Should I get a 3 jaw chuck or a 4 jaw chuck for my dividing head?

I'm leaning towards the 4 jaw independent. Thoughts.
Wes

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    --Depends. Heh.
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My first chuck was a 4 jaw. I thought "It's more useful - it'll hold anything." True enough, but the diddling around with 4 adjustments every time got tiresome. Especially since 95% or more times it's round stock that I'm turning. I now have a 3 jaw also & I wouldn't be without it. I.e., I'd give up the 4 jaw 1st.
BTW, they are both Bisons & the 3-jaw centers to .001, IIRC.
Bob
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How about an adapter so you can use your lathe chucks?
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Ned Simmons

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wrote:

Exactly what I made to use my rotary table as a dividing head.
Steve R.
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Mine arrived yesterday. It looks well made and turns smoothly without roughness or tight spots, but a neighbor needed help cutting up and hauling some trees so I didn't get to check it out further.
I previously made a 1-1/2 - 8 adapter to hold a Jacobs 58B lathe chuck in a 1" 5C collet. I think I could make back plates for borrowed lathe chucks and center the work on it in the lathe first. As others have noted, centering in a 40 turn indexer is very slow and tedious. I made a non-plunger handle for my small rotary table to speed it up some.
The center taper is Morse 2, not B&S 7.
The Jacobs lathe chuck looked too good to pass up but I haven't found much use for it. I leave a 1/2" chuck on the 1/2-20 spindle of my small lathe to use for polishing, since the jaws aren't as dangerous.
jsw
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wrote:

My smallest chuck is too big.
Wes
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It's the only rational choice. What if you have to chuck up odd-leg parts?
It takes a few "lessons" to learn to quickly true up the work in a 4- jaw independent chuck, but you can work to much better tolerances with it than with a self-centering type.
LLoyd
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3jaw, if you want to get anything done without spending huge amounts of time in setting up.
reasons? a)using it as a rotary table, you will centre it under the mill spindle with the x-y mill controls. so it doesn't matter if its out a bit in the chuck. b)using it horizontally as an indexing head, the major fixed adjustment is determined by your tailstock - you adjust the other end to this, and check levels/eccentricity. There is adjustment in the divider head to get it really close.
And unless your machinery is in absolute, first class, no wear, calibrated condition - slop in othere areas is going to be the limiting factor, not the 3 or 4 jaw chuck.
(the above I learnt at trade school, in 'Basic Milling")
Andrew VK3BFA.
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On Wed, 13 May 2009 19:38:14 -0700 (PDT), snipped-for-privacy@wia.org.au wrote:

True only if you don't rotate the rotary table.
The mill's x-y controls can't take out centering error in the chuck because the x-y position of the mill table remains constant while the x-offset and y-offset of the piece in the chuck vary with the rotational angle.
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A 4-jaw will enable more precise centering and is more versatile, 3-jaw is quicker and more convenient. With a bit of practice it only takes a minute or two to get something centered in a 4J to under a thou.
You know what accuracy you seek. In my case, I have a 3J on my indexing head and have a 4J for it that I've never used.
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Time spent indicating work in a four jaw chuck on a dividing head will be quite a bit longer than on a lathe. 40:1 turns ratio makes for 20 handle turns every time you adjust a jaw and check with an indicator. Repeat a few, maybe three times for jaws one and three, three more for jaws two and four.
My past experience has been that a three jaw is adequate for the class of work done on items that require plain indexing cuts. For more complex and higher tolerance dividing work like cutting gears the job was placed between centers and driven with a dog. I've not had a project that required the hold of a four jaw or accuracy exceeding a 3 jaw with a bit of shim to correct chuck runout.
I'd vote for a 3 jaw for starters. Get the 4 jaw later if you have a project that can't be gripped in the 3 jaw.
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My vote is to go for the advantages of both. Get an "adjust true" three jaw chuck. I got one for my lathe and I'm plumb spoilt.
If you haven't seen one, this chuck has an extra plate that can be loosened and then four set screws to adjust the part right on center.
Karl
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wrote:

