Laminate Gluing Pressure

[Overs Pianos]

>----- Original Message -----
>From: "Farrell" <mfarrel2@tampabay.rr.com>
>To: <pianotech@ptg.org>
>Sent: August 26, 2003 6:50 AM
>Subject: Laminate Gluing Pressure
>
>. . . I am toying with the idea of laminating spruce ribs with Titebond and
>providing clamping pressure with the vacuum bagging technique.
>
>
>At 9:12 AM -0700 26/8/03, Delwin D Fandrich wrote:
>. . . I would not trust the clamping pressures generated by vacuum-bagging
>to adequately form any structural joint having a relatively small contact
>area.

I could understand your doubts if one were to try it with the panel 
on the vacuum table with the ribs being pulled down onto the panel 
with the vacuum bag, but in our case the ribs are fixed to the gluing 
table and the panel is pulled down onto the ribs. This multiplies the 
gluing pressure between the ribs and panel to around six times the 
applied vacuum, depending on the spacing between the ribs.


>I know of at least one piano maker who tried
>vacuum bagging ribs (against my advice) and gave it up after the associated
>problems became overwhelming (which were consistent with my advice).

I'd be interested to hear about the techniques the manufacture used, 
and your reservations about the process.

>Second, the recommendations given by adhesives manufacturers are very
>generalized and are usually based on tests using hardwoods and they assume
>certain shop and material standards and practices. Spruce is not a
>hardwood. Nor do we follow industry practices as we assemble piano
>soundboards. The adhesive problems we encountered with our
>rib-to-soundboard tests when using higher pressures was primarily related
>to "dry" joints. Basically when the clamping pressure got above the
>(approximately) 60 lb (27 kgf) range there was too much glue squeeze-out
>and too much of the adhesive solvent (usually water) was forced into the
>very porous earlywood leaving the adhesive without enough solvent to
>properly cure. We talk of glue "drying" but it is actually a curing
>process. Water is necessary for the glue liqueur to fully cure. An adhesive
>can be dry but not cured--there will be a considerable difference in
>bonding strength. (And let's not bring epoxy into this discussion--it's a
>whole other technology.)
>
>Finally, while this problem showed up in our tests, which were conducted
>with the wood samples dried to 4% to 4.5% MC, it may not be a problem with
>wood samples at some higher and much more reasonable MC. Regardless of the
>clamping pressures, gluing up wood samples at this MC is problematic.
>Within the woodworking industry it is understood and accepted that the
>ideal MC for most wood processing operations--including adhesive
>bonding--is around 7% to 8% MC. The practice of gluing up any wood
>structures at 4% MC would be enough to get most woodworking plant managers
>fired.
>
>Del

Thanks for the post Del.

Ron O.
-- 

OVERS PIANOS - SYDNEY
      Grand Piano Manufacturers
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