Double backs

[ScottA]

Hello all,

I'm new to the forum and very happy to find this post.
I'm planning a double back and sides for my next build and this is just what I need.
I have a set of highly figured flat sawn EIR I want to use, I also have a set of Alaskan Yellow Cedar to use for the inside laminate.
I'm thinking .045 for the EIR and .025 AYC, I would appreciate any thoughts on those dimensions, I've never done any doubles before.
Also, I'm thinking either Fish or Hide glue to join the laminates. I would greatly appreciate your thoughts on that as well.

Kind regards,

Scott

[Rienk Ayers]

I'm planning a double back and sides for my next build ... I have a set of highly figured flat sawn EIR I want to use, I also have a set of Alaskan Yellow Cedar to use for the inside laminate. I'm thinking .045 for the EIR and .025 AYC, I would appreciate any thoughts on those dimensions, I've never done any doubles before.

Scott, it sounds like you're intending to do a lamination (two plies glued together) and not a double back.
Just to clarify, I believe a "double back" is like "hollow back" of the McNight style which is two separate back boards, spaced apart, so that the inside back is not in contact with the player, and free to vibrate without said dampening (though I don't know how the air space between the two backs is relieved).

[TonyinNYC]

Hello all,

I'm new to the forum and very happy to find this post.
I'm planning a double back and sides for my next build and this is just what I need.
I have a set of highly figured flat sawn EIR I want to use, I also have a set of Alaskan Yellow Cedar to use for the inside laminate.
I'm thinking .045 for the EIR and .025 AYC, I would appreciate any thoughts on those dimensions, I've never done any doubles before.
Also, I'm thinking either Fish or Hide glue to join the laminates. I would greatly appreciate your thoughts on that as well.

Kind regards,

Scott

I think using hide glue would be rather difficult on such large surfaces unless you do a hammer veneering process. DO a Google search to find out how that is done. I would personally go with regular TiteBond or something similar.

[ken cierp]

Welcome Scott,

Ditto Rienk what said

We have some pretty good info in our adhesive section. Bottom line is not to worry about alleged acoustical properties (there is no evidence) but rather to use a product suited for the task HHG in my view is a poor choice for a lamination since it dries so fast – there are slow dry adhesives designed specifically for this purpose.

[Dave Bagwill]

If you are making a lamination, either West Systems Epoxy or an equivalent; or Weldwood Plastic resin glue, which is used by some good luthiers would be a better choice than Titebond. Epoxy takes the water factor out of the situation; and the resin glue is a known great glue for the application.
Double sides are not gapped; double back are; backs can be laminated though.

BTW I have used Titebond for a double side, and so far no problems have come up. But that is not the same as a lam, where thin veneers are used on the inside, and can cup and warp like crazy with the Titebond.

[Rienk Ayers]

Whatever you do, stay away from using polyurethane glues for laminations.
We used it all the time with composite laminations we did for telecom radomes - because it was the only thing that worked for gluing structural foam to plastics - but PU is essentially a foam, and deadens the joint.

I toured the Gibson plant in Nashville a while back, and I think they used regular wood glue.

This is a bit of a rabbit trail (since the thread is supposed to be about double/hollow backs), but has anyone seen HHG used in an oven type of arrangement for large pieces? In composites, autoclaves are often used to heat activate resins while the parts are in vacuum bags... it would be an interesting experiment to do something similar with wood using HHG.
But then again, if you're doing laminations, it is either for strength, shape, or cost reduction, so any 'perceived' benefits of HHG would probably be a moot point... just thinking out loud...

Now I return you to your regularly broadcast program!

[Ken Hundley]

I am curious, Ken...how would you clamp the inner back here? I would think a gobar deck would be perfect, though you'd have to be careful not to push too hard towards the center. Maybe a clamping caul that fits inside the sides, that basically rests on the edges of the linings?