Replies

1. 
   JBD | Jan 14, 2004 06:17pm | #1

   I assume you mean a curve of varying radii or an inside curve somewhere.  A compound curve is 2 dimensional like a sphere, otherwise you are talking about a 1D curve which is cylindrical.

   Backing veneer on panels are necessary mostly to counteract the warping caused by shrinkage of the glue during cure of the face veneer, and secondary is the function of sealing.  Two reasons why you don't need a backer veneer, just seal with finish:  One, is that you are attaching at points on a curve - if expansion/contraction occurs it won't  be noticeable.   Second, if you use the glue I use in all my bent laminations, you won't have the shrinkage to counteract - Polyurethane glue.  I'd seal the back for sure though with Poly or something.

   As far as glue, bent laminations seem to be perfectly suited for low viscosity polyurethane glues like Gorilla Glue.  Unless you use a vac press (and risk deforming), you will always have some kind of gappage somewhere.  The expansion of PUR glues are just the ticket.  I've never had a failure.  They don't spring back either.  They don't introduce moisture to the wood either like PVA's.  Longer working time - essential for bent laminations.

   As far as thickness is concerned,  It all depends on your radii.  A 3mm ply should be great, but if "tortured" or applied over a non-faired substrate, it will telegraph the imperfections more the 6mm.  6mm will "smooth over" the high/low spots better.

   Ever consider bending plywood?  Pro shops call it "wigglewood".  Its basically inside out plywood.  The core veneer is like 1/32" (long way on 4 x 8 sheet), and its faced with 3/16" Lauan on either side, grain going accross the width.  This stuff is great.  It'll take a very uniform radius down to like 6".   Double layer this stuff and glue, and very little clamp pressure required. 

   Are you using band clamps?  Is this an inside radius?

    

   "The furniture designer is an architect." - Maurice DuFrenes (French Art Deco furniture designer, contemporary of Ruhlmann)

   http://www.pbase.com/dr_dichro

1. 
   CCaughlan | Jan 14, 2004 11:30pm | #3

   Thanks Doc for your comments.

   What I meant by a compound curve is one with inside and outside radii, like a french curve. The smallest radius is about 1 mtr. I don't see the 9mm MDF being "tortured"" if I run kerfs on the back side. I haven't seen any of the "wigglewood" you described here in Mexico City but I'll make a point of looking for it.

   Your point on the PUR glue is well-taken. I notice Titebond offers a PU.R

   I was thinking of using cauls when applying the veneer. You've a better suggestion?

2. 
   JohnWW | Jan 14, 2004 11:25pm | #2

   A word to the wise:

   I've seen a lot of people get in trouble on their first few veneering jobs, and the job you described is especially risky because of its size and visibility in a public place.  Presumably you are going to be trying to make money on it, or at least not lose any. 

   The learning curve on veneer work is fairly steep, this is a big job to tackle on your first time out.  The questions you are asking suggest to me that you are getting way ahead of yourself.

   You should do a lot of research and be very conservative in designing and executing this job.  Asking a few questions in a forum like Knots won't give you nearly enough knowledge to make a project like this come out well.

   John W.

   
   

   ---

   Edited 1/14/2004 3:41:06 PM ET by JohnW

3. 
   eddiefromAustralia | Jan 14, 2004 11:50pm | #4

   I echo John and John's comments,

   When you kerf the 9mm MDF, you're looking at an (actual) thickness of about 5mm remaining in the MDF.

   Attaching a 3mm veneer to one side of this will not produce a stable job.  The MDF will move with moisture changes.  The timber will move with moisture changes at a different rate.  There's not enough stiffness in the veneer or the MDF to avoid this assembly from buckling.  You can't back the MDF because you've kerfed it.

   I support John B's suggestion - go to a local specialty panel supplier and get a sheet of 6-10 mm bending ply (wigglewood). Get them to glue a 0.7mm veneer onto both sides, giving you a show maple face and a cheaper hidden backing face.

   The supplier will give you minimum bending radii.  Confirm it will do what you ask with regards to bend radius.

