I am in the process of making a veneer press with commercially available veneer press screws.
What is the proper spacing for these screws? I guess what I’m asking is, how many square inches under the screws has the correct psi pressure for a good glue bond.
Thank you,
Don
Replies
Don,
It's more of a question, how the cauls are situated. For instance some big manual presses may only have 2-3 big screws and they use a steel caul framework to spread the load. The pressure from your screw decreases as it goes out from the screwhead in a concentric motion. Best way to describe, is it's cone shaped. And you need the cones to overlap.
A furnituremaker I know uses a manual press, 3x7 and he uses 4x4's approx. for cauls and 1-2 sheets of 3/4 melamine between them and the veneered piece. You need a similar setup top and bottom.
When I had my homemade 5'x8' press, made from steel and weighed 1,000lbs. I used 4x4's across, 2x2's running the length then 3/4 melamine. Finally threw it away I bent it so bad. Went to vacuum, so much easier to just flip a switch.
I doubt that we have any idea how much pressure we apply to a glue joint no matter how we clamp. It would take far too much resreach and analysis to figure it out. Your goal is to distribute the pressure as evenly as possible. To do that, your caul needs to be very rigid. I use 3/4 MDF backed up by a hardwood frame underneath and 4X4s above where the screws make contact. Sometimes I use two layers of MDF. I use 1 mil or thicker polyethylene film to prevent glue contamination of the MDF. My press screws are not quite a foot apart on center. I have no idea how much pressure I apply. Too much will actually break my press structure. It is my sense that not much pressure is required, but it needs to be fairly even. I go by feel as to how hard I turn a screw. It works so far. Hope this helps. LOL!!
Cadiddlehopper
Every nine inches both directions. A lot of that has to do with how much material you put underneath the face of each screw to spread the force evenly.
You may want to contact the Adjustable Clamp Company at http://www.adjustableclamp.com/
Edited 12/13/2006 11:47 pm by gb93433
DonC gave you the same answer I was going to give you.
This is the basic formula in the industry, but you do need a few sheets of something to spread that pressure. I would use a sheet of 3/4" melamine, with a sheet of 3/4" plywood on top of it. Two would be better, and good quality plywood is best. Not CDX!
And when you are gluing, screw in the clamps in the center first, give the glue a couple of seconds to spread out a bit, then do the outer ones.
Hal
http://www.rivercitywoodworks.com
More exact info: My screws are spaced 9.5 inches on centers, close to 9! The advice about starting pressing at the center is sage advice also. Melamine covered MDF would have been a good choice for the cauls except that I expected to use polyurethane glue. It might have stuck to melamine. There is also the extra cost and availability. I planned my cauls such that I could remove or add layers for different thickness projects. The throw on the screws is quite small. Switching out cauls gives more adjustment. More LOL!
Cadiddlehopper
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