Completing the base casting of the CNC mill. The lower weldment is complete with anchors and embedded in the precision grout mix. I used Sika 212 Precision Grout with ~40kg of aggregate. The full casting is 6in (150mm) thick. The main cutting loads of the will be supported by the steel weldment, the cast structure provides added mass for vibration damping and increased stiffness as well as a structure for mounting the mill on a moveable frame in the future. Wasn’t able to capture footage of the pour, too many things were going on.
Sika 212 is a non-shrinking grout, designed for machine footing and base applications. It has voilume compensating elements, but does still have some expansion, upto 0.3%. For an 800mm length segment, that is an expansion of upto 2.4mm. Which isn’t great, with a higher volume of aggregate, that expansion is reduced, but I will have to relap/grind the top bearing rails mounts. I plan to do that with a small granite surface plate, abrasive paper, and some precision dial indicators and straight edges. Sika 212 HP is an even better option, but wasn’t commonly available where I am. It’s hardened expansion is only 0.04%. Epoxy mixtures would be even better, but was not within my budget for this build.
This is not the best method to build a precision metal woking machine. However, I think that if the shortcomings are understood and compensated for, you can get a fairly accurate and stable machine. My hypothesis is yet to be confirmed as the machine is not complete yet. Originally, prior to casting the grout, the steel weldment with lapped 25mm linear guideway mounts was flat to within about 0.004″ across it’s full length on both rails, with even less twist. There are stresses in the welds, the plate stock, and the grout, I will measure the assembly post curing, and remove material as needed to re-establish and flat, parallel, untwisted datum for the X-axis linear rails and rotary trunnion.
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