best chuck for a 12 inch lathe

[Ray C]

-And once you get your expensive round metal plate, you need to spend a good bit of time making it perfectly fit your lathe and chuck.

For screw-on types, the work is not as complicated because there's not much you can do but D1-x types need a good bit of work. When I purchased several new backs, I was amazed at how far off things were. The nose tapers weren't even close -or all that round as a matter of fact. I reset the geometry and surface ground first the spindle side then lightly went over the chuck side. After getting the back's taper to match the spindle nose taper, I cut it just deep enough to allow the backs to come into contact just right.

I knew that my chucks were square because I'm the original owner and surfaced them when I got them. Anyhow, after tuning-up all the D1 backs, all of my chucks re-mount either dead-on or within a half thou.

Can someone explain why a 6.25" round plate with a few holes in sells for so much? $188 sounds like a lot for a backplate but i have seen them priced like this as well I am sure that I am missing something. Thanks!

 

[george wilson]

An 8" chuck is fine for a 12" Sears lathe. I had an 8"4 jaw chuck for mine. It won't ruin the angular ball bearings in the lathe's spindle. Indeed,the front wheel bearings in my old 1950 Chevy were not as large as the tapered roller bearings in a 12" Sears Atlas lathe(my first lathe). The important considerations are: Will the jaws clear the ways enough to crank them out sufficiently to use the chuck's full capacity? On a Sears lathe,you may or may not be able to have the power or rigidity to turn a real large piece of steel anyway. Brass or aluminum will likely be workable. I had lots of trouble with my 12" Sears lathe being rigid enough to take a decent cut. The bed also warped under load during cuts,causing it to not turn accurately.

Sears advertised .001" accuracy ONE INCH from the chuck. And,that was about right unless I babied my cuts along,taking a lot more time than should have been necessary to make something. My subsequent Jet 10" x 24" bench lathe was SO much better it was a revelation.

 

[kd4gij]

Here is an 8" chuck on my 12" craftsman 101.07403

 

[rafe]

I'd definitely recommend a 4 jaw (partially because you have a 3 jaw) 6" would likely be big enough not sure if the 8 would be too big to gain much over the 6. You can really dial in a 4jaw for accuracy ove a 3jaw. I picked up a used skinner 8" for my 14 1/2" online(E-bay) and had to machine a back plate that had the correct thread on it (shout out for backplate to tools4cheap) and I could not be more happy with the set-up. If you are patient you will find a 4jaw for that lathe for a reasonably investment. The independents are reversable, the scrolls are not unless they have 2 sets of jaws or bolt on (2piece) jaws ...I think that is an accurate statement

 

[kd4gij]

For a 4 jaw I would go with 8in for shure.

 

[projectnut]

The real advantage of a 4 jaw chuck is that the work can be removed repositioned and recentered. Since each of the jaws moves independently all you need is a good dial indicator to adjust the work to well within .001". Keep in mind there are several different styles of 3 and 4 jaw chucks. I have 2, 8" chucks that are worlds apart. The smaller one has a body only about 3" deep and is open on the back. It weighs about 20 lbs. The second one is nearly 6" deep and is totally enclosed on the back. It weighs nearly 50 lbs.

A 3 jaw chuck on the other hand has a typical run out of .002" to .003". In the commercial machining world they aren't considered to be precision chucks. They're typically used where all operations can be done with a single setup or where the finished product is parted from the original stock.

As far as how wide the jaws are supposed to be opened the rule of thumb is that they should not extend beyond the chuck body more than 3/4". On chucks with jaws less than 1 1/2" long only half the length of the jaws should extend beyond the body. The reason for keeping the larger jaws to within 3/4" is safety for the operator. On the smaller chucks it's to make sure enough teeth on the jaw are engaged to the scroll to provide sufficient gripping power.

 

[Steve M]

The real advantage of a 4 jaw chuck is that the work can be removed repositioned and recentered. Since each of the jaws moves independently all you need is a good dial indicator to adjust the work to well within .001". Keep in mind there are several different styles of 3 and 4 jaw chucks. I have 2, 8" chucks that are worlds apart. The smaller one has a body only about 3" deep and is open on the back. It weighs about 20 lbs. The second one is nearly 6" deep and is totally enclosed on the back. It weighs nearly 50 lbs.

A 3 jaw chuck on the other hand has a typical run out of .002" to .003". In the commercial machining world they aren't considered to be precision chucks. They're typically used where all operations can be done with a single setup or where the finished product is parted from the original stock.

As far as how wide the jaws are supposed to be opened the rule of thumb is that they should not extend beyond the chuck body more than 3/4". On chucks with jaws less than 1 1/2" long only half the length of the jaws should extend beyond the body. The reason for keeping the larger jaws to within 3/4" is safety for the operator. On the smaller chucks it's to make sure enough teeth on the jaw are engaged to the scroll to provide sufficient gripping power.

Was searching for exactly this info. Just bought a Smithy import 12x37 and am in the process of working out the kinks. The three jaw chuck seems to have about 0.0015 +/- runout using a brand new piece of 0.875" drill steel. I checked for roundness with a mike as best I could, getting no more than 0.0002' variation (operator error?) and chucked it up. When I take a light cut, I get 0.0005 - 0.001 runout on the turned part. Seems like the out of roundness should equal that of the chuck, not sure why it is less. Got similar results with the mike. Haven't tried the four jaw yet. Was having some surface finish issues turning CRS - not there with the drill steel just as I expected.


Steve in Central TX

 

[rgray]

Was searching for exactly this info. Just bought a Smithy import 12x37 and am in the process of working out the kinks. The three jaw chuck seems to have about 0.0015 +/- runout using a brand new piece of 0.875" drill steel. I checked for roundness with a mike as best I could, getting no more than 0.0002' variation (operator error?) and chucked it up. When I take a light cut, I get 0.0005 - 0.001 runout on the turned part. Seems like the out of roundness should equal that of the chuck, not sure why it is less. Got similar results with the mike. Haven't tried the four jaw yet. Was having some surface finish issues turning CRS - not there with the drill steel just as I expected.


Steve in Central TX

.0015 on a 3 jaw is awesome. Wise mine were that good. Once you cut a piece you should have 0 run out though. As long as the cut compleatly "cleaned" for lack of a better discription...left no spots uncut....even if you had .003 TIR it would be 0 after the cut. That's why once you cut it round in your 3 jaw you don't want to remove it until it's done. Even if you mark it you may not get it back in to the same location and then it will have run out.

You may want to make a test bar by turning a piece of that drill rod between centers. It may be very round but not quite straight.

 

[george wilson]

I will say that the 8" chuck I bought for my 12" Sears lathe was made FOR that lathe. It was lighter than the 8" chuck shown just above. Only about 2" deep,with integral threads to screw on,and had a hollow casting on the back side that followed the contour of the parts. Probably nearly half the weight of an 8" chuck with added back plate.

I still don't think an 8" chuck would ruin a lathe with angular bearings. Now,bronze bearings on a very old model are a different case. I am not a student of early Atlas lathes,so I do not know if they had plain bronze bearings or not. Of course,old South Bend 9" lathes had the WORST possible setup: Cast iron bearings that were INTEGRAL with the headstock(As do most Myford lathes,except on SOME of the very last models). When those bearings wore,you were in trouble! Only some fancy line boring and re lining with bronze could fix them. Probably beyond the ability of most home shops. I WOULD recommend lighter chucks for such lathes. And,OIL the spindle bearings every time you use them,too!!

By the way: I always custom fit my back plates and true them up when ever I fit a new chuck.