3 jaw chuck

All discussion about lathes including but not limited to: South Bend, Hardinge, Logan, Monarch, Clausing and other HSM lathes, including imports

Moderators: Harold_V, GlennW

hoppercar
Posts: 244
Joined: Sun Jul 22, 2018 4:09 pm

3 jaw chuck

Post by hoppercar » Fri Nov 15, 2019 9:12 pm

My 3 jaw chuck on my south bend lathe does not run concentric very well. This is a non adjustable three jaw that is threaded directly to the spindle. The jaws screw into the chuck scroll plate, is there any way to regrind these jaws to get them to run true again ?

User avatar
10KPete
Posts: 395
Joined: Wed Sep 30, 2015 5:29 pm
Location: Nordland, WA, USA

Re: 3 jaw chuck

Post by 10KPete » Fri Nov 15, 2019 10:04 pm

If the chuck scroll is worn, each different size piece chucked will come to a different center. If the jaw gripping surfaces are worn, changing the size of the part will not change which jaw shows high/low. Most new 3 jaw chucks aren't going to get better than about .003" TIR overall anyway unless one pays way more than average. A 4 jaw chuck is what is wanted for dialing zeros.

All that said, Yes, chuck jaws can be trued up a bit with a tool post grinder. The jaws must be 'loaded' with pressure in or out depending upon which surfaces are being ground. I've done many of many sizes..

Cheers,
Pete
Just tryin'

hoppercar
Posts: 244
Joined: Sun Jul 22, 2018 4:09 pm

Re: 3 jaw chuck

Post by hoppercar » Sat Nov 16, 2019 7:43 am

Yeah, thats what im thinking, they need ground....trying to find a way to load the jaws, so I can grind them

User avatar
SteveM
Posts: 6967
Joined: Mon Jun 27, 2005 6:18 pm
Location: Connecticut

Re: 3 jaw chuck

Post by SteveM » Sat Nov 16, 2019 9:43 am

First, I would try to find out more about the problem.

Have you cleaned it? If not, I would first do a complete teardown and cleaning. Trying to correct by grinding when the problem is dirt would be unnecessary surgery.

From your post it wasn't clear - can you unbolt the backplate from the chuck or is the threaded part integral to the chuck?

Try indicating the chuck body - both on the face and the side. If that is off and the backplate is removable, I would look at fixing that first before grinding the jaws.

After all of that, if you still have problems, then consider grinding.

Search the forum for "grinding chuck jaws" and also for "spider" (that's the tool you will use to preload the jaws, as Pete mentioned).

Keep in mind that if the reason it is out is wear to the scroll, you can grind the jaws at one diameter and they can be off at others. You would want to grind at a diameter you use a lot

Steve

Harold_V
Posts: 17842
Joined: Fri Dec 20, 2002 11:02 pm
Location: Onalaska, WA USA

Re: 3 jaw chuck

Post by Harold_V » Sat Nov 16, 2019 5:12 pm

It has been mentioned, but not necessarily with a good explanation. Here's something to consider.

The scroll dictates how jaws run. The scroll, which is most likely to have been heat treated, can be eccentric, and most likely is. It also has the ability to shift on its spigot. There may be a place where the jaws run true, while any rotation causes eccentricity. That's why changing diameter of gripped items varies the chuck's gripping accuracy.

If you make the choice to grind the chuck, assuming it has the typical three hole option, mark one of the holes and use ONLY it when using the chuck. The reason for this is that the scroll usually isn't a dead tight fit on the spigot, so the chosen hole will always shift the scroll on its spigot the same way. That, in turn, will shift the jaws in a consistent pattern, removing one of the reasons universal chucks may not operate with repeated accuracy.

Small three jaw chucks are often sprung, the result of over tightening of the chuck when only the tips of the jaws are used for gripping. That process applies greater leverage to the chuck body, often resulting in distortion of the jaw ways. When this happens, material inserted in the chuck is supported only at the rear of the jaws, in effect, resulting in chucking items beyond a reasonable length in regards to the three times diameter rule of thumb. This often translates in to cuts that are three sided, as the material moves away from each jaw as it rotates, due to the short engagement. Chatter and poor finishes are often the result of this, along with the less than round cut. Grinding the jaws after being properly supported by a spider often markedly improves the chuck's performance, even if the chuck body has been distorted.

H
Wise people talk because they have something to say. Fools talk because they have to say something.

