Cloth press design: rough draft- please critique!

[webboy]

Hi chrismp. Thanks. Good points. Obviously your design would be a lot easier to build, so in the end I might well end up doing basically the same thing. However, with a flatter arch shape, I worry that the forces won't be equalized enough and I'll risk splitting down the long axis. My press needs to be close to 50cm wide so my monster hose wont bulge out the sides.

One thing I wonder about though- since I have the blue boards configured in a true semicircular arch, most of the forces will initially be transferred to the lowermost blue boards, right? (Which in itself defeats the purpose I think!)

Something like this:


On the other hand, since they will flex as the load increases, then some of the load will begin to transfer onto the vertical plywood, but still not all of it. Logically if there were zero flex in the arch, then the plywood wouldn't see any load at all, correct? If my assumptions are correct, then it would seem that most of the force would remain on the arch, and a smaller percentage would end up on the vertical supports- depending completely on the amount of flex in the arch. Am I totally off in space here?

I'd love to be able to calculate this, but it's so far beyond me it's not even funny.

[chrismp]

your hose is another concern i have.
actually i would make the press a little narrower than your flat hose so the hose does bulge out the cordura. this way your wood structure is supported by the hose across the whole press width.
otherwise i think your structure would bend along the hose (the shopvacs had that problem with their double wide press).

have a closer look at our press again. we use five small hoses that exceed the width of the wood frame so the whole frame is supported.

[webboy]

Hi again. I hadn't thought of that. Of course you're right and I agree. Thanks!

[chrismp]

cheers.
seems like i've been the only one to address that issue in my press design. i think it just wasn't that obvious until the double wides popped up.

maybe the other cloth press guys could chime in on that topic?

have a look at this video: http://www.youtube.com/watch?v=a4AMjXalupo
at about 1:17 you can see the top mold bend. it gets better when they move the top supports though.
still they've cut down their press to a single wide later.

[webboy]

Yep, you can really see it. While I agree with you 100%, I've tried to address this sort of flexing with 1) the vertical supports, which I believe will help since they are oriented crosswise and have fairly beefy "feet," and 2) the outer edges won't get pinned by the cordura like they did on shopvac's press since they have a near vertical slope, (which seems like a factor to me anyway.)

So... Hopefully all this (your idea plus mine) means that my design won't much flex along the longitudinal axis.

[webboy]

By the way, I drew an interlocking "spine" for this thing.

[skidesmond]

Great drawing, but I am not sure if the ribs are the way to go.

I am not an engineer by any means, but here are my concerns.

If the ends of the top mold terminate between two of the ribs then the pink layer of plywood may end up taking quite a bit of force at that point. Probably would be o.k. but, it sticks out to me. On the bottom there isn't much mold material above that layer of plywood and it may deform around the ribs.

If the green ribs are not oriented perfectly vertically then I could see them collapsing by getting pushed to the side, although based on the design of interlocking in to the orange pieces that probably wouldn't happen. Each rib though is still going to see about 4,000lbs of pressure. Not sure if that is within the materials capability or not.

Why not just build the molds with rounded tops and bottoms. I think it would be easier and then you wouldn't have any open spaces to worry about compressing.

As I look at your drawing (which again, is awesome) I think I see that you are trying to make it so that you can change out the molds and keep the weight down. Pretty cool design really, I am just not too sure about the ribs. Looking at the other fabric presses none of them have any open space.

Good work and good luck!

I'm not an engineer either.... But I was wondering if it would be stronger if the green rounded tops/bottoms ran the length of the press instead across. It would look like a torsion box with the red/pink plywood on the bottom and the green objects run the length.

There's another post where someone used a number of wood I beams together for the top and bottom, basically making a torsion box. He said it was strong with minimum deflection.

http://www.skibuilders.com/phpBB2/viewt ... 24&start=0

[webboy]

Hi SD- Is this what you meant? What do you think?



You might be right- and this would be way easier to build.

I wonder how the interface between the blue and outer green parts would behave though? I suppose I could leave the tops square and route a matching groove so the tops are more stable- but I can't really visualize it well enough to know if that would be needed.

