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)?
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.
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.
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.
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.