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    Default machine alignment / set-up

    Hi,

    I have build a CNC milling machine and I'm nearing the stage of putting it in it's final installation position and starting to level it all up and set the axis perpendicularity etc.

    Until now I have roughly set things up with a 2ft spirit level. That's a good first approximation but is probably only good to within a millimetre or so over the length of the axes ( 1m20 x 0.70m )

    It seems the starting point would be to get the two X guide rails parallel. They are SBR20 , chromium plated ground steel bars on an ali support, bolted to an I-beam. My chasis has adjustable feet for this purpose.

    I thought of finding a thick plate of float glass long enough the span the 70cm between the guide bars and adjust until there is no rocking motion. I suspect that could be better than my initial spirit level method.

    Is there another way I could check that the two bars are co-planar and parallel ?

    Thanks for any tricks of the trade or good ideas.

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    Default Re: machine alignment / set-up

    You might be able to find a machinist's precision level or you could seek a surface plate on which to adjust the rails to a common height.For a router,either would probably suffice,but for a mill intended to do very precise work it might not be enough.There have been machines which have been calibrated by laser to establish the amount of sagging that occurs as the turret moves across the gantry and on which there is a layer of compensation software that applies a correction factor.Not really a hobbyist's option though.



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    Default Re: machine alignment / set-up

    Quote Originally Posted by routalot View Post
    You might be able to find a machinist's precision level or you could seek a surface plate on which to adjust the rails to a common height.For a router,either would probably suffice,but for a mill intended to do very precise work it might not be enough.There have been machines which have been calibrated by laser to establish the amount of sagging that occurs as the turret moves across the gantry and on which there is a layer of compensation software that applies a correction factor.Not really a hobbyist's option though.
    Well I have already measured the sagging in these X guide bars with a dial gauge as the weight of the gantry moving. The gantry is large section ali extrusion so over the length I doubt that will sag measurably. It weighs about 50kg. The sag is about 2 thou at the mid point. I should have evaluated this better at design stage and use larger section steel.

    I had considered adding a software correction. That is more for the table flatness and levelling. I have a 16mm steel plate which is pretty good but not perfectly flat.

    I'm using a STM32 port of GRBL and I could modify that to take a map profile of the work area using the z-probe. Running a decent ARM processor instead of Arduino means there is probably plenty of time to do on the fly corrections. That's something I want to do once I have a working machine.

    For the initial working I want to get the machine set up as well as possible without going to software calibration.

    Machinist's precision level sounds like the right answer to my question but I'm not sure where I could "borrow" one of those. Nice thing to have but the larger ones seem to cost as much as it cost me to build the machine !



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    Default Re: machine alignment / set-up

    Quote Originally Posted by reg.miller View Post

    Machinist's precision level sounds like the right answer to my question but I'm not sure where I could "borrow" one of those. Nice thing to have but the larger ones seem to cost as much as it cost me to build the machine !

    Grizzly has a reasonably priced one. It's currently on sale. Here's a link: https://www.grizzly.com/products/Gri...-Per-10-/H2683

    I have the 12". They also have an 8". I had to adjust it for level. Very fussy and difficult, but once set, it is about as good as you will find. I am currently building an extrusion based machine. Got it level in all directions to less than .001" over the length in all directions. Actually, the level is only accurate to within .0005" in 10". Nevertheless I got measurements of less than .001" over 60" in one direction and 74" in the other. I probably could have gotten it closer, but the extrusion 45 degree angle braces I used. when tightened down, wanted to caused minor flex. It was terrible getting them all aligned so they didn't alter level. FWIW, 80/20 extrusions have a cut tolerance (square) of .002"/inch and length tolerance of .015". Every piece I've ever purchased was off square and inconsistent in length. For most things it doesn't matter, but IMO, it matters a lot when building a CNC. I got the tolerances down to about perfect on my benchtop mill. It was a lot of work and a lot of thought coming up with supports, but it was worth it.

    FWIW, A 40mm x 160mm extrusion 74" long should sag no more than .0006" under its own weight, if fastened down at both ends - at least according to 80/20s tech toolkit. Loading with 100 additional pounds, the tech toolkit says a max deflection of .0014" should be expected. Adding 200# would would give .0021" max deflection.

    Gary



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    Default Re: machine alignment / set-up

    Reg,

    Sorry. I didn't realize you were in South Africa. No doubt you source most of your parts outside of the U.S. Worth measuring your extrusions for square and for length, though.

    Gary

    Last edited by GME; 04-24-2019 at 01:51 AM.


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    Default Re: machine alignment / set-up

    Thanks for the Grizzly link. That's a pretty good price, though they'd probably want as much again to send overseas if they accept non US orders at all. I'll give them a call.
    I will have to measure how sensitive I can get detecting changes with my standard level.

    The flex on that extrusion seems quite close to what I got for my 80cm I-beam which is 1.5m long with a 50kg ( circa 100lb ) load. Interesting. Maybe they design their sections to give a similar rigidity to an equivalently dimensioned I-beam.

    I only use extrusion on the gantry, and that's big enough that it is not going to move. He's a view of what it looks like to give you an idea of what I'm describing.
    video links.



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machine alignment / set-up

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