Vacuum Blower Temp / Bleeder Board Material

[M2@ASI]

We have a Multicam MG204 with a 20HP vacuum table system that we have
recently had some concerns about. We installed the machine last year and
when we first put it into use we were using 3/4" LDF as a bleeder board. Being
that this material is hard to source in small quantities and that we had heard
that it was fine to use MDF we went ahead and switched over to using 3/4"
MDF for the bleeder. The vacuum we are getting through the MDF is excellent,
comparable to what we were getting with the LDF. One significant difference
we have noticed is that the exhaust coming from the vacuum is much hotter.
The vacuum also sounds like it might be working harder but does not sound
like it is struggling by any means, but the temperature of the exhaust may be
as much as 50% higher than when we were using the LDF.

Has anyone had a similar experience? Is MDF indeed safe as a bleeder and how much additional stress are we putting the blower through? I am going to contact the blower manufacturer to see about an operating range for
temperature but if anyone could relate any experience it would be greatly
appreciated.

Mike

[ger21]

We use MDF, using two 25HP pumps on a 5x12 table. Make sure you skim .01-.02 off both sides before using it.

I've never used LDF, so can't compare the two.

[M2@ASI]

Thanks Gerry. I actually milled all the way to .05" on both sides and still
ended up with the same results in so far as temperature is concerned.

Although it was hot the machine was still operating within its limits as stated
in the manual, however in the interest of reducing long-term wear and tear
on the blower we ended up switching back to LDF. The vacuum is once again
operating significantly cooler than when using MDF, approximately 50 degrees
cooler.

[Geof]

... The vacuum we are getting through the MDF is excellent,
comparable to what we were getting with the LDF. One significant difference
we have noticed is that the exhaust coming from the vacuum is much hotter.
The vacuum also sounds like it might be working harder but does not sound
like it is struggling by any means, but the temperature of the exhaust may be
as much as 50% higher than when we were using the LDF....

I think a higher pump temperature coupled with better vacuum through the MDF board makes sense. Better vacuum suggests that this board is more porous, you don't lose as much vacuum across the board because air passes through more readily. Concomitantly the pump is pumping more air, it is working harder so it gets hotter.

Were you sealing the edges of the board and blanking off any area on the surface that was not occupied by stock to reduce leakage? If you are careful to minimize leakage with the MDF you could benefit from improved holding power with less increase in pump temperature.

[keebler303]

We have I think a 10HP and a 15HP pump on our 4x8 table. The vacuum system has a relief valve so that when it reaches a high enough vacuum level, it opens the valve allowing some cool air to flow through the pump to cool it while still maintaining "good" vacuum on the table. Anyone else have one of these?

Matt

[M2@ASI]

Geof - It is my understanding that the LDF is more porous, therefore the
pump can pull the air through the board more easily and operate at a cooler
temperature.

Although I do blank off any unused area of the bleeder when necessary, I
have never taken the time to seal the edges. Can you tell me how much
more hold down power I may get from this? I have an ongoing argument
with our shop manager on this. He maintains that because the sum of the
exposed surfaces on the edges only equals a couple of square feet it wouldn't
make much difference. I counter that because vacuum is being lost around
the overall perimeter of the board the impact is much greater than it would be
if only a small section went uncovered.

Matt - We do have a relief valve but it is manual unfortunately. Something
automatic would be great however I am not sure if it would switch on very
often. I check the vacuum gauge often during operation and it is always
within what the manual states are its operational limits.

Mike

[keebler303]

LDF, MDF, etc. are made a bit like paper. The fibers are predominantly in the X,Y plane. When it is pressed, the top and bottom surfaces are compressed to a greater degree while the core is a little less dense. Due to the lowered density and the fiber orientation, the board may leak significantly more out the edges than through the thickness. Its easy enough to paint it with a couple coats of polyurethane.

The relief valve is not necessarily there to protect the pump from pulling "too high of a vacuum". Assuming you have a good seal, your pump will be pulling 0 CFM at lets say 25 in hg. If you could crack open a relief valve (either manual or automatic) and let in say 1 CFM of ambient air, your vacuum level may not change significantly, but now the pump is pulling a little air through so it will run cooler.

Matt

[Boltz]

An oil sealed, vane-type vacuum pump will run coolest when fully blanked off. A positive displacement blower, on the other hand will run hotter at higher vacuum levels, and needs a relief valve to protect it. My Cylcoblower runs quite a bit hotter at 17" than 12". I think the heat comes from the friction of the air leaking back between the rotors.

-Jim Hart

[M2@ASI]

Great info Matt, thanks. I was wondering what would be best to seal the
edges with and I was thinking a polyurethane would work well. I figured it
wouldn't be absorbed as much into the board as a lighter substance would be.

Jim - We have a TurboVac 425 on our MG204 with a Roots blower. Like
I mentioned previously we do have a manual valve that I can open. I think I
will go ahead and try opening the valve slightly during operation and checking
the difference in both vacuum hold and operating temperature.

Mike

[Boltz]

Mike, does your MG still have the original H-board controller? Do you have steppers or servos?

-Jim Hart

[M2@ASI]

Jim - The machine still has its original control board, which I assume is the H Board, with servo motors. We were considering replacing/upgrading the board later this year or early next.

[Boltz]

Have you figured out how to do a slew move with the spindle off? Also I haven't been able to get my spindle to turn CCW.

It's a pretty good controller, but impossible to find out anything about. Multicam couldn't even come up with a schematic for my "very old" (early 2000) vintage machine and Extratech (who wrote the firmware) won't even talk to me. Nonetheless, the speed and accuracy in normal operation is quite good (I doubt I'd say that if I had steppers), but there are other features like edge-probing that I'd like to add, and there is zero documentation.

There are 2 retrofit options that look viable.

One would be an industry standard control like a Fagor. Nobody seems to like Centroid or the other non-standard retrofits.

From what I can tell when working with servos Mach3 just outputs step/dir and makes the drives do all the work of closing the loop.

The other would be LinuxCNC (formerly EMC2). The performance I've seen on Mori retrofits is pretty impressive. The trajectory generation is high-end.

-Jim Hart