I have recently purchased a cnc router with an 8 x 4 bed with the intention of cutting 3/4 ply, it came equipped with a 5.5kw spindle and a 5.5kw suction pump which I believe equates to an 8hp. The company I purchased the machine from said I needed to use a sacrificial board with a thickness of 9mm (slightly less than 3/8) which I am doing but after cutting 2 sheets of ply the sb curls up on the edges and I lose suction, so I flip the board and it's ok for another 2 sheets of ply then it curls up again, can I use a thicker sb to stop it curling up and would it work with the suction I have ?

I use 3/4 inch ultralite mdf for mine

Gary ,

What kind of mdf are you using ? You should be using an mdf with a 40lb density .( at least that's what I use with no problems )

Additionally , do you surface both sides of the spoil board before using it?

Gary,
All thicknesses of mdf will curl when flycut. You can try alternating side of the board that you use, that will help it stay more balanced. Your big issues are the small vacuum pump and thin spoilboard. I counterbore holes in the corners of my board and use nylon screws to keep it held down and in place when the vacuum is off, but with only a 3/8" thick board, you wont be able to do that. Try flipping it and make sure your gasket between the vacuum table and spoil board is in good shape.

Thanks for all your replies, so yesterday I bought some 18mm mdf lite, I didn't flycut it as wanted to see how it performed with nothing done, so stuck my 8 x 4 ply on it and it held it better than the 3/8 ever did, I cut out everything I needed and nothing moved, perfect I thought, as soon as I took the ply off, the sides of the mdf were curled up and I couldn't achieve any suction, so now I have turned the board over and taped up the edges. What am I doing wrong as surely I should be able to cut more than 1 sheet without flipping the mdf ??

Gary, take .005 or so off both sides, you will see its not actually flat to start with anyway.(affecting your precision) we do this and never flip until the sheet is used up, usually around .080 thick, then we pitch and repeat.

Where are you located? Humidity has a big factor in this. When pulling the vacuum you are also pulling the moisture out of the spoilboard creating an imbalance causing the warp. We have the same issues but are running a 35 hp pump which overcomes it for the most part. We usually run 40-50 sheets a day and flip the spoilboard everyday. That's what works for us.

KenW

Once again many thanks for your replies.
I am located in the UK so perhaps not the best place to be as it's been mostly wet and damp. I have skimmed the board but still the problem persists. The machine I have bought was new but it doesn't come with a vacuum gasket so I presumed it doesn't need one, or is it standard that all machines come without one and they are retro fitted by the end user ?

Gary ,

The entire perimeter under your spoilboard (mdf) should be sealed with a gasket material . Depending on hour machine and table the size of the gasket varies .i would contact your CNC manufacturer to get the information on the exact size .

Once you get that , you must surface both sides of your mdf . One reason is to get rid of the slippery sheen that you see so air can full penetrate the spoilboard without obstruction . It is also to calibrate the sheet, as others have said above you will see how the sheet is not uniform across .

Also know that too thin of a spoilboard (like the 3/8) will cause vacuum loss because the thickness allows the suction to pull through to easily .

Best of luck

Whether you vacuum table is not full table,i think it with T-slot mixed,right?If so,you can try to put one thin plywood on the table,abosorb it with the plywood you need to cut.

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4x8ft wood cnc router