I am cutting a lot of 5/8 Chinese plywood. It has melamine laid up on both sides. I am in NC where we have been having average mid to low 90s and high humidity. This stuff starts to curl up as soon as you break the band. I usually cut melamine with particleboard core, so I am new to plywood. I have a Becker vtlf-250 pump, but the curl is too much for it. Is it the Chinese plywood or just the heat and humidity? Or do I need to look at a stronger pump? Cutting on Biesse Rover 30.
From contributor J:
I would think that the plywood is the problem. I cut a lot of the 3/4 Chinese birch with UV finish on 1 side and 2 side. Some is real crap and will not suck down. I have the same Becker pump but have 2 of them. I mostly use 1 pump, but on a warped sheet, or if the spoilboard has been cut on a lot, I use 2. The supplier I get the Chinese birch from has been giving us some pretty good stuff. Other suppliers have crap and there does seem to be a difference even though the supplier who has crap says it is all the same and everybody gets it from the same place. I had just about given up on the stuff until I got some from this other supplier. If the stuff is curling after you cut the bands, you've got problems and it will probably not suck down even with 2 pumps. The Chinese ply is a crapshoot.
I could never sell something like that to a customer. Not even for paint grade work. Due to the fact it would delaminate randomly and would barely hold a screw, I wouldn't even use it for an applied base that would never be seen.
I certainly hope there are better products out there from Chinese suppliers. I haven't come across any yet. I also bought some 1/2" plywood in a pinch from Home Depot. I don't think it was Chinese, but I am not sure of the origin. One of the sheets warped to the point where it was unusable before I got it back to my shop. The veneer was so thin you could not sand it at all without sanding through. I should have known better when I saw that they had almost sanded through in spots from the factory. Ended up being a complete waste. I don't even know why they sell stuff you can't even use. I suspect a lot of reject material makes it onto store shelves and into the inventories of suppliers when it should have been sent to the landfill to begin with.
A rapid change in humidity may affect the first sheet or two in a lift, but after that there is little moisture imbalance between the faces of a given sheet. Warp is caused by unequal stresses and that is indeed often built into the board at the time of mis-manufacture.
Thermwood and others make a pressure roller system that rides along with the spindle and gantry to deal with situations like this, and for stack cutting too. I do not know if Biesse offers such an option for the Rover.
If your volume is low you may be able to utilize an edge clamping system either separately from or in conjunction with your vacuum. Some inexpensive routers use edge clamps instead of vacuum. It can be as simple as a piece of aluminum angle or bar stock attached to a set of toggle clamps or as sophisticated as a pneumatic actuated bar or rod engaged with G code level commands.
I sometimes need to nest my parts avoiding the corners of a sheet. I can then place a 1-inch screw for 1/2 inch material or a 1-1/4 for 3/4 material in the corners manually and help hold down a sheet. It helps a lot to place the sheet concave side down if possible. This does not help much with saddle shaped sheets though.
I just needed to know if this was normal, as the material supplier is saying I don't have a large enough vacuum. Being new to plywood I did not know it was my vacuum, the humidity, or the board. I can usually get most of the boards to suck down by flipping them over. But some is wood grain on one side and white on the other. So flipping them over is not an option.
I can add another vtlf 250 if I really need, but I don't think it will do any good.
Your pump is a 10 HP rotary vane. Excellent quality unit, perfectly capable of holding work. You have not got a lot of extra pumping power, so you can't waste it. Covering unused areas of the table or zoning the table and sealing the edges of the spoilboard are important, but I ran a similar unit for many years on a 5 by 10 table and was fine with it, and hundreds of other people do every day.
I agree, switch to a better board. You may find the cost to be not too different in the end, and the savings in time and waste will likely make the better product cheaper.
The stuff we bring in now is referred to by the two companies that supply it as "calibrated core." Which basically means it has a nicer core than the bottom end stuff. There are way less voids than the cheap stuff, the veneers peel much less, and the cores have fewer plies than the really low end material. I have some salesmen who say their material is the same and we usually have them pick it right back up after we cut the banding.
Also, the price is about 2-3 dollars per sheet more than the low grade stuff, so still much cheaper than domestic. I would change over to domestic but I have yet to find that customers want to pay the extra money for that, no matter what the job size is. We typically work in million dollar homes and even they want the cheaper options these days.
You should tell your customer to let you purchase the material for him so you can get your hands on it and make the right decision. Don't be afraid to ask for 2 or 3 test sheets to cut up before you buy a unit. We do it all the time now when someone is promising us the moon. Not one supplier has declined to send the sample sheets for free yet.
One last thing... Though I use import core material for my boxes, I do not use it for exposed ends, cases, or anything that is on the outside of the cabinetry. That is either domestic or MDF core material. The inconsistency in the cores shows through finish and makes for a service nightmare!
I too would buy a domestic plywood if the price were right, but haven't had any luck finding anything economical. If anyone knows of a low cost domestic plywood, let me know.