Sheet hold-down

[RoboChair]

Originally Posted by MrBasse

If you have any holes to run on your parts you can run a drill program fist and use those to screw/bolt down where individual parts will be. We've had good luck with this, but we don't have a sacrificial board, ours is all slots for clamp mounting.

Originally Posted by Jared

thin double sided tape seem to work well.

We on 1678(and me personally at work) use the above methods as well as leaving behind tabs on some of the more troublesome parts.

I have a few things to say about sacrifice boards.
1678 is using a fine particle board as our sacrifice, it takes half a 4x8 to cover the table of our VeloxCNC and the full sheet only costs about $35. We use small sheet and wood screws straight into the sacrifice, in holes where possible, holding down edges otherwise(takes 4-8 screws to do it this way). We also have several blocks of plastic to the side of our sacrifice that have been machined in place that we use to clamp extruded square/rectangular tubing to to drill and machine(PM me if you want pics). When we plane down the board another couple times or so and machine into it enough to risk hitting the bolt heads holding it down we will take it off, throw it away, then bolt a fresh piece down. We have gotten our mister working with WD-40 at a VERY FINE rate that keeps the sacrifice in relatively good shape even after heavy use. If you see droplets you are using WAY too much lube. You cannot use a water based lube(even with a fine mist) with the stuff because it swells and warps, requiring much more frequent resurfacing of the sacrifice.

Greenbotics where I work uses a 1" thick PVC foam sheet(now about 0.85") that is bolted to the table, it is far more expensive up front but has many benefits. The biggest upside is being able to use any lube that you want as it won't adversely affect the sacrifice, the water based stuff is much cheaper. The PVC foam is also much more dimensionally stable in areas that have changing humidity levels or many other environmental factors. That means that you can machine parts that require a reasonable amount of depth accuracy such as +-0.005". Another big upside is that once you have planed the sheet down enough you can just glue on another sheet on top of the original and have WAY more material to plane off than the first go.