Gantry Not Parallel to Base Plate?

Hello all,

I have been tasked with running an MR1 that someone else built. I noticed that when squaring stock opposite faces were about .006" out of parallel, always in the direction of the stock aligned with the X direction of the machine. Removing the vice and mounting a dial indicator on the spindle and running the indicator along the clean base plate in the X direction is indicating about .005" from one side of the base plate to the other. This would lead me to conclude that somehow the base plate and gantry are not parallel in the X direction. The base plate was surfaced per the assembly instructions when the machine was assembled. What could cause the gantry and base plate to become misaligned like this?

You can re-surface the base plate. Or shim of of the the Y rail castings. However…

If you are facing small stock (ie what can fit in a vise, a few in across) and it’s ending up with a 6 thou deviation on one side, that doesn’t sound like the same source of error as if you are measuring 5 thou over the whole X travel.

If I were you I would put the vise on a surface plate and check it with a surface gauge for flatness. I’d also check that there isn’t debris under the vise when it’s clamped down. Finally id suggest making sure you using an indicator correctly (sorry), and not getting weird results due to cosine error or other human error problems.


Sorry, I forgot to mention that there was debris stuck under the vice that I believe was causing most of the inaccuracy. However there is still that .005" over the X travel that I feel is a bit much. I know I can resurface the base plate but I’m questioning how the gantry and plate became misaligned. If I surface the plate again but don’t correct the root cause of the misalignment it will just drift out of square again.

I was having a similar issue when facing my base plate and made a post about it. The conclusion is that there is thermal expansion in the spindle as its traveling it heats up. If you run your spindle at 3500 for 5 minutes then 25 at 7000 and then surface the results is much smaller of a difference got mine down to .0015. You also have to let it warm up before cutting parts or at least rezero tool height before final surface passes

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Keep in mind that 5 thou over 23 inches of X travel is only 8.7 tenths (0.00087") over a 4 inch workpiece. That’s really not terrible.

My X gantry has a ~1 thou “bow” - keep in mind that your X beam itself will have some error baked in. I handle this by limiting precise cuts to the center of the bow, and my machine is trammed normal to the midpoint of that curve.