Mounting a 4 Jaw in a 3 Jaw chuck on a lathe.

Roger Williams · Dec 23, 2012

Hello, is it good practise to mount a 4 Jaw chuck in a 3 Jaw for the sake of convenience ?. I've seen this done a few times on Utube and was wondering if this is a common 'trick' in the machining trade. It looks as though it would save taking a bloody great chuck off for just a small 4 Jaw job. It doesn't seem to look right , but perhaps with discretion ?. Cheers. :confused:

oldbikerdude37 · Dec 23, 2012

Iv done it for light cuts on smaller parts. I would not if I need to turn a long piece of large square stock round part way. Not for hogging.

Its OK but not something I would do often, I just swap out the chucks, its not that big a deal to change a chuck.

JH-Q · Dec 23, 2012

I do this all the time on my Nakamura. As i make a lot of one-off parts and only light cuts are needed on the 4-jaw. Good enough for making small acme threads like 20x4mm.

I also sometimes clamp a 160mm cheap 3-jaw on the 300mm hydraulic chuck jaws, for second ops and delicate small parts which the big hydraulic chuck would deform.

Roger Williams · Dec 26, 2012

Hello, thanks for your replies. The main reason I asked was that Keith Fenner did this in one of his excellent videos and Itherefore wondered how common it was. Cheers.

Marty Feldman · Dec 26, 2012

Isn't it simply a matter of what kind of tolerances and degree of accuracy you need?
Every piece of workholding apparatus has its runout, and these multiply as you increase the number of them in the train.

Lakeside53 · Dec 26, 2012

Maybe so, but that's why it's in a 4 jaw so it can be adjusted deadnuts. You do need decent jaws on the "holding" chuck and body on the "held"; some setup fiddling to be correct in all planes.

To me it make more sense to mount a 3 jaw in the 4 jaw. At any one scroll position they will repeat pretty well if dialed in by the 4 jaw.

I use an ER32 straight shank collet chuck mounted in a 4 jaw. Works great for repeating work.

Nmbmxer · Dec 26, 2012

My power chuck has tapped holes around the face, I assume for bolting counterweight. I made several plates that register in the center of the chuck that bolt into these holes. To put an adjust true 3 jaw on I just remove the softjaws and bolt the chuck backplate over them. It works very well, I have them for an adjustable 3 jaw and a small 4 jaw. I can get more RPM on the 6" steel 3 jaw than I can on the 6" pneumatic power chuck. The small 3 jaw lives on the lathe 95% of the time, I can adjust the runout easily, it repeats, it doesn't smash the bejesus out of plastic parts, and most importantly to me -- there is no pressure to bleed down and throw parts out (I hate my pneumatic chuck.) It is important to make sure that your power chuck doesn't bleed off, you might leave the small chuck clamped in the spindle and when you power down for the night it can fall out and dent your way covers...

Cole2534 · Dec 27, 2012

Lakeside53 said:
To me it make more sense to mount a 3 jaw in the 4 jaw. At any one scroll position they will repeat pretty well if dialed in by the 4 jaw.

You lake a good point, I'd not thought of that.

Turn a proper gage in your 4 jaw, install 3 jaw, indicate the assembly and go to town.

Mechanola · Dec 27, 2012

It is not good practice, it’s dangerous. It’s been done during my apprenticeship and I always found it frightening. Chucks are not made to be put into one another. Sorry, tinkering to me

Philabuster · Dec 27, 2012

oldbikerdude37 said:
Iv done it for light cuts on smaller parts. I would not if I need to turn a long piece of large square stock round part way. Not for hogging.

Its OK but not something I would do often, I just swap out the chucks, its not that big a deal to change a chuck.

Depends on the size of the chuck.

I have a large 21" 3 jaw in the manual lathe with D-11 back. I have a half-dozen different 4 jaw chucks I will use which range in size from 6" to 12", not including the monster 24" chuck which actually fits the lathe's spindle nose. No way in hell am I going to use the monster 24" chuck--especially when an 8" chuck will allow me to finish the job with less time dicking around. The smaller chucks are better suited to some jobs as the jaw sizes are smaller. Soft jaws allow you to clamp securely on the chucks without fear of damage or spitting out the chuck. I also have a few chucks with steel back plates for direct clamping in the hard jaws.

I also chuck up a 4 jaw in the CNC lathe using soft jaws when necessary.

