CNC Table Flatness Specification

I know this has been discussed in other posts, but I still don’t understand.

Just how flat should the table be for my 30x30 Longmill? I don’t have the equipment to build a torsion box and even those are dependent on a relatively flat building surface to start.
Instead I opted for a simple 2x4" table with a 3/4" plywood top. I have hand planed my table beams and shimmed to get to about a 1.5mm dish in the center measured corner to corner with a precision ground straightedge. I also pretty much know that the y-axis gantries are not going to be coplanar without shimming the feet (planning on fishing line test once the feet are on). I feel like perhaps I should cut my losses and just buy a decent table saw to make a torsion box. Any advice appreciated!

1 Like

Hello and welcome to the forum @Jowilmei. I, too struggled with “what is good enough” regarding table flatness and squaring my 30x30. While I would never tell you NOT
to buy new tools, instead I will tell you that spending the time to surface your work surface does really well. I built a decent bench for my LongMill, but no matter how hard I tried, I couldn’t make it “perfect”. That’s where the surfacing came into play. So think about it for a moment, if you have both axis of your machine secured to the surface and your MDF secured down, and run a surfacing tool pass (included in GSender) across your top, in theory, the work surface will be perpendicular to the gantry overhead. I ran mine a couple of times, then used my random orbital sander over the top ever so gently to take down a couple of ridges (had a cheap surfacing bit) and that was that! I did put both leveling feet as well as drop down casters for the rare time I need my CNC moved from the wall. I have been very pleased with my LongMill and knock on wood, haven’t damaged my surface yet. Once again welcome Josh! This forum and more importantly the people here is the reason I purchased from Sienci. Maybe someone else can chime in with their words of wisdom.
Also, check out the bench build thread here. I also documented my build in a separate thread. Some good posts there, and not everyone made a torsion box bench.
Oh, I almost forgot, get a good surfacing bit. Those 10.00 ones from Amazon are junk.

@Jowilmei Welcome, Josh. I will second everything that @Jake advised.

I don’t fault anyone for trying to get the table as flat and rigid as possible. I have all the tools, and I like to think the ability, to build a torsion box. I didn’t. I have a very sturdy 2 x 4 frame table. Cross members ensure that the top is well supported and the table is rigid. Mine is on 4" wheels as I need to move it to access cupboards beside it. My table top is a single sheet of 3/4" MDF. On top of that, I have 3/4" MDF slats separated by t tracks. I surfaced then with a good bit when I first installed them. I replaced them after a year or so of hard use. That was well over a year ago and I have not replaced them since.

I now plan my projects so that I don’t need to surface the slats very often at all. If anything, the only reason to surface them is the change in their thickness that humidity causes.

I strongly advise you not to overthink this. We are dealing with wood, largely. It moves, even as we carve it. Trying to obtain sub-mm accuracy on a wood table for carving wood is, IMHO, chasing your tail.

I may well be taking a different approach to this. I bought my LM to use as a hobbyist. I bought it to learn new woodworking techniques and to have fun. So far, my approach has let me accomplish those objectives. My approach has allowed me to carve some quite intricate two-sided jobs with excellent precision.

Your objectives, and therefore your approach, may be very different. There are many members here who will disagree with my approach.

Please be sure to post your projects.