Bessey K-Clamp bar flex

[Michael Marcinelli]

Hey all. I just got some 50" Bessey K-Clamps online. I chose Bessey since they seem to be better liked than the Jorgensen Cabinetmaster and I'm surprised to see that there's quite a bit of flex in the bar when you press on it. I know that during a glue up the pressure is applied evenly across the bar so flex probably won't cause the bar to bow but even a slight bow is a problem.

I know that steel bar clamps won't flex but then you don't have pads that clamp perpendicular to the bar and you don't have the nice wide pad.

Anyone else have concern over this flexing? Any wisdom will be appreciated.

Thx
Michael

[Steve Wargo]

The way I see it, is if you have to apply enough clamp pressure that it bows the clamp... any kind of clamp... you probably don't have your joint cut well. That being said, I've never had a problem with any Besseys I own. Very stout.

[glenn bradley]

Steel bar clamps flex too at that length even with 3/8" x 7/8" bars. The amount of pressure used doesn't make this an issue for me. You just need to clamp the pieces for gluing, you don't need to crush them or squeeze all the glue out although you certainly can. DAMHIKT. Many folks use the aluminum bar clamps without a problem.

[Jim Becker]

'Never have been concerned with this issue. I somewhat agree with Steve on this, too...over-clamping a joint not cut accurately resulting in some deflection of the bar isn't really a problem with the clamp.

[Anthony Anderson]

I'll agree with the others, the joint/board edges should be flat and should require little to no force to close any gap, and definitely not enough force to flex the bar. Now with that being said, on my 84" and 96" the flex is there and is pronounced over the length of the clamp because the bars are so long, and no so much a result of the pressure applied. The only situation where you may have a gap in between boards with mating faces is if you are, say, gluing up a table top and intentionally put a small gap in the middle of the boards being glued up, so as to keep the ends together when/if shrinkage occurs. Clamping pressure should be light, and just enough to hold the boards together and force the glue into the cells/fiber of the wood to create a strong joint. HTH. Bill

[Gary Keedwell]

Not starting anything here but we had a thread a few weeks ago in response to magazine article (I think it was WOOD) in which the recomendation was xxxxPSI and the discussion was very lively. I don't have the article handy but they said you needed alot of pressure and alot more clamps then most people tend to use. Maybe I will look around for that Magazine this morning.
Gary

[frank shic]

gary, i saw that article as well and my first thought was: who in the world is going to actually measure the PSI per clamped joint? the second one was: how do you measure the PSI? i guess i must be just doing it all WRONG! ;)

[Ryan Bess]

I hope I'm not too silly for saying this, but I always use the flex of my long Bessey's as a "gauge". I apply pressure with the clamp until it begins to bow, then I back off slightly so the clamp is straight. Somehow I've always told myself that this creates the proper pressure. I'm not sure if it's good advice, but it has always worked for me.
Ryan

[Michael Marcinelli]

thanks for all the good advice. my initial concern was not so much having to apply a lot of force to correct poorly jointed edges but more a concern that the bars potentially flexing under more extreme conditions.

say if i were gluing up a 48" wide panel composed of several 2" thick oak boards. that's a lot of weight on the bars and i'm concerned that when a normal amount of pressure is applied to glue up of well jointed edges that i might see a, say, 1/8 inch bow when applying a normal amount of pressure.

[Gary Keedwell]

gary, i saw that article as well and my first thought was: who in the world is going to actually measure the PSI per clamped joint? the second one was: how do you measure the PSI? i guess i must be just doing it all WRONG! ;)
I hear ya Frank. But if you go by the glue manufacturers and Wood magazine's recommended pressure, most of us are not even coming close. Most of us just put the boards together and apply enough pressure for a little glue squeeze out. Glue makers say that we are under applying pressure and starving a joint is nonsense. What is a guy to do?:o
Gary

[Bill Huber]

gary, i saw that article as well and my first thought was: who in the world is going to actually measure the PSI per clamped joint? the second one was: how do you measure the PSI? i guess i must be just doing it all WRONG! ;)

I just checked on the Titebond site on clamping pressure and this is what it had for Titebond III.

