Glynne":23ht29wc said:
My question is what is likely to be the maximum thickness I can use as a veneer before I have to start worrying about movement?
I too cut most of my own veneers and this is a question I continue to wrestle with.
To clarify though, when you say "movement" I'm talking about micro-cracking, when the bottom or glued surface of the veneer is prevented from shrinking and expanding across the grain by the substrate, but the top or show surface obviously isn't held rigid. At some point of increasing veneer thickness the top surface of the veneer will start to move differently from the bottom surface, might take a few months but minute little fractures and fissures will open up along the grain and it looks nasty.
With a burr veneer the wild grain pattern means there's rarely much width across the grain, so you can generally get away with a thicker veneer, indeed you normally need a thicker veneer to provide enough structural integrity for the burr to be able to withstand handling.
But for normal, reasonably straight grained veneers this is a real issue. Cutting your own veneers it's tempting to make them very thick to give the option of heavy sanding and re-finishing in the future, but you'll often regret it if you do. American woodworkers often talk about 1/16" as being the maximum thickness, so 1.5mm. John Makepiece ran an exhaustive series of tests with Hamish Low and they concluded 1.2mm was the safe maximum. Many British craftsmen I've spoken to aim for 1.0mm. Having said that I once had a few exceptionally rare and beautiful boards of rippled Oak, I reckoned that being quarter sawn and with Oak's very coarse and open grain I could get away with 2.0 mm veneers, and sure enough ten years later they're as good as the day they were laid. However, I ran my own experiments on some crown cut Pearwood (with very tight grain) and within six months using 1.8 mm there was clear micro-cracking along the grain, but the 1.2mm veneers were unaffected.
Another possibility is "cladding" rather than "veneering", where you lay a veneer on a ground of solid wood but with the grain of the veneer and the ground all running in the same direction. You might do this to eek out some particularly fine boards that you have. Then you can go much thicker with the veneer because the ground and veneer will both shrink and swell together, I'll sometimes clad Oak with something special like Rippled Black Walnut for say the under-carriage and legs of a table for example, where no end grain is visible and I want to stretch my precious stocks of an irreplaceable timber that bit further.
Cascamite is a good choice for vac bag veneering as long as you've got the time to keep the pump running and the bag occupied for 18 to 24 hours. I often use a slightly different UF adhesive (Cascamite is a UF) such as Bordens or F120
http://www.airpress.co.uk/295-f120-urea ... kg1kg.html
These are liquid UF glues and come with a choice of fast or slow hardeners, so I can use a fast hardener with a heated blanket on top of the vac bag and the job's done in a couple of hours.
Incidentally, you can't usually lay a veneer that's straight off the bandsaw
on both faces as the rough saw cuts will telegraph through the veneer and look terrible after a few months. i bandsaw my veneers a few tenths over thickness and bring them to final thickness with a drum sander, but at a pinch you could run a board through a planer thicknesser, bandsaw off a veneer, lay the
smooth face down, then sand the rough face afterwards. The problem with this strategy is that you can't really do book matched veneers this way (and once you get into veneering it won't be long before you'll want to book match).
One final point, with thicker saw cut veneers at say 1.0mm thick, you really should
edge glue the veneers with PVA when jointing them together and before laying them. I know some furniture makers who even edge glue 0.6mm thick commercial veneers. That's optional, but for 1.0mm plus thick veneers edge gluing is mandatory. It sounds hard but it's really not, you shoot the edges as normal, then apply tape to the joint
on the glue side. Open up the joint like a hinge and run a minute bead of PVA to the joint, then close it up, wipe away squeeze out, and apply tape
on the face side. After half an hour remove the tape from the glue side and you're ready to lay the veneer. After just a bit of practise you'll be laying veneers with totally invisible glue lines, which is the standard you should aim for.
Good luck!
