How Important is a flat/level surface for a CNC?

I recently build a workbench for my Shapeoko HDM out of 2x4s, only problem is that it’s not completely level and the top is not completely flat. I’ve tried shimming the piece of 3/4 plywood that acts as the surface, but I still haven’t been able to get it to be flat enough
Another problem is that the concrete floor in my garage itself is not flat, I’ve already shimmed the workbench so that it no longer wobbles on two legs, but I fear that the workbench has already flexed to take the curve of the concrete floor.
The bow in the plywood is not huge or visible, but when I put a 1m long straightedge along the top, there’s around a 1/16th of difference on the two edges compared to the center.

My plan is to put the 3/4MDF base that I cut to act as the base of my enclosure, and use the weight of the HDM to “flatten” the plywood, then shim the remaining spaces between plywood and workbench and finally screw on with brackets to make a level surface.
Would it work or would it end up causing a twist in my cnc?

I could try and spend more time on shimming the plywood, but it already has a slight bow in it without anything holding it down.

The easiest way to address this is:

  • ensure that all 4 corners of the table are at the same height/level to each other (a spirit level is the traditional way to ensure this) — shims at the corners of the MDF should be fine for this
  • put a sheet of foam on the table and put the machine on that — it will address any imperfections in the table — as an added benefit it will help dampen vibrations and somewhat reduce noise

Possibly a pair of foam sheets — one under the MDF, the other on top of it which the machine rests on — would work well.

1 Like

Thank you for the suggestions,

I think measuring the heights of the table would be a good way to level the table further, only the concrete floor itself has a very very slight bow to it. I think leveling it that way would help but I’m not sure if completely remove the bow from the table.

I think having two layers of foam would be the most effective way. Would 1/2 EVA foam work? Or would something thicker/thinner work better?

Get the threaded leveling feet for all the legs and adjust the feet as needed to the slope of the concrete. Then do the adjustments to the table top by adjusting any or all of the feet to get level in all directions.

A good way to get the bow out of the plywood is to add braces under the plywood but space the braces lower then the bottom of the table top and use screws to pull it down or shims (door jamb) to raise it as needed. Be sure the bracing is secured enough to handle the pressure of the shims or screws.

You should only need one layer of foam then.

Anthony

2 Likes

The easiest way to make a dead flat surface is a torsion box assembly.

If you have a tablesaw they are straight forward to make. Not sure you need that for your CNC as a piece of foam will take care of imperfections.

Bill

2 Likes

I built a torsion box for my SO3. Attached are the instructions. I used a Rockler Cross Lap jig but I include instructions to make your own similar to a finger joint jig.

torsion_box.pdf (2.0 MB)

This is for a SO3 XXL so just modify the measurements for your machine. The torsion box is a great addition and I made mine so I have about 8 inches in front of the machine for putting a tray for bits to drop into and a place to put my calipers and other miscellaneous things like pencils and rulers.

3 Likes

I think a piece of foam might be the way to go.

I should have invested in a chop saw and better wood. Some of the 2x4s have a slight bow in them, and trying to cut all the pieces with a circular saw got me some wonky results that I had to fix later.

I’m thinking one layer of 10mm EVA under the MDF, and a few sorbothane pads from HDM to MDF

Is there any specific type of foam that works best? I’m currently looking at EVA foam

When you buy 2x4 material look through the pile and try to get the rift cut material. Most 2x4 pine material is cut from the outside of the tree and is flat sawn. So that means the material has the rings of the tree going mostly across the wide part of the board. That flat sawn lumber is most likely to cup and bow. The rift sawn boards are the ones that the rings of the tree are 45-90 degrees across the board and that part of the tree is more stable. It is a pain to look through a big bin of boards but the outcome is worth it. When a small pine tree is cut in the flat sawn method there are a few pieces that are rift sawn. Most pine lumber that is made into 1x4 and 2x4 are cut from small trees. The big pine trees are saved for the larger sizes and are usually cut the heart of the pine tree to get 4x4 and 4x6 posts. The example below is oak and pine does not really have the rays that oak has but quarter sawn and rift are the best for stable boards. Saw mills do not make rift or quarter sawn pine on purpose because it is wasteful over cutting flat sawn lumber. Most 2x4 lumber is not used for fine wood working so they do not care what happens after they cut it and ship to stores.

2 Likes

The answer to your original question is that the table doesn’t have to be flat and level.

But it sure will make using your machine easier. As has been stated, there are several ways to pad or adjust the surface to at least get your machine stable.

While your machine is running a program, it will want to move around. Any solution you choose must hold the machine in a stable position.

1 Like

Thank you for the tip, I’ll keep that in mind the next time I buy wood.

I placed mine on spongey rubber… it levelled out and the rubber absorbs most of the noise on the frame.

I have also added dampening car insulation on my cnc frame with great results.

Get adjustable feet…

Both problems solved…

1 Like