Centering aid for polygonal stock

[Toolmaker51]

Hi mklotz
Thank you for the explanation all makes sense now.
I often put a 3-Jaw chuck into the massive 4-Jaw. As I cannot be bothered to get the crane to change the chuck. This is a common practice in the UK. This is also why the chuck keys are normally square on a 3-jaw and hexagon on a 4-jaw. Just put the hex one out of reach (in a cupboard) and then you can’t operate the wrong chuck and have the 3 jaw, job, etc. all crashing out of the 4-j. It is amazing how often you naturally go to the chuck that is, attached to the nose of the lathe spindle and not the one being held.
Thank you again
The Home Engineer
.

At one employer, our 2 different 16" Clausings often held smaller 3 or 4 jaw chucks for precise reason Marv, thehomeengineer, and RalphXYZ observe. They had no 5C closers. Larger chuck capacity, means proportional increase in minimum size too...Even a passable 3 jaw can be clocked for better concentricity in a 4 jaw - provided the same key socket is used, consistent part size, and jaws are tightened in a uniform manner.
We made .250 and .400 ⌀ offset 'cams' periodically, combining a 3 and 4 jaw made it quick work.

[Ralphxyz]

Now that is what I call a real explanation!!

Thanks Marv

[Bony]

This is slightly off-topic, but surprisingly (for me at least) hexagonal stock can be mounted in a 4 jaw self-centering chuck and remain centered with the lathe axis. No care need be taken about placement in the 4 jaw chuck. Try it if you have a self centering 4 jaw.
My "day to day" chuck is 200mm self centering because it affords better grip on the workpiece, AND it allows square and hexagonal stock to be mounted axially.

[Ralphxyz]

3 Jaw in 4 Jaw now that is interesting, thank you!

Ralph

[thehomeengineer]

This method is great if your 3-Jaw chuck jaws are worn as you can true it up in the 4-Jaw so it again runs concentric once again.
But, I like this method for eccentric turning. When you have, several components all needing the same offset this can be achieved by adjusting the 4-Jaw to get the 3-Jaw to run out by the required throw/displacement and then each component can be put into the 3-jaw one after another knowing it has to be the same as the previous machined offset.
One note: when using a 4-Jaw you may need to slow the spindle RPM right down due to the eccentricity of the work. Larger chucks can have weights added as counter balances to overcome this issue.
The Home Engineer

[mklotz]

Mounting a square collet block in the 4jaw or a hexagonal collet block in the 3jaw allows you to use collets without removing whatever chuck is mounted on the lathe.

This can be particularly handy when the work sequence demands going back and forth between the mill and the lathe.

[Ralphxyz]

Mounting a square collet block in the 4jaw or a hexagonal collet block in the 3jaw allows you to use collets without removing whatever chuck is mounted on the lathe.

This can be particularly handy when the work sequence demands going back and forth between the mill and the lathe.

Marv, in order to remove/change the stock when using collet blocks you have to remove the collet block at least on the 5C collet block I just received. Mounting the 3 jaw chuck in the 4 jaw avoids this problem.

Possible you know of some other collet blocks?

Ralph

[mklotz]

Marv, in order to remove/change the stock when using collet blocks you have to remove the collet block at least on the 5C collet block I just received. Mounting the 3 jaw chuck in the 4 jaw avoids this problem.

Possible you know of some other collet blocks?

Ralph

Collets can handle diameters far smaller than any lathe chuck can close to. If you're going to be doing a lot of collet swapping you really need to build a proper collet chuck for your lathe. That's what I did so the collet-block-in-chuck thing is only used very occasionally.

[Ralphxyz]

I have 5C collets and collet blocks but the block has to be removed from the chuck to change the workpiece.

I have ER25 collets that use a screw compression nut so these could be mounted in a 3 jaw chuck mounted in a 4 jaw chuck and multiple pieces could be turned
with the exact same placement easily, of course I do not have any ER25 collet blocks.

I guess once I learn how to cut threads and turn tapers I will have to make up collet blocks and nuts.

They are on ebay but kinda expensive for me.

Ralph