The 5" Bison Set-Tru is ~6" long including the backplate, which seems like a lot of overhang for a hollow 1-1/2 - 8 spindle.
I like the idea of a slotted backplate so the work can be supported out near the cut, partly to protect expensive gear cutters and hard to make custom formed tools. Indexing jobs are often machined on the OD and have to be clamped elsewhere anyway.
Large old wheel lathes had a faceplate with the drive gear on the back side of the rim. There were blocks that bolted into the slots for use as temporary chuck jaws, with a radial setscrew bearing against the work. They would center and hold a front-mount lathe chuck too.
jsw
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Couldn't you use one of those indicator holders that mount on the mill spindle, and first indicate on the chuck body. So you get the spindle directly over the dividing head center of rotation.
And then indicate on the part so you get the part centered.
That would be twice the number of times one has to indicate, so twice the chance for error. But I would think one would usually need to get the mill spindle directly in line with the center of the dividing head anyway at some point.
Well I think that would work when using the dividing head horizontal. Or you could center with the head horizontal and then rotate it to vertical.
Dan
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The BS-0 could be aligned with a 1/2" rod in its MT2 center hole. If your collet springs open a center finder point is convenient, it also works with 5C collet blocks. But they are difficult to remove from tight collets.
jsw
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    I don't know about your dividing head, but mine had a knurled thumbscrew which will disengage the worm from the spindle, allowing you to turn the chuck much more quickly by hand. It also has a ring behind the chuck with a lever-controlled pin for simple dividing (2/3/4/6/8/etc) so you don't even need the plate for simple dividing.

    And -- you can also set things up on an arbor between centers to get plenty of accuracy.

    I don't even (yet) have a 4-jaw for my index head. Between the 3-jaw and the between centers I've been able to do what I need to do. But a 4-jaw would be nice, and if I stumble across one which fits it, I'll pick it up.
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The exploded drawing shows a screw behind the index plate that may lock the eccentric worm engagement.

Personnally I've mostly done either lengthwise grooves or flats on a shaft, between centers or in a collet, or cuts on or near the OD of a disk which was centered on a mandrel and clamped to a faceplate. Two examples are the tractor steering gear and a circular tee slot. I've made a number of long adjusting screws by milling a hex on threaded rod, which doesn't fit collets very well.
I bought some 5C-mount lathe chucks to index a turned part accurately and found that they didn't hold securely enough unless I took very light cuts. I didn't want to overtighten and damage them. The instructions for the Sherline chuck warn that it's for light duty only. If the shaft rotates while cutting flats one side jams into the end mill. Then the belt slips and I weld up the gouge and regrind the tool, but that would be expensive on a geared-head mill.
Some commercial jobs I've worked on were more difficult, the most complex was a rotating ink jet print head somewhat like the crankcase of a radial engine. I think the machinist made a fixture with centering plugs to locate both sides.
If you use a faceplate you can clamp all around the edges and remove each one to cut under it. You could center the work on a rod in the center collet by making a cone that slides on it, or a slightly tapered bushing for more precision.
jsw
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    That could be it -- but IIRC, it is near the other end of the worm screw from the hand crank and index plate. It both allows the screw to be disengaged (it is in a bearing which is pivoted near the index plate), and to be adjusted for minimum backlash when it is engaged.

    Hmm ... I've made some by using a hex collet and hex stock, turning the OD down using a box tool (in a bed turret) and threading using a Geometric die head, then parting off. Very quick if you need to make more than one -- slow to set up for the first one.

    Yep. My dividing head has a center with a T-bar just behind the point. The T-bar is slotted parallel to the axis on each end, and drilled and tapped for a square headed setscrew on each end. You set up the work between centers, clamp a dog (also using the same style of square headed setscrews) around the arbor or some direct part of the workpiece, and then tighten one setscrew in the T-bar against the tail of the dog so there is no give in the connection of dog tail to T-bar.
    It also has a hefty three-jaw chuck which screws onto a 1-1/2x8 (or something similar -- I really need to check that again and make a mount for the smaller 4-jaw chuck which I used to use on the old 2-1/4x8 spindle for the Clausing before I converted it to L-00 spindle nose.)

    And, of course, something to prevent rotation of the workpiece relative to the dividing head.

    Right -- but I don't have a faceplate for the dividing head. Perhaps I should make one of those, too. The thread should not be difficult to turn using the Clausing.
    Enjoy,         DoN.
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4 jaw, definitely - unless you spend so much time using the dividing head that dialing in the 4 jaw gets to be a drag. In which case, you'll probably be using collets anyway.
You can do things with a 4 jaw chuck that you simply can't do with a 3 jaw. I can't think of anything that the 3 jaw can do that the 4 jaw can't. Holding hex stock comes the closest, but the 4 jaw will do even that with some patience. If holding hex stock is your primary objective with the dividing head chuck though, I'd opt for the 3 jaw. Or hex collets.
Assuming that your dividing head allows for disengaging the worm gear, dialing in the 4 jaw is quick. As others have already mentioned.
John Martin
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