Vacuum and Part Size

      A question about how to relate vacuum requirements to minimum part size. January 2, 2012

Question
Is there a rule of thumb regarding how many inches of vacuum and minimum part size?

Forum Responses
(CNC Forum)
From contributor T:
A lot of machines are set up so if the vacuum drops below a specified level it shuts off. For flat sheet applications I was involved in a project where the guarantee was 4" x 8" rectangles with a 1/2" Bit over a 5' x 12' foot sheet - optimum nested, through cuts to the spoil without parts moving. In order to get it we had to do a lot! On that particular machine I believe the vacuum was set to trip an alarm at 14 or 15 inches so it was a constant concern. It is really application specific, on flat tables look at what you can do to recover the vacuum you lose when you cut. On pod and rails if the surface is too small you probably need a mechanical assist anyway.



From the original questioner:
Nice idea re the vacuum interlock. My current pump only pulls six or seven inches of vacuum (maximum rated is ten inches) during operation. Any guesses as to the smallest part I should be able to cut? Does it make any sense to try and measure vacuum at the sheet rather than at the pump inlet? I have a flat table. What do you mean by recovering vacuum lost?


From contributor M:
The more inches cut from any material (hardwood, plastic, metal, plywood etc.) during a cycle the longer and larger the area for vacuum to escape. If you stick a sealant just to the inside of your tool path you will create a barrier wall so that full vacuum pressure remains against the part (whatever size) and the cut created by the tooling has no bearing on the pressure regardless of the number of parts being cut.


From the original questioner:
I have used some of your product when cutting repetitive components and it's worked well. Most of what I do is random cabinet components though.


From contributor T:
Vacuum lost means nested applications and through-cuts to the spoil (slightly into the spoil .2mm .006-.008 thousandths). A lot of components in the nest equal a lot of tool paths (swathes) in the sheet to allow vacuum to be lost at the spoil the more you cut. In other words if you put a 5'x12' sheet on and cut it into four equal sections is much less likely to move than if you cut it into 40 equal sections (more through tool pats for vacuum to escape).


From contributor B:
There are many questions and even more factors to consider with this post.
Six-seven inches of mercury seems very low and certainly will be an issue for all but the largest parts. Are you cutting smaller parts from full sheets? Hoping to hold smaller blanks for processing? In either case, there are lots of work-aroundís and solutions that I've seen and used over the years. Onion skinning, holding tabs, fixtures, and methods of isolating the unused vacuum areas are all options based on your specific set up and needs.


From contributor B:
Cutting strategy can make up a lot. My Fuji pump only pulls 7" max but is very tolerant to leaks. Never the less the best I can get is seven inches and I frequently cut 2" X 12" parts without having them move. First I always cut from one side of the table to the other. The last part needs to be somewhere around three square feet. When I cut I leave small tab on the part connected to whatever is left on the sheet. Then go back and cut just the tab. In the end the more vacuum the better. Cutting strategy can go a long way as well.



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