Hi all, I'm new to CNCs. I purchased and set up my first machine, a 6040 from China CNC Zone. I paid a good amount extra for a sturdy one with linear rails. It's heavy and beefy that's for sure. https://www.ebay.com/itm/39394372632...UAAOSw52phFOrs

I've been using the machine for a bit now, learning a lot, and decided to make my own build plate. This is where things got weird. I got a precision ground plate from midwest steel (flat within 0.015", thickness tolerance is 0.005"). I was thinking that meant once leveled with the machine, there should be 0.005" max deviation in thickness when jogging across the bed with an indicator. What I observed was quite different: when jogging along y, the indicator reads a line roughly (very rough approximation) like this:

0" 12" 24"
.. +0.005
..... ..
.. .. .. 0.000
.. .. ..
.... ..
.................................................. . -0.008

^^ if the graph looks messed up see the attached pic

I did place a shim (shown above) below the y linear rails roughly halfway to see if it helped with deflection. It did indeed create that nice flat area in the middle of the y axis, but I still have the issues shown at either end of travel.

If the machine wasn't level with the bed, I'd expect some linear change to show on the indicator over distance traveled. Now I'm trying to understand if what I'm seeing is variation in thickness of the plate or if the machine's linear rails are nowhere near straight.

Along the x axis the machine is level within about 0.005", but shows a similar pattern where, when in the middle of the linear rail, the readings are very consistent, but things go haywire towards either end of the rail.

Spindle is trammed within 0.001" (as far as I can tell with the surfaces I have to work with).

This leaves me with an area in the middle of the bed, about 6"x6" where x and y are level within about 0.002" which is great at least.

I understand that I can finish the top of the plate with the machine and get it "level" -- but then it won't be straight if the linear rails ain't straight!

I originally assumed the thing would be within 0.001" throughout the whole build area (noob thoughts gathered from YT content creators) but I realize that's not realistic -- I'd be happy with 0.005" at this point. Are my expectations too high? If they're not, how do I go about getting this damn thing level? Rebuild it? Really trying to ease the buyer's remorse right now ha!!