Again, thanks for the feedback! This is quite the community!

[skidesmond]

In looking at your latest drawing, my original thought was to get rid of the orange blocks and just use the green running the length.

But I think what you have in the latest drawing will work just as well if not better. I would add another green board along the outside on each side.

Perhaps have the outside boards a bit shorter in height than the center boards. Then you would need only 3 blue boards, 1 down the center to cover the green center boards and then 1 blue board on each side. So you'd have a 3 sided blue board top instead of the many multi sided blue top in the pic. Does this make sense? I can try to draw pic if it will help.

What were you thinking of using for material? I think MDf is too week, no long fibers. Plus it will be really heavy if you're going to be assembling the press every time you make skis. OSB plywood or a good grade exterior 5ply plywood will be much stronger.

[webboy]

But I think what you have in the latest drawing will work just as well if not better. I would add another green board along the outside on each side.

Perhaps have the outside boards a bit shorter in height than the center boards. Then you would need only 3 blue boards, 1 down the center to cover the green center boards and then 1 blue board on each side. So you'd have a 3 sided blue board top instead of the many multi sided blue top in the pic. Does this make sense? I can try to draw pic if it will help.

Haha- I was about to start making a drawing and then I realized that I didn't really understand. Maybe you could do a quick and dirty sketch in MS paint or something if you're into it.

To me it seems like you're describing a square cross section, but I'm pretty certain that's not the idea you had in mind.

[skidesmond]

Excuse the lame drawing but it's the best I could do. I don't know how to use Goggle SketchUp so I used MS Word then dopped in Paint so I could save it as a jpg file. (Photobucket won't let you upload non-media type files ). Probably would have been better if I did by hand... Anyway... This is an end view of what I was trying to describe.

Hope it helps

[webboy]

Thanks for the drawing. The way I see your suggestion, it becomes pointless to maintain the height. If I was going to go down that route, I would probably just omit the side boards like this:



Anyway I believe the blue arch configuration is one of the strong points of my design, and it seems to me that it should be strong in reality as well.

[Alex13]

Hey Webboy,

Good to see the progress you're making. I haven't read all posts so excuse me if it has already been addressed (in a bit of a rush atm).

The arc idea is definitely true, you'll direct a lot of the forces through the sides by cutting it the way you have. This is similar to stone arcs in churches etc, it was a closely guarded secret of certain stonemasons for a long time.

To be honest if you did this arc well enough, you could eliminate the centre vertical supports altogether. You'd just need something to keep it from pushing outwards at the base (which the cordura may do itself).

Your idea of supporting along the length would work quite well, much better than my angle proposal (though also heavier, of course).

As far as material choice goes, avoid plywood, it's very weak in that direction, I'd try to find a structural hardwood and cut those pieces with the grain running vertically if you can.

[webboy]

To be honest if you did this arc well enough, you could eliminate the centre vertical supports altogether. You'd just need something to keep it from pushing outwards at the base (which the cordura may do itself).

Interesting- All along I have been worried that the sheet materials would split lengthwise if I did it that way.

For clarity's sake, is this what you're suggesting?

As far as material choice goes, avoid plywood, it's very weak in that direction, I'd try to find a structural hardwood and cut those pieces with the grain running vertically if you can.

OK, no plywood. I've started another drawing (mainly since I decided it should be narrower,) this time using only 45mm (1.77 inch) thick pine for the blue arch and for the vertical stuff. I would think that would be much stronger than plywood, so do you think I would really need to go to hardwood though? (expensive! )



The gaps between the bottom arch boards and the verticals are now about 150mm x 100mm, I was thinking I could add a few more light green ones and bring it closer to 100 x 100, but I have no idea if that would be necessary. The gray thing below is my 12" diameter hose, BTW.

One more question- for the pink and purple sheet material- what would be the best of the following: OSB, plywood, or MDF? In this drawing I have 2x21mm plywood.

Perhaps I ought to convert this thing to a mould and pour the press in concrete.

[webboy]

Another view showing the interlocking "spine."

(I need to get out- been freakin' sitting on the computer all morning.)