Wide glue-ups, cupping. Back bracing needed

[dirk martin]

I'm making some wide glue ups for a customer in glued up maple and also some in cherry.
Finished size of 18" x 22", and 1/4" thick.

Some are cupping, after final sanding.
My customer is fine with "back bracing", to attempt to keep them flat.

Do ya'll have some suggestions on what type of back bracing I could use? Would strips of MDF work? Glue them on?

[John TenEyck]

Why do you need such thin panels, what's the application? Knowing that would help.

John

[Kent A Bathurst]

Why do you need such thin panels, what's the application? Knowing that would help.

John

Yeah - and how/in what will they be installed? It its something drawer bottoms, I should think that glue-up followed by install right off the clamps would take care of it.

[Steve Jenkins]

One thing that will make panels cup is leaving them laying flat and uncovered. The top of the panel will change moisture content with changes in humidity. Either cover them or prop them on edge so air can circulate all around them. They may flatten out if you set them on edge for couple days or may not. The problem with gluing a brace cross grain on the back is just that. Cross grain.i would expect the panel to split at some time in the not too distant future due to expansion.

[Peter Quinn]

What your making there is called a problem. Wide solid panels don't like to be 1/4" thick, they always cup, at least until they crack to relieve the stress. It's why people use plywood. Regardless of what customer wants....it doesn't typically work, and you have the pudding to prove it! From here, I'd clamp the. Together in stacks between stuff cauls until they are ready to ship. What are they for? Can't advise on strongbacks without more info. Honestly they should be milled much thinner and glued to a piece of plywood!

[dirk martin]

Why do you need such thin panels, what's the application? Knowing that would help.

John


They are being engraved, with laser art.

[dirk martin]

.... Honestly they should be milled much thinner and glued to a piece of plywood!

Except with plywood, you get unsightly edges, and edgebanding isn't desired.

[Prashun Patel]

I assume you are cupping along the width. Gluing bracing cross grain will not allow panel movement with the seasons.

Your back brace would need to allow lateral expansion. I'm struggling to see how any fasteners will have enough purchase on the 1/4" panel to be very effective in keeping it flat, unless the fasteners go through the whole panel.

Is your customer averse to a frame and panel? That will keep the panel flat while allowing expansion inside the slots.

I suppose you could also make breadboard ends that are thicker than 1/4". Even so, you'd have really skinny splines.

[dirk martin]

I assume you are cupping along the width. Gluing bracing cross grain will not allow panel movement with the seasons.

Your back brace would need to allow lateral expansion. I'm struggling to see how any fasteners will have enough purchase on the 1/4" panel to be very effective in keeping it flat, unless the fasteners go through the whole panel.

Is your customer averse to a frame and panel? That will keep the panel flat while allowing expansion inside the slots.

I suppose you could also make breadboard ends that are thicker than 1/4". Even so, you'd have really skinny splines.


Help me with your mis-type of the word "purchase". What did you mean to write?

[Prashun Patel]

I'm using 'purchase' to mean 'grab' or 'bite' or 'penetration'.

[ryan paulsen]

Help me with your mis-type of the word "purchase". What did you mean to write?

Purchase, in this instance means to "grab." In other words, 1/4" is too thin to try to screw a brace onto from the back without coming through the front.

EDIT: oops, looks like I was too slow...

[John TenEyck]

They are being engraved, with laser art.

OK, then I would take a piece of 1/8" plywood, edge it all around with your wood, and then veneer a 1/16" thick layer of your wood on both sides. That will give you a stable 1/4" panel. The other choice is to make a frame and panel, as already mentioned.

John

[Ted Reischl]

An obvious question here is:

Is the panel(s) getting engraved and then used full size? Or is the engraver cutting pieces out of sheet and needs custom sizes to maximize yield on the laser? If it is being used full size I would make up the panels, put them on a mini pallet and strap them down to keep them flat during shipping. What happens to them after that is the customers problem. Sorta like what happens if one buys a pallet of plywood and does not store it properly. The folks who made it are not responsible.

[scott vroom]

How about if the OP used a cross grain brace but only used a dot of adhesive at 3 points: both ends and one in the middle. This would hold the panel flat but would allow for fewer stresses from wood movement. Yes/no?

[Brian Tymchak]

How about if the OP used a cross grain brace but only used a dot of adhesive at 3 points: both ends and one in the middle. This would hold the panel flat but would allow for fewer stresses from wood movement. Yes/no?

