Not aware of any sadly, but never mind - can order some more stuff next week from alu warehouse.

So given some doubts raised about the accuracy of the shoulder bolts as a way of aligning the mounting plates, I thought I had better check what I was getting. Given the gantry arms will sit on a saddle plate, these mounting plates do dictate the squareness of the Z to the Y axis, so the closer they are to perfect the easier time I will have making sure the gantry is squared up.

I figured this was a good method of checking - in theory if the mounting plate is perfectly level then the reading on the indicator should stay at zero as the plate moves from front to back (indicator is metric, 0.000mm)







Magically, this side was 0.000mm both at front and back, the middle showed -0.006mm which I would guess is due to non-straightness in my current machine's X-axis gantry meaning that there is a slight bow in the top of the plate, but 0.006mm is only slightly larger than the some bacterium is long so I'll forgive it that!

Sadly, my joy at that stunning level of alignment was short lived - the other side showed one end about 0.072mm out between front and back. Not that bad over a 200mm or so span, but not really at the level of what I'm after.

So two options - either I just skim down one of the shoulder bolts and surgically tap the mount into alignment, or I make life difficult for myself and add a shoulder for the carriages.

I decided to try the shoulder method, since i can easily change back and forth between either method then depending on the results of my experiments. The only catch is that since my current machine isn't exactly perfectly accurate, I was largely relying on the flatness of the eco-cast plate to give the required alignment and tolerances... that means machining a pocket to then give the carriages a shoulder to rest on is not ideal as I've then lost the flatness of the plate and am back to relying on my machine's tolerances.

I decided to try machining a slot, then making a counterpart to go in there which would provide an alignment rail whilst still having the carriage mount to the ground surface of the plate.

This is the design:




Doing it retrospectively, I would have machined the whole thing back side first, then flipped over for pocketing the heads of the screws... that would have ensured the absolute best possible alignment between slot and mount plate. Seeing as that option is no longer possible, I used some 6mm H7 dowel pins and pre drilled a pattern into the bed so that the plates are aligned as best as I can manage to the machine.






Then machined the inserts:






They fit very snugly, can't feel any play even without the screws in place.







I then re-ran my indicator test, and while an improvement over the poorer side of 0.072mm, when compared to the perfect side it got significantly worse - it is approximately 0.020mm higher at the back than the front. It is fairly easy to knock it into shape to within around 0.004mm between front and back by loosening the bolts a touch and giving a little tap. I may try just using a little shim material to give a more precise fit. I think aiming for anything more precise than that really is just chasing my own tail since the first test showed the surface isn't flat to more than 0.006mm anyway.