shims
I am curious about the process to use shims. If the surface the rail is bolted to is not flat and straight, then it would need to be made so before bolting the rail down - that I understand. But shimming the trucks can only correct for a table that is twisted (the truck surfaces not co-planar) or two rails that are flat, straight, and parallel but not in a level plane (i.e. one is higher than the other). If the rails are skewed, shimming the trucks will not correct it. Granite parallels could check for flatness and skew in the rails, but it's harder to imagine how to check the rails for level (other than by using a level), or to check the mounting surfaces of the table to see if they are coplanar and parallel to the table top. I guess that can be done on a large enough surface plate with a little setup. Can someone educate me?
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the rail surfaces can be shimmed but getting shims in .0010, .0015, .0020 it makes it difficult to get shim
differences of less than .0005"
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i have used a electronic level connected to a computer and it will see if surfaces are planar, straight, twisted, etc
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as i machine castings on a big CNC the easiest thing is to put casting on the 8 by 14 foot table and put a
.0001 indicator on spindle and use the machine to measure it. i use a double column gantry mill so
we have the ability to make one side of the Y rail higher than the other using the two W columns.
this is so we can get 2 rail surfaces perpendicular (top to side).
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sometimes a rail surface is not straight. sometimes it goes up or down .0005" over 5" with the rest of a
40" or longer rail surface is straight. hardness variations, casting deflecting, machine error. hard to say.
we often cut another .0010" so casting deflecting from cutting pressure is not the cause of error. some
castings just distort after seconds, hours, days, weeks, etc or after hold down bolts are loosened. we usually
torque only 30 inch lbs which is close to finger tight when we want best accuracy.
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as a machine is assembled the linear rails slides are checked for straightness of travel and
perpendicular to another with a granite square as machine is assembled. gravity and how a
machine is bolted to the floor sole plates can easily twist or distort a machine frame