Hold-Down Problems with Small Parts and Pre-Finished Plywood

Troubleshooting difficulties keeping small parts still while cutting pre-finished plywood. March 26, 2009

We use a Busellato Jet 3006 RTR CNC with a Dekker 25hp vacuum pump for our nested based operations. Melamine works well but we have trouble keeping pre-finished plywood from moving. Any suggestions? The backer board is 3/4" MDF.

Forum Responses
(CNC Forum)
From contributor M:
My first suggestion would be to tailor your nest in a manner that each part stays attached to the rest of the panel until the tool has completely made it around the perimeter of the part. For instance, let's say I have a square part that I am going to cut out of the bottom left corner of a panel. If I were to start in the bottom right corner of my part and cut in a CCW direction, then my part will be effectively severed from the rest of the panel once I have cut the first two sides of the part. As the router cuts the next two sides, there is a high probability that the part will move. Now... if I were to start in the upper right or left corner of the part and cut in a CCW direction, then the part would remain attached to the rest of the panel until just prior to the cutter finishing the cut. I have never used Jet Nest, so I don't know how feasible this will be for you to do.

My second suggestion would be to give LDF a try instead of the MDF. We have used LDF for a long time and have tried MDF many times in the past because so many people insist that it works better. We've never seen MDF work nearly as well as LDF for a flow through spoil board. The only downside to LDF is that it can be difficult to find sometimes.

Another suggestion would be to seal the gap between the edge of the panel and the surface of the spoil board with masking tape. This helps the panel to initially get sucked down to the spoil board. Yes, you will immediately create a "leak" as soon as a router bit or drill penetrates the panel, but I promise this will help keep parts from moving.

You could always do the old onion skin trick. This may be the only thing that will work for many of your smaller parts. Also, surface your spoil board often; as the surface of the spoil board gets rutted up, your part holding capability goes way down.

From contributor I:
What size parts are moving?

From contributor K:
Keeping material stuck to your spoilboard is one of those problems that have many threads to the solution. If you are cutting well with melamine and getting the results you want, that tells me a lot. It says you are doing a number of things right. There is always room for improvement in any system, including mine, and our battle is balancing all of these factors to our best advantage. I use a similar pump and an MDF spoilboard; Panfiber is my choice for the spoilboard.

Melamine is usually very flat, and nonporous. It is one of the easiest things to make stick to the spoilboard. There are many ways to go wrong - low vacuum, bad flow through your spoilboard (too high or too low are both bad), but you seem to have this part down pat.

VC ply is notoriously difficult to make stick to a spoilboard mainly because it is warpy. If the material does not touch the spoilboard, it has zero holding power at that point. Look closely at a sheet as you put it on the router. Note the areas if any that are curling up. Many times simply turning the sheet over can solve the sticking issue so the high spot is in the middle of the sheet - the vacuum sucks the middle down. Onion skinning is great, essential, I think, and gaming your cuts as contributor M said are both excellent strategies, but if the lift of the work off the spoilboard exceeds the thickness of the skin, as it often does, you are still out of luck with the part in that area.

I have had best luck addressing pre-finished by first choosing my material and supplier as carefully as possible. Flatness is the biggest factor. There is the number of plies in the core to consider - 5 through 13 plys are common - the more plies (in general) the flatter the board. I also find that a fir core is a little flatter than a poplar or aspen core, but this may have more to do with manufacturing process than species. I usually choose a 9 or 13 ply fir core if it is available. This is also a tough call, because veneer quality also comes into play. You need a good-looking veneer for your project too.

As an aside… more plies also goes a long way toward fixing a ridging problem that many people run into. As a slightly dull cutter cuts VC ply, it usually cuts better along the grain and slightly compresses the cut against the grain. After the bit passes a point the cut against the grain bounces back, leaving a ridge on the alternating layers of material, making it difficult to edge band. This end grain, now fuzzy, sucks in moisture more quickly and you will actually notice the ridge grow over time. Using very sharp cutters minimizes this in a 5-ply poplar core, for example, but using a sheet that has 13 plies also serves to minimize the ridging.

There is also the option of choosing an MDF or flake core instead of VC. If this is unacceptable, there are composite cores available, though they are harder to find. The best arrangement is VC center with a 1/8” MDF skin below each veneer, but I haven’t seen that available for several years. The ones I have seen lately are backwards, using an MDF core and veneer at the outside and that doesn’t work as well.

If all else fails the last sure bet in my own bag of tricks requires a little more work, and more waste. Nest your parts with a gap at the edge of the board and put a few 5 mm holes along the edge of the sheet as a first operation. You can also throw a few holes in strategic areas between gaps in the nest, say for instance in the toe kick of an unfinished end or between through-boring operations that are not seen in the finished product. Pause the run after these holes are drilled and throw a screw into the spoilboard at these locations as needed. This gives a few positive touch points to the spoilboard that will not move where you know for sure the cutter will not hit the screw. The edges are nearly always where your problems occur. Start the program back up and remove the screws afterward.

From the original questioner:
Thanks for your input. We might be asking too much. The parts we have the most trouble with are drawer parts. They would be impractical to modify the rout sequence because there may be 50 parts on the nest. We onion skin and use a 1/2" diameter bit. LDF is hard to get unless you want to order a unit of it. The flatness of our panels is probably the culprit. I'll check that. We might also try the 3/8" diameter bit.

From contributor E:
Sounds crazy, but you might also check the brushes on your dust shroud. I cut dovetail drawers out of 5/8 Baltic birch and when we do, I have a shroud without brushes we swap out. I caught the bristles dropping down into the groove and scooting small parts enough to break the vacuum and then you know what happens. On really small stuff I will even shut off the dust collector and let the dust fill the cut groove back in. Sounds crazy and definitely does not add any life to your bit, but I know it has saved me a couple times.

From the original questioner:
Our shroud has plastic strips that do drag across the parts during the machining process. We'll experiment with this idea of yours and see if that helps.

From contributor F:
Another way to keep small parts from moving, even on a vacuum table, is double sided tape. We use a really good double sided masking tape that is only about .006 thick and by the time the vac pulls the sheet flat on the table, you'll have to pry the parts up from the spoil board with a chisel after cutting. Taping to pre-fin is a little tricky, so you want to make sure the tape doesn't pull off or damage the finish. We always tape down drawer parts, and one or two pieces of tape is all it takes. It's about $8 a roll from Bron Tapes in Phoenix. Try a local tape supplier.

The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contributor L:
You can also try Vil-Mil. It creates an adhesive bond that holds the parts to a sheet that are easy to remove afterwards.