Work hold down?

[CNCRob]

We use LDF alot for forms at my job. We seal the edges with duck tape and skim just a hair of the coating off for better suction.

[Chamm128]

How much CFM does it take to get enough suction to hold something down?

[2muchstuff]

It's not the CFM that holds things down, it's the "strength" or amount of pull. It is generally measured in inches of water, inches of mercury or mm of mercury. You can have a perfect seal on something with no CFM and still hold it in place if you have enough pull. CFM is used to size the pump, how fast a vacuum can be pulled down or maintained. CFM will also allow you to have leaks in the system (open holes in the table) and be able to hold a set vacuum level holding your parts in place.

It is always a good idea to get a pump at least twice the size that you need. Installing a vacuum regulator on the suction side of the pump will allow you maintain the level you want and allow for a level of reserve vacuum. Also with a vacuum regulator your pump won't work as hard and last longer by keeping a constant flow of air thru it keeping things inside cooler.

[xairflyer]

So is a shop vac any good or do you need a pump.

[Bloy2004]

My experience with shop vacs was....
when there is not enough air flow through the motor , the motor would race faster from less loading of air through the cooling fan and thus the motor would have no cooling and heat up melting the brush holders causing them to stick and a resulting arcing with soon motor burnout.
shopvacs need constant good air flow (lots of it) to function well under long use.
If you've ever placed your hand over the intake of a shopvac (or it got plugged, you notice right away the increase in rpms. Running it for an extended time like this will result in eventual burnout.
I used one with an extra bag inside to filter out finer particles having the motor reduced in air flow and although it worked great for a while, after a lengthy operation the motor gave out.

Bloy

P.S contributing factors also incuded a probable "cheap" shopvac.