I just drew up our press design in proe. I think it should be pretty solid. Our biggest concern is that the two ibeams on top are a slightly different thicknesses the one is .25" the other .375" so the one might difflect differently and cause uneven pressure. What do guys think?
If the I-Beam webs are proportionally that large and they are steel, it will weigh a ton, but they aren't going anywhere with a mere 50 psi. I bet they would be solid at twice that.
I recommend using grade 5 bolts on any bolted connections.
I have an ibeam press very similar in design to the one you picture above. My beams are w10x49, so they are very heavy, but I was worried about underbuilding the thing. I press at 60psi in the hoses. I noticed very little deflection of the beams upwards, although I haven't measured to see how much there is. I have however noticed that the pairs of beams will separate from each other by as much as 5-7mm in the middle. There is a lot of pressure there, so in the end I am glad that I used such heavy beams.
It sounds like you must be getting a deal on the beams if you are considering this option, which is good because they are f#*king expensive. In your picture it looks as if the beams on the top are the same and the beams on the bottom are the same. If you are using a pair of identical beams for the top and another pair of beams for the bottom I wouldn't be worried. If you are mismatching your pairs (ie using two different beams side by side) then they will deflect differently, although probably not that much because it sounds like you are using heavy beams. I would watch the seperation in the middle because that seems to be the spot with the most visual movement. Perhaps a support holding the two beams together would also help?
Are your lower beams really as small in relation to the top beams as shown in your drawing?
Keep in mind that the load on the lower beams is identical to the upper beams so the lower beams will deflect much more than the top. Unless your mold is really strong then the deflection of your lower beams will tend to flatten out your camber. If possible you should try and get 4 similar beams for the frame unless you think the lower ones are strong enough as they are. If you tell me the pressure you will work at and the exact size of the lower beams I can check the deflection and stress for you.
Tony
My preference would be to design the top and bottom beam sections eseentially the same as they are both going to see identical forces. Your bottom T sections look like they are going to deflect a lot more than the top I beams. Depending on sizes this may be a moot point.
I also like the idea of tying the beams together to avoid the deflection differences you will see with the 2 different web and flange thicknesses.
We would love to have everything be unifrom but were working with free steel we got here at school and what we got is what we find. I like the idea of tying the top two beams together to try and even things out. The T's we we're planning on making from one big ibeam 16x7X.5 making them 8X7X.5 or about that. There is one more beam we could use which is quite substantially larger but I don't think we could move it to get it into the shop to cut it.
I checked your lower beams with 50psi and assumed that your frame was 6ft long and each lower beam was loaded up by a fire hose pressure vessel about 6" wide, you will get 2mm or a bit less than 1/8" deflection however the stress in your lower beam is 27000psi and the yield stress of steel is about 36000psi. The reason for the high stress is the long thin vertical members of the lower frames which concentrate the tensile stress on the lower end of the beam so don't exceed 50psi.
Thanks for letting us know. I've been meaning to do some analysis on it but hadn't got around to it. I think we're gonna try and find some more steel for that bottom section. If we're going to make a press that big and heavy we want it to be able to withstand a lot of pressure.
I keep wondering why everyone goes for so much pressure, most marine type epoxies caution about too much clamp pressure and I now know that it is not a matter of forming the ski since it is completely pressed to the mold at as low as 0.6bar or about 10psi. I get plenty of squeeze out of the glue and the skis are light so I know I am not overloading with epoxy. Any good or bad experiences with lower pressure would be interesting.
maybe it's overkill to go with more pressure than that...I don't really know what kinda pressure is needed cause this is our first attempt at making skis. We just figured we wanted to make it able withstand 100psi so we could experiment with different pressures.
looks a LOT like my press, which I am getting very close to testing. I will post when I do it and we'll see if any big problem arrise.
so is there anyone that has actually used a vacuum pump to press the skis? if so where do you find the bags to seal it with and what kinda pump did you use?
i haven't used a vacuum setup to make skis yet, but have been using it for various other things like sidewalls and mold construction. I'm liking the vacuum more and more as I use it and am going to use it for skis soon.
I'm using a vacuum generator and bag from https://www.qualityvak.com/index.html and it is the venturi type. Electric ones work just as well and are nice if you don't own a big air compressor. I have a 2hp compressor and it is not really up to the task.
