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Clamping Pressure (Not the same old question!!!)

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Forum topic by JRBackus posted 04-26-2012 07:37 PM 1399 views 0 times favorited 12 replies Add to Favorites Watch
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JRBackus

5 posts in 907 days


04-26-2012 07:37 PM

Topic tags/keywords: question

The background before I ask my question: I’ve been building waterskis for several years out of mostly oak and walnut. I have mold that I put 4 layers of 3/16” together to add up the the 3/4” waterski that I need. I have threaded rods and bars to provide the pressure for bending the wood and clamping it to the mold.

I really want to use a vacuum bag to make these skis but I’m worried about the integrity of the glue joint. The reason I’m thinking this is because if I look up the “proper” clamping pressure for hardwoods, it’s in the 900 psi range, and obviously a vacuum can only pull 14 or so PSI max. But thinking about that, if my waterski is 8”x72”, that means I should have over a HALF A MILLION pounds on the ski in order to get a “good” glue joint, which, in my opinion, is absurd.

Now, under that same reasoning, a vacuum would put about 8000 pounds on the ski which seems more reasonable, but I still don’t know.

Here is the question: Does a vacuum bag provide enough pressure for clamping hardwoods? Does anyone have any experience bending SOLID hardwood layers (not ply or veneer) with a vacuum bag?

Thanks!

—Jeremy


12 replies so far

View Loren's profile

Loren

7723 posts in 2334 days


#1 posted 04-26-2012 08:17 PM

You’ll probably need a top and bottom caul/form to do it
with a vacuum bag unless you steam bend the layers
first in which case you may be able to vacuum glue with
a bottom form only. 3/16” boards are quite easy to bend
with a backer bar and form or on a luthier’s bending iron.

-- http://lawoodworking.com

View DS's profile

DS

2131 posts in 1106 days


#2 posted 04-26-2012 08:41 PM

+1 what Loren said. Put the mold/cauls in the bag too. You might be surprised how strong a vacuum is.

-- "Hard work is not defined by the difficulty of the task as much as a person's desire to perform it.", DS251

View David Kirtley's profile

David Kirtley

1281 posts in 1684 days


#3 posted 04-26-2012 09:00 PM

It depends on what kind of glue you are using. That 900psi clamping pressure does not look right to me. That is a huge amount of clamp force. Most glues are at about 175-250 for hardwoods. Much less for epoxy.

Structurally, you would be better off using more layers that were thinner but I assume you are going for the appearance. How tight is the bend? If you are going for more than a 12in radius or so, you are going to have to put the clamps to it for cold molding. It will also be hard to gauge the shape as it will have more spring back than with thinner layers. Unless you are doing some exotic skis, I don’t imagine there will be that much of a radius in the bends so it may not matter.

-- Woodworking shouldn't cost a fortune: http://lowbudgetwoodworker.blogspot.com/

View crank49's profile

crank49

3456 posts in 1657 days


#4 posted 04-26-2012 09:35 PM

Your initial logic of 8” x 72” = 576 sq.in.
Then 576 sq.in. x 14 PSI = 8064 Lbs of force is exactly correct.

I know it would work fine for glue pressure and will probably be better than you can get with clamps.
The trick is how to do the bending. As Loren said, steam or a bending iron. There was a video link on here a while back showing the process. I’ll try to find it.

-- Michael :-{| “If you tell a big enough lie and tell it frequently enough, it will be believed.” ― A H

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JRBackus

5 posts in 907 days


#5 posted 04-27-2012 11:58 AM

Thanks for all the advice.

The mold I have is a top mold for a ski, so the wood is bend down over the mold (bottom of the ski is up).

If the vacuum is enough pressure to get a solid glue joint, it will be MORE than enough to bend the wood. The curve of the ski is probably closer to 48” radius (very light curve), and with 3/16” lams, you can almost bend it down all the way by hand. I think the total amount of bend is maybe 6 inches over an 18” run.

I’m using Gorilla glue as it’s about as waterproof of a glue I can get without going to a specialty store.

I’ve got all of the vacuum equipment I need from my composite skis i’ve built, I just need a bigger bag to fit the whole mold. I wanted to get some opinions before I spent $150 on a bag that had no chance of working! I think it’s time to just go and try it!

Thanks!

—Jeremy

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crank49

3456 posts in 1657 days


#6 posted 04-27-2012 01:31 PM

Gorilla glue will really foam in a vacuum. The water it requires to cure will be boiled (at room temperature) by the vacuum. I would use epoxy.