Required clamping pressure http://www.titebond.com/images/clear.gifEnough to bring joints tightly together (generally, 100-150 psi for softwoods, 125-175 psi for medium woods and 175-250 psi for hardwoods)

So would that mean white knuckles or just turning white on hardwoods. Do we all need to buy load cells for our clamping pressures or will the new clamps have them built in. (need more batteries)
I can just see the new clamps now with an LED readout on the side for clamping pressure. The cheap ones would have lights on them, yellow for soft, green for medium and red for hard woods.
Then we would have to have some way to calibrate them (more batteries) so we would know they are right.
We could then start threads like "Is my clamps really reading the right pressure".

Well it never stop.....:D:D:D

[Mark Gordon]

Actually, I bet that even with medium force on the clamp we are reaching 200 PSI. Because of the mechanical advantage of the screw, a 10 lbs force on the handles could result in a 10-25 times higher force on the clamp face.

If you want to try it, just put your finger in the clamp, and tighten the handles a little.... there see!

I have another question: for people who don't white knuckle it.... have you ever had a joint fail?

Thanks



Required clamping pressure http://www.titebond.com/images/clear.gifEnough to bring joints tightly together (generally, 100-150 psi for softwoods, 125-175 psi for medium woods and 175-250 psi for hardwoods)

[David Weaver]

Titebond is saying that because their optimal joint strength is with that, and they're probably accounting for some slop in your joinery.

What the rest of us don't know is what percentage joint strength is there if the joint is clean and there is 1/10th that in clamping pressure. I'll bet the difference really isn't that much in ultimate joint strength. It's about the adherence to the wood and the thickness of the glue between the woods, right?

I haven't seen anything I make break, even stuff that had gap-filling gorilla glue put in it before I read from Bob Smalser and that FW article that it was very low strength at that level, but I would fit into the category of the others here, that the joints have to fit tight and flush or I throw the piece away or into the scrap tub and start with another piece. As a hobbyist, the financial consequence is very little.

After Bob's article, though, titebond only gets used on stuff that I don't care if it is or isn't repairable - like things my wife wants me to throw together quickly, or items that will go in a house.

From the FW test showing liquid hide glue, it looks like hide glue performs a lot like titebond with different joint tightness - in terms of reduced strength for loose joints, as well as in ultimate strength. I'm getting kind of excited to try some hot hide glue the next time I build something I want to "keep" for good.

At any rate, I have the same bessey clamps - they flex, it's no big deal. That's factored into their load rating. If they flex so much they touch your material, then you either want to reposition them or put a shim between the bar and wood to make sure they don't bruise your project. All of the bar and K-body style clamps of that heft will flex if you twist them good.

I'm still blown away that the FW reviewer only cranked them to 300 pounds of pressure.

[frank shic]

So would that mean white knuckles or just turning white on hardwoods. Do we all need to buy load cells for our clamping pressures or will the new clamps have them built in. (need more batteries)
I can just see the new clamps now with an LED readout on the side for clamping pressure. The cheap ones would have lights on them, yellow for soft, green for medium and red for hard woods.
Then we would have to have some way to calibrate them (more batteries) so we would know they are right.
We could then start threads like "Is my clamps really reading the right pressure".

Well it never stop.....:D:D:D

LOL that's exactly where i see this heading, bill!

[Michael Marcinelli]

another point... i use bar clamps for both the top and bottom of the boards. i'm not so worried about the top clamps, i'm more worried about the bottom clamps since the boards are resting on the bar, the bar becomes the level surface for the boards. so if the bars bow slightly, so does my panel.

thoughts? w/ parallel jaw clamps are they typically used only as top clamps? if so, what's used on the bottom?

[frank shic]

michael, you can bar clamps to clamp from above and below the panel.

[Josiah Bartlett]

If you throw a couple of extra clamps in there but just barely tighten them so they don't move around, then you get a pair of straight bars to hold your panel from bowing along with all the clamping pressure from the bowed bars, so it shouldn't be a problem.

As long as you jointed the gluing surface well any slight bow when clamped should spring back when the clamps are released.