Here is a site that describes how to build one:
http://www.joewoodworker.com/veneering/welcome.htm
I've got a giant bag that fits the mold and everything, but I wouldn't recommend that since its a pain to wrestle the mold into the bag. A better way is to get vacuum film and seal it to the top of your mold. You can get the vacuum film, breather material and other stuff from http://www.fiberlay.com/
-kelvin
Vaccum is a very common pressing system in working with composites. It is standard in aircraft industrie and as my experience with compsoites comes from building aircrafts it was naturally to use it for my first ski.
As i use a relatively small mold (1900x200mm) i put it into a vaccum bag. The pump i use is a laboratory model from KNF (810) :
http://www.knf.co.uk/products/index.htm
(It is the one on the lowest picture.)
In my opinion vaccum is the simplest an safest way to create a relatively high pressure (-0,8bar means 8000kg/m2 pressure!).
thanks guys so does any one have any pics? so the bag would go over the bottom ski shaper and the mold?
Here is a pic of a quick test that I did. The entire mold goes into the bag. You don't need a top mold for a vacuum press
another pic of vacuum pressing the sidewalls.
-kelvin
I use vacuum as well, we have a pump in the lab ans it is seldom used.
This is serial production...
A friend of mine made two pairs of skis without using neither the vacuum pressing nor the press.
He has used several clamps. I had the feeling it could be possible, but I have easy acces to the pump, so I never pushed the clamp thing further. I have to aks more details.
This is a good option if somebody does not have a compressor, a pump or if he does not want to spend time and money to build a press.
so where do you get those big bags? plus is it kinda hard to get it into the bag when it is all being epoxyed togeather? i have seen it done with just with like half a bag and taped to the top of the mold would this work better or worse? thanks guys for all your help!!!!
I think that most here have seen the page on graf about using a refrigerator compressor to use vacuum bagging, though it's a little spare on the details.
I found this page which has slightly more detail. But an old Scientific American Amateur Scientist article shows how to turn a refrigerator compressor into a vacuum pump that'll generate and hold a pressure of about 1-2 mmHg, or basically -1 atm relative. So that's 20% better than just using the compressor alone and getting -0.8 atm with very little effort/expense. But maybe it's not really worth it, just throwing this out there.
(warning ramble: SciAm used to be so cool in the 60's when this article was published. Telling you how to make things like a 2Mev linear particle accelerator, a nuclear magnetic resonance spectrometer, or how to measure the temperture in a cow's stomach. Now it sucks, which is sad. Ok, ramble done.)
davide - Is that cardboard in your pic? Are you using it as a breather fabric or just above the breather fabric?
Hi in there. first I'd like to thank you for your site and wish you a very good year from france.I'm half french and finnish skier and wood carpenter and I wanted to ask you if a vacuum bag and vacuum pump from the retailer roarockit.com would be suitable and enough powerful to give shape to a telemark ski.
best regards
samuli
Yes, it is card board used as breath fabric.
I have also this special fabric, but most of the time I use cardboard. Actually I have the feeling now that the breath fabric allow a better pressure distribution. And it is possible to use the breath fabric several time if it is protected from the epoxy with a plastic sheet.
I have pressed four sets of skis using a vacuum pump. It accually works, but it took me 3 pairs to accually get a ski that I could use. I get my vacuum pump from United Rentals. Just ask them for a the vacuum pump off of a concrete coring machine. You can get the bags at www.fibreglast.com, It is also a good site to buy fiberglass and epoxy.
Don't know if you folks have seen this.
http://www.joewoodworker.com/veneering/welcome.htm
I plan to build the V2 venturi pump soon. They have a kit with all of the parts except the PVC pipe and end caps. The price is right and all available in one place.
My parts came in for my vacuum press Friday and I was able to put it together Saturday. It hasnt snowed in a week or two so staying home and working on this was a good thing. Im using the Project V2 set up with my 20 gal compressor that I already have. It took me a couple of hours to prep everything, and another hour to assemble. I still have to get a couple of end caps and make it a four chambered vacuum reservoir.
I was able to draw 20 in. HG easily at 4500 ft above sea level. It would go to 21 in HG with a bit of time. Thats .7 bar or ~10.29 PSI. As near as I can tell I have no leaks and was able to hold the vacuum for over an hour.
Ill be running some shake down tests this week on a couple of small projects, and get used to bagging my layups.
Hafte
i've never made a pair of skis before but i have repared snowboards before, i need some help on designing a skipress, getting snowcad2 to work and all the other things that go with making skis, thanxs alot
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