Bolting to solve Bow

[beiben (Blaine Eiben)]

I’m new here and been reading lots of posts about issues with XXL cross braces either sagging or bowing. I built a solid one piece 3/4” MDF top then built my own t-track with 3/4” MDF slats that go in between. The XXL sits on the frame (no leveling feet) on top of a rubber pad. I used shorter M5 bolts so it sits entirely in the frame. The worktable is a 3/4” melamine top which is mounted to wall on 3 sides w/ a 2x6 front brace with 4 2x4 cross braces underneath. I can stand on top of it with no deflection so I know it’s NOT the table. My MDF is bowing up in middle at least 3/16”. Was thinking about drilling through bottom MDF/metal cross brace/rubber mat/Melamine table top. Would drill 9 of these holes (4 corners 4 middles 1 dead center) Then using a 5/6” carriage bolt starting from middle tighten to pull bow out to the extremities. In theory I could put constant downward pressure into the rubber mat. I could then adjust each bolt until level all across.

Thoughts from those who have been there and done that?

My frame is square, flush and true. Friggin cross pieces just have a heck of an upward bow.

Thanks in advance

[mikep]

Did you measure before you put it down on the assumed flat table? If not, and it was sagged a bit when you attached the top, then bade the bottom flat, the top will now be bowed the other direction, as you see. Surface it, and you’re done.

[DanoInTx (Dan Nelson)]

Without the leveling feet the bottom is not flat, there are some gaps. I shimmed mine before bolting on the wasteboard, then added my secondary wasteboard slats/ttrack, and finally trammed the top surface with the machine. Mine was pretty flat prior to that final surfacing, not perfect, but nowhere near 3/16” off, maybe 1/16” or less. Really long forum post here:

Sounds suppression, XXL bed flatness, and other musings

So I’ve gotten more into the new house, as in it’s just called “The House” now, and where I was living is “The Old House”. I’ve been doing a bunch of house work lately, but have finally been putting some effort into “The Shop”, which has brought me back here. I’ve gotten my dust collection worked out well, gotten the SuperPID working well, and I can now say without the leveling feet, laying on rubber mats, the XXL noise is tolerable, meaning I can have decent conversation in my shop without sc…

Dan

[beiben (Blaine Eiben)]

I know I can just mill the top and make it flat. What I’m wanting to do is actually take the bow out. I ran a straight edge on the table prior to setting up XXL and it was perfectly flat. The sheet of MDF was flat. I drilled the holes a little larger to allow for squaring and not binding when assembling. So I know that can’t be pulling up the center. I replaced the M5 bolts with shorter ones and can see daylight between mat and cross supports. I’ve also noticed a fair amount of downward deflection in the center when pressure is applied. The supports have to be bowed don’t know what else could be causing unsupported span with two sheets of MDF on top to be the highest point.

What I’m trying to figure out is if there would be any issues with removing 9 of the M5 bolts, drilling them out, then replacing with 1/4” bolts. The bolts would run through top of first wasteboard and come out the bottom of the actual table top. These I could then tighten down to force entire XXL frame into the rubber mat from center and work way outward to straighten the bow and also in theory stiffen up the entire XXL by making it more rigid. This would be checked using a dial indicator.

I thought about shimming but don’t see how raising outer front and rear sides will cause the center to push itself down.

Never used a CNC so not sure if bolting it down to table top to stiffen it will cause any issues or problems that I’m overlooking. I have it setting in back cove in the shop so wasn’t planning on lugging it around anywhere.

Thanks again for advice

[DanoInTx (Dan Nelson)]

The idea of shimming is that the bottom of the machine is not flat if you remove the leveling feet, it’s higher on the cross straps than it is at the ends. So if the bottom isn’t flat, you’re not starting with a flat base for your wasteboard. I’m sure you could bolt everything and pull it into position, but you’ll still likely have to tram the top, MDF isn’t 100% flat, and humidity changes can cause it to swell also.

Dan

[mikep]

Wellll… what I did was to remove the feet, add separate strips of MDF underneath the table (between the straps) and glue the strips to the bottom of the SO3 table, and the top of the table the machine is sitting on (use silicone). This will -NOT- be totally flat. Then, I put bolts into the foot threads from the top, and adjusted those until the table went flat -this dropped the middle a touch and raised the ends a tough (it was bowed up in the middle)

Then, surface the wasteboard, and anything left will be gone. There’s nothing to really flex at that point. In my case, it’s a tool cabinet, it’s 2" top, then 3 layers of MDF in a stack - the frame is only there to hold the rails in place, and that’s all the weight that is on them - which is why the foot adjustment is needed.

As far as I’m concerned the whole thing is really solid, doesn’t flex, and it’s very flat. I’ll have some trouble getting it off the cabinet when that eventually becomes necessary, but I can accept that.

To your direct question, no, there’s no problem with using longer bolts all the way through the tabletop, but I strongly recommend that instead of leaving a gap under the wasteboard that you put something in there for the wasteboard to sit on. The underlying straps go past the bottom of the frame, and without the strips like I did, the machine will not sit on the frame, it will sit on the straps. In my machine, because the strips are under the wasteboard, and they are thicker than the straps, the whole thing is sitting on the wasteboard, not the frame or the straps, which I think is a better way.

There are a lot of different ways to achieve the same end results, and you’ll see lots of them here. One of the great things about this is exactly that, there are a lot of ways to solve the problems, and some work better than others, but you’re free to pick and choose the ideas that you like and go from there. There is certainly a lot of try-fail-learn-repeat

The forum is pretty active, and there are a pretty good number of really experienced people here, so if you get stuck, keep asking questions!