|Thread scoring run|
|Reworking a 12 inch plate|
Cushman has been a major player in lathe chuck manufacture since the 1862. They have made scroll plated chucks at least since the 1870's which opened and closed all jaws together. The chuck featured here is an 18" 3 jaw chuck. Usually, 3 jaw chucks are scroll plated chucks and the 4 jaw chucks have all jaws operating independently. The later have screws behind the jaw that advance the jaw to tighten it. Cushman is a survivor and at this writing is still in business although imported chucks are flooding the US market sometimes for as little as 10% the price of the Cushman chuck.
Chucks are made in all sizes usually ranging from 6" on up to however large your lathe may be, though usually beyond around 30" to 36" users tend to use a face plate with 'face plate jaws' instead of a chuck as such. Three jaw chucks have a reputation for not being particularly accurate, but rather 'quick and dirty', because they do what they do--all jaws close together, and there is nothing the user can do to improve on what they do. By contrast a 4 jaw chuck has independent jaws and the operator can pick up his work as accurately as his patience and skill permits, but of course patience and skill are required.
Scroll chucks are usually more expensive than 4 jaw independent chucks and it is customary if a lathe is equipped with 2 chucks for one to be a 3 jaw and the other to be a 4 jaw with the 4 jaw being the larger of the two. An 18" Cushman Scroll plate chuck is a real prize. As you can see from the photos, the jaws have 2 positions, and can be reversed. The first photo shows a 5" piece clamped with the jaws in the 'small' position. The center hole is 4" which allows 'runout' of the cutting tools into the center for work that is less than that size. In the first two photos I was threading a 3.75" thread so I could use the center hole passage for the threading tool runout.
Cushman now offers a newer design of 3 jaw chucks which can be adjusted for accuracy largely putting to rest the classic argument of machinists for using the 4 jaw. Of course, the new computer controlled equipment uses power chucks so the whole idea of manually setting something in a lathe is dated. The most obvious limitation of the larger chucks is that they don't work well on small objects. Indeed around 2 inches is as small as this chuck should be closed as the jaws get off the inside of the scroll plate making it easy to damage the plate. For this reason if you have a chuck larger than about 12 inches, it makes sense to also have access to a smaller chuck for little work. Sometimes this is in the form of a smaller lathe with smaller chucks or just smaller chucks for the big lathe.
See also: American Lathe