1100MX Tramming and Milling 6061 T651

Hello, I have project on my 1100MX where I am milling 18"x5"x.5" pieces of 6061 and I am getting all sorts of material thicknesses after the job is done. I was hoping for a +/- .005" tolerance but instead it is looking more like +/- .01" which is a lot considering that I get (target thickness is .315") anywhere from .308" to .320". It would be one thing if the part more or less be uniformly off by that amount but instead I am getting one extreme on one end and another on the other with gradients in between that are not consistent.

I have an SMW plate on top of my table that I have trammed to within .001" in the X and Y direction.

On top of the SMW plate, I have 3 SMW vices lined up to hold the part. I have trammed the vices, all 6 pieces to within .001" of each other by using shims.

I have probed Z from the top of the plate being machined at first, but now I am just going from the mating surface of the vice which made things a bit more consistent.

My biggest problem right now is that I am getting a .005" gradient (.320" on the near side) in the Y direction even though tramming the SMW plate in the Y gives me .000" gradient and vices maybe .001" not in favor of the gradient. This side that ends up thicker, has the moveable SMW vices that get tightened. I also use a rubber mallet and ensure that the plate is touching all the vices flat.

If anyone has any idea where these extra thousandths may be coming from, then I would greatly appreciate any advice. I suppose it could be the metal warping. Perhaps I need to be more methodical with how much torque I put on the vices with a torque wrench. That’s all I can figure. But how could extra torque on the part lead to extra thickness?

@Michael_Nesterak It sounds like you have a bit of twist in your base castings. This can be mitigated by performing the following proceedure. The Commissioning and Calibration Guide and the Commissioning Series - Leveling and Squaring Your Mill video cover the steps to square your mill against it’s stand.

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Thank you for that great suggestion. I went ahead and bought a precision level and was able to get the table to be within .0004"/10". What I found though through this process is that the gibs were not tight enough to prevent the table from sagging left to right about .006" which I was able to take down to .0015".

Next I shimmed the SMW vices so that they align within .001" with each other and milled another part and saw that for whatever reason the side of the table closest to me was causing the part to be thinner by .005" versus the side that is further away from me (in the Y direction of course). Still can’t figure this out. I believe that I am tightening the vices pretty well in order to prevent any lift.

My next attempt to fix this irregularity is to heat the coolant to help get the machine up to temperature. I already do a warm up routine for it but I want that coolant to be at least 70F.