User avatar
10KPete
Posts: 395
Joined: Wed Sep 30, 2015 5:29 pm
Location: Nordland, WA, USA

Re: 3 jaw chuck

Post by 10KPete » Sun Nov 17, 2019 1:17 am

Thanks, guys, for filling in the gaps!! :oops:

Pete
Just tryin'

earlgo
Posts: 1468
Joined: Sat Jan 29, 2011 11:38 am
Location: NE Ohio

Re: 3 jaw chuck

Post by earlgo » Sun Nov 17, 2019 10:30 am

Hah. I had the 3 sided experience just last week. I have a 1958 Atlas 3 jaw that has been used hard. I chucked up a piece of 1/8 dia brazing rod to make a 1/16 dia pin. Floppity-flop, 3 sides. I ended up making it from a piece of 1/4 dia brass. Right on, gentlemen.
--earlgo
Before you do anything, you must do something else first. - Washington's principle.

pete
Posts: 1759
Joined: Tue Feb 10, 2009 6:04 am

Re: 3 jaw chuck

Post by pete » Sun Nov 17, 2019 4:31 pm

If you look at the history of lathes 3 jaw scroll chucks are a fairly recent convenience but not an absolute necessity. Before 3 jaws were invented turning between centers and shorter work got bolted to the face plate using various methods. Probably it didn't take long before screw adjustable jaws were then attached to those face plates and the early design 4 jaw was invented. Even today some larger 4 jaw chucks are available that have a T slot between each jaw. And for extremely large industrial lathes they don't even make 3 jaw scroll chucks that large.If the work can be fully machined and then parted off at a single chucking it doesn't really matter if the chuck is out or not. But if the scroll is that worn then it's likely the jaws holding faces are also well worn. And as others have pointed out that is important for how well and rigidly the work is held. Because of the required clearances even on brand new chucks to allow the jaws to move, those jaws will very slightly tilt in there retention slots in either direction as the chuck is tightened depending on which way the jaws are loaded when holding by the OD or ID of the jaws. Because of that issue those OD and ID jaws as others have mentioned do need to be pre loaded in the correct direction before grinding so the jaws will be parallel to the work surface when fully tightened. There's numerous YT videos showing how to do it. Some better than others as it's not a job that should be done incorrectly. Probably the best I've watched was done by Suburban Tools.

I got hung up about getting good concentricity out of a 3 jaw when I knew a lot less and I spent quite a bit on a couple of high end Emco heavy duty chucks to get it. In hind sight I would have been better off buying a very accurately made 4 jaw independent and learning how to quickly dial a part in.With a bit of practice it doesn't take long. For smaller diameter parts within there size range that might need re-chucking, collets are faster than the 4 jaw. There slower than using 5C but I use ER's when I need collets on my lathes. Those collets and the collet chuck obviously need to be well made and concentric or there's no point in using them. I've seen quite a few Youtube videos showing cheap off shore collets and chucks with some pretty high run out numbers. Most average quality off shore 3 jaws and even a lot of drill chucks can easily beat what some of those ER collets might have. I still own 3 jaws for each lathe but use them a lot less than I used to in favor of a 4 jaw or the collets. Other than there speed and maybe holding octagonal stock, I can't think of anything that couldn't be machined just as well or better using a 4 jaw. So in a home shop those 3 jaws are mostly a speed of use convenience. And I'll sure agree about pulling that chuck apart and cleaning it first. Even a .001" speck of swarf in the wrong place can still throw a jaw off on a really good chuck, so it's just good general maintenance to pull them apart for cleaning once in awhile.

User avatar
Rich_Carlstedt
Posts: 1487
Joined: Sat Dec 21, 2002 12:16 am
Location: Green Bay Wisconsin USA

Re: 3 jaw chuck

Post by Rich_Carlstedt » Fri Nov 22, 2019 10:21 pm

Go review this thread
viewtopic.php?f=42&t=84427&hilit=grinding+chuck+jaws
Look at post 9
The Photobucket pictures don't show and I can't find the originals at the moment
but I can show you the chuck. note the drilled holes for pins to hold the small ring that loads the jaws properly for grinding
Also the master socket for tightening ( always) is painted white and is at the top.
Rich
Chuck.jpg

hoppercar
Posts: 244
Joined: Sun Jul 22, 2018 4:09 pm

Re: 3 jaw chuck

Post by hoppercar » Wed Nov 27, 2019 1:57 pm

Mine is exactly the same on my south bend lathe. Only one chuck key drive, and the jaws screwinto the scroll plate. I think my.jaws are sprung enough, from clamping near the ends of the jaws so much over the years. I would need tohave pin hole in the jaws like this to be able to clamp on a chucking ring. .....otherwise, im just gonna scrap it, and get a new chuck and start from scratch

Harold_V
Posts: 17842
Joined: Fri Dec 20, 2002 11:02 pm
Location: Onalaska, WA USA

Re: 3 jaw chuck

Post by Harold_V » Wed Nov 27, 2019 4:47 pm

Keeping the old chuck for use when holding by the jaw tips would prolong the useful life of a new chuck. Might be worth the effort to drill the holes so you can restore the old chuck to a reasonable degree of precision for just such use.

Rich's idea of the pin holes is very good. It places the load on the jaws where it is needed, and is out of the way, assuming the added length isn't troublesome.

H
Wise people talk because they have something to say. Fools talk because they have to say something.

User avatar
SteveM
Posts: 6967
Joined: Mon Jun 27, 2005 6:18 pm
Location: Connecticut

Re: 3 jaw chuck

Post by SteveM » Wed Nov 27, 2019 5:37 pm

You can make a fixture like this to grind your jaws.

Downside vs drilling the holes is that this will only work at one diameter, but if what you are trying to correct is the grip front-to-back, then this could do it.

Image

If you were tossing that into the trash, I would intercept that. It still has useful work left in it.

Steve

Post Reply