[John Karas]

I do use Frontline clamps for panel gluing & i never look back or use any other system again..

Others agree see bellow some of the comments from another woodworking forum bellow.

I grabbed four of them. A bit exxy for some, but if you are a cabbie or just trying to make a few $$$$ it is the best money ever spent (and that includes Domino) These clamps are something else. I can make a 2000 x 1000 table top at a good pace (not rushing) in no more than 20 minutes, and it is perfectly flat. I would have no trouble working just by muself in my basic workshop in making 20 table tops a week. Cabinet doors (dowelled, domierd or tennoned) what a bonus, dead flat. Just awesome.

They clamp top and bottom from 10mm - 75mm thickness and up to 1200mm wide The unique bit is 2 monster springs, so when you tighten the clamps they act first and bring together the clamps with huge pressure before the boards come together.

Everyone who has seen them (Like Sheddie above) are blown awayhttp://www.woodworkforums.ubeaut.com.au/images/smilies/smile.gif.

[Peter Quadarella]

How long until we can buy clamps where we just set the PSI we want, press a button, and it tightens up for us? ;)

[Will Blick]

I fear responding to this thread, as for some crazy reason, clamp pressure topics strike an "attack" cord with many.... so I will be as brief as possible...

For hardwoods, the 200 psi recommendation does account for a small amount of uneven jointing surfaces. However, keep in mind, unless you joint the edges near perfect, then glue up within an hour or two, your jointed edges are no longer square enough, and will require the psi Titebond recommends. This data comes from Titebond engineer.

Also, as mentioned, the strength of the glue joint is also an issue, not all joints need to be at max. possible strength. Many joints can be assembled with rubber bands and they will have sufficient strength. (certainly not your application though) OTOH, its not just about strength, it's about aesthetics. A 2 - 3 mil glue line is nearly invisible, and therefore, it kills two birds with one stone - i.e. max. strength and invisible joint line. The ~ 200 psi stated pressure by Titebond (and many other sources) is required to achieve this 2 - 3 mil glue line.

In your example, 48" x 2" boards (doesn't matter how many you glue together), you can reverse engineer your way into the number of clamps required. Feel free to change the numbers to suit your taste.....

48 x 2 x 200 psi = 19k total pounds of pressure, or force (not psi)

Most parallel clamps deliver 750 - 1100 lbs of force... assuming a person is above avg. hand strength. (most ww's are) Subtract 25% if you are avg. or below avg. strength. I have tested many of the popular brands which I own. The Stanley's being the best of the "parallel" types. Bar clamps have the highest force, ~1200 lbs due to their handle design. Since you are concerned about buckle, which is a a valid concern, I would tighten the clamps down 80% of max., which should yield on avg. 500 lbs of force per clamp.

19k / 500 = 38 clamps...

Seems absurd, huh? But, if your boards are very freshly jointed and perfectly flat, you can achieve the same 2 - 3 mil glue line, with probably 50 psi, so 1/4 the number of clamps required, = 9. Don't need a max. strength glue joint? It doesn't need to be 2-3mil? Heck, you can cut the number of clamps in half again, so maybe 4. So lots of variables for you to consider. Apply this to your situation and you should produce a glue joint that meets your objectives.

Also, I have used these with great success to prevent bar buckle from deforming my straight boards.... Woodpecker only make them 12,24,36". Of course, you can make something similar.

http://www.woodpeck.com/media/main_2flat-bars.jpg

Another thing to think about... many of the bar clamps will slip after fully tightened, so the joint does not experience the full pressure during drying times. This is especially true with the Rockler nickel bars sold with their clamps. And the QR rears make the clamp very vulnerable to slippage. Parallel clamps are superior in this regard.

Hope this helps...


As for clamps that go to a certain pressure and stop automatically.... most small production cabinet shops use such clamping systems, you set the pressure, push a button, and you are assured the proper clamping pressure on every glue up. Of course, they are designed for certain size items, such as entry doors, cabinet doors, etc. For the hobbiest, a tiny bit of math and little common sense will get the same good results.