You have two different cases.
ie; You have a pump or you are going to size a pump system for the system.
If you have a pump then you will reduce the available negative pressure to address the variables associated with the system.
If you are sizing a system then you are using the same calculated system requirements, will be conservative, and optimise the system.
The available hold down force is simply the negatve pressure available times the applied area. In the end, the available hold down force required must consider pump efficiency, line loss, and leakage, etc.
Additionaly the hold down force required must address the machining forces. The machining force can be approximately calculated, but since the calculated value is very subjective, practical experience with a particular machining circumstance is
very valuable and would override calculated. Thus judgement comes into play and is very difficult to address different or numerous senerios.
Now note that i say "SYSTEM". The porting of the table is different for a small piece as compared to a large piece or maybe you have a table desgned for both small and large pieces. The leakage rate varies and again subjectively one must
provide appropriate values in the calculations, and size other parts accordingly.
I would strongly suggest that you have a hard look at vacuum systems which can be purchased ( be attentive to what they are used for ) to get a feel of commercialy available designed systems, use any practical experience from folks that are actualy using one, do some calcualtions within your capabilities to see what you want to do.
There is more to calculating than one would think.......
RICH
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