substitute for MDF

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This question didn't get any takers at rec.running, so I'll post it here in case anyone is interested.
The running deck on my treadmill broke. Dismantling the unit I see that the deck is 3/4" MDF 48" x 27" Thinking that rather than pay 30 bucks for a 4x8 sheet of the stuff that I'll never use for anything else I could construct a replacement deck from the copious amounts of 3/8" plywood and 1/2" plywood I have on hand. I know, the thickness dimension won't be the same, but there is definitely leeway in that. I can adjust the belt tension to compensate for the slight increase in height.
My question/s are such. Is MDF much better for this application, a situation where the deck will encounter someone running on it for an hour or more every other day. That is, is plywood too flexible (too much flex will strain the bearings for the motor)? Is MDF stronger than plywood assuming the dimensions are the same, more durable? What would you do in this situation, just buy the MDF and figure you'll use the rest for something else?
This is a 3 year old treadmill, rather expensive model and cost to have the vendor ship me a replacement board would likely be more than a new 4x8 sheet of MDF.
I already have a prototype using a single 1/2" plywood sheet, it's definitely too flexible to use. So now I'm wondering if laminating it with a 3/8" would stiffen it up enough.
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"Eigenvector" wrote:

How many plys do the 3/8 & 1/2 have?
Lew
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That's an interesting question, hadn't thought of it.
the 3/8ths is 3 ply, the 1/2" is 5 ply
Technically I don't think it's 1/2", 9/16th or similar actually.
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"Eigenvector" wrote:

If you can tolerate the added thickness, two (2) layers of 1/2 glued together will provide 10 ply which would be worth a shot.
By comparison, 3/4 Birch cabinet grade has 13 plys.
Lew
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I do appreciate yours and everyone's responses.
For some reason I'm thinking that 2 3/8th together would be too flexible, but I can potentially cure that by adding an additional cross piece to stiffen the entire assembly. Yes I know, I'm turning what should be a simple matter of calling the vendor and requesting a replacement part into an engineering exercise - but what can I say I might as well have some fun figuring this out.
I'll try the 2 3/8ths and see what happens when I add 3 additional 2x2 cross braces, then I'll have to sand and paint the top surface to smooth it out, as another poster reminded me. If I can get another year out of this treadmill I'll treat myself to a new one next year around tax time - a good treadmill is about 1500 dollars, but it's worth every penny if you use them.
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*snip*
Got a Home Depot around? They sometimes have cabinet grade 3/4" plywood for sale for about $25 a sheet. It's usually the same as their $40 birch, but more defects are allowed (and there's many times repairs made.) Good stuff for what you won't see anyway.
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Where are you? I have a deck you can have if you're close enough to pick it up. Art
"Eigenvector" wrote...

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Why not just glue together a couple of layers of the 3/8" plywood you have so much of?
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After thinking about what you said, I see your point, 2 3/8ths sheets is 3/4". Still the remaining question is whether that would be as strong and/or durable as 3/4 MDF.
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than MDF. That is assuming that the plywood has had all voids repaired. The only good attribute about MDF is that ir is very flat. Jim
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Isn't there some sort of laminate on the top under the belt? The top side of the deck on my treadmill has laminate of some sort. You need a slick, durable surface for the belt to slip on, otherwise the belt will wear quickly. I'd be worried that plain MDF or plywood would wear quickly, and wear the belt quickly too.
You might want to check with www.treadmilldoctor.com. (Click on Doc's parts link) They will fabricate you a deck. Don't know whether it will be cheaper than buying from the manufacturer. I bought a replacement control board for my Spirit from them a few years ago, and had them replace the bearings in the rear roller when they failed. Their service was OK. They also sell a deck reinforcement kit you might want to try to avoid breaking it again.
HTH,
Paul F.
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Eigenvector wrote:

My guess is that two sheets of 3/8" glued together is much more resilient than MDF for a lot of reasons.
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Not the least of which would be that 2 sheets of 3/8" plywood would be less than 3/4" .(unless they're very old sheets of 3/8)
Max
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Have you checked with the manufacturer about warranty? Some expensive treadmill manufacturers warranty their products for 5 years or more.
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Oh I know the vendor will supply me with a replacement, but that takes the fun out of doing the exercise (pun intended).
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But at what price? And shipping won't be cheap. Check out the home store. Some sell mdf in half sheets so you'd have a lot less waste. OTOH, if you know any woodworkers in the neighborhood, they maybe jump at a deal on the excess.
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snipped-for-privacy@yahoo.com says...

No contest. The ply, unless it's totaly crappy quality, is going to be considerably stronger and more durable than mdf.
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It would be 3/4", assuming 3/8" ply was actually 3/8". These days, if you're really lucky, 3/8" plywood is 5/16". And 1/2" ply measures 7/16" on a good day. My vote would be to laminate a 3/8" and a 1/2" together. They should add up to just about 3/4".
todd
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On Sat, 23 Feb 2008 15:27:07 -0800, "Eigenvector"

My wife's Life Fitness 9100 HR uses an oak deck and an automatic waxing system. The deck is similar to hardwood flooring. It has a really nice feel as you walk or run.
This is a commercial unit, top of the line when new, designed for continuous use, as opposed to typical retail model. If MDF were the ideal material, I think the 9100's deck would be MDF.
I know a local guy who repairs gym equipment in commercial settings, and he hooked us up with the 9100 from a local university remodel.
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wrote:

Kinda sounds like overkill, but I bet you'll never break that deck. I'll bet using oak makes it wonderful to run on, very little flex, nice solid feel, good sound dampening.
Really I think the benefit of MDF is that fact that it's smooth and CHEAP. But if I can pull this off with plywood then so much the better. I'm not quite at the point where I'll get oak planks. Smoothing it out will be a bear, that is certainly not plywood's strength.
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