Nothing Ground Breaking Just Another CNC Router Table Build

[HootBuilds]

I will introduce myself first.
Back in 2008 or 2009 I had the urge to build a CNC Router. This was to do some simple cutouts, roughing out plugs for molds, or doing some simple carving. I then became a member of CNCZone, and found it to be an excellent source of very professional and knowledgeable DIY builders/makers. I have been quietly lurking and admiring builds now and again. I have not posted anything until now, and needless to say it has taken me until now to start my first CNC Router build. Now I have time and lots of desire to start my build.

The plan is to build a sturdy well built table base from wood, and pour a concrete top (like a pour in place countertop). The table itself would be 3" thick 5ft x 5ft because I would like a full 48" of travel. ( I live in the United States, so I use freedom units) I am thinking I can get the top of the table very close to level and then grind the concrete close to flat while it is still semi green, then finish level / flatten it with self leveling epoxy.
The base that holds this will be made from pine. 6x6 for the legs and 2x6 for support (joists).

I did manage to start drawing it up.




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[wizard]

Interesting concept that I kinda like. However I have a concern or two.

1. I really don’t see 3” being thick enough. You will want to reinforce the concrete with rebar or wire mesh or both. And frankly I think 3” is a little thin for that.
2. The other thickness problem is that you will likely want to cast in place mounting hardware that is well embedded in the concrete. In some cases you will need solid concrete support around those inserts. The inserts could be J bolts or bars to tapped holes either way you need thickness to assure security. While you might be able to use threaded inserts or anchor bolts after the concrete has hardened it might not be the best solution for every mounting need. Locating and drinking precision holes in concrete will be a huge challenge.
3. I really don’t see the point in leveling and grinding if you are Going to epoxy level. If you are going to grind why not do it with the precision needed for your rails. My understanding is that companies do grind concrete to particularly nice flatness so that is always a possibility. Terrazzo floors for example can be ground rather flat. (They have to be otherwise you would see the defects in the polished surface). The big advantage though is that your rail system gets bolted directly to the hard aggregate filler.
4. By the way I’m not too certain grinding in the green state makes sense, I’d wait a good three months for the concrete to harden and stabilize.
5. Your wood base needs refinement to avoid collapse under weight. At the very least add some X bracing or other structural improvements. By the time you are done that table will weigh in at several hundred pounds, it’s rolling over onto somebodies feet will be painful. In fact the top heavy nature of the table is a real concern.
6. While it might not be a concern at the moment moving the table will one day become an issue. The lower runners connecting the legs do not look like the could handle a pallet jack under them, raising the table to move. When your build is finalized and comes close to a ton (maybe far more) you need to think about how you will handle it.

[ger21]

I agree that you need diagonal bracing on the base, or it will sway back and forth.


If you are going to go the self leveling route, than I'd build the table from an epoxy sand mix, rather than concrete. It won't have the shrinkage issues that concrete will have.

[HootBuilds]

Interesting concept that I kinda like. However I have a concern or two.

1. I really don’t see 3” being thick enough. You will want to reinforce the concrete with rebar or wire mesh or both. And frankly I think 3” is a little thin for that.
2. The other thickness problem is that you will likely want to cast in place mounting hardware that is well embedded in the concrete. In some cases you will need solid concrete support around those inserts. The inserts could be J bolts or bars to tapped holes either way you need thickness to assure security. While you might be able to use threaded inserts or anchor bolts after the concrete has hardened it might not be the best solution for every mounting need. Locating and drinking precision holes in concrete will be a huge challenge.
3. I really don’t see the point in leveling and grinding if you are Going to epoxy level. If you are going to grind why not do it with the precision needed for your rails. My understanding is that companies do grind concrete to particularly nice flatness so that is always a possibility. Terrazzo floors for example can be ground rather flat. (They have to be otherwise you would see the defects in the polished surface). The big advantage though is that your rail system gets bolted directly to the hard aggregate filler.
4. By the way I’m not too certain grinding in the green state makes sense, I’d wait a good three months for the concrete to harden and stabilize.
5. Your wood base needs refinement to avoid collapse under weight. At the very least add some X bracing or other structural improvements. By the time you are done that table will weigh in at several hundred pounds, it’s rolling over onto somebodies feet will be painful. In fact the top heavy nature of the table is a real concern.
6. While it might not be a concern at the moment moving the table will one day become an issue. The lower runners connecting the legs do not look like the could handle a pallet jack under them, raising the table to move. When your build is finalized and comes close to a ton (maybe far more) you need to think about how you will handle it.

Thank you for your concerns, and these are all valid points and will be taken into consideration. It is great to get some feedback.

1: I have busted up concrete that was 3 - 4 inches thick before and it was a real PITA. 3 inches should be plenty. When the machine is running, there will be at most 200 pounds pressure in one spot. The concrete slab mainly has to hold together across the spans where nothing is supporting it. I will definately be using some remesh for reinforcement. On some automotive repair forums where they talk about installing a car lift. Some of the forum members talk about installing the lift on 3" concrete. This seems a bit sketchy to me, but also suggests that 3" should be plenty.

2: Cast in place anchors are a great solution to attaching the ways to the table. Another solution would be to use the epoxy in anchors. Although the only anchors that I will need will be the ones holding a subplate for the Y axis ballscrew and Y axis ways to the table. But then again another thought is running these fasteners all the way through and sandwiching the concrete between the way and the ball screw / stepper mounting plate. This will allow me to be a little less accurate with my fastener placing to get everything aligned.

3: Grinding to a precise flatness could definitely be done. I guess it depends on how well the concrete form is leveled and how well the concrete is finished while wet. I will definitely keep this in mind. Grinding to fairly flat before using the self leveling epoxy, is so that I do not exceed the max allowable epoxy layer level. The epoxy I am looking into, the max allowable per layer thickness is 1/8". It would also allow the leveling layer to be more uniform.

4: Grinding in the green is easier on tools. A rough pass could be done in green (48 to 96 hours after the pour), and then I could wait 3 months and do a finish pass. This could be probable since I am not in too much of a rush to get-r-done.

5: Racking of the wooden frame is a bit of a concern to me as well. When the slab is poured it will weigh in at 960#. I plan to do notch joints where the 2x6s meet the 6x6s. Along with the 3/8" bolts. I was hoping this would be enough. I do plan to build the electronics cabinet under the table. Maybe I could use a 3/4" sheet and use that as some reinforcement.

6: Moving it would be a PITA. Although I think I could find a way. Right now We do not plan to move away, and if we did I think I could find a way to get it out of the shop. I once unloaded a 9x42 bridgeport with a 2 ton cherry picker off a trailer, and positioned it. That was a sketchy experience, but it was done successfully, and no one was maimed.

[HootBuilds]

I agree that you need diagonal bracing on the base, or it will sway back and forth.


If you are going to go the self leveling route, than I'd build the table from an epoxy sand mix, rather than concrete. It won't have the shrinkage issues that concrete will have.

Epoxy mix is many times more expensive than concrete. Many times more expensive and from what little will be gained, I do not feel the cost is justified.
After the concrete is cured I will have very little shrinkage after the first 100 days. After that I will have more shrinkage or size changes when the temperature fluctuates compared to the shrinkage that will occur after those 100 days.