3 Jaw Vs 4 Jaw Chuck

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Bray D

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I have a PM 932M with an 8" Vertex rotary table that I'd like to outfit with a chuck. I've never used one in real life, but I realize the benefit of being able to clamp on the outside of a piece rather than flat clamping to the rotab surface.

What are the pros/cons of a 3 jaw and 4 jaw chuck? I'll likely be clamping on square and rectangular stock frequently. Intuitively, this would drive me towards a 4 jaw chuck.

Any thoughts?
 
Given that much of your work involves square and rectangular stock, then the independent 4 jaw would be the best option. The 4-jaw is more time consuming to center but is more versatile. If it's in the budget, buying both a 3 and a 4 jaw might speed things up.
 
The only advantage to using a three jaw scrolling chuck is rapid fixturing. There are four jaw scrolling chucks available. The four jaw independent chuck can do anything the three jaw can plus mount workpieces off-center, precisely center a workpiece, and work with irregularly shaped workpieces.

One thing that I would like to see is a thin X-Y table that could be mounted to an RT. It would allow you to perform many of the 2D machining operations that would otherwise require a CNC. The DRO on mold mill/drill has an arc cutting feature but it requires a large number of small X-Y steps to make a reasonable arc. I also have a 12" RT on the mill and if I could precisely move the workpiece to the center of the RT, cutting that arc would be a piece of cake.

Years ago at work, I had programmed cutting an Archimedes spiral in G code (we didn't have a CAM program at the time). My recollection is that there were about four thousand lines of G code by straight line interpolation. We later investigated having an outside vendor make the product with a CNC diamond wire saw but their machine was limited to a few hundred lines of code. I rewrote the G code, changing to arc interpolation and the same precision was obtained with only a dozen lines of G code.

In theory, this operation could have been done fairly painlessly with a manual process an X-Y table on an RT.

(I got a Tormach, which ended my quest for the slim X-Y)
 
Thanks for the responses. The quick fixturing that comes with a three jaw is appealing, though the versatility of a 4 jaw has its benefits as well.

I'll ponder it a bit more before I pull the trigger. I'm so new to machining, it's difficult to ascertain what jobs will lend themselves more to the rotab than others.

I agree that having one of each would be ideal; I'll just have to decide which one I'd like to add to the arsenal first.
 
My suggestion is to not fight and just join the club with us other tool junkies. I am new to machining and have had to buy another rolling tool chest to fit all of,the new toys I have acquired. If you don't want to join the club then get,out now before your hooked. It is a truly debilitating condition that leaves you perusing craigslist and eBay almost constantly...
 
Oh yes, Enco and others have been draining my bank account regularly. I've become a self proclaimed tool junkie. The mill purchase was simply feeding the addiction, which I've found has now compounded the issue, haha. The chuck is a relatively large purchase, so I'm doing a bit more legwork before picking one up. Im sure I'll have one of each flavor before too long. Im leaning towards a 3 jaw right now, simply due to the auto-alignment property. I think I could get by with the square and rectangle stock in my vise. We'll see what I decide in the next few weeks.
 
If you already know yo are going to be doing a bunch of square stock, the four jaw is a better place to start.
Second- are these pieces that have been worked on before getting to the lathe? Or is it just starting at square stock? If these are second ops, DEFINITELY go Independant four jaw.

Bernie
 
One other thought, that I didn't see mentioned. With the 3 jaw you will always have the 3 to 5 thousandths run out ( more or less) which is normal (or so I've read) for a 3 jaw. With the 4 jaw you can dial it in "dead nuts", but you do have to dial it every time. Good luck with whatever you decide, JR49
 
Well depending on what you really need of doing all the same size you can leave two jaws in place only losing the other two & change the stock out. I do this often on the lathe when a 1/16th is close enough I don't even remeasure. I can set a 4 jaw up about as fast as I can true a piece in a 3 jaw. Now if I'm doing something like junk pipe I'll through that in a 3 jaw. I also usually use a 3 jaw if I'm going to be turning a concentric shaft for another part to ride on but that probably won't be likely on your rotary table but you never know. A 4 jaw independent can do almost anything a 3 jaw can but the other way isn't true.
 
If you're doing square/rectangular work, a four-jaw is a must.

Let me also put this out there - four-jaw chucks hold far better than a three-jaw, even without biting the work. At the shop I work at, I've seen ol' Dave do things with a four jaw I could never do with the three jaw on the Haas TL-2 I run. We've had a lot of big work in - 10, 11, 12" diameter pucks, usually 4 to 5 inches long. He can put one in his four jaw chuck, holding by the ID jaws - I think they're 3/4" thick - and take .050+ deep facing cuts with no problem, plus well over an eight-inch deep on the OD...

I have tried that with a three jaw, and have in fact had disastrous results. I've launched 10" parts before. The only time I've praised the obnoxious chuck guard on the TL series lathes!
 
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