benchtop problems

[Mike Steinhilper]

My benchtop is 2" of MDF sandwiched b/w 2 3/4" ply. The lamination went well, and I'm starting to attach a skirt around the whole thing. Noticed right away that the end of the new top that has been hanging off my old bench is already sagging. So obviously I'm going to have some problems. The new top is only 19" wide, as the skirt width brings it up to 24". I cannot have an apron for support because it would make the face to thick for clamping to the surface. Any ideas on support? Would it make sense to run a couple metal bars underneath the length and width? I'm surprised and really pissed off that the MDF in the middle is sagging already. I thought the ply would support it, i guess i was wrong. :mad:

Maybe a simpler question would be do you think my skirt/edging would serve as support enough? The skirt is 2-by material glued up to form 2 1/2" wide edge (2 pieces on end). I think that may keep it flat front to back, but the 50" span between legs is my concern.

[michael osadchuk]

.... I would think that the on edge skirt pieces would some SOME rigidity...... but I would think that most of that rigidity gain would be from the long side skirt pieces on the end to end rigidity of the benchtop; I would not think that the edge skirts on the two short ends of bench would not have much effect on the back to front rigidity of the bench beyond "a few inches" from each end...

.... I have virtually no experience with how well MDF takes glue but the two edges of the plywood won't make a strong glue joint; perhaps an expoxy that has good gap filling properties; others, I'm sure have better hands-on experience on making a strong joint here

..... people have used right angle iron (eg. as in iron bed rails supporting box springs) to keep the top of router platforms flat; what people do is loosely bolt the right angle iron to the bottom of the platform every x inches and then insert washers between the bottom of the platform and the flat part of the iron in those areas where the platform is sagging and tighten up the bolts to shim up these areas..... one right angle iron running lengthwise under under the your benchtop might provide futher ridigity and perhaps even some ability to correct a sag...... bolting into 3/4' ply and then 2" of mdf might pose an issue getting a solid grip for the bolts (threaded inserts?)

good luck

michael

[Doug Shepard]

I take it you dont yet have it sitting on the legs? I'm thinking that this might not be sag so much as the way it was clamped/glued and things might change slightly once it's mounted on the legs. Anyway you can attach it to the legs and recheck it before you do anything?

[Mike Steinhilper]

Doug, I hope you are correct. Michael, I wonder if the angle iron is rigid enough along a 7' length to make a difference. I guess I can give it a shot.

[Mike Cutler]

Mike

I think you're going to be fine once you get the top placed on the bench proper. That's a very heavy top you made there.
Both of my benches use MDF for tops. One is 2 layers, 96"x36", screwed together like a subfloor, and the MDF overhangs the front and back by about 4" along the 96" length to facilitate clamping. The other MDF bench is two full sheets made into a torsion box. It was a little "potato chippy" until it got installed on the base also.

[Greg Cole]

Without seeing, it's hard to tell... but I'm inclined to agree with the "other" Mike here. You should be able to get it flat when you mount it to the base.... providing the base has enough support to keep from sagging again.
I'd caution against trustinggeneric angle iron to be truly flat along any length too.
FWIW, I used epoxy to laminate the MDF top for an assembly/finishing table I'm working on now. In some non scientific testing or gluing some scrap pieces at 90* angles letting the adhesive cure & then break'em apart, epoxy holds MDF better that any glues I had at the time of testing (TB1, 2 & 3, Gorilla Glue and construction adhesive), the construction adhesive was 2nd.... then again thats what the screws etc are for.:rolleyes:

Greg

[Mike Steinhilper]

Ok, that is encouraging.... .however, remember there are no aprons (other than the pine skirt around the edges), so do you think that middle span will be ok? Mike, what sort of base do you have?

[Matt Ranum]

Well here is my experience with my MDF top. I made it 30x60, 3 layers deep with 1/4 tempered hardboard on the top and sides. I have it on 4x4 legs and the legs are flush in front and back and are 36" apart with the balance overhanging the ends.

After about a year it settled noticeably in both directions. I still have it up after about 5 years or so and it didn't get anyworse after a while but it did reach about 3/16" dip in the center. When my new one gets done this one will stacked in the lumber pile upside down and I hope it will straighten back out again. Its a great solid top and I love it but I will put it on a full cabinet base next time.