Vacuum Chuck (table/bed) Pump Sizing

[dpw3868]

Background:

I have a need to size a pump for piece holding on a "cnc" style "grooving" machine.

The operation uses saw blades to cut slots (what we call grooves) into a sheet of a polishing material.

To hold the work piece we have a vacuum table that utilizes an ancient compressor plumbed in reverse. This doesn't actually work and we require tape to hold down the piece.

My goal is to specify a proper size pump that will do the job. The foam sheets are porous.

Sizing a Pump:

Pressure - Determines holding force. this is determined by the machining needs and not of much interest to me because its a trivial calculation

Flow Rate - because the material is porous there is "leakage" which we must have adequate flow to compensate for and actually draw the vacuum. This is the main concern.
The material has open and closed cells and from scans of thin gauge material the open cell porosity is about 3% and the bed has 2500 .125" holes in a 60"x60" array.

I used bernoulli's principle for flow through an orifice to estimate the required flow rate. I took a conservative view that the material was 20% porous. Assumed there was an opening of 20% of the area of a .125" D hole and a preassure difference of the vacuum capability of the pump. This gave me the flow required for 1 hole.

Does this seem like a realistic approach to this problem?

 

[quark]

Why can't you use positive pressure to grip the sheet? For example, you can press down the sheet with pneumatic cylinders.

In the absence of actual data, your approach is ok. Once you arrive at vacuum level required to grip the sheet, calculate acfm at the required vacuum and this will be your pump capacity. If you have enough patience, search for sterling vacuum booklet (I might have referenced this link in about 5 to 6 threads, earlier).

Normally, pulling against vacuum is more difficult than shearing. As your load is porus and you need to groove the sheet, keeping it in place seems to be not right, but I am just guessing.

 

[3DDave]

It's much easier to block off the unused areas with plastic sheet. By blocking off the flow you can easily achieve as much vacuum as the pump is capable. If not, you are limited to the sustainable pressure drop, which is going to be more expensive. For example - blocked off one could get nearly 14.5 psi hold down with a little pump; not blocked off you will need to sustain sonic flow through the holes to get near 14.5 psi.

 

[dpw3868]

So let me explain further.Its a high density polyurethane foam, relatively low shear stress in comparison to metallic materials.

What I did was take a hypothetical vacuum available on a pump. A becker 175 CFM 25" HG pump.

Using 25"HG or 12.5 PSI as vacuum pressure and taking .125" diameter hole *.80 for 80 of the material that is not porous.

I get a holding surface of .0098 in^2 per hole. The holes are spaced 1.2" on center in a square matrix. a 23"x55" pad has about 878 holes covered.

This gives an apply area of 8.615 in^2 and an apply Force of 107 lbf. (i don't know if that is enough to machine the material, although it has a low profile of .118")

That part i think is reasonable assuming we obtain the 25' HG of vacuum.

The next part i take the same .125" hole *.20 for the effective opening. This is very conservative as the material open cell porosity is ~ 3%

I get an effective opening of .00245 in^2 delta Pressure of the same 12.5 psi. I used a flow factor of .6

Q = C*A*sqrt(2*delta_P/density_air)

I get a .120 CFM per hole or 114 CFM for the same 23x55 pad assuming the other holes are effectively sealed.

Is this reasonable? If the table and piping is at vacuum do i need to assume the air entering it expands? This would greatly increase the flow rate requirements.

The current system is using an old compressor plumbed backwards with no documentation. I have no basis for comparison.

Quark i was able to find the sterling book. And if i understand the problem correctly I think i determined the leakage in my example and now i need to use the pressure change to calculate the ACFM required.

Thanks for the feedback look forward to any more discussion and responses.

 

[3DDave]

I missed that you are trying to vacuum chuck a porous material.

The values you have are probably conservative as the path length through the material hasn't been considered. Thre may be some experimental data available, but it should be easy enough to do a test with a pressure gage, flow meter and a shop vac to estimate the holding power.

One problem is lateral leakage under the foam. Ordinarily a dam is built into the vacuum table that can seal well against metal, but the foam could prove more difficult.

The second is that, locally, the foam may plug the holes so the holding power is just the hole area times the pressure delta.

I'd try placing a waxed paper sheet over the foam and then tape around the edges to seal the paper to the table. Then the leak path will only be where the saw cuts are.