After presenting a few applications i use vacuum for HERE, specifically for holding parts, now it’s time to see some vacuum gluing applications.
The basic setup is pretty simple: a bag of a suitable plastic material, in which you insert the parts to be glued or laminated, usually over a supporting base. Seal the bag and pump air out of it with a vacuum pump, and you’re applying athmospheric pressure to your parts.
And her’s the first caveat: on planet earth with vacuum you have a maximum theoretical pressure available of 1 atm (roughly 1 bar) at sea level; that’s about 1 kg per square centimeter, or 14,7 psi. Definitely not a lot, compared to the pressure you can apply with a regular clamp. The big difference is that it is applied to ALL the surface of the part in the vacuum bag; so for example gluing a rosette, with a surface of almost 70 square cm, you are applying a pressure of more than 60 kg to it. Moreover, since all the air is being pumped out of the system, the glue will occupy it’s place in all the tiny pores and holes in the soundboard and the mosaic. Here is an example:
You can see most of the elements of the setup: the bag, the supporting base for the top, the cedar top itself, paper to protect the bag from the glue, a breather mesh, the air outlet and the pump’s hose; the only things missing in this picture are the closing bar that seals the bag and the vacuum pump. Here you can see all of the elements:
Obviously the vacuum pump (in this case a Venturi system, no motor; runs on compressed air) does not have to be so ugly, but if a prototype works well, why throw it away?!
Compared to the vacuum pump i use for holding, that has to work continuously, the one used for gluing can be turned on (automatically) only when the depression inside the bag goes below a certain level; this way you save a lot of energy and wear.
Another serious limitation is it’s incompatibility with hide glue; this is the glue i use on almost everything! The late Rolf Eichinger, one of my mentors and a very meticulous artisan, always stressed the importance of running tests for onself, in order to avoid bad surprises or perpetuating false myths. In this case, i tried to make hide glue work in vacuum in several ways, including some techniques that supposedly would work, suggested by other makers, but the net result is that the bond is not very strong at best.
So i just use vacuum for gluing in those operations where i would use a different adhesive than hide glue anyways.
In the case of the rosette, i much prefer a yellow glue; hide glue, with it’s shrinking quality that’s usually desirable, adds too much tension to an already delicate region of the soundboard: all the tiny pores in the mosaic and pourflings of a rosette are first impregnated with the glue; if this shrinks, as it’s the case with hide glue, the contraction of the parts is very severe, stressing and deforming the soundboard.
Other glues suitable for use in vacuum are epoxy resin and poliurethane; both have their specialized applications in guitarmaking.
So, to summarize:
- Even distribution of pressure.
- Elimination of air bubbles or pockets.
- Convenience; can glue as many parts simultaneously as fit in the bag.
- Faster and lighter than a ton of clamps.
- Limited maximum pressure available.
- Incompatibility with hide glue.
- Need for a dedicated setup.
Overall i find it a good addition to my workshop and for the parts i use it for it’s definitely better than the equivalent operation done with clamps; yet the use of it is relatively limited especially because the incompatibility with hide glue. The only applications in guitarmaking where i suppose it’s indespensable are some steps of double top construction.
If you’re going to use vacuum both for holding and for gluing parts, then to me it’s clearly worth the investment in time and money.