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Topic Archives => How To - Resources => Topic started by: Flem on March 19, 2019, 11:20:30 PM
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So last summer I said I would share some info on how I press my bows. Thought I better keep my word and since I am semi documenting my swap build, thought this is a good opportunity to do so.
First I want to say that this is only one way to press a bow, no better or worse than any other method, just where I have ended up.
So anyways I start with a vacuum pump.
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There are lots of plans for these, Joe Woodworker has a big section of examples people have made and they are all much nicer than mine. You need a vacuum pump(bottom left), pressure switch (#1), solenoid valve(#2) and the tanks. You could just run a pump without anything else, but the pump would almost have to run continuously. The tank setup allows for a approx. 10min on/off cycle. I've yet to get a perfect seal with no pump cycling. Thats not to say it can't be done. I'm content with this one, but if I built another, I would not use any plastic tubing or PVC.
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Speaking of tubing. I started with reinforced vinyl, no good, it doesn't like heat. The green stuff is heat tolerant reinforced rubber (250F). I put a quick disconnect and a check valve in the line, because it nice to be able to pull the vacuum on your bench before going in the hot box. Originally I used the little brass connector to attach the vac line to the bag, but found that it got too hot and weakened the interface with the bag. I replaced it with a nylon bung for a plastic barrel (250F).
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Pretty high tech there, Flem.
Looks cool.
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Thanks Flem I have been waiting on this one with great interest I have multipal uses for a vac press , that Joe wood workers page is very good I have been looking for a decent vac pump fo the right $$$ but a little skeptical of some I see for cheap & re builts I know you got lucky on the E box store but I'm not much of a gambler looking for some thing new !
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Pretty high tech there, Flem.
Looks cool.
Hillbilly tech
So where was I when I ran out of beer...
Bags. You need some sort of tubing or membrane to squeeze with. I was using 4mil LDPE tubing, because I had a roll left over from when I was making skis
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Its a bit wider than I need, but it works. Its about the most common plastic sheeting and its cheap. But above 160F it starts to get soft and distort.
Then I discovered this stuff;
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Its for food vacuum sealers and its good to 250F. If you look on websites that sell composites, there are lots of materials, some reusable for bagging. I'm still looking for the perfect reusable system. I hate to throw away all that plastic after 1 use.
Since most of you guys make curvy bows, I thought I better lend some relevance to this process, so I got out a old recurve form and put four 3/16" slats on to see how they would bend. They were too long for the form so all 4 overlap in the center, making it 3/4" thick. It looks like it did not pull the tips down all the way, but thats because the excess bag material pinched together before it could suck onto the form.
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I noticed later that I had left the pressure on the vac set to 12hg for another project. At my elevation I can pull about 25hg.
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Can you reuse the bags? Would you mind showing some pics of how your hose connects to the bag?
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Ritchie, I don't blame you for wanting to buy new. I will say my used pump is probably 10yrs old. Guess I got lucky!
Skeater, I have tried to reuse the bags, but it seems like they always get compromised somehow. It really sucks to try to find pinholes! Might have a better chance for reuse if you used a heaver plastic than 4mil.
Stupid bag won't work if you don't seal the ends and get a port in it!
I sharpened a piece of copper pipe to cut the port. Its nice when it just barely stretches over the fitting
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The bung adapter has a shallow seat on the inside and I replaced the hard rubber washer with 2 nestled O rings, for more grip on the bag, the outside is a soft rubber washer, hard washer and nut.
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To seal the ends, there are a few options. Some people use a roll closure, didn't work for me. The setting compound tape works good, but the best closure and cheapest/no waste solution is the impulse sealer.
$15 cheap Chinese POS, but its lasted a long time
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Thanks
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Cool Rig you have there Flem... Love to see someone else using vacuum bagging... It is so useful in many applications... I glue up my forms using a vacuum bag... No screws to worry about being in the wrong spots... A lot of guys would cringe at the fact that you only got 12 psi going at 25 hg... Just goes to show you that you don't need a lot of pressure to make a good bond with epoxy...
