3 jaw chuck soft jaws

[bill stupak]

The 3 jaw chuck that came with ny newest lathe (see https://www.hobby-machinist.com/threads/atlas-mk2-10100-project.113034/ ) has .008 TIR which I’m not crazy about. I had a 3” mini lathe 3 jaw hanging around and decided to mod it to use soft jaws. The ring in the pic is to preload the jaws to bore the soft jaws. It came out great. The first attempt got it to less than a thou TIR.

 

[projectnut]

The problem with most scroll chucks is that the “scroll” isn’t consistent over the entire length of travel. You might have .001” at one point and .003” at another If you intend to primarily use the chuck at the diameter you bored the soft jaws you should get consistent results.

However that’s no guarantee you won’t significant runout at other diameters. Soft jaws are primarily used when making multiples of the same part, or in situations where you’re using soft materials and it’s important not not to mar the surface.

 

[Karl_T]

If you want top accuracy, a spiral plate works best. Then you can bore it to the exact diameter.

Here's my home made one. I know Shars tool sells them.

 

[WobblyHand]

If you want top accuracy, a spiral plate works best. Then you can bore it to the exact diameter.

Here's my home made one. I know Shars tool sells them.

View attachment 498679

How does one use this spiral plate?

How did you make the arcs in the plate?

 

[Karl_T]

Put the plate on the softjaws with them set just bigger than your bore. The jaws need drill and tap spots for bolts. Tighten jaws against the the spiral. Then bore the jaws at the diameter of your part.

Now you are ready to run parts after removing the spiral plate. I have found this is even more accurate than a collet, at least on my old equipment. Works really good for any part where you turn one end then flip the part around to turn the other end. Or any part that needs to be mounted "dead nuts" on.

To make one yourself, The spirals are just large arcs with an offset center. I just played in my CAD program to get one arc I liked, the copied it two more times with 120 degree rotation offsets. The arcs can be machined with a rotary table Or, in my case, on the CNC mill.

Karl

 

[rwm]

The spiral really just maintains tension on the jaws while you bore them to the correct ID. Hence perfect concentricity.
How would you cut spiral on a rotary table? I have never considered this?

 

[WobblyHand]

I was hoping that it wasn't done on CNC. I don't know how one could get consistent arcs manually. I know I could get one of them ok, but all of them matched manually, hmm.

 

[rwm]

They don't have to be consistent or precise. But why a spiral? I suppose you could use a plate with 3 radial slots?

 

[WobblyHand]

They don't have to be consistent or precise.

Good to know, but it would bug me if they weren't consistent!

 

[Karl_T]

I just brought up my CAD file on this. The center of one arc is at X0.5 Y-0.7 and the radius is 3.25".

Just set this on the rotary table at 0.5,-0.7 and cut a slot wit radius 3.25. rotate 120 degrees and repeat.