Stable spoilboard material
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 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.
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