No. It would still crack, or maybe just warp since we're talking ~1/16" growth/shrinkage on each side of the tack in the middle. You can tack in the middle, but the ends have to move as the panel shrinks and grows. In fact, the ends will move more relatively than the middle since the whole panel will grow and shrink. Typically, on a thicker panel, we might run a screw into it with an elongated hole in the cross brace so that as the panel moves, the screw will move in its hole a little bit. On a thin panel, we're going to have to get creative. Like maybe guide blocks glued to the thin panel that ride in slots in the cross braces. Or something. Haven't done it before so I'm just blue-skying here...

I vote for the frame and panel approach here that someone mentioned earlier. I recommended it on another recent post where the OP was trying to frame a large wooden panel, oddly enough for a piece of laser art. Hmm, I smell a trend here..:rolleyes:

[dirk martin]

An obvious question here is:

Is the panel(s) getting engraved and then used full size? Or is the engraver cutting pieces out of sheet and needs custom sizes to maximize yield on the laser? If it is being used full size I would make up the panels, put them on a mini pallet and strap them down to keep them flat during shipping. What happens to them after that is the customers problem. Sorta like what happens if one buys a pallet of plywood and does not store it properly. The folks who made it are not responsible.

Sorry, but that's not the way I do business.
I guarantee all my wood. If they can't use it, then I'll replace it, or give a full refund.

[Peter Quinn]

Funny thing about customers is they often paint you and them into a corner. They don't want edge bands, they don't want frames, they don't want cupping......I'd suggest stone or corian. My last boss wouldn't do panels skinnier than 5/8" in solid, for flat panels and solid drawer bottoms we ran the raise to the inside, bottom or back. I guess plywood may be out for your client for engraving, unless it's a thin engraving and a thick veneer? Perhaps bread board ends of sorts might keep those panels flat or close at this point if that's acceptable.

[scott vroom]

Yeah - and how/in what will they be installed?

I have the same question: What is done with the panels after engraving?

[dirk martin]

I have the same question: What is done with the panels after engraving?

They are hung on the wall....unframed.

[dirk martin]

.... Perhaps bread board ends of sorts might keep those panels flat or close at this point if that's acceptable.

What's a "bread board end" ?

[John Schweikert]

What's a "bread board end" ?

The google: https://www.google.com/search?q=breadboard+end&es_sm=91&source=lnms&tbm=isch&sa=X&ei=PWIkVcHvEMaTsAX8zYHABw&ved=0CAgQ_AUoAg&biw=1777&bih=963

[Robert LaPlaca]

Dirk, I hate to a buzz kill, but a bread board end isnt going to be practical in a 1/4" panel.. I would say 3/4" is low end of practical limit for bread board joint

[Ted Reischl]

Sorry, but that's not the way I do business.
I guarantee all my wood. If they can't use it, then I'll replace it, or give a full refund.

Wonder how come my hardwood supplier does not have that policy? Maybe that is why they have been in business for 60 years.

[Allan Speers]

I'm making some wide glue ups for a customer in glued up maple and also some in cherry.
Finished size of 18" x 22", and 1/4" thick.

Some are cupping, after final sanding.
My customer is fine with "back bracing", to attempt to keep them flat.

Do ya'll have some suggestions on what type of back bracing I could use? Would strips of MDF work? Glue them on?

What's the application for the panels? Will the backs be visible?

IMO, if it's for the back of cabinets, which will always be against a wall, I'd use square-tube steel, or maybe aluminum if weight is a concern. The metal supports can be much thinner than even the hardest, most stable wood.

MDF? Yuck. I'll never understand the fascination with that junk for shop jigs & such. In my experience, it ALWAYS warps badly over time.

If it will be visible, and thus must be wood, I'd use anything with high resistance to bending (hard maple, maybe) and screw it in with little longitudinal slots, so it can move slightly against the screws.

------------------

EDIT:

OK, I just read the whole thread. I see what you're doing.

Heck, why not just use a rectangle of plastic, and laminate a thin face of veneer onto it?

[Jerry Miner]

This is not going to sound very helpful, but....

If you are doing "business" involving making objects of wood, you really should get an understanding of the properties of the material before you get yourself in too much trouble.

A 1/4 x 18 x 22 inch panel is practically GUARANTEED to cup. Especially flat grain. It's wood.

You didn't say whether you used quarter-sawn or flat-sawn material, but QS is much more stable. But still, you're asking for trouble, IMHO.

I do have one suggestion you might try: take something fairly stiff like aluminum channel (or you COULD try a couple strips of MDF) and mount to the back of your panel with mounting tape (the double-stick tape with a foam core)---it might give enough flex to allow wood movement and enough grip to hold your panel flat.

