[NordicCnc]

Hi. I have a home made cnc machine built out of heavy duty 80x160 aluminium extrusions. I had them milled flat by a local shop, but for some reason they werent as flat as I wanted them to be. I resorted to shimming and in the worst location I had to put 0.3mm shims. This works fine, but the shims are not perfect, as they create "hills" due to the bow/warp of course. Also I suspect there may be some twisting going on, but it is difficult to tell because the HGR 20 linear rail carriages are flimsy as hell in my opinion (easy to force 0.1-0.2mm indicator movement by pushing the right/left edge. Anyway, I now have access to a 1500x50mm assembly straight edge DIN874/00. I was thinking if I could somehow try to lap the linear rail surfaces by moving that back and forth over thr surface, using some abbrasive or something.. Any ideas folks, did you ever try something similar?

Oh and I migth switch to 25mm linear rails, the 20s are just mehh. My machine is 1500x1200mm in Y/X so I could use the added stiffness.

Thanks!

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[pippin88]

What brand rails and what preload?

Some rails are meant for rapid travel and loose positioning - they have no preload and have a bit of play when force applied.

[joeavaerage]

Hi,
I would suspect its not the rails moving so much as it is the aluminum underneath that is moving. I use 20mm HSR cars and rails on my machine, and those rails are screwed down to the
cast iron axis beds and they don't budge when you lean on them. Don't blame the rails or cars if the underlying frame is flexible.

Craig

[NordicCnc]

Thanks for the replies. The preload is ZA and precision is C grade. I bought them from BST Automotion store on Aliexpress.

I really think it is the carriages, because on my Z-axis the rails are mounted directly on a 20mm cast aluminium plate. If I press on the carriages on the right or left side edge, the indicator moves..

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[NordicCnc]

Here ard some pictures of the rail mounting that I found on my phone.

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[Fella13400]

How did the shop machine them flat? To get the bow out properly each one would need to be done separately, dialed in and machined, flipping over and repeating the process atleast twice for each side to start getting a decently flat part. If the extrusions were just clamped flat to the table and had a skim coat off each side then they wouldn't be much flatter than they started out as. I would have expected better than a 0.3mm flatness after machining, the parts are quite long though. You can likely get an idea of how tbey approached it by cost. If it a couple hours or less machining charge then they probably just got a quick skim cut on each side.
The 50mm x 1500mm straight edge would need to be very thick for it to be even somewhat useful for lapping. If it's on the thinner side, it will likely deform as much or more than your extrusions while trying to use it. If your part is the same length as the straight edge it's not ideal to say the least. If you need the part to remain parallel top and bottom then lapping will land you in more trouble than you have now.
If you do find a way to "lap" them i would use 120 grit wet /dry sandpaper to start and 400 grit to finish. Lots of water on the paper to keep it cutting. The straight edge would need the paper taped or glued to it.
Shimming accurately at every bolt down location 9n the worst ones is probably your best bet. Continue to dial and shim unit it's within your tolerance. 1" rails are more robust but will still sag over those lengths. If your push your axis so far out of alignment with a finger and it's definitely at the carraige I'd say you have the wrong type.