First Proper CNC Router

[diycnc]

After welding the frame, i did a quick check to see how straight and planer the two Y axis tubes are using a small surface plate with a DTI on a long arm. The surface plate was placed on a piece of plywood with three jack screws used to level the plate to three reference points on the tubes at the front left, rear left, and rear right.

Both tubes are now convex, and not planer. Measurements are below. Max deviation is 3mm!
I cant tell you how much is from the welding, and much was there before, but given that both are now convex my bet is its dominated by the welding.


My original plan was to use shims under rails to bring them into alignment, but given the level of error I'm having second thoughts about that.
Is my only option epoxy?

[JAZZCNC]

I'm not sure I'd trust your method of measuring as it won't be very accurate, just the length of the arm its self will deflect a fair bit. Unless you have a flat reference surface to measure from then you'll never truly know for sure how far your measurements are off.

To me you have 2 choices, Fit the rails as it is and see how it cuts and then shim out any issues. Or just bite the bullet and go with epoxy.

What size machine and what's the main usage.?

[Neale]

The long rails on my machine are 1.8m, 100x50x3 box steel. One dipped in the middle by about 1.5mm, the other side a little bit more. Like you, don't know if that was welding or how it came, but in my case epoxy filled the hole

[diycnc]

I appreciate the concerns about the measurement method.
Did some more measurements today to check the repeatability and accuracy of the method.
I deliberately rotated by plywood platform 180 degrees, and positioned the surface plate differently to really test the approach, and pretty surprised to the 1st and 2nd measurements vary by only a max of 0.1mm, that more than good enough for me.
I also used the same method to measure the polished edge of a piece of granite upstand, its the straightest thing I have to hand. I got less than 0.1mm deviation along its 500mm length. Again I'm happy with this.

All in all im very happy with the measurements, and im confident they are accurate enough for me.



X and Y axis are 750mm.
Intended for wood and plastic, and hopefully aluminium. I know ill have to take it easy with aluminium, and I'm not expecting it hog out anything more than the smallest of cuts, or the highest of surface finish quality.

Going back to shims, my concern is will there be a noticeable lack of stiffness with the rails supported on just a handful of points?

[Boyan Silyavski]

Going back to shims, my concern is will there be a noticeable lack of stiffness with the rails supported on just a handful of points?

No problem if you use epoxy putty. I have never tried it but if you think of mounting the rails and then pouring epoxy, i believe it will be difficult to take the air out, though probably will work if you use West System epoxy as its quite thin.

To be 100% sure, best method is pouring the epoxy 5mm deep and 40mm wide, you could use only one channel to connect but make sure channels extend additional 20cm from the frame, there is a lot of info on forum. In my second build from signature i have explained in great detail how to do it properly and most economically for that matter. It cost me a couple of hundred $$ to understand the process in detail.


Another cheap option is use calibrated steel bar like 40x5mm or something, put epoxy putty on it and straighten it on place with straight edge. When dry, dill and mount. Its precise and cheap flat surface

At the end all depends on what shape you rails are in, in most of the cases they are perfect and easy to mount and shim, plus its a small machine, so easier




But epoxy makes that even easier, if you do the epoxy right . Most of the people i am 100% sure do the epoxy in the wrong way or have no way of measuring the result. As funny as it seems i went to that conclusion, measuring my results on a 3m rails and reading the build logs at that time





Similar machine like yours done with 100x100x3mm box , 40x100cm x/y , was well under 0.05mm frame bend when finished.