Bent Lamination Questions

[Tony Falotico]

I'm going to make a form for bent laminations for a bow front nightstand. The nightstand will be 19" wide and I'm thinking of 1" bow. I plan to use 5 ply's of 1/8" poplar with one ply 1/8" maple on the outside face.

My question is about springback.......... Do I make the bending form with the bow I want or do I make it with extra bow to account for springback ?? If I need extra bow, is there a rule of thumb for how much extra ??

I'll need three pieces for each unit, about a 2" front rail, a 2" cornice, and a 5" drawer front. I'm figuring on making two 6" wide bent lam's and cutting the three pieces from them.

[Charlie Plesums]

There was a formula for estimating the springback in Fine Woodworking a few years ago, but I couldn't find it. Bottom line, if you have just a couple fairly thick layers, count on springback. If you have lots of thin layers, the springback is negligable.

An important factor is the glue...
don't use PVA (white or yellow) because of creep
don't use Polyurethane (Gorilla) because it pushes laminations apart
Best is urea formaldehyde - such as DAP/Weldwood Plastic Resin - the glues often used for veneering.

[Alex Berkovsky]

Tony,
Here is some info from the May issue of American Woodworker on bent wood laminations that may be helpful.


Lamination ...........Ply
Radius ...............Thickness
------------------------------
2-4" ..................3/32"
4-8" ..................1/8"
8-12" .................3/16"
> 12" .................1/4"

Note: Hard or stiff wood such as maple may need to be cut thinner than
soft woods such as pine.

As far as springback, they say:

"You can never predict exactly how much springback will occur. But you
can compensate for springback in your form design with this formula.

Rfinal is the radius you want for your bent lamination; (n) is the
number of plies; Rform is the radius of your jig.

Formula:

Rfinal(n-1)/n=Rform

Example: A 5" radius, 10 ply lamination

5"(10-1)/10 = 45/10 = 4.5"

[Doug Shepard]

What Charlie said. With urea glue, 1/8" plies, and the shallow 1" bow I kind of doubt you'd get any springback at all. Is making a 10" glue lam for each unit feasible? At least that way if you do get any miniscule springback all the pieces for each unit would be consistent and easier to match up.

[Tony Falotico]

Is making a 10" glue lam for each unit feasible?

That would be great, I could get matching grain patterns on each unit......
BUT........ Unless the Band Saw Fairy makes a delivery, I'm limited to 7" resaw capacity. A new (real) BS is next on the list, but other obligations keep getting in the way.

[Jamie Buxton]

I'm guessing you don't have a vacuum press. When you're using mechanical clamps, one tip I learned the hard way is to use cauls which run the 6" direction on your glue-up. Put a clamp at each end of the cauls. To see the reason for this, try bending a laminate around your form by putting just one clamp in the middle of each end. Notice that the corners of the laminate pull away from the form, even though you have the middles of the ends clamped firmly to the form.

[Doug Shepard]

That would be great, I could get matching grain patterns on each unit......
BUT........ Unless the Band Saw Fairy makes a delivery, I'm limited to 7" resaw capacity. A new (real) BS is next on the list, but other obligations keep getting in the way.

Sounds like a darn good excuse for buying new tools.
... Honey, I'm sorry. I just cant make those nightstands you wanted so desperately unless I get a 16" bandsaw...
... Well sure - I guess we could return those plane tickets to go visit your family if you really think I should order one...
:D :D

Also - all the bent lams I've ever done have used both male & female forms with the lam stack sandwiched in between. Jamie's reply sounds like real good advice if you're just using a single form.

[Richard Wolf]

Tony, with a small bend and 5 ply's you should not experience any springback. But on the safe side, bend the door first and build the cabinet to fit.

Richard

[Tony Falotico]

Tony, with a small bend and 5 ply's you should not experience any springback. But on the safe side, bend the door first and build the cabinet to fit. Richard

Thats what I plan to do Richard, make the bent lam's first and cut the rest of the cabinet based on the lam's............

