Help Maintaining Z accuracy with vacuum table

[fozzyber]

I am having issues maintaining accuracy with the z with my vacuum table,
about +- .020"... My Z axis is good and tight adjusted properly.
My vacuum table is a sheet of 3/4" maple ply base, with a mdf plenum (painted) then a mdf spoil board.

What I am seeing is I surface the table and start cutting and in as little as 20 to 30 min latter Z accuracy is going south. I am assuming the wood / mdf is gaining and losing moisture.

What I am thinking is pulling all of that off and useing a 4ft x 8 ft x1/2in sheet of steel :eek: as my base, then using a 1/2" sheet of type one pvc as my plenum then mdf as a spoil board.

Anyone have any comments or advise as to how this may or may not work....


Jerry

[gerryv]

I think before I would do that I'd try to either verify or rule out that assumption just to be sure.

If you can mark a few points on the lower V-track of the gantry with an marker and gauge the distance from those points to matching dots on the table below you should be able to determine if you're getting swelling, shrinkage or some other deformation.

A cheepo dial indicator attached immovably to an appropriate length of steel rod would do the trick as a gauge as long as the end of the rod touching the V-track is cut square and is very smooth. Doing this as a test job progresses would simulate your situation I'd think.

[fozzyber]

OK, i'll bite. sounds like a good test. I am surfacing the table right now. when it is done I will mount an indicator in the spindle and run it around the table with out moving the z, and see how much variance I get on a newly surfaced table.
Then I will run the vac for a while and retest....

[fozzyber]

after surfacing the table I indicated .002" of variance in the table. I am now running the vacuum on only half the table. that might tell me if it is from vacuum or just ambient...

[ken_rychlik]

How much off are you getting? Mine will contract about .010 over the course of 3 or 4 hrs cutting. I adjust the z down after 6 or 8 sheets to compensate for it.

You may also try a straight edge across your top. Hold it there without the vac and then turn it on and see if it moves. If your spoilboard is not down all the way when you surface that could be it.

My spoilboard is glued to the plenum.

[bleeth]

Baton Rouge?
With the swinging humidity levels there you could be seeing the expansion and contraction of the spoilboard as it gains and loses moisture. I get that too.Closed up shop all night and humidity going up. Mid morning sun and breeze blowing through and board contracts.

[myxpykalix]

This is just a thought...why not try cutting your part twice an an experiment.
Cut it once with the vacumn on, then cut it again just using some type of hold down without the vacumn. And measure at both times.

That would tell you how much the vacumn is contributing to the equation right?
Also are the edges of your mdf sealed? I take silicone caulk and coat the edges and it turns to a thick rubbery coating and seals the edges. I would think that maybe that flow of suction from the sides may cause part of the issue instead of a straight flow from top to bottom?:confused:

[coryatjohn]

I had some mysterious issues on one end of my table. I'd see the end of the table heave slightly after several hours of cutting with the vacuum.

Turned out the outflow from the vacuum pump was blowing vertically, which impinged on the metal under the table at the end with the issue. By redirecting the flow away from the table, the problem vanished.

Do you get any heat buildup under your table?