Stable spoilboard material

Choosing spoilboard material that won't shrink, grow or warp. January 3, 2001

We use our router to machine relatively small solid wood parts on dedicated spoilboards. We pocket out areas to locate small pods, which in turn hold the wooden parts. I use two-sided melamine for the base and MDF for the pods and then locate it on our bed.

One of these spoil-boards appears to have grown .1 of an inch over the three months between uses. Each of the 8 pieces located over the 8 spoilboard had a progressive error. When we re-ran the base pocket program the error went away.

What spoilboard materials remain flat and have minimal dimensional change due to moisture variation (wood) or temperature variation (plastic)?

Forum Responses
I assume the melamine is particle board core. My experience is that a 4 x 8 sheet of particle board will swell close to 1/8" in each direction. The melamine faces will slow this expansion, but not eliminate its occurrence.

I believe MDF is much more dimensionally stable.

The most stable material that I have found is Phenolic Canvas (Ryertex). Unfortunately this stuff is heavy and very expensive. A good grade of 11 or 13 ply Baltic birch is also reasonably stable.

One way to minimize the impact of spoil board instability is to use several small fixtures instead of one large one. These small fixtures are located individually on the bed with pins and bushings. Error increases with the distance from the machine reference, and the smaller fixtures simply minimize this distance.

I agree with using Phenolic or Baltic birch. With continued use you may find your vacuum system can assist in breaking down MDF and particle boards. If constrained to MDF, try sealing the fixture by applying finish. This can also increase your vacuum levels by restricting air flow in unwanted areas.

Watch out using plastics--UHMW is very slippery, and PVC can emit gases during machining. Any wood composite used can take on moisture that will effect its accuracy. It will change with the weather.

Don't forget to check the level of your machine. If you are in a frost area your plant floor can heave, causing movement in the machine base.

You might try laminating 2 pieces of tempered masonite together and then milling the face flat. This takes a little time but provides you with a very stable spoil board.

The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contributor A:
Since you are essentially using the spoilboard as a fixture, I recommend the following: After machining your spoilboard from the most stable material you can find (MDF or baltic birch), try encapsulating it in marine epoxy such as is available from System 3 or West System. They build offshore racing yachts using this technique and they hold up forever.