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Distorting Milling Table While Clamping Work to Table


Oct 25, 2021
Manitoba, Canada
I was wondering if anybody has had any experience with their milling machine (or horizontal boring mill) tables distorting from clamping work down to them. I have searched through the forums and could not find any discussions on this topic.

I have a Wotan B75 HBM with a built in rotary table. The table is 30" by 40". About two months ago I was clamping some 6" x 6" x 30" long bar down on the front edge of the table in order to face the four sides square to each other. (this was a typical job of squaring blocks on a horizontal machine. i.e. machine face, remove part, deburr, place back on table on just machined surface, clamp, machine next face etc. until all four faces are done). After machining three of the sides I decided to measure the width of the block at the top and bottom to see if the faces were parallel. The result was that the top was about .006" narrower than the bottom. This was the opposite of what I was expecting as the table slopes away from the column slightly and the column leans away from the table slightly and if anything the part should have been wider at the top, not the bottom. I could not figure out the cause until yesterday when I had a similar job and got nearly exactly the same results.

I have a set of long (16") clamps and I was using these to clamp the part. The distance between the part edge and the heel block was 12" and the stud was roughly in the middle. My theory was that the downward force at the ends of the clamp combined with the upward force of the stud was causing the table to bend into a convex shape and this tilted the part towards the column. I checked this by placing a dial indicator on the just machined surface near the top and then releasing the clamp studs. The part leaned back away from the column .003". Since this occured on both sides it accounts for the .006" difference in width between the top and bottom. I had one more surface to machine and this time I clamped close and not quite as hard. The result was almost no light visible under a known good square. (previously you didn't need a light to see how bad it was, you could see it from a distance).

Has anyone else run across this problem and do you have any advice or rule of thumb you use to prevent this. Or have you used this to your advantage by slightly bending an out of flat table to improve squareness/paralellism? I will definately put more thought into how I clamp parts on my boring mill from now on.


Yes, this is a real thing that happens and you should always be aware of. Nothing is completely rigid.

Doing large parts in horizontal with big heavy bar clamps is a sure way to bend the table - chain clamps mitigate this to a great extent.
How so? Just better placement options?

Yes - you can anchor them wherever puts the least strain on the table. You can tuck them in under the part if it's in vee blocks for example.

But also because it's very easy to massively overtighten a big steel bar clamp and studs, and not so easy to do with a chain clamp.
I set up a table vice (two pieces, "fixed jaw" bolts directly to table, "adjustable jaw" bolts separately to the table at the other end of the part) on a horizontal mill once and gave it a good hearty tightening. This bent the table enough that the X-axis travel was definitely too tight. Backed off the clamping pressure and all was well.
Irrespective of however you are clamping the work the column should lean forwards towards the work table by roughly 0.001” to 0.0015” in its length. It shouldn’t lean away. The is to counter the forces created by the cutting process.

You really need to get the table level to the ways also in an ideal world. Trying to work around known errors can be frustrating.

Regards Tyrone