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Thanks Shredd. I concur with everything you stated. I'm always looking for stuff to squeeze, cause it works so well.
12psi sounds so wimpy! I prefer to brag that it generates 1750lbs per Sq. ft :goldtooth:
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I think epoxy will bond without pressure.
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That's about 1,100 lb on a 60" bow... Seems enough to me... :thumbsup:
Stic... Don't think it... Know it... Matter of fact the more epoxy you squeeze out the weaker the bond... Talk to any boat builder about that or just look up info on epoxy... I think that there is only so much that you can learn from other craftsman and hearsay and then there is the reality of the situation...
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Really interesting setup. Thanks for sharing !
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144 x 35 = 5,040 lbs.
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A 60" x 1 1/2" bow with 60 psi on it is 5,400 lbs...
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A 60" x 1 1/2" bow with 60 psi on it is 5,400 lbs...
Shredd if you are talking hose forms, then I don’t believe this is completely true. :dunno: Just because the hose has 60psi, doesn’t mean all that pressure is being transferred to the bow. Some of that pressure is used to fill the gap in the form. If it takes say 5-10psi to expand the hose to the form, then you are only applying 50-55psi.
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So I'm not going bore anybody with a glue up. I'll get right to the baggage.
I don't go to much trouble pre-bag. I do use some disposable cauls on the sweeps, just to make sure to get good pressure on the one area that the bag stretches most. Also no need for any tape or strapping to keep things in place, except on the ends to keep things from sliding down.
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Once that done, its into the bag. Helps to cut the port hole first, not much room after the blank is in.
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Heat seal the ends and its time to suck, which can be done one of two ways. You can close the ball valve, pre-charge the tanks and then get a fast draw down. Or what I prefer, the slow draw down. Turn the pump on and while its pulling, make sure everything stays lined up.
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The green material is breather fabric, allows the vacuum to pull along the entire length of the blank. It also sucks up any excess resin. You can buy fancy breather/bleeder fabric, I just use whatever I have laying around that fits the bill.This stuff was some furnace filter material.
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I'll say it again, this method is not superior to any other, I like it because its easy and I'm lazy.
I do think it has two advantages over other methods
1)no need to strap or tape the crap out of the blank, the vacuum pulls the blank together from all sides.
makes it real easy to keep things lined up.
2) For the style of Bows I make, no form needed. For other styles, bottom only. No form= less mass to bring to heat. I preheat to 160deg, after the blank goes in the box it takes about 5min to get back to temp.
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6 pics max per post.
5min after going in the oven
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Baking
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I got 15min pump cycling on this one, thats good for me!
Roy, I think you get 1400lbs per sq.ft with C-clamps. You got too much math there for me :knothead:
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A 60" x 1 1/2" bow with 60 psi on it is 5,400 lbs...
Shredd if you are talking hose forms, then I don’t believe this is completely true. :dunno: Just because the hose has 60psi, doesn’t mean all that pressure is being transferred to the bow. Some of that pressure is used to fill the gap in the form. If it takes say 5-10psi to expand the hose to the form, then you are only applying 50-55psi.
There is very little resistance in the hose to inflate it... Not enough to be worth mentioning when you are estimating 5,000 lbs what is an extra 1-3 lbs for the weight of the hose??? I would think that it would take less than 1psi to inflate a hose... I am no Scientist but whatever area the hose comes in contact with and and your gauge reads 60 psi. you are pretty much putting 60 psi on that area... Thats what psi means, "lbs per sq. inch".
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OK, last installment.
Done cooking, 5hrs@160deg
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Out of the bag, got some clean up to do!
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Looks like the squeezing was successful
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I normally use a peel-ply layer in a addition to the breather fabric. I had run out. I use polyester liner fabric from Joann. Its a lot more work to clean up without it. The peel-ply lets you remove the squeeze out by tearing it off the blank. Spent some time cleaning on this one :knothead:
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Nice dude.
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Came out nice, glue lines look good.
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Very nice looks good !
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That sucked up pretty darn good. :o
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Awesome build, Flem.