vacuum chucks

[kwhite2]

anybody using grid type vacuum chucks, or any type of vacuum chuck i guess, on vertical mini mills, whats your opinion on them for work holding, boss wants me to look into them, thanks.

[Matt@RFR]

They work really well for parts that have enough surface area. For hold down force, take the surface area of the part in square inches and multiply it by 14.7 for a perfect vacuum at sea level. The formula I use in a spreadsheet is as follows:

(inHg * .4913 ) * Sqin = total hold down force

So if you are pulling 25inHg and have a part with 10sqin surface area exposed to vacuum, you will have about 123psi actually holding the part down. In that example, the biggest endmill I would personally use is maybe 3/16" if roughing very aggressively.

Hold down force is strictly normal to the vacuum table. Sideways force is not near as high so if using larger tooling to rough the part with, pins may be needed to keep the part from sliding sideways.

Concerning the table itself, they are really easy to make. Certainly don't buy one! I have several vacuum plates that I made for specific parts so I can machine through holes and not lose the vacuum source.

[Geof]

As Matt says they are fine if you have enough area. We machine 1/4 polycarbonate pieces held down on an area of about 60 square inchs and need a vacuum of better than 20" Hg, that is about 10 psi or 600 lbs of force holding the part down. Less than that and running a fast cut around the perimeter with a 1/4" high helix cutter will lift it off.

[gpcoe]

We use them a lot where I work. As long as you have a pin or something to hold them in shear, they're great. We had one project that was roughing with a 1in rougher in aluminum, 12k rpm, 200 ipm. Held by two 1/8 dowel pins and vacuum. I had a bit of trouble with one of the parts i programmed with a vacuum chuck, because there wasn't a good way to hold it (pins, etc.) and I had to mill the outside of it. Just had to slow things down a bit.

Greg

[Ydna]

I occasionally use them for engraving. The vacuum system I use is absolutely as rudimentary as one could imagine I've also done flycutting with them before, but only in very very small depths (enough to dust off the grind marks on material stock before the engrave)

[kwhite2]

think we could hold coplanar .0002 flatness and // on 5" x 10" x 1.25" aluminum plates that have about half the thickness channeled out leaving .5 walls with this type of system.

[Geof]

I think your biggest challenge trying to get that flat and parallel will be distortion in the material due to pre-existing stresses be unevenly relieved. My experience with 6061 plate is that if you take away a lot of material from one side it will finish up bowed.

If you are dealing with stress relieved material you might have a good chance but if you hold it down on a typical vacuum chuck with rubber seals around near the outer edge there may be enough distortion from compressing the seal that you do not meet your specs.

I would finish the perimeter first and then make a vacuum chuck which sealed around the perimeter so the underside of the plate could be pulled down evenly onto the bottom of the chuck. This would mean the vacuum and sealing forces are not trying to distort the plate in any way. The bottom of the chuck would have to be "perfectly" flat with a cross pattern of shallow grooves to distribute the vacuum.

[kwhite2]

the material is thermal stressed relieved, hey thanks for the input, gives me something to work with, not sure this is route i want to go yet

[underthetire]

We use them daily on some machines. The one draw back is once the part starts lifting off the vac fixture, it looses all work holding all at once. I built a vacuum panel with a cheap plc to monitor it and stop the machine if it detects a small amount of vac loss at the part. Just FYI.