-- Michael :-{| “If you tell a big enough lie and tell it frequently enough, it will be believed.” ― A H

View Ripthorn's profile

Ripthorn

768 posts in 1671 days


#7 posted 04-27-2012 01:57 PM

Some clamps can do 900 lbs of force, but they typically don’t specify the psi, though it is easy to calculate. Just divide your 900 lbs. by the area of the clamp face. Then you have to remember that you are only getting that psi rating on the actual surface area contacted by the clamp. Say each clamp has 3 sq. in. of surface are and you use 10 clamps at 900 lbs. each. You are getting 9000 total pounds of clamping force, which is not much more than your 8000 lbs. from a vacuum. Most of your quick clamps will do 300 or 600 lbs. of clamping force, which would produce (you guessed it) 3000 or 6000 lbs. of total clamping force for your 10 clamps which is significantly less than what your vacuum would produce (under ideal conditions, which you won’t achieve).

-- Brian T. - Exact science is not an exact science

View JRBackus's profile

JRBackus

5 posts in 907 days


#8 posted 04-27-2012 02:33 PM

Crank,
I was a little worried about that too, but I actually called Gorilla Glue and they said there was no problem using the glue in a vacuum. The moisture shouldn’t be a problem at all, because the moisture in the wood isn’t going to be pulled out un the time it takes for the glue to set, and as for the foaming, it may cause a mess around the edges, but ihe actual glue joints should be fine.

I may still give epoxy a try too though. I initially used gorilla glue because it’s waterproof and easy to find, but I HATE working with it. Last time I had to get it off my hands, I used my orbital sander!

Here are some picutures of the skis i’m building…

—Jeremy

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JRBackus

5 posts in 907 days


#9 posted 04-27-2012 02:38 PM

Rip,
I totally agree about the psi of a clamp vs force of a clamp. I think if you look up “clamping pressure”, the values given are taylored to typical clamping methods (bar clamps, c-clamps) used for joining small areas. When you use a vacuum, the pressure is obviously evenly distributed across the entire surface so my feeling is that less pressure is actually required.

—Jeremy

View David Kirtley's profile

David Kirtley

1281 posts in 1684 days


#10 posted 04-27-2012 07:22 PM

I think that this would be a lot better done with epoxy. The open time alone would make it easier. Add to that the fact that it is cheaper than Gorilla Glue is only a bonus. The only thing to be careful of is that you can over-clamp epoxy and starve the joint which makes it weaker.

I prefer the System 3 epoxy and in this case, the GelMagic would suit well. It has thickeners already and is dead simple to mix up. They even sell it in a caulking tube that mixes as it comes out. (That is a bit pricey though) Do the math and see how much the glues cost per oz.

I would also think about putting a real light glass on the bottom. Maybe even down to 0.5oz cloth. Just to toughen up and prevent dings. fiberglasssupply.com has a good selection and fair pricing.

Another glue to consider would be Titebond III It is waterproof and a lot nicer to work with than the polyurethanes. I can’t say it would do as well as epoxy long term for prolonged wetting but so far people building boats have been having good success with it.

-- Woodworking shouldn't cost a fortune: http://lowbudgetwoodworker.blogspot.com/

View MrRon's profile

MrRon

2859 posts in 1929 days


#11 posted 04-28-2012 04:17 PM

I would use a resorcinol glue, as used in wooden boat building.

View JRBackus's profile

JRBackus

5 posts in 907 days


#12 posted 04-30-2012 01:57 PM

After some thinking, i’m going to go with epoxy. The resorcinol is a really good option as well, but I have a lot of epoxy on hand and I would have to order the resorcinol.

A note on Titebond III… My experience with it is that it’s about as “waterproof” as toilet paper.

As for the fiberglass on the bottom, the skis i’m building are used on a waterski show team, and they are beat up big time. No matter how much glass you put on the ski, it’s always going to get dings, so we just sand down the skis and refinish them over the winter. And not to mention I hate working with fiberglass. I’ve build several honeycomb core, fiberglass skis, and I just don’t have the right ventalation and vacuum systems to deal with the dust. I was itching for about a week after trimming the ski to size.

Once I finish the stairway and hardwood floors in the house, I’ll get back to ski building and let you all know how it goes!

Thanks!

—Jeremy

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