Doug, I agree whole heartedly, and LOML has approved the BS purchase.... Unfortunately, she needs a new vehicle, hers is pretty high mileage, getting old, the A/C is out and I just can't bring myself to putting $800 to $1200 into getting the A/C rebuilt. It's served us well, we got our money's worth out of it, and It pains to think of sinking $$ into it when I know it will soon be a money pit. Soooo, I guess some of the tool budget needs to go towards a vehicle if I can ever get past the sticker shock. I don't know that I'm cheap, but we try to be careful when spending our money, We are looking at retirement in the next 10 to 12 years ......... and don't want to retire broke ....

[M. A. Espinoza]

Thats what I plan to do Richard, make the bent lam's first and cut the rest of the cabinet based on the lam's............



Springback should be negligible if you let them set up overnight before pulling out of the forms.

One ply of maple shouldn't be a problem, but you might consider soaking the maple in hot water and prebending it if you want to be extra sure. Kind of like steam bending, but you will need to let the maple dry some before gluing up.

I would just try it without the pre-bend, that is a pretty minimal curve.

Good luck.

[Jamie Buxton]

Formula:

Rfinal(n-1)/n=Rform

Example: A 5" radius, 10 ply lamination

5"(10-1)/10 = 45/10 = 4.5"

Alex, I haven't seen that particular article, but usually that formula has a squared term in it. The result is that a seven-ply lamination is supposed to have something like a 1% springback. A ten-ply lamination would be much much less than that.

That said, my experience is that the formula isn't very accurate. It might be a nice starting point for an estimate, but it isn't good enough to design with. Like the other posters say, the best strategy is to build a form and see what you get. Maybe is will be close enough to what you wanted that you can go ahead and build the rest of the furniture with it. If not, build a new form and try again.

[jim chambers]

Tony,

The formula I use is the height of the bend divided by the number of plys squared equals the springback at each end. In your case it would equal less than 1/32" at each end. I have found it to be pretty accurate, especially when using epoxy instead of water based stuff. So as most everyone here agrees, you will have very little springback on such a shallow bend.

As Jamie mentioned, with 7" wide stock you should use cauls with clamps top and bottoms, and you may want to leave your form a little low so you can spring clamp the plys themselves together between the cauls on the exposed edge of your drawer front if it is not to be painted.

I prefer to bend into a form when possible rather than around one, and in your case I wouldn’t bother with male and female forms.
I don’t know what you mean by "cornice", but I presume from your plan that all the radii are identical. Flush (or "inset" depending on your location) drawers the same radius as the face frame, and the same for the cornice.

The last bending job I did, I took a bunch of pics and made an article out of it. Here’s a link to the bending section if you are interested.

http://www.miterclamp.com/radius/pages/Bendingforms.htm (http://www.miterclamp.com/radius/pages/Bendingforms.htm)

Good luck and have fun.
Regards,
jimc

[John Mihich]

I've used that formula for several projects and it's very accurate. Unless you are using several very thin laminates you will get some spring back albeit a little the thinner the pieces. I have used poly and yellow glue with similar results.

[Martin Lutz]

use a rigid glue like UniBond 800, urea formaldehyde. PVA's (yellow and white) will remain flexible and may continue to move with time.

[Tony Falotico]

Best is urea formaldehyde - such as DAP/Weldwood Plastic Resin - the glues often used for veneering.

Charlie, I got the DAP / Weldwood Plastic Resin like you recommended..... The formaldehyde part worries me. What is the stink factor and how long will it linger in the shop ?? Or do I need to do this outdoors ??

I remember when my sister bought a brand new mobile home (25 years ago) the formaldahyde smell was so bad I couldn't stand being in it. Took several weeks for it to finally air out......

[Jamie Buxton]

All the walls in that mobile home were full of formaldehyde-laden foam. That was a lot of formaldehyde. For your little lamination job, you'll be using only a few tablespoons of glue, if that much. That's lots lots less formaldehyde.