I know you don't want to hear it, but a veneered mdf panel (cabinet-grade 1/4 plywood is typically mdf core) would stay flatter, engrave well, and not show much of anything on the edges but a neutral tan mdf edge. Better material for the application--- again, MHO.

[dirk martin]

What's the application for the panels? Will the backs be visible?

IMO, if it's for the back of cabinets, which will always be against a wall, I'd use square-tube steel, or maybe aluminum if weight is a concern. The metal supports can be much thinner than even the hardest, most stable wood.
.....

How would you fasten that to the 1/4" panel?

[dirk martin]

....

I know you don't want to hear it, but a veneered mdf panel (cabinet-grade 1/4 plywood is typically mdf core) would stay flatter, engrave well, and not show much of anything on the edges but a neutral tan mdf edge. Better material for the application--- again, MHO.

I might try gluing a panel to some MDF, to see what it looks like. I'll try Titebond. Will I need to glue something on the backside of the MDF, to equalize the pressure? Or does that only pertain to veneer glues, and not Titebond?

[Allan Speers]

How would you fasten that to the 1/4" panel?

I'd use some kind of adhesive that dries flexible. Maybe silicone caulk?

Heck, you could probably use any glue: With 1/4" stock, you probably don't have to worry too much about movement, especially across the narrow dimension, which I assume is how you would brace them. In other words, As long as you use enough bracing, that bracing would probably stop the cross-grain movement without it splitting. It would depend some on the species of wood, of course.

- But a flexible adhesive would certainly be safer.

[Kent A Bathurst]

Sorry for the negative waves, but 1/4" solid wood panels, with backers glued to them........I don't see how this can possibly turn out well.

Depth of laser cut? Possible to have veneer on top of a panel? Is there any "give" in the 1/4" dimension?

[Jerry Miner]

No no no. Gluing your 1/4 solid wood panel (which will expand and contract with humidity changes) to a piece of mdf (which will not, appreciably) is encouraging cupping. It may lie flat at first, but with seasonal humidity changes---you have those where you are, right?--- the two materials will fight each other and you will have a problem.

I'm suggesting a thin (< 1/16"---commercial veneer is more like 1/40") veneer on each side (this is how plywood is made, and for good reasons)-- for a balanced construction and enough "weakness" in the veneer to keep it from pulling too hard on the core material.

[Peter Quinn]

I was thinking the ends of a bread board approach could be 3/4". So you plow a 1/4" groove, sort of like a frame and panel with no stiles, creates a wooded "scroll" like visual, which may be an improvement over the Pringles chip type visual.

[Britt Lifsey]

Since you are considering a backer that is fairly thick anyway, you could re-make the panel thick enough to avoid the cupping, 3/4", and then cut steep bevels on the back to give the illusion of a thin panel from the front/sides.

[Jim Matthews]

A sliding dovetail would serve. Instead of routing the recess in the thin panel, apply three cleat pairs.

http://www.woodworkersjournal.com/wp-content/uploads/sliding-dovetail-5.jpg

Glue on three sets of paired cleats such that the "North" and "South" poles are on the same strip.

Cut them at an angle, like the sides of a dovetail joint. Make a mating batten to fit in the recess
to span the width ('East to West') of the panel.

Glue the batten to the center strip and cleat pair to fix it in place.

Wax the extremities of the batten so it can move with the Seasons.

[J.R. Rutter]

I would go thicker (solid) or thinner (veneer). 1/4" is in the danger zone.

[Kent A Bathurst]

I would go thicker (solid) or thinner (veneer). 1/4" is in the danger zone.

JR - you hit it - good evaluation. KInda like "you can't get there from here"

[Prashun Patel]

Duh. Sometimes we can't see the forest for the trees. Nice point.

[Raphael Weil]

Hey guys,

I'm facing a similar issue and was warned today about my board dimensions, wanted to see what you thought with regards to that.

I'm working with 4/4 cherry that's 12 inches wide. I guess I get it close to 3/4" thick after I plane it. I was warned today that engraving on a board 3/4" thick was going to expose me to cupping. The boards are otherwise going to be 12" by 15" or 12" by 17". I thought about back bracing them, and the suggestion I got was simply to use oversized holes for screwing in a back brace. The oversized holes would allow for some lateral movement but still prevent cupping.

Am I worrying for nothing about 3/4" panels cupping? They'd be in people's living rooms, and I could warn against exposing to humidity. If there is a risk, what do you think of the back brace idea?