I’m mostly finished building my lowrider v2. The torsion box table’s surface is a nicely flat 12mm MDF board. I bought a lot of threaded inserts I was planning to put right into that surface, but am now wondering whether it would be best to get a separate MDF board of the same thickness to use as a waste board. Can you please share your thoughts about the two options I’m considering?
Use the table’s surface with a thinner wasteboard
Put a 3mm or so mdf wasteboard on top of the surface. Then drill a grid of 8mm holes into the table through the wasteboard and the table surface using the lowrider and probably hand-drill one more row around where the lowrider can’t reach. Take off the wasteboard, flip the table surface and hand-drill wider, shallow holes around the holes with a Forstner bit to get the back of the inserts flush with the surface. Then screw in the inserts, flip the board again and reattach.
Advantages:
When I change the wasteboard, I don't have to go through unscrewing and inserting all the inserts again. I just have to put holes into the wasteboard using the same pattern.
Buying new wasteboards is cheaper (since thinner)
I can simply take off the wasteboard since it wouldn't need to be attached when not used. Then I have a flat table across the whole length of it to use for other things.
The work piece is a bit lower, so I don't have to use some of my Z height just to clear the wasteboard.
Disadvantages:
I have to partly take the LR apart to flip the table surface
Use the wasteboard
Wasteboard would be 12mm MDF. It’ll be slightly oversized so I can get inserts around the LR’s cutting area too. Mostly analogous to the other option in terms of how I put in the inserts. The wasteboard would be attached to the table with wood screws (outside the cutting area of the LR to be safe).
Advantages
Don't have to take the LR apart to install it the first time around.
Planing the surface might be easier?
Disadvantages
[Pretty much the inverse of the advantages of the alternative]
Yep, it is pretty much a ‘pay me now or pay me later’ approach.
Suggest having the LR drill holes for you, in either case. If drilling in the support table, then use the same drill pattern to drill the spoil board.
One suggestion look into using dowels or Miller Dowels (stepped wood dowels, available on Amazon and elsewhere) for some holddowns or fixtures. Not much lost if hit by your CNC and can be perminant if you glue them in.
Thank you Jack for your thoughtful comment. Definitely agree on letting the LR do the work. I already verified it’s square, so that should work.
The Miller Dowel system sounds great, but doesn’t seem to be available for purchase here in Germany, at least not for a price I’d be willing to pay. Shouldn’t be too hard to 3d print something similar, although layer adhesion might not be strong enough to withstand the forces and without such a step drill it’s a bit more work to drill the holes. I will likely use or modify some of the clamps and jigs available on Thingiverse, e.g. this or this - of course those contain a metal fastener, so the risk is higher. I could also try to use Nylon bolts and washers.
In fact another idea I had is to use a the head of a nylon cap screw similar to a bench dog, e.g. I could make cam shaped rotating clamps that have the screw head as a pivot point, e.g. like this one.
You also don’t have to have holddowns everywhere. I think having them along the x and y and maybe one line at x=y x=0.5y would be enough, and all you work would be as close to the origin as possible.
A thinner wasteboard’s other disadvantage is that it either needs to be secured more, or it won’t be as flat. When you chew up a 6mm piece of MDF, it won’t let your workpieces stay flat, unless it was really well secured to the next layer.
I put inserts in the table and put a regular thickness waste board on top. You don’t have to worry about hitting an insert with your bit and you can always resurface the waste board if it gets chewed up enough to lose level. With that thin of board you’ll just end up replacing it after a while. For drill operations I have enough Z to drill through the table so I just run the same code with different Z0 depths to drill the waste board itself.
Thanks Jeff and Bill for your thoughts. I think I will go with a thicker wasteboard either way. Since I have to change the belts anyhow (I had gotten steel enforced ones originally), the disassembly won’t be too bad, so I’ll likely put the inserts into the table surface.