I do veneering and bent lamination with UF glue on much larger pieces, and am not bothered by it. Put it this way.... Pop open a half-empty gallon can of latex paint, and you'll get a whiff of some chemical brew. That stinks worse, IMO, than the UF glue you'll be using.

[Charlie Plesums]

Charlie, I got the DAP / Weldwood Plastic Resin like you recommended..... The formaldehyde part worries me. What is the stink factor and how long will it linger in the shop ?? Or do I need to do this outdoors ??

I remember when my sister bought a brand new mobile home (25 years ago) the formaldahyde smell was so bad I couldn't stand being in it. Took several weeks for it to finally air out......
The data on that glue is at www.dap.com/product_details.aspx?product_id=42 (http://www.dap.com/product_details.aspx?product_id=42) - I get it from a fellow woodworker who buys in bulk, so I don't have the label other than what is on the web. The instructions say a slight formaldehyde odor, but I use it indoors and don't remember any odor.

The amount of formaldehyde in glue and insulation years ago was huge... this is more like the glue used in plywood these days.

[Tony Falotico]

Well, I got one put together...... slight change in plans, this is for a leg on a table. It is 11 3/4" radius inside, thirteen 1/8" laminations (maple and black walnut) for a thickness of 1-5/8". Spring back was, to my amazement, 0, nada, absolutely none!! It's got several problems to it so it is not going to be usable, but at least I know I can do it. Lessons learned --


make your forms user and clamp friendly. I tried to do too much at once, didn't plan for good clamping angles
mix enough glue the first time


I will re-design (re-build) my form to be easier to use and clamp. I need to cut the outside form into pieces and not try to bend the entire thing at once. I will get my glue consistency correct the first time, it makes for easier application. Also, I'm going to review the design, maybe cut back to 7, 9 or 11 layers for a slimmer look.

Bottom line, I'm excited -- I know I can do it successfully, and I can't wait to get started again------

[Charlie Plesums]

... It is 11 3/4" radius inside, thirteen 1/8" laminations (maple and black walnut) for a thickness of 1-5/8". Spring back was, to my amazement, 0, nada, absolutely none!!...Also, I'm going to review the design, maybe cut back to 7, 9 or 11 layers for a slimmer look.
The more laminations, the less the springback. 13 laminations I would expect none. 3 laminations, expect some. So be prepared for possibly a little springback if you do, for example, 7 laminations.


...I need to cut the outside form into pieces and not try to bend the entire thing at once. ...
I don't even use an outside form. Clamp the center to the form, then work out from the center, one clamp at a time, pulling the laminations into the form. If you still want to use pieces of your outside form as you do this, fine, but if "not try to bend the entire thing at once" means some today, more tomorrow, forget it.

[Tony Falotico]

Clamp the center to the form, then work out from the center, one clamp at a time, pulling the laminations into the form. If you still want to use pieces of your outside form as you do this, fine,

That's exactly what I plan to do, cut up and use pieces of my outside form and clamp one piece at a time working my way from the center out. I don't know that I have enough good clamps not to use the outside form piece.


but if "not try to bend the entire thing at once" means some today, more tomorrow, forget it.

No, No, that's not what I meant :) :)

Charlie, I really appreciate your comments, help and advise........... please keep 'em coming, Tony.

[Tony Falotico]

These bent lam's are fun....... making legs for a shelving unit, wanted a 2-1/2" bow centered in a 68" chord -- computes to a 240 ft radius. Five ply's, run the formula gives me a 192 ft radius for my form. Calculated offsets for 192 ft radius, comes to 3-13/64" bow in center. So I rounded to 3-1/4", re-computed my offsets every 6" and made my form accordingly and glued up some legs. Let 'em set up, pulled from the form and .. WOW !! 2-9/16" bow in the center.

Had I not rounded I could have been dead on !! Moral of the story ... The equation very accurate !!