Lawrence M's Table Bed Support
Lawrence built the scratch build CNC machine from the book and he came up with a very nice addition to the underside of the table that provides extra support so the table will not sag over time.
The scratch build CNC Machine calls for two sheets of MDF that is sandwiched together to strengthen the table. The way the book CNC Machine is designed, there cannot be much structure underside the bed of the machine because the gantry spans underneath the bed and is where the gantry connects to the x-axis lead screw. This only allows for the front and back of the table to have support. Lawrence fixes this with a metal structure that is ribbed and runs along the length of the table.
Here is Lawrence's information on this fix:
I had mentioned to you that I had removed the sag in the 48" suspended bed by inserting an iron grate under the bed and pressing it up underneath with supports that were about 1/4" higher than the end pieces from the original design. This made the entire machine weight bear down on the grate as it was lifted off the table by 1/4". I also added 8 bolts/nuts to ensure the new supports didn't collapse with vibrations.
You can see the end of the grate supported by the piece of phenolic plywood behind the original end supports. It lifts these old ends up 1/4" off the table and brings the weight of the machine down onto the new plywood and grate (1 inch thick.) There is about 1/8" space between the grate and bottom of the gantry. I thank you for your planning for my addition in your original drawings! The pattern of 8 screws is the bolts that keep the plywood from twisting or falling out. The grate is pressed in and not fastened.
I mentioned to Lawrence that the iron grate would also be a great addition if only one layer of mdf (or plywood) were used for the table. That configuration would recoup 3/4" of space for better gantry structure and reduce the weight of the table. It would also make the table stronger by reducing the dead load above the iron grate.
(1) Be sure bolts on the bottom of the plys will align with the bars of the grate. Either that or the bolts will need to be recessed on the bottom. ...I may have actually removed one small section of bar to made room for a bolt in my retrofit.
(2) Cut the grate (I got my 1" grate at a lumber yard) with a circular saw and a metal cutting blade. Slowly! It makes lots of sparks. And the sparks represent minute pieces of hot metal. Wear eye protection (that seals around the eyes.)
(3) Cut large circles in the end supports so you can replace the lead screw and reach the nut locations for the x-axis motor mount.
(4) I put the cross welds of the grate up because I was worried about the small tolerance I had and potential touching between the grate and the gantry bottom. If only one ply is used, or if the gantry bottom is dropped by 1/4" or so, then I'd put the cross welds down and use the flatter surface on the up side against the ply. Since I have several layers above the grate and I shimmed the last disposable layer, I will leave mine as it is.
(5) I also made two chunks of plywood that fit VERY snuggly, on edge, under the grate, between the grate and the table. I set these temporarily under the grate when it sits idle for a period of time. I have them tied together with a string, that drapes UP and OVER the cutting surface so that I will be reminded to remove them before I use the table the next time. I assume these chunks of plywood will help keep the grate from sagging from the weight of the gantry over the long time.
(6) To retrofit:
(a) Remove lead screw and gantry bottom.
(b) Lift machine at the ends, about two inches, and support with small blocks that do not extend under the table (where the new supports will be placed.)
(c) Slide in the grate from the side. Let it rest on the table (it is heavy.)
(d) Lift the grate at one end and slide in a new support, which lies flat. Do the same at the other end.
(e) While lifting one end of the grate up to the underside of the machine, twist the support so it flips up onto an edge. Clamp it to the original support. Let the grate rest on it.
(f) Do the same on the other end.
(g) Be sure that any protruding bolts on the underside of the machine will fit between the iron bars when you lower the machine. I actually had to remove the grate, cut off another inch of it, so that I could slide it to one side a bit and keep from hitting the nuts. Then I re-installed it.
(h) Pull blocks from under the original supports and slowly lower the machine onto the grate and new supports. The machine should be about 1/4" higher than before (due to the over sizing of the new supports.)
(i) Adjust and re-clamp the new supports and drill new 1/4" holes through the original support and the new support. Bolt these together for stability.