I am currently making an adaptor plate to mount a 3" 3 jaw chuck on a 4"
rotary table. Its pretty seems straight forward if it works.

1. I am CNC milling the back of the plate to be close to an exact fit to
the surface of the table. That job is running right now. Because of
backlash in the Taig I machined exact size to the t-slot grooves. After its
done I will measure and remove additional material as needed. This should
put the rough form very close to center on the table.

Here is the rest of the plan.

2. After the form fits I'll go back and center spot to drill holes to mount
the adaptor to the table.

3. I'll drill mounting bolt holes for the adaptor, and mount it on the
table. If there is any slop, I'll try to rough adjust it to the center of
the slop when I tighten the bolts into the t-nuts.

4. Then I will mount the table on one of the manual mills. Its slightly
too big for either of the CNC mini mills without a base adaptor mount.
(That is the next project) Then I will rotate the table while cutting to
create a register for mounting the chuck. This should give me exact size
and center. (pretty close anyway). I will also cut an outside surface
reference on the plate. It should be concentric with the register, and
centered the same. This will make later steps easier.

5. Dismount plate from table.

6. Coat back of chuck with grease and slide onto register.

7. Drill chuck mounting bolt holes in the gaps in the grease.

8. De-Grease and mount chuck.

9. Mount plate back on table.

10. Indicate reference surface on adapter and final tighten plate mounting
bolts.

(All bolt holes are to be counter sunk for the bolt heads. I am sure there
must be a flaw in my plan, but it will have to do for now.)

HERE IS THE QUESTION.

How would I approach the problem if I wanted to mount a 4" chuck on the 4"
table? With the 3" there is just enough room to tighten socket head bolts
to mount the plate to the table, but no clue how I would approach it except
maybe with 2 adaptor plates or with a spool shaped adaptor plate. I can
already see projects where I would like to have a 4" chuck on this table or
a similar one. There is no central taper in the table. If there was I
would have just bought a chuck with a taper on it already for the current
job.

(I am also working on a stepper mount design for this rotary table, but that
can come later. Right now I just need it to index a 3D part that needs to be
cut on 4 sides to remove all voids.)