MDF Spoilboard Effect on Vacuum Performance

Variations in panel porosity can affect the suction through your spoilboard, but other things could also be happening. July 29, 2012

I am running a Biesse Rover 30 flat bed with a Becker VTLF-250 vacuum pump. I am new to this vacuum setup. I am getting about 22 cm of mercury with a new open spoil board. Before I changed the spoil board the old one ran about 5cm. It was close to the same when new. I think the first one we had on was not dense enough. It was from the same supplier just purchased at different times. I think the newer one is running better.

Is 22cm (about 8.5" of mercury) on an open spoil board normal? With the old one we were lucky to get to 28cm on a full sheet. If the board had any warp to it we had a lot of trouble getting to 15cm. With the new one we get to 55 pretty easy (this is close to the full power of the pump).

Forum Responses
(CNC Forum)
From contributor R:
I have clients with all sorts of different machines and pumps. Obviously, the gauge is vacuum draw. The higher the number, the better the "grip". Five seems quite low, 22 would be good.

It sounds to me like your initial setup was flawed somehow, probably a gasket was rolled or some other leak. All else being equal, regular MDF, lightweight MDF, or whatever should be at least close to one another when a sheet of ply in on the bed and set properly.

From the original questioner:
I am leaning in the direction of the first on being ultralight. Its odd because I use ultralight on my other machine and have to buy it from another vendor because this vendor doesn't carry it. I also had three boards from this same lot. They all performed the same way. Learning the new machine cost me a couple in the learning process. As I understand it, the holding pressure is the ending pressure less the starting pressure with an open spoilboard. I am just trying to get a reference of others starting point with a VTLF-250, a fairly common pump.

From the original questioner:

I guess I am not clearly stating the question. Someone correct me if I am wrong. The actual pressure on the working board is the pressure read on the gauge less that of the reading with an open spoil-board. Assuming this is true, then there is a big difference in reading a 5cm of mercury to start vs. 22 on an open spoil board on the same reading on the working board. At the same reading, say 40 cm, I would have 35 cm of pressure on the first case and only 18 on the second. I am getting all sorts of readings on open spoil-board reading on different boards. We have gone through about six with readings from 22cm to 12cm to as little as 5 cm.

If someone out there has a Becker vtlf 250 running on a 4X8 table I would really appreciate the reference. Are your readings all over the place or steady? Or am I just looking at something that does not really matter? One consequence of having low pressure was that when the spoil-board got to less than a half inch thick the pump would not hold is down enough to surface properly.

From contributor R:
First, even full thickness spoil boards can curl up after a few plannings due to moisture. I'm not sure about your machine, but I used to have my clients create a program to cut holes and counter-bores in the spoil board perimeter. Matching holes were drilled and tapped into the phenolic bed and nylon bolts were used. This kept the seal tight, and the nylon bolts would not damage tooling it you happen to hit one.

As far as the gauge readings, they should read about the same under similar conditions. Does the gauge read the same whenever the bed is open or does it change? You can test the gauge function by running the pump with the bed open and sequentially cover the bed. Each time you lay on a piece of stock the gauge should rise. There are a number of issues that could be causing problems.

From the original questioner:
The gages seem to be working fine. I have two zones and both read very close to the same. The spoil-boards read the same over time. Each one I put on has a different reading. I can take one off reading 22cm open and put another one on and read 12cm open. If I switch them back it goes back to 22cm. Is this variance in spoil-board porosity normal?

From contributor Y:
In listening to operators of all types of CNC Routers who use MDF as their "suck-through" plenum that each board varies due to the resin content and the variations in either the amount or size of the fibers. I was informed that if you keep notes on the amount of vacuum drawn you will see a "bell curve" as the MDF becomes thinner and thinner after leveling the surface.

From contributor R:
The short answer is yes, MDF can vary greatly in density. I have had panels that seemed like LDF, and others I thought were made of iron. Buying from the same manufacturer can help, if it is possible. Not the same vendor, but the same manufacturer, but even that is not a guarantee.