Are there any schematics for a true 4’ x 8’ cutting table?
It doesn’t need to be fancy just use the Calc and no more than 4" thick.
From what I have gathered the ongoing theme when designing is strong and square. My biggest concern is because the table length (56.00 x 111.00) exceeds the sheet length of a standard piece of plywood. That leads me to worry about the seams that will have to be on the surface.
Maybe I am worrying too much about it.
I am about to draw out my plan, I will post it here in a bit. Any advice will be appreciated.
You can build a nice frame (bessel points are something I just learned about), and then put a full sheet right where you need it. Remember the table is only supporting one sheet, really no need to get crazy. Hard to explain but the machine require less accuracy than you think, and you will have a full sheet CNC, if you do not like your hand built table you can CNC the next one.
c = 1/sqrt(n^2-1) so for two supports c = 1/sqrt(3) or 0.57735.
So if you are supporting the table side with two legs, putting them 0.57735 * the length of the table apart would be the least amount of sag?
So if you had a 2m table, you would put them 1.15m apart, or 0.42m from each end. (Metric was easier for the math).
Or for a 9ft table, 22.8in from each end.
I wouldn’t stress about hitting that exactly, since wood is a natural material, and pine 2x4s curve more than they sag (in my experience), but it’s cool to have at least some reason to put the legs somewhere.
Hmmm… So should the Airy points be based on (and under) the whole table, or just the spoil board surface?
Thanks for pointing that out Ryan. Now I’ve got something to think about for the rest of the day.
That is why I think bessel is “better” for this application. Airy is flattest ends, bessel is flattest overall. do in X and Y and the table is supported at the best possible locations, with the 4 corners probably being the worst possible.
In reality with this do anything on such a small thin light duty table…probably not much as we already over build the hell out of them. I think it is cool to know this is the “right” way for a “precision” table. Ox tools…dude is full of stuff like this. I have to pause and look things up when he talks.
So I am also working on my table design. Since I have a tendency to overthink things, I am going to put this out there. When I am working on the Y sides of the table I am thinking of putting a piece of 3/4" angle down the side of the board. I want to put a full sheet of ply for my spoils board, so I was going to build the table around that and this is the long side side rails. Once I have the Y sides done, I would cap each end with a 2x4. Then drop the spoils board in so that the top of the spoils board is even with where the bottom of the wheels on the Y axis are. Would this work? or is this just overthinking the table aspect of this?
I was thinking that I would make my own 2x4 out of plywood. They would be 3.25" by 1.5". This may be overkill, but my thought is the multiple strand orientation would limit warpage, and I just don’t know if I could get the same precision with 2x4 lumber.
I am building my table to be 56" x 112" rather than 56" x 111" as requested by the calculator. The thought here is that math is just easier with the even number, and when the width is twice the length.
I still haven’t figured out the best way to skin it with a top layer. I am trying to wrap my head around minimizing waste. This maybe largely because I was planning on using the expensive plywood for the top too. I could use MDF and decrease my costs, or I could add other struts along seem lines to allow for optimization.
Anyway, I have some thoughts:
And if that doesn’t work then I have some pictures…
My table is something like 3’x5’ and I used plywood inside. If you are comfortable with it and you know how to rip it straight, it is a very good choice. I had a similar torsion design to yours, but I didn’t double anything up, and the supports were about 15" on center. I used 1/8" or maybe 3/32" hardboard on the top and bottom. I added a piece of 3/4" plywood to the top, and it overhangs the torsion box by a foot, so my total size is about 4’x5’ for cutting at least 3’x4’.
Your design looks good, although you’re cutting it close on thickness and it will probably be more than strong at 2.5" width on the supports.
Re: your questions about seams and full sheet tables, you can order in 10 foot x 4 foot sheet stock here so I made my table 10 foot and just had one seam on the skin down the side lengthwise where I butt-joined the skin sheets. I went with a 4" high table made out of ultralight MDF with 1/4" skins and 1/2" webbing spaced 11" apart. If I could have gotten 1/8" in 10 foot I might have done the skins with that like jeffeb3 but I didn’t want 3 seams on my skins. I only had a couple mm sag with this suspended from the ends so if you had 4 legs positioned as discussed above ( in from the corners not at the corners) I think you would not have any measurable sag at all. I think the most stress on the skin is in the middle so I put my skin seam as far to the side as possible… no problems so far for me. These tables get crazy heavy pretty fast… even with my setup I am at 150 lbs just for the table top.
I haven’t regretted making the table 10 feet long it gives me a parking area and room for a clamping/registration strip around the edges… if you are constrained for space you can always make it to the calculator specs and extend the belt holders past the end of the table and/or make a longer wheel rail to get more room later if you need it though.
Re: MDF vs ply I don’t want to start a religious war but it might not be worth spending extra on ply:
Depending on how you construct the box you don’t have to skin everything over. You only need skin where you are going to have work, so a 4’x8’ sheet with outriggers for the mechanicals to run on… Get 10’ 2x4s and rip them down a half inch or an inch to make the flat surface.
That was very helpful.