Wetzler Veneer Press

I have picked up 6 sections of an old Wetzler Veneer press. It is built from I beams and large screws. These press frames have (5) 1-1/4" 5tpi screws with a 3" pad each. These frames will allow 50" wide glue up. The questions is how much pressure do you put on each screw? It came with 2 T handles that fit over the 7/8" square on top of the screws. I used a torque wrench and set it to 70 foot pounds and with a measuring devise it reads about 7000 pounds. I read that a vacuum bag puts 2117 pounds per sq foot. Is that a good number to use to figure how much pressure I should put on each screw? I also tried a Jorgensen 4512 F clamp and could only put 1500 pounds of pressure. The Jorgensen clamp has a 1-1/4" dia pad. I noticed that I can bend the I beams if I put to much pressure on them.

Any advise on what is the best way to set the right pressure? What is the right pressure?
I am gluing Plywood 3/4"+1/2"+3/4" together with Tite Bond cold press glue.


Thanks,
Cory

The best place to look is the manufacturer's data.

Most wood glues work best with about 100PSI, or 14,000 to 15,000 lbs/ft^2
Of course very few of us have hydraulic or firehose presses with a density that can attain that. So manifestly, glues must work "well enough" with a good bit less. Go to the FPL and browse for a bit.

Here's the Titebond spec:
Titebond - Product

Application Temperature
Above 50°F.

Open Assembly Time
15 minutes (70°F./50% RH)

Total Assembly Time
15-20 minutes (70°F./50%RH)

Minimum Required Spread
Approximately 6 mils or 250 square feet per gallon

Required Pressure Application
100-250 psi, depending on substrates

Method of Application
For most consistent coverage a mechanical spreader is recommended.

What is true is that most of us in small shops compensate by spreading much more glue than would be necessary with "ideal" pressure and thus compensate in a fashion, for using less pressure. Without roll or curtain spreaders and a fast means to lay up sheets, we deal with too much moisture, but get a longer open time, and a less critical press force. FWIW, epoxy is one of the few that does not require much pressure at all. Vac bagging started when epoxies made low pressure gluing practical. People now try to extend that to other glues.

It is impractical to count on a full 14.1 PSI from a vacuum press. The pressure varies with barometric pressure and altitude, and the typical rotary vane or venturi pump is no way going to get down under 13PSI with normal conditions. Add some leaks & pump wear and it could be reasonable to estimate about 12 or so which would = almost 1800 lbs/ft^2. I round that down again to 1,500 for quick estimation purposes & potential vagaries in the plumbing.

You have correctly pointed out that the bending stress of the I beams will the limiting factor for your press. That type takes a lot of caul sheets to spread the force uniformly over a large area. Simplified, the force is spread downwards and out on a 45° cone from the point of the screw. Using stiff/rigid platens (thickish steel plates) spreads it farther with more uniformity. If there are only plywood or MDF caul sheets, thicker is the way to go to spread point loads observing the 45° cone effect.

The other limit of pressure for your set up will be to mitigate the point pressure effects through using adequate caul thickness or stiffness or both, commensurate with the pressure you decide to apply. Basically, it is easy to press a visible grid into the work, if you over-tighten and/or under-caul.

Should you worry about ultimate pressure? Probably not for veneer. Again, the downside is that you will probably automatically used much more glue and hence add more moisture than "technically" necessary. But that is the trade off that will happen anyway without a metered glue spreader & extremely rapid ("instant") assembly and clamping process. There are a lot of people vac bagging that stuff even though the manufacturers specs recommend against it. So as you note, that would be the low end pressure value.

smt

As usual, Stephen has put together a detailed, comprehensive reply, based on his deep knowledge and experience. Well done smt!

My simple answer is the more pressure you can get, the better. No way you can get too much with a hand screw press, unless you're pressing a 6" square.

As you have a measuring device, would be interesting to see how force changes as you tighten all the screws in a section. I've had both cast iron and massive wood frame screw presses, and it was easy to see the screws loosen as adjacent ones were torqued.

Also, laminating plywood is a lot more forgiving than pressing veneers. You're just not going to get bubbles with 3/4" ply! Stephens comments re using press plates or cauls are very important for getting even pressure. My standard procedure for veneering was 1/8" aluminum sheet, followed by 8/4 softwood panels, with big beams (6"x9") over then with the grain crossed, and if necessary another layer of beams with grain crossed yet again. Like cribbing under a big load.

Your big issue will we the time from spreading to pressure if you're doing large panels with yellow glue. I saw a Wetzler press at the Vladimir Kagan factory in Queens, NY and they used a pneumatic driver for the screws, WAY faster than doing it by hand. Epoxy works great too, and introduces no water into the panel. Urea glues (plastic resin glue) have a longer open time also, but require 70 deg shop temps. I had problems with this in my first shop and switched to epoxy, and never looked back in spite of the cost.

For getting correct glue spreads with yellow glue, I made a spreader by cutting notches in a plastic squeegee. Calibrate with a scale and a 1' square panel. For epoxy, where I found spread was really critical, I weighed out the proper amount in plastic cups for each surface, also avoided having the mix kick prematurely in the big bucket.

Veneering is a pretty fussy operation, takes some practice to get it right, especially if you want to wind up with flat panels.

What are you actually making Cory?

As said, glue spread and time are important. Even pressure seems more important than massive forces. Epoxy doesn't need much pressure and can be easily over pressed into a weak layup. Since I know I can get a good rubbed joint with no pressure and can't have too much pressure on Titebond type glues, most anything in between that brings the surfaces into uniform contact probably works fine. I did a test with Titebond on hard maple, 10 square inches, 40,000#s of force in a hydraulic press, 1" steel culls. Next day the joint was forced apart by driving a steel wedge(in the press.) The joint held, the wood broke. We regularly vacuum press MDF, 3 layers of .75". Titebond rolled on. The glue line is actually stronger than the MDF.

Thanks for the information. I am gluing up the three layers of plywood to make a very large wall unit. They call out for them to be 3/4" + 1/2" + 3/4" = 2" thick plywood shelf. I did not want to cut every part twice line them up and nail a 1/2" spacer between them. So I am going to glue them up as one solid piece and cut them on the Hendricks Panel saw. One cut and ready for assembly. I made a platen for the bottom out of 2 layers of 1-1/2" MDF glued together and then laminated both sides with laminate. I also glued up another 2 pieces of 1-1/2" MDF and laminated it then cut them into 16" wide pieces for the culls. After squeezing the crap out of them the frame bent and created a slight bow in the MDF. So I flipped them over and put the crowns facing each other.

I also use a torque wrench to set the screws so they are close to equal. I set the torque wrench to 40 pounds and my Dillon force gauge reads 3000 pounds. Every 10 pound jump on the torque wrench the pressure goes up by 1000. At 70 pounds on the wrench I get 6000 pounds on the 3" pad at the end of the screw.

I used the cold press glue to assemble the plywood and learned you have to move fast. After spreading the glue it dries really quick and you have no time to mess around. I used a foam 9" roller and my buddy was pouring the glue from a 1 gallon bottle. I know Titebond makes an extended time glue and I may need to go for that. I have had good results with the cold press glue. I used to use a leveled up platform and put my layers to be pressed then stack a full hack of 5x8 3/4" mdf on top of it. It worked really well. I don't have a full hack of MDF anymore.

Thanks for all your help.
Cory