MDF, Pine, and Bondo

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fourrings wrote:

You'd probably be OK.
I was thinking more along the length of the stringers.
If you properly glue the skin to the structure, you'll have an amazingly strong and rigid structure. There shouldn't be movement at the splices, because you will have placed crossmembers under them that pick up both sheets. Each area within the table becomes a box girder.
Did you ever think of wrapping the whole thing in plastic laminate? You could use one sheet on the top, running from the bottom of one leg, over the top, to the bottom of the other, and again on the bottom side.
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One more thought...
As a design element, you could leave the top and bottom skins slightly wide, creating two interesting, parallel lines, when the table is viewed from either side.
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That's definitely an idea. Though I will have a control plate flush- mounted in one rib, which will make access less convenient under an over-sized skin.
As for laminate, I figured that's one more specialty product that will require prior experience to apply properly. I figured if I mess up on painting, I can always sand down the troubled spots. This will also allow me to have a seamless surface that I'm looking for.
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fourrings wrote:

Don't forget that holes in the rib will compromise strength. A "doubler" rib forming another box behind the controls will help.
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There will be five 1/4" holes (and 1" apart) in a horizontal arrangement. You think that will be a problem?
I just came back from HD. Found three possibilities for 3/4" plywood: Sandeply, Oak, and Birch. Sandeply is $32 per sheet and others are $40. Which would be ideal for the ribs and cross-sections?
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fourrings wrote:

Probably not. Round holes help the stress travel right by. I was thinking of a square box placed into the rib. In a load carrying part, round holes are better than square holes with round corners, which are better than square holes.

The choice with the highest number of plies and fewest voids. Ideally, you'd get cabinet grade Baltic or Finnish birch plywood. There are versions of this with "paint grade" face veneers that aren't that expensive, but you may need to order it at the Pro Desk.
More plies equals a stiffer rib and more edge wood for the skin to glue up with.
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It's not exactly a box, just a metal plate that will be flush-fit into the "fascia' rib. But only the five 1/4" holes will ever penetrate the load-carrying rib. Good to know about the effects of various hole types though.

Sandeply has seven plies, but the surface is not MDF-smooth unfortunately. I figure that I could fill any voids with Bondo and work the surface with an OS to get it smooth? Provided sandeply is strong enough for this application. If I can get away with $32 ply instead of a $50+ special one without any side-effects, that'd be great. I'll have to call some local plywood distributors to see what I can find though.
(At the rate I mention Bondo, I should be doing auto body work... I just keep reading all the praises of its use on wood. I also expect to fix a lot of mistakes in this project).
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fourrings wrote:

I don't have any experience with Sandeply.
Be aware that most home centers can get cabinet grade plywood and some of it defaults to 5' x 5' sheets. 5x10's are often available, if you need continuous parts longer than 5'.
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Stepping in here, from the other side of the country...
I've used Sandeply, sourced from my local, pro plywood supplier. It's not what you think of when you say baltic birch, or wasn't in my case. I made bookcases out of it for the sister-in-law school teacher, and painted it with latex semi-gloss. It worked, but just barely.
Were I to build something that takes the level of thought that fourrings is putting into this job, I'd go with Finn birch or Appleply from one of the pro suppliers. Spend the extra $25. Stronger. Cleaner. More stable.
YMMV.
Patriarch
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yes, but it's not too bad. there are a few kind of important things to know, but it ain't rocket science. also, because of it's density it makes an especially suitable surface for your application. it will add stiffness and strength to your torsion box.

Laminate is available in some pretty long sizes, and it presents a professional looking finished surface from the start. getting a really true looking surface is a lot about doing the prep work, which is: bondo/sand/test/bondo/sand/etc....
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you'll have to test that hypothesis on YOUR plywood with YOUR screws and YOUR skins. test pretty agressively, too- bang it around some to make sure.
me, I'd probably start with 1/4" crown x 1-1/2" staples, partly because it's what I have and partly because the staples displace so little wood.
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unless the hardwood is laminated or steam bent to the curve. then it would likely be much stronger than a sawn plywood rib.
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snipped-for-privacy@yahoo.com wrote:

Agreed!
I have a stack of ash that will become a toboggan this year to enter this event:
<http://www.camdensnowbowl.com/tobogganNat.cfm
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Thanks for the heads-up, Patriarch. I'll skip on Sandeply. I've called a few suppliers yesterday and gathered that imported Birch and Appleply is hard to come by. And the default 5x5' will not work for me. So if I don't find anything, I might have to settle for HD Birch.
Thanks again for everyone's help! I now have all the info I need to start.
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Many carcases similar to your description are made by constructing a framework of 3/4" plywood. The plywood pieces are cut to the cross- sectional shape you want and then layed out in the desired pattern. Its something like the wings of a balsa wood airplane model. Then the bending wood is applied, giving you the smooth curve. This becomes the substrate to lay up the P-Lam. This method is very strong and should hold your equipment Joe G
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fourrings wrote:

How about bending plywood?
You can get it with many different facings, and it's pre-kerfed to facilitate bends. Check with your local hardwood dealer, as it's usually a special order item. Plywood wholesalers usually stock it, so the special order won't take long to get.
You MIGHT be able to order it from a home center, as they often deal with the local plywood wholesalers, but you'd need to know exactly what you want, as the home center won't.
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