   If you don't have a panel saw, get the supplier to cut to final size too - supporting bending ply while you cut it is like supporting soggy spaghetti.

   Hope that this helps and sorry to rain on the parade.  Just that I see a recipe for disaster in your original suggestion, in my opinion.  Probably end up far cheaper in the long run too.

   Cheers,

   eddie

1. 
   JBD | Jan 15, 2004 01:01am | #5

   Please allow me to clear up some confusion I see.   I believe the "veneer" in question is not veneer, but rather thin veneer plywood, as the spec was 3mm (1/8") or 6mm (1/4"), and I or any cabinetmaker would assume ply.   

   That being the case, expansion/buckling will not be an issue as Eddie was concerned with, and the ability of an inexperienced ww'r also will not be an issue, as John W was concerned with, because thin ply skins are much easier to do than real veneer (less than 1mm) due to its ability to "bridge" well, and air pockets won't be noticed if there are screwups. 

   However, with the inside AND outside radii, coupled with the lack of longitudinal stringers, this may be more difficult than first realized. 

   Will there be curved ply stringers (top edge, two middle and bottom within 28" ht) in addition to the furring on the steel?  You will definitely need them if the vertical strips are more than 8" o.c. (100 mm on center).  

   Then you will need to create a simple clamping form for the inside radius to avoid flat spots.  Outside curves won't require a form, just glue a cleat to the end of the kerfed MDF and use clamps to "pull" the MDF tight against the stringers n strips. 

   Do the MDF first, then later do the outer ply face.  Glue the inside radius first, then when the glue is set up, you can "pull" the outside radii.  Might have a little trouble getting the glue in later for the outside radius glueup, but its possible. 

   "The furniture designer is an architect." - Maurice DuFrenes (French Art Deco furniture designer, contemporary of Ruhlmann)

   http://www.pbase.com/dr_dichro

1. 
   JohnWW | Jan 15, 2004 01:38am | #6

   Dr. D,

   Rereading the original question, I see that your interpretation is probably correct.  I read the part about a back veneer without registering that 3mm and 6mm were far to thick to be true veneers for the front face, so my warnings don't apply.

   John W.

1. 
   eddiefromAustralia | Jan 15, 2004 02:17am | #7

   ditto - I read 3mm veneer -  (btw - I am a cabinetmaker - cultural differences at play here  - thought that we were referring to a resawn maple flitch)

   eddie

   Edited 1/14/2004 6:21:42 PM ET by eddie (aust)

4. 
   migraine | Jan 15, 2004 02:48am | #8

   Why not make/laminate the  the convex/concave face first.  Fill all imperfections and sand smooth.  The wiggle wood is a great idea.  Then, finish the front of the panel with a real veneer faced plastic laminate(as in formica) and contact cement.  Make sure you apply a consistant/even coat of adhesive on both sides to prevent telegraphing of the adhesive.  You can also mask off the ends of the veneer/panel and aplly titebond to the last 1" or so and then cold press this with various types of spring clamps/bar clamps

   My feeling on the appling the veneer directly to the wiggle first and then applying  the completed part to the frame might not work.  The veneers applied to the wigglewod create a 5-ply panel and make it alot more rigid. 

   Investing in a vacuum press system would be my recommendation

1. 
   CCaughlan | Jan 15, 2004 09:20pm | #9

   Thanks everyone. There are some good thoughts here. I stand corrected in referring to the 6mm ply as veneer, but FWIW Ernest Joyce in his Ency. of Woodworking, mentions thickest veneer being #8 at 0.160"/4.064mm.

   When I finish I'll post some pics and comments.

   Craig Caughlan

1. 
   Norse | Jan 16, 2004 08:12pm | #10

   You might want to check out pliable MDF but it sounds like your curve is fairly gentle.

   http://www.outwater.com/mdfboard.html

   Also, the SanPly's are very stable and flexible.

   http://www.jacaranda.com/products.html#sanply3

   Good luck,

   Norse