View Full Version : Lathe chuck sizes
11-11-2006, 09:12 AM
Does anyone know why the 4 jaw chuck supplied as normal equipment with a lathe is always bigger than the 3 jaw? As an example, my Myford has a 4" 3 jaw and a 6" 4 jaw and my Colchester has a 10" 3 jaw and a 12" 4 jaw. I have yet to think of a logical reason for this. I'm sure there must be a good reason and look forward to enlightenment from the expert contributors to this forum.
11-11-2006, 09:19 AM
Best I can offer is that it may be to take advantage of the lathes swing when handling uneven shaped workpieces and uneven jaw extensions. The 3 jaw is probably sized to handle "most" round work. Just my $0.02 Den
J. R. Williams
11-11-2006, 10:16 AM
The size of the chucks supplied all comes down to one thing---$$$$$$$$
11-11-2006, 10:40 AM
4 jaw chucks tend to be shorter than 3 jaws of a similar diameter. I've had a gap bed lathe where the jaws of the 4 jaw (when the "normal" way round) would swing in the gap, but the 3 jaw's jaws were over the bed. A larger diameter 3 jaw would have had the jaws striking the bed, not a problem with the 4 jaw. Handy to have the extra capacity of the bigger 4 jaw.
Of course, this doesn't apply to straight bed lathes, where the larger 4 jaws often put dints in the bed...
11-11-2006, 12:05 PM
I wondered about this a while ago when I got my little Atlas 618 and some books that came with it.
The Atlas manual, or South Bend's HTRAL says that the independant (4 jaw) chuck should be "sized to handle the largest work that could be expected" and that the jaws should not foul the bed when extended....(gee, surprise...)
That usually means the 4 jaw will be a little bigger than the typical 3 jaw.
11-11-2006, 12:05 PM
There are several factors involved. Ian B hits one of the more important, the capacity of the chuck with the jaws extended. Too large of a three jaw chuck will not be capable of handling as large a workpiece as a smaller chuck, as the jaws will hit the lathe bed when opened too far.
Otherwise, the three jaw is more likely to be used on smaller parts, where it's smaller size makes it more convenient to get up close.
A four jaw has greater gripping strength, and will give a more secure hold on larger material.