Spoilboard Material and Vacuum Strength

Variability in MDF material may result in problems with consistent vacuum hold-down. Here's advice on how to compensate. September 8, 2007

The MDF we have been using has changed characteristics recently. Bottom line is, we're not getting the hold down on smaller parts that we used to. In the past we were using standard 3/4 MDF, fly cutting both sides, and would have 10-12 differential on the vacuum, which worked well for small parts such as nesting dovetail drawers. The MDF now seems to be much denser and we only get 5-7 differential on the vacuum. We have a 40hp Travini pump, so that shouldn't be the issue. I was recommended to try the ultra light MDF, but there's so much vacuum pull through we can't fly cut, since the machine (KOMO) errors out. We're in the Northwest and maybe our MDF is a little different. Any thoughts would be appreciated.

Forum Responses
(CNC Forum)
From contributor S:
Do you seal the edges of your spoilboard? Maybe you lose more vacuum through the edge than you used to. Put some iron-on edgetape.

From the original questioner:
We're already sealing the edges with edgebanding. Still too much flow through.

From contributor C:
This may be an over obvious answer, but have you cleaned or replaced your filters? On our Andi, we found after using it for a few months that there was a second filter inside the Becker pump that our Andi rep didn't show us. That and the fact that MDF, when cut, is such a micro fine dust that no matter how often you blow out the filter, some dust is left clogging the pores. Try a new filter and see if that helps. As for the MDF itself, you are right that Ultralite is a joke for use as a spoilboard. If the 3/4" you are getting from one vendor is too dense, either try another vendor or a different thickness to start with, like 5/8" or 1/2".

From contributor T:
Try to get double refined MDF. We go through the same thing here where some of the units of MDF we get in make horrible spoilboards. As for the error codes with low vacuum, you should be able to adjust the safety settings. We had this problem after the spoil board had been surfaced many times. We're running a Komo Mach II. The setting adjustment is in the back of the operator console.

From contributor C:
Just for reference and curiosity, I looked at our current vacuum readings on a job we are cutting now and here are the results.

Unloaded spoilboard = 11.5 inches of mercury
Loaded spoilboard (Lyptus Veneered Medex MDF core) = 18.25 inches
After cutting average nest = 16.5 inches
Nest included 3" x 24" parts with out issue
25hp Becker pump
3/8" comp spiral bit, ccw cut 15,000rpm, 750ipm
Nesting with CabinetVision's Advanced NC Center, small parts using Onion Skin, very small parts using Return Onion Skin and cutting first
Even our toe kick notches don't move.

Another thought... Might your flycutter be dull and polishing your spoilboard's surface? For good holding, you need lateral friction as well as good holding vacuum.

From contributor R:
All good responses, but I think I would rather have too much flow-through than too little. I was a field service guy for Komo for 6 years and I never installed a new machine where I didn't have to reset the vacuum safety switch to a lower level. We have a 1605TT in our arsenal and I had to adjust the switches on that machine also when it was new. Your machine manual has the switch location and it will take a little experimenting to set it for the density of MDF you are using, but it should take care of the problem.

From contributor C:
Ultra Lite MDF or any low density MDF as a spoilboard is a bad idea because every kerf that you cut through your material causes a kerf leak where too much air flows. This then has an immediate effect on hold down pressure. Search the Knowledge Base for "spoilboard" - there is a great write-up on this that explains it fully.

UL MDF also when vacuumed down unloaded (without product on it) is not the same thickness as it is when loaded. It compresses under pressure. As the kerf leakage increases, the spoilboard gets thicker and has less hold to your parts. In my opinion, never use Ultra Lite or low density fiber board.

From contributor G:
As someone mentioned, you may be polishing the spoil board. If you are using an insert type cutter, set it at 1000IPM. What this will do is leave a little fir on the top of the spoil board, which in turn provides better hold down.

From the original questioner:
Some follow up since I started the post. We are now using 1/2" MDF from Plum Creek, which after fly cutting both sides twice, was starting at 12 inches of mercury. When we put a panel on, we're at 22 - 24 which is giving us a 10-12 differential. When cutting nests of dovetail drawers, which results in a lot of kerf openings, we end up at 16-17. Thanks for all the feedback, folks.

From contributor H:
When I am making a spoil board, this is how I set mine up. I start with a 3/4" sheet of ultralight MDF. I plane down the top side by 1/16th, then flip it over and plane the other side by 1/16th, leaving me 5/8". Then I'll edge band that 5/8" MDF. Next I'll take a piece of 1/2" regular MDF and do the same, 1/16 off the top, and 1/16 off the bottom, leaving me 3/8. Now I'm ready to cut.

The ultralight works great letting the air pass through and the resistance is in the 3/8 regular MDF. I set my tools within ~ +.001" - ~ +.002" and my program to cut through by ~.002", so when I am running, I nick my board by about 1/64th of an inch, leaving me with my .5mm spoil board program to run when necessary.

I spoil my board before every job, and run the sheets with the smallest parts first. I have found with that spoil board setup and clean filters (excellent point above), I lose very little.

Also, how far is your pump away from your machine? I know of people who have had issues by placing their equipment too far from their machines.

From contributor HL:
I use LDF only for spoilboard. For bolted down spoilboard. (If sb can not be bolted down or you will not do same discontinue reading this method.) Flycut both sides to cleanup (No set amount removed). DO NOT cut through parts. Program or set tools to leave .006-.015 material in Z axis on ALL parts and material. This eliminates troughing and reduces flycutting. I have used this method to cut at least 15000 sheets of MDF doors and many thousands of other wood parts since 1997 and have found this method works best. For Smallest parts screwing down a fence and blocking blanks in, leaving material in Z axis, eliminates part loss if done correctly

From contributor HL:
To increase vacuum: Cover ALL the top of the spoilboard (sb) not being used. I use 1\4 styrene insulation cut to fit with plastic on at least 1 side. Also 0.01 thick plastic sheeting. Sheeting covers all sb imperfections, some debris, can be lapped over edges, and if there is an overcut no damage to cutters.