I understand what you are trying to build now. I think getting the table flat will e a challenge, nothing wrong with it though, but you are going to be facing a bunch of stacking / compounding errors that will add up over time - you’ll eventually probably find you’ll need to have a spoil board that you surface to be level with the machine.
Since the table is going to be wood, it is going to change size / flex a little with changes in humidity. Also, the machine itself is made up of a number of parts that are 3d printed, and while pretty accurate, I know from my own 3d printing experience, that you need to build in tolerances. This along with the fat that all screw holes have a little bit of slop in them means your gantry and rails are going to have a little error in them as well.
I guess what I am try to say is that, even if the table is completely flat, you might have a difficult time getting the machine trimmed to the perfectly flat surface of your table.
However, no reason to not go for it, the more accurate you build the surface, the easier it will be to get your machine dialed in.
When I built my flip table I built it very simply - I had a 3/4" threaded rod run through the middle, and then build a 3 layer 3/4" plywood sandwich around the rod so the table would pivot around the threaded rode. I used screws so I could take it apart if I needed to. However, this is just to put tools like a lathe and sanders on so perfect level was not a concern for me.
Good luck interested to see how it turns out!
The torsion box table will not warp or twist the way he is building it. It may not perfectly flat but it will be as flat as anyone here can get it. Typical torsion box is 100% plywood so no expansion or contraction will be had in the build of it. At least that is the theory.
Mike, you’re absolutely right. There is no garranty that the mill will be dimentionally perfect and it may still require surfacing the spoil board to get it right. What can I say, I’m a perfectionist. Besides, I’m bound to learn something along the journey.
You can never achieve perfection but the closer you are the better the accuracy and the products produced with the machine. Sometimes the extra accuracy will not be needed or even noticed but I’ll know and that matters to me.
I’m going to give it a shot and if necessary I have machinist’s straight edge and some card scrapers to fine tune it if required.
My plan is to mount the machine on strips of plywood on top of the torsion box so the machine doesn’t need to be disturbed to replace the spoil board.
Hey, those dovetail nuts are pretty cool. I’ve never seen them before. They are cheap enough but shipping would change that, if they even ship to Canada.
They would be very easy to make. You might even be able to 3D print them.
My only concern would be over tightening one and splitting a chunk off the spoil board.
My plan at the moment is to use T-nuts for the spoil board but I’m open to other ideas. Like adding a t-nut to each square on the dovetailed spoil board you have. Another idea is to be able to slide out the existing spoil board and slide in a vacuum table in its place.
I actually have an enclosure I built some time ago for my previous CNC machine which will fit the LongMill (12x12). My plan is to use a base board of 3/4" MDF with 1/4" nuts installed, topped with a replaceable waste board that will be bolted into the 1/4" nuts to keep it in place. The Longmill itself will be mounted on 4" strips of 3/4" MDF that are glued to the base board. In the future, I’ll put in T-tracks and the waste board will become strips of wasteboard instead of one piece.
I am not expecting my longmill till March, but I have been kicking around an idea for my table. I really like your design and like how you are going to put the Mill on strips to make changing the spoilboard easier. That is exactly what I was thinking of doing, it just seems like a wise thing to do. I was thinking of doing two sheets of 3/4 plywood for the main table, but now I might go with a torsion box.
A torsion box really will provide the flattest surface if done right. If you’re assembling plywood with wood screws, I would recommend using screws that are long enough that all the treads are in the partitions between the top and bottom layers. This allows the screws to pull the pieces together eliminating any gap between the parts. So if the partition pieces are properly cut your torsion box will be as flat as the material allows. Using shorter screws that would result in thread in both the top layer and the partitions and will not close a gap unless they are tightly clamped prior to driving the screws.
If you’re using MDF you should use particle board screws which I believe are fully threaded. That means you must fully clamp the parts before driving the screws or drill pilot holes in the top and bottom that are as large or larger than the major thread diameter. The partitions will also need pilot hole but these will be smaller so the fastener threads engage the MDF. So you can see that this complicates thing a little. Take the time to get good results and your spoil boards will need little if any surfacing.
I have been thinking about using dados to build my torsion box and I think I’m going to pass because it adds another layer of complexity. Getting the dado depth consistent would be critical to a flat surface. It might work OK if you cut dados with router as it would ride along any curvature in the sheet stock giving fairly consistent depth. On the other hand, cutting dados on the table saw could be much more problematic. If you cut with the concave side down it would require considerable downward pressure to get consistent depth. Cutting with the concave side up would require the operator to rock the material to keep the material in contact with the table at the peak of the blade arc.
Of course, you could use any method you want and just flatten the final product or ignore the torsion box and just flatten your spoil board. I’m of the opinion that the more accurate your initial effort the easier your life will be going forward.