OK, here’s a weird one. Turns out that this insert is so long that it’s hard to get it to be perfectly flat. There are grub screws, but they obviously only work in one direction - pushing up, except at the front and rear where there are fixing screws.
Now, if the middle naturally wants to bow down, then the grub screws can be adjusted to push the middle up. However, if the middle naturally wants to curves up, then there’s nothing that can be done to level the plate.
I checked the original plate, which is aluminum, and it appears to have a slight bow in it such that the middle is naturally down. The plate then bends as you attach it front and rear, and the grubs screws do their job.
How can I get the same kind of bow in my machined plates? I don’t need much - just a couple of mm’s.
One thought was to place a 2 mm spacer under the middle of the plate when attaching it to the spoilboard (we’re machining the underside). That would push the middle up temporarily, so when removed from the spoilboard and flipped so the milled side is down (the actual position), the middle would be recessed by the thickness of the spacer.
But, that’s just the first step, Next we have to get the currently unmilled surface (the installed top) parallel to the bottom. So, attach the bottom down to the spoilboard, forcing it flat (the ends will need tighter fixing), and do a simply facing operation.
When removed from the spoilboard the plate would then spring back to a now slightly bowed composition. This, of course, assumes I can get strong enough bonding to the spoilboard to do these milling operations. The Acetal (Delrin) plastic is pretty bendable at the 11mm thickness, so maybe.
But, perhaps another way would be to somehow map the existing Fusion 360 model onto a slight curve. Since I’m milling all the areas that would contact the tablesaw top, that would essentially force the box in the bottom as needed. I’d still have to mill the topside of the piece with the corresponding curve as well, but that wouldn’t be harder than the first “mapping.”
Is there some “easy” way to warp all the existing milling operations onto a slight curve like this instead of the existing flat plane?
If not easy, perhaps another way is to manually change the heights for the 8 leveling areas (4 along each side). If the leveling areas near the ends were milled deeper, then attachment there would make the item curve up in the middle since the leveling areas there aren’t as deep. The trick then becomes how to mill the other side to be flat when installed. That’s a bunch harder I think.
The way I don’t want to have to do this is to drill holes in my tablesaw top and have clearance holes in the insert, so using a screw would force the plate down.
Yet another option is to figure out how to permanently bend the Acetal a couple of mms. I’ve seen it done with thin plastic, but this finished piece is 11mm thick, so I don’t think a heat gun would be enough to get the plastic into temp range it needs to be, but maybe?
Thoughts, ideas welcome.