of the jaw). Seven steps in all.
Jaw1: steps up linearly to be 0.26mm high at the nose.
Jaw2: steps down linearly 0.11mm before rising slightly at the nose.
Jaw3: steps down linearly to be 0.31mm low at the nose.
Jaw4: steps down linearly to be 0.24mm low at the nose.

Those measurements were made with a light gripping force being generated i.e. the jaws being firmly driven against their respective worms, but not sufficient to begin generating a bellmouth effect.

Tightening the jaws further on a workpiece to a secure level (i.e. a good 'grunt' force) generates an additional 0.1mm of bellmouthing.

There are other problems with this chuck, but they can be overcome.

Norm, how do see a convex contact face being generated? I have considered external profiling , but that would entail making a grinding table, guides, etc. I think it's six of one, half a dozen of the other.

Paul, if you check the link you'll see that the Makita is a router.

John, true, the radial position of each jaw doesn't need to be identical, but setting them in that position makes the 'spider' loading plate symmetrical.

Thanks for your comments.

Roger

steamdave23/11/2015 11:40:57
521 forum posts
45 photos
Posted by norm norton on 23/11/2015 09:23:19:

Usually, you only need to remove a few thou from the tips of the jaws to get them aligned. I have successfully ground a couple of three jaw chucks by the established method of: place a centre in the headstock spindle, remount the chuck, support a MS bar of around 3/4" between centres, apply grinding paste to the bar and gently apply jaw pressure between rotating the chuck under slow rotation, hold the bar in a leather glove. This way you are closing the jaws on the scroll and will hopefully get a TIR of 0.0010"-0.0015". All the usual warnings of grinding paste and thinking how you CAREFULLY go about this apply.

I've not heard of that method before. Sounds easy for correcting minor imperfections.

Dave

The Emerald Isle

Ian P23/11/2015 15:21:43
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2594 forum posts
114 photos

Roger

Two things. I think the concave face referred to was if the tips of the jaws are ground in situ their gripping edges are part of a 'cylinder' so concave in the longitudinal axis.

With an independent chuck I dont see an point whatever at grinding the jaws in the chuck. I have trued up and transformed both independant and self centering jaws by stoning (diamond type) the jaws out of the chuck.

Obviously much easier with independent chucks, all you need is a decent bit of ground bar, a source of light, and a little patience.

Ian P

David Clark 123/11/2015 16:37:20
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3357 forum posts
112 photos
10 articles

Put a washer at the start of the jaws (near tailstock). Grind the outside diameters of both outer steps. Set each jaw parallel to the surface, just grind the inside face of the jaws (the bit that holds the work. You should be able to do this in a milling machine with a grinding wheel on the arbor. Don't forget the wheel guard.

Roger Head24/11/2015 01:02:39
209 forum posts
7 photos

Hi Ian, I was querying why Norm thought there was a possibility of getting convex tips. As you say, concave is the usual.

The benefit that I see of internal grinding the jaws is that all four are done at once (and are in the 'loaded' condition). Also, there is a fair bit of meat to come off several of the jaws, so altogether I think machine over manual is attractive.

DavidC. Ok, grind the outside to establish reference surfaces...

then

"in a milling machine with a grinding wheel on the arbor. Don't forget the wheel guard."

A grinding wheel in the spindle of the mill? How do you arrange a guard for that? A clamp around the nose of the quill?

Hopper24/11/2015 06:55:58
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6652 forum posts
347 photos

I ground my three jaw chuck in situ using a "clover leaf" to put outward pressure on the outer ends of the jaws. Used a Dremel tool at a slight angle. Dressed the grinding stone with a diamond dresser but did not worry too much about getting it dead parallel to the job. It is only the leading edge that "cuts".

I did it a couple of years ago and the chuck has worked perfectly since.

The Dremel is a bit light for this kind of duty but we got there with small cuts, so the idea of using a router is probably a good 'un.

And rather than making up a spider or clover leaf, which needs the holes precision located to work properly, it might be easier to use the other common method of using three (or in your case four) pieces of flat bar in between the side faces of each chuck jaw -- sort of like a spider but made in four pieces.

Ajohnw24/11/2015 09:57:41
3631 forum posts
160 photos

It sounds to me that the best and quickest option would be to grip some pieces of strip between rather than in the jaws and then grind them individually by rocking the chuck by hand.

Very light cuts, maybe 1 thou at a time to help reduce grinding wheel wear as you may not be able to feed the wheel entirely through the jaws. Dressing the wheel somehow with a diamond point fixed in place wouldn't be a bad idea.

John

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Roger Head24/11/2015 11:39:49
209 forum posts
7 photos

Hi Hopper, yes, I'd already decided on the four bits of steel if I do it in-situ. I had tried it knowing that theoretically there wouldn't be any forces trying to pop them out, but it was good to actually see it in action. I think I'd put a large worm-drive hose clamp around the periphery, just for safety's sake.

That's a very impressive spider you've made - knurled and all...

John, yeah, the diamond point might be a good idea. I saw a photo where someone had one mounted on a mag-base sitting on the carriage, and their grinder on the top-slide.

I'm not really committed either way yet...

Roger

Hopper25/11/2015 04:31:47
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6652 forum posts
347 photos

If you use the strips, you can do the same as I did in on the spider, drill a hole and hacksaw a slot in between each pair of jaws. That gives the strip some spring to take up any small differences in dimension between them. I ran my lathe on back gear at about 30rpm while grinding. I wouldnt bother with hose clips etc at that speed.

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