Securing large sheets of plywood

I’m making a very large object which involves cutting large parts out of 1/2" plywood. By the time I’m done, we’re talking about 40 sheets.

Needless to say, plywood has a tendency to warp and buckle, especially as you cut through it. Strategically-placed holding tabs are critical, but they don’t address the problem that the surrounding material will buckle and warp during the cut.

You can and should screw the plywood down into your spoilboard, but the question is where? You want to add hold-down screws near your cuts, but not at the risk of hitting them during the cut.

Solution:
Add shallow 2mm drill marks to your GC program that indicate where it’s safe to put a screw. Works great! Make sure to put them a few inches outside the cut line, so when the MDF won’t hold a screw in that spot anymore, you can just move over a little.

ESTL has intersection and center of circle features available with the Drill function which make this quite easy. Worth a few minutes to figure out how those work.

After you’ve done this, the first thing your cut will do is go around the sheet and indicate where you should put screws. Totally worth the effort.

3 Likes

This. I do this. If I am doing a smaller job, I use cncjs preview:

  1. Put down the workpiece.
  2. Put in one screw, usually right behind where I want the origin.
  3. Make it alignedish. I just jog 100mm or so and see that the router hasn’t moved relative to one edge…
  4. Put in another screw, again in the negative.
  5. Go set thw origin.
  6. Jog using the preview to empty space, especially in hole I’m cutting (but not pockets).
  7. Turn in the router and come down 2mm
  8. Jog away and put a screw in there (I still predrill again).
2 Likes

Yep, exactly. Turns out that it’s quite easy to just place the markers into your gcode and then follow the gantry around with screws as it marks the spots. I’m all about production efficiency right now :slight_smile:

Swapping the next sheet like a NASCAR team.

2 Likes

40 sheets. It must be a house.

1 Like

I believe there are also plastic nails you can get. You still try for the negative space, but if you are off, your bit won’t be damaged.

I just add screws after the first pass. I rarely do full depth through cuts. I’ve cut too many screws.

1 Like

Greenhouse. I hope :slight_smile:

1 Like

…brilliantly simple. I like it.

I’d love to see the design, finished, or even in progress pictures. It sounds like a great project.

This was my first foray into steel carving on the mpcnc…

2 Likes

I have a couple screws like that. I keep using them again on the cnc as hold downs.

It’s probably very dangerous, but I stand there watch with eyes down the line to the stock and spoilboard how close I end up getting to a screw after I forget to take a photo of the layout of the stock offsets and where I decided to put the screws.

I do that, but have my plastic eye shields on.

Generally I’m looking over the top of them…

Good to see this posted for reference. I tried the plastic brads (metal ones too) and found full-sheet stock isn’t held in place firmly enough to reduce chatter. Programming in work holdings with screws is the way to go.

I did a full sheet cutout of 1/2" plywood by clamping along the edges then doing auto bed levelling with a sensor in 9 spots to account for any warping. Worked well but I don’t think it’s any faster than your screw technique. It’d be interesting to see if all the sheets warped the same way when doing multiple I might only have to bed level once.

I can’t imagine doing 40 sheets post some pictures when you’re done!

Yes, this techniques lets you add them earlier in the process, so that first pass can be nice and clean.

Sounds like you need to turn it into a downdraft table. You probably wouldn’t need a lot of holes. Maybe some 3/4" holes every 6 inches. You could go with smaller holes more tightly spaced in a smaller area that you could use for downdraft while sanding. Just cover the larger holed areas with 1/8" ply or mdf when you want to do some sanding.

My bet is a wikihouse :smirk: