Lathe chuck size

[bambuko]

This just a sanity check ;D
For a 4" centre height lathe (8" swing) what would be the max three jaw chuck you would be prepared to fit and use?
I am talking here EMCO type lathe and it's many clones. They normally are delivered with 4" dia chuck and I would have no hesitation to use 5" chuck, but what about 6" chuck?
Of course I know I can do it (ie it will fit), the question is though - is it a good idea, or should I be concerned about something? (like for example it's weight?).
I am good on theory but often short on practical :-\, so your experience and recommendations would be appreciated - thanks.

Chris

 

[steamer]

The Problem you will run into with a large chuck is that if you limit the opening of the chuck jaws. With a 6 inch chuck, you can't extend the jaws very far before you foul the saddle or the bed.

A 6 1/4" 3 jaw chuck fits my 12" lathe nicely in that respect, so using the same ratio, I would say the 4" would also work well. You might be OK with a 5" though. I have found that most lathes have a slightly larger 4 jaw chuck supplied with them than the 3 jaw.


Dave

 

[Bill Gruby]

You may also run into a weight problem. This will cause excessive belt wear on start up. Once up to speed you still may have belt problems.

"Bill Gruby"

 

[Mainer]

A 6" chuck is about right on my 10" lathe. Anything else would really be too big. As steamer points out, as you open the jaws they begin to project beyond the diameter of the chuck, effectively making its o.d. larger. If you put a 6" chuck on your 8" lathe, you could open the jaws only an inch before the effective o.d. became 8", thereby eliminating any increased capacity advantage of the 6" chuck over a 5" chuck.

 

[DICKEYBIRD]

I have a 160mm (6.3") 4-jaw for my Emco Compact 8 clone and use it all the time.

Yes, it's big & heavy but it works very well. I only use it with the belt in the indirect drive positions and always start up with the belt tensioner off, spin the chuck by hand and then apply the tensioner. Been using it that way for 7 yrs. and have suffered no ill effects that I know of.

I like that it allows a bit longer lengths of larger stock to be chucked that won't go up the spindle's 20mm spout. I've only run into 1 case where the jaws would have hit the bed....I just need a bigger lathe! ;D

 

[BillC]

The way things happen....I bought a 24" lathe many years ago for a Marina I owned and as it happened; the first job that came in the door required a 26" lathe. So, lathes are the biggest compromise that one will ever make - (as far as machine work goes) and the largest lathe that you can acquire is not going to be large enough!

BillC

 

[TroyO]

I have a 6" chuck on my HF 8x14" lathe.

It is about as big as I would want to put on there. I'm not sure you lose much by having the 6" over the 5" really, though.

The limitation of the jaw opening fouling the bed I'm not sure applies... you could likely do that with a 5" as well. The difference being how far you are hanging the jaws out of the chuck body to get there.

If you look at it this way.... what you can hold in there is limited by the thickness of the jaw between the workpiece and the bed. A 6" chucks jaw is the same/marginally thicker than a 5", I would assume. It's not even hugely thicker than the 4". The difference is my jaws aren't hanging ~2" out of the chuck.

More to the point, mine has detachable jaws so I could turn up a set that was only 1/4" thick and be able to fit 7.5" (roughly) if I wanted to try it. That was my big reason for going 6" as it was the smallest I found that had removable jaws.

More of a concern might be the loss of length... LOL, it is thicker then the smaller chucks.

I can clamp a 6" round on there with outside jaws. I think it was around 2" with the inside jaws?

 

[Jasonb]

I don't get the point about not being able to use the holding capacity of a larger chuck, reverse the jaws and a larger chuck will always hold bigger work. eg the max safe work dia for a 4" is about 4" with about 1/3 of the jaw out of the body and a 6" will hold 6". And if you have got more than 1" of jaw sticking out of a 6" chuck to hit teh bed you have wound the jaws out too far anyway!!

The other advantage of the bigger chucks is the hole in the chuck body is usually larger so you can fit larger work in without too much overhang. When I had an Emcomat8.6 (the original geared head 8" lathe) the 3jaw would only take about 25mm in the body but the 6" 4jaw would take 45mm

Having said that a 5" 3 jaw and 6" 4jaw independant will cover all you really need and the 4jaw can be used for round work and will be more accurate as well.

The only issue is it takes a few moments for the chuck to get upto speed which does loadup the machine and again takes longer to slow down.

Jason

 

[Niels Abildgaard]

A big chuck makes big overhang and can tempt to make things that bend spindle and self confidence.
My Boxford (4.5/9) Inch swing looks right with a 5 inch and dangerous with a 6 inch.

Regards

Niels

 

[bambuko]

Thank you!
I didn't get this chuck because I wanted to get one for my lathe - I got it simply because it was a good deal I couldn't resist
If I don't use it for the lathe, it will be just great for my rotary table on the mill, but ... I still think there might be times when it will be useful for bigger jobs in the lathe.
To address some of your comments (in no particular order):
"...jaw opening fouling the bed..."
Only if you extend jaws beyond what is recommend (and good) practice. If you see picture attached below(extracted from Röhm) with jaws extending no more than 1/3 of their length outside chuck body, max swing for 160mm (6 1/3") is 194mm (7 2/3") - still clear (just) of the bed.

"...not being able to use holding capacity of larger chuck..."
As Jason points out, and as shown in the picture attached below, with 160mm (6 1/3") chuck, max dia I can hold externally (A4) is 160mm (6 1/3"). It is similar for internal chucking (J2). So there are times when bigger chuck will be very useful.

Thank you for all the other comments - I tend to agree, that probably 5" (125mm) or at most 5 2/3" (140mm) chuck would probably be better choice.

I will wait for the chuck to arrive and will make final decision than, in the meantime thak you for your help guys!

Chris

 

[bambuko]

Well ... here is 4" chuck next to 6" on my lathe:

I have concluded that just because it is possible doesn't mean it's wise
I am now waiting for 5" chuck and the big one will either be for sale or might use it on the rotary table, as previously mentioned.

Chris

 

[BillC]

Yes, having the jaws exposed past the chuck body is asking for trouble. Lathe chucks of the proper size are important but dealing with work too large for the lathe will be cause for a compromise in the choice - and being very careful and alert will prevent a bad accident. But then, that's the nature of machine tools anyhow....being careful and attentive. I cannot say honestly that I have not done precisely what you took pictures of - to get a job done - but it wasn't smart at all. We do things in our own home shops that would get man dismissed from a commercial machine shop.

BillC