Cole2534 · Dec 27, 2012

Mechanola said:
It is not good practice, it’s dangerous. It’s been done during my apprenticeship and I always found it frightening. Chucks are not made to be put into one another. Sorry, tinkering to me

In my idea above one would use a thread mount chuck screwed onto some bar or thick wall tube so youre essentially chucking an arbor. That doesn't seem so bad to me.

James H Clark · Dec 27, 2012

Roger: Although I agree with Old Biker Dude and others that it is not a recommended practice, some times it can be done . Using Lakeside53 's idea of mounting a 3 jaw in a 4 four jaw, remember to only chuck on the back plate of the 3 jaw, the rest of the body on a 3 jaw can be hollow, thus easily damaged. Good luck.

JH

mach2 · Dec 27, 2012

Be sure to wear steel toe shoes for when the chuck falls.

moonlight machine · Dec 27, 2012

mach2 said:
Be sure to wear steel toe shoes for when the chuck falls.

Why?, It's hard to get your toes out when the steel toe cap is smashed down and pinching them!

Abom79 · Dec 27, 2012

I've never mounted a 4j in a 3j, but have done just the opposite many times. Mostly it has been a smaller 3j chuck needed for small parts. Just indicate the 4j true. I also mount my Jacobs collet chuck in the 4j quite often, mainly because it doesn't fit the spindle nose. Works great!

adama · Dec 27, 2012

This came up in the cnc section, net result is a few people have done it, most successful but a few disastrously! Like any quick bodge it allows for a whole new bunch of things to go wrong in bad ways. That said i have done it, but in my case its gripping my baby 3" 4 jaw in the 8" three jaw on my Harrison to grip small square work (big 4 jaw does not close enough). That 4 jaw mounts on a clock makers size lathe, hence its probably significantly better held in the big three jaw than on its own lathe! You just need to adjust speeds + feeds appropriately. With a cnc though you also need to reconsider all your clearances + origin shift! Equally there is a bunch of other safety features that this by passes on a cnc which could catch a operator out (they normally look for a closed chuck signal before starting for starters).

Hence this really needs some care and a bit of a common sense risk assessment before doing this outside of a home shop enviroment.

stuarth44 · Apr 7, 2020

just mounted a 10inch 3 in a 22 inch four, changing the 4 is not easy, esp one handed

Phil in Montana · Apr 7, 2020

I would never mount a 4 jaw in a 3 jaw chuck, however I mount a 3 jaw in a 4 jaw all the time...Phil

Trueturning · Apr 7, 2020

I would not do it. The three jaw is secured very solidly by itself yet holding the four jaw Chuck with it relies on the scroll and of course how deep the 4jaw can be held and whether the Chuck would bottom out solidly against the face of the three jaw Chuck. Clamping and machining forces matter.

The very smartest thing mentioned earlier is to just remove one Chuck for the other use the right thing. No one will lose their shirt just changing it out.

Many over the years like to go two miles out of their way to show how they can do flaky work. I do feel they are much more a danger to safety when they try this. Sure there are things that are good ways to do things I just would go with the 4 jaw secured by itself. Can’t think any other way would be as acurate and rigid as that.

There is a lot of weight to using the best tool for the job. Even the best way to measure. I think to get serious with it using the proper Chuck is better than playing with it.

Bobw · Apr 7, 2020

Phil in Montana said:
I would never mount a 4 jaw in a 3 jaw chuck, however I mount a 3 jaw in a 4 jaw all the time...Phil

How about a 3jaw in a 4 jaw bolted to a 3 jaw?

Yes, I really did this. I have a 6" 4 jaw that I bolt to the front of my 3 jaw
for when I need it, and I was using it, and a "SUPER HOT" job came up, it was a
Saturday night into a Sunday morning thing, hundred parts or so, nothing critical,
if there was actually a print on it, there would have been a napkin, a sketch, and
the tolerance block would have said "Has to WORK".

So I just grabbed a POS 3 jaw I had sitting around and tossed it in the 4 jaw.

Not the best thing to do.

I see nothing wrong with grabbing a smaller 4 jaw in a 3 jaw for a quicky. I've done
it many many times until I made a plate that could bolt to my 3 jaw.

I've also chucked up a straight shank ER holder in a 4 jaw bolted to a 3 jaw.

Sometimes you just gotta do whacha gotta do.

Mounting a 4 Jaw in a 3 Jaw chuck on a lathe.

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