|Forum topic by Nicholas Hall||posted 12-06-2012 05:46 PM||2895 views||2 times favorited||88 replies|
12-06-2012 05:46 PM
In the course of researching my split-top Roubo workbench build, I came across an article “Get Serious About Clamping” in Nov/Dec 2007 issue of Fine Woodworking written by Roman Rabiej, who is an avid woodworker and a professor of industrial engineering at West Michigan University. This is the guy Titebond calls when they have a glue question. Roman spent an inordinate amount of time testing glue joints for failure using a engineering testing equipment to determine the recommended glue clamping pressure as a function of wood species and grain orientation.
Here is quote from the introduction of the article:
“Most woodworkers have only the vaguest idea of how much
Obviously I can’t really post the whole article because that wouldn’t be right. If you subscribe to Fine Woodworking online access you can get the article, and its definitely worth a read. If you don’t, you should, it’s only $3.00 per month and you get access to every article written and tool review from the last 30 years! The tool reviews alone have paid for 10 years of subscriptions by helping me to get way better tools for much less money. Anyhow here is the table of clamping pressure:
I was actually quite surprised that the species of wood can vary the required clamping pressure by 1000% (in the case of flatsawn Ponderosa Pine vs Sugar Maple). I was also quite surprised to learn that the grain orientation can double the required clamping pressure. Now, let’s assume that someone who has tested tens of thousands of glue joints like Dr. Rabiej has learned a thing or two about gluing. What does this mean for the glue-up of a workbench top?
In my case I’m doing a 4” thick by 96” long top, so the total square inches of glue face is 384 sq inches. The boards I have are flatsawn, with the flatsawn faces in the glue joint. I’m using ash, which is most similar to red oak in density and grain structure, so we look at the chart and see that I need 900 Pounds Per Square Inch (PSI) in order to get the recommend clamping preassure. That means I need to exert 345,000 pounds of force in order to get the recommended 900psi distributed across the glue joint (900psi * 384 sq inches). Further in the article, the author shows that the average woodworker can extert 1050 lbs of force with a 3/4 inch pipe clamp (which is what I’m using). Luckily I have 10 pipe clamps already, so that gets me 10,500 pounds of force. Now I just need to find 335 more sets of pipe clamps and devise a way to get them mounted on the wood. Thats probably not going to happen…
Now I know that most people use 10-15 pipe clamps for their workbench top glue-ups because I’ve seen the pictures. I only have 10 pipe clamps myself. What I’m wondering is whether or not I should bother to get more clamps or if I should just go with the 10. After all, it’s not just the workbench tabletop that gets glued together. I do conference room tables as well that are 1.5” thick. I also often make table legs by gluing together two 2.5”x2.5”x30” pieces of 4/4. I’ve found that no matter how I prepare my glue joint (jointer + smoothing plane) I’m rarely satisfied with the glueline. I’m wondering if the problem isn’t pressure, I’ve obviously been seriously underclamping just about everything.
I can accept that I’ll never come close to the recommended pressure for the workbench top, there’s too many square inches. But for a $5,000 conference room tabletop, I really want a perfect glueline. I’m wondering if its worth it to rig a glue table up with bottle-jacks, which is the only way I can think of to generate 100,000 pounds of clamping pressure. At any rate, it’s my lunch break and this is the sort of thing I think about. I thought I’d see what you folks think over your own lunch/coffee breaks… :)
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