Vacuum and Spoilboard Issues with Small Plexiglass Pieces
Spoilboard and hold-down suggestions for cutting plastic on the CNC. September 28, 2009
Question
What are the advantages of an LDF or an MDF spoilboard? Does edgebanding the spoilboard help with pressure for the vacuum? I am cutting 1/8" x 48" x 96" plexi into 2" x 4" pieces. I have been double stick taping the underside, and it takes too much time.
Forum Responses
(CNC Forum)
From contributor A:
In my experience, LDF is the worst you can use. Stick with regular MDF or spend a little more money and use what I am using which is called Extrida. Basically it is a waterproof MDF. Mill about .015 off each side and seal the edges and you should get the best vacuum. Also, cover whatever part of the spoilboard is not being used. I use thin acrylic or even thin Sintra. Doing it this way, I am able to just about peg the vacuum gauge every time. Keep in mind every machine is different, just because it works for me doesn't mean it will work for every one. Every machine is different. This is a good starting point. Instead of using regular double sided tape, try using carpet seam tape. It is the thinnest and strongest on the market and can be used over a few times.
From contributor B:
Others may disagree, but I'm with contributor A. I have never been happy with LDF as a spoilboard. I have never really felt compelled to seal the edges of the spoilboard, and I've not had problems with holding. Keep in mind that the holding power of your table is a direct function of the part size. The bigger the part, the more surface area for the ambient air pressure to press down on. 2" by 4" parts will prove very difficult. Many software systems will allow you to add tabs between parts. This would be a big help in your case. You could try onion skinning, where you make one pass not quite all the way through (leave 1/32" or so) and then re-trace the pattern in a second pass cutting through the thin layer. There will be very little side pressure cutting the thin skin. Also, try a downshear bit if you can. The cutting pressure will push down on the part, helping to maintain your seal.
From contributor C:
Using a cleanly milled spoilboard, you should be able to set surface to the top of the peel coat paper on the bottom and then set a max depth for that surface. Basically, you're using the peel coat on the sheet like the double stick tape. When done, cut the paper and peel off the parts.
From contributor D:
I'm a huge advocate of maximizing the potential of a CNC, but in this case I would really consider cutting the 2" x 4" plastic parts on a beam saw. With the right blade, cutting speed and maybe some coolant on the blade, your cut quality and accuracy would equal or surpass your router and the speed would blow away the router. If this isn't an option and you have to cut multiple sheets of these parts, I would make a dedicated spoilboard for these parts so you could firmly hold them and only make one pass.
From contributor E:
You are in the same boat on this job as everyone. Charge more for the parts you produce. I recently lost a job (based on price) to another vendor cutting small parts like the ones you mentioned. He's having a hard time holding the parts and the customer is asking for my input on how to get the job to run better.
From contributor F:
I've also routed 1/4" deep pockets into the spoilboard that are offset inside the perimeter of the finished parts. This increases the suction underneath the part. To further increase the suction, you can drill a hole through the pocket. With such small parts I would also suggest cutting the sheet into four 48 x24 pieces first and routing the pockets in the spoilboard only for this size piece. The rest of the table would be sealed off. Obviously, this requires a few more cuts and some more set-up time but it would be better than all that 2-sided tape if you're making a large volume. Also, depending on the quality you require for the edges, cutting the sheet into a more manageable size on the CNC and then chopping it up on a table saw may even be quicker.
