Shimming

Moglice
Moglice enables the moulding of complex shapes and structures with micron precision, without machining. This method reduces the process times and costs in modern production technology many times over.

https://www.diamant-polymer.de/en/products/moglice/ as alternative to JBweld. ergizmo, that squaring process is also used in the professional world, though usually with moglice or something similar rather than JBWeld. The other difference is that rather than build it relative to the surface plate, they would square the column on the machine itself and then inject the material between the two pieces with a dam to hold it inside.It works very well.

leveling
https://www.cnczone.com/forums/vertical-mill-lathe-project-log/96410-cnc-forum.html

1) I stand the block up on the granite surface plate, with the down side being the end I want to square up.

2) I "test fit" shims of various thicknesses under the end that is down and check perpendicularity with a square off the granite plate. I keep moving the shims around, and using different thicknesses and checking with the square both off the main flat surface and my reference side surface. After the end is square I note the positions of the shims with a marker.

3) The shims are super-glued in the spots noted down with a marker.

4) The granite plate is covered by saran wrap, stretched tight and the ends taped down to the sides of the plate with masking tape. I keep stretching the saran wrap over the plate by pulling and re-applying the tape until the top is a smooth surface.

5) The saran wrapped surface plate is sprayed down liberally with non-petroleum based silicone spray.

6) JB Weld is mixed up and applied to the end being squared up (The one with the shims).

7) The granite plate is then placed on top of this end. The weight of the plate squeezes the JB weld down to the shims. The silicone covered saran wrap allows me to remove the plate later.

8) After about 6-8 hours, the plate is peeled off and the excess JB weld that squeezed out over the sides of the block is trimmed off with an exacto knife. The saran wrap is removed from the plate.

9) I wait another 24 hours.

10) The end is checked for perpendicularity on the granite surface plate (JB weld side down) with a square and feeler gauge on both the flat sanded top surface and reference side.

11) Any filing that needs to be done to get it to where I want it is done. This was pretty minimal, the process itself does most of the work.

links
Lathe

Machine design