That's very good info. If you are doing mass production and starting
with blanks or pre-machined parts then pods are the way to go, no doubt.
However if you are machining parts out of sheet then you want vacuum on
the whole table. Table vacuum is generally more versatile but usually
needs some creativity when it comes to holding smaller parts.
I see a few suggestions to skim your spoil board to improve flow. In my
personal opinion that is not always a good thing. The higher the
permeability of the board the more flow you get in exposed areas. Flow
rate and pressure drop are directly related. If you decrease the flow
rate the pressure drop across the board decreases.
Take two areas of the board, one is covered with an impermeable object
and the other is open. The open area has no limit to the available air
so the pressure drop across the board is high and the flow rate is high.
However in the case of the covered area, little or no air flows through
the covering so there is very little pressure drop across the board.
Your part sees nearly the full vacuum.
If you are cutting impermeable materials such as plastics or alubond
you want a very restrictive spoil board to reduce lost vacuum in the cut
areas. If you are cutting permeable materials such as MDF you need less
restriction in the spoil board to compensate for the flow through your
work. It's a complicated subject and the only real solution is to
experiment and see what works for your setup.
Les
On 09/06/2021 21:00, Todd Zuercher wrote:
There is a wide discrepancy between the levels of vacuum that different types
of vacuum pumps and blowers can generate. Some kinds are rotary screw pumps,
liquid ring pumps, rotary vane pumps, positive displacement blowers like a
roots blower, or regenerative blowers (which are more like a fan). You need to
consider both the level of vacuum you need to attain and the amount of air you
need to move to get there. Compare a 10HP regen blower, vs a 10HP rotary vane
pump. The regen blower can move 4 times as much air as the positive
displacement vane pump. But the vain pump can draw more than twice the vacuum
level. Which would hold a part better will depend a lot on table design and
area. An open hole table with small pockets over the holes can work very well
with the blower, but the blower will struggle trying to hold small parts on a
fall board. The vane pump would work great on closed pods and jigs or a
smaller fall board, but if the table is very large or an open flow design it
won't be able to keep up and vacuum levels could fall too low.
A 5ft x 10ft fall board (sheet of MDF that has been skinned can flow nearly
300acfm (maybe a bit more) of air when a 15inHg vaccum is applied to it. But
if you need 20inHg to hold your parts, you're going to need a much larger and
more powerful pump to get there (probably at least double). But if you only
need 10inHG the flow through the board will be much less probably closer to
100ACFM. The greater the pressure differential the higher the flow and
leakage. This is why gasketed jigs and pods work so much better for holding
pieces. You don't have the flow to contend with, you only need a pump capable
of generating the pressure level you need and the ACFMs of the pump are less
important.
Todd Zuercher
P. Graham Dunn Inc.
630 Henry Street
Dalton, Ohio 44618
Phone: (330)828-2105ext. 2031
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