Pump Capacity Requirements for Vacuum-Table Work

Necessary pump size varies depending on many factors. November 12, 2005

Question
I have a 15 hp 300" vacuum pump. Should this be able to hold a 4x8 sheet of stock for machining? Any suggestions, tips or guidelines would be helpful.

Forum Responses
(CNC Forum)
From contributor D:
300" of vacuum should hold it pretty good.


From the original questioner:
It is 13" of Hg. That sounds better.

From contributor H:
Is 13" the differential or the total vacuum being pulled? Differential is the vacuum difference between what is being pulled with just the spoilboard on the table and vacuum pulled with the spoilboard and material on the table.

Also the vacuum needed to hold your parts is dependant on your material, the size of parts you are cutting, whether you onion skin or tab, size and type of cutter, your feed rate, etc. If 13" is your total vacuum then you are probably in trouble.



From contributor M:
13" is way to low; a 15 hp 300 c/m pump should pull much more than that. Are you at a high altitude? Have you fly cut both sides of the MDF spoilboard? Also, are you using gasket material to close off the grid under the spoilboard ?

Consider edgebanding or painting the 4 edges of the spoilboard to eliminate bleeding of the vacuum. Is the sheet size the same as the spoilboard, if not place a non-porous material on the spoilboard to close of this leakage? Concerning the pump, what brand is it? Is it a oil flood or oil less, or is it a graphite vane type and if so have the vanes been checked/changed?


From contributor G:
Your vacuum requirements depend on the size of your parts, part permeability, total leakage in your system, inrush requirements to "make" vacuum on warped parts, and cutting stresses. I have customers that do one side machining on full 4' x 8' sheets of melamine faced MDF and do very well with a 5 hp blower pulling 10" Hg.

On the other hand, machining nests of small parts (less than 25 square inches) on a bleed board will require a minimum of around .8 hp per square foot of bleed board with a pump capable of pulling vacuum in the range of 25" Hg. If you only have 15 hp and 13" Hg. you will only be able to do heavy machining on fairly large parts with a reasonably tight fixturing system.


From contributor K:
Your cutting strategy will also play a big part in what you can effectively hold down. I have a 6.2 hp pump that holds quite well when cutting small drawer box parts out of Baltic birch. I use a .25 compression bit down to .0625 and follow up with a final cleanup pass to clean up tooling marks. This allows me to get by with a weak pump until I can upgrade.

From contributor S:
The 300" does not tell us what gauge you are referring to. Is it inches of water? Or maybe open flow of the pump?

With additional information - pump manufaturing, model, number of holes in hold down fixture, diameter of holes, if any holes will be left open to atmosphere (if so how many) we can then give you a hold down force that will be applied to your material. A fudge factor will need to be taken off for leakage - that will be dependent on porosity of the material.

If you have tried to hold with your current setup and have found it not to work, you may try placing a thin sheet of plastic over your material (food wrap will work). This will allow you to help eliminate some of your leakage. You probably should make sure you clean out your inlet filter after you have tried this.

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