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The newest Luthier Tips du Jour instructional video is on Youtube. The topic is the double top. Subtitles are available in both English and Portuguese and you can translate them into any other language by using the CC option on Youtube.
As always, comments and discussion are welcome.
Enjoy!

Superb video as usual. Thank you for your efforts and continuing promotion of guitar building on the cutting edge.

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"Try to be the person your dog thinks you are."

http://sites.google.com/site/bertsguitarshop/

Robbie,

Thanks for these on the Double Top. I've been studying this every since Charles Fox showed us his double top that he uses in the Ergo Guitars when I took his SS building course. It is very impressive and fascinating. One day when I have a lot of extra time (HA!) I plan to try my hand at it. The vids as usual were superbly done BTW!

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Bill Hodge


One does not simply, own enough guitars!

Fantastic. Thanks!

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It's not what you don't know that hurts you, it's what you do know that's wrong.

Excellent tutorial!!!!

I am a bit surprised at how much epxoy is used with nomex. From the video, I gather that with all skins glued it comes to somewhere at 35 grams, is that correct?

I would also like to know how much the finished top weights (after final thicknessing), and how much Alan's traditional tops weight on average for comparison.

Thanks Robbie!

Pat

Thanks everyone. I am glad you enjoyed it. Some folks have asked about Alan doing a masterclass on the subject. I will try and set it up.

I just finished the sound box on my first double top and it's very responsive. I used the routing method of inlaying the Nomex and the pockets are only in the lower bout. I love the video but now I'd love to see how you brace it

I made the bracing very light on my classical double top because once the top was glued up it almost felt like it was braced.

wow!
thanks robbie!

Your usual great work, you are making us all a lot smarter.

Fred

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Fred Tellier
http://www.fetellierguitars.com
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Howdy Gang, sorry to come in late on this, I was off fishing when Robbie put it up

Pat, notice that although I mix up 35-40 grams of glue most of it is left on the palette. Only a little bit of it on the legs of the Nomex is actually used. But there are a lot of those little legs in there. Run the video again and I believe Robbie discusses the actual weights we came up with as we went along.

In general I'm finding that the double tops are about 1/3 less weight than my standard tops. That is on average, for a lighter wood like Engelmann it isn't as much as for the difference with a nice dense Adirondack. But that is a good ball park. Also there is no "final thicknessing" as such. The face skin is only 0.060" so you can do some clean up of dings and such but no perimeter sanding like with a solid top. I use the same plastic film I used on the palette to cover the completed top to protect it until finishing, that avoids lots of dings and other uglyness. It also is great when taping in the the binding, no pull-up of the top wood at all.

Another question that came up was about binding/purfling. Nothing special needed, I just do it the same way as with a solid top even though I don't have any hard parts around the rim. I just glue in with a syringe of Tighbond and just enough to hold it in place. I use a fly-cutter with two angled blades and bottom-bearings. I cut the purfling slot first, then follow with the binding cut. If you don't get aggressive it cuts clean and doesn't pull out the Nomex or anything like that.

Alan D.

Right you are on the bracing. For SS guitars I use an open X about 95-97 degrees and a single symmetric tone bar. No finger braces. All are 1/4" wide and the X is only about 5/8" tall at the center, all are scalloped. You can go to my web site
http://dunwellguitar.com
and click on the Luthier Pages button to see more in the guitar and bouzouki building pages there.

Enjoy,
Alan D.

Thanks, Alan!

Thanks Robbie! That was so good I feel like even I could make a double top.

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"An adventure is only an inconvenience rightly considered. An inconvenience is an adventure wrongly considered." G. K. Chesterton.

Alan said:



I am basing this amount on the actual difference measured between weighting the components (128 gr) and the 148 gr with the epoxy, for a difference of 17 gr. Since you have to do that twice (for gluing the other layer on), you get about 35 gr total. Is that correct?

The reason I am surprised is that my version of the double top uses balsa wood instead of nomex. Since the epoxy (I use T-88 also) has to cover the entire surface (unlike the nomex, which is full of holes), then more epoxy is required. After weighting the top components and checking the final weight with the epoxy, I get a difference of 27 to 30 gr, which is slightly less than what you have on the video (since I make classical guitars, the top is a little smaller so I figure in the end the amounts are pretty much the same).

Any thoughts on that?

Many thanks,

Pat

When using PU glue, do you use the same method for applying it to the nomex?

I've always had PU glue foam up on me quite significantly - does it not do that when using the minute amounts required for a double top?

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Pat, you have the numbers right I believe. The 15-18g is typical for the way I do it. With my small body Serenade it is closer to 13-15 but the demo was for a full sized dread. In your case, once your layers of wood are clamped there will be some squeeze out, etc so the actual film of glue left is pretty thin. Whereas with the Nomex each little leg gets a fillet of glue at each interface. So you have these little beads of glue at each leg and that adds up. That is why it is important to limit the amount of glue rolled out so the fillets/beads are as small as is reasonable and still hold. I did test strips when I was first figuring out what was the minimum amount of glue per/in-sq and when the glue weight got down too low the fillet almost disappeared. With those the Nomex ripped off when I ran the strips through the drum sander. THAT was exciting!! Nomex and spruce shrapnel all over!

A test I have not tried is to roll out a very thin layer PU glue directly to the wood and vac clamp the Nomex to that. PU foams a bit and maybe it would get up on the Nomex and then run back down and form the fillets. This is just a gedanken experiment, I have no data to justify this so if you go that route do extensive testing first. We all need to be VERY careful with putting out double tops that are sound and stable products, it is fundamental that we do not want them to get an undeserved bad reputation in any way. Nuff said.

Also, I have only used the T-88 epoxy glue, it works so well I have not experimented beyond that. I have not used PU glue because of the foaming issue. I want each cell to be an open empty space, not partly filled with foamed glue even if it is light weight. Again, I have just a gut feeling on that and I have not tested it at all. Perhaps a PU filled cell structure actually sounds better, I don't know. The PU glue does have the advantage of faster drying time but I just plan my glue-ups well ahead for overnight set times. With a vac system that has a tank and doesn't leak the pump doesn't run much.

Alan D.

Alan-
Thanks for the video and your follow-up comments.

Can you give some details (name, source) about the (pink?) plastic film you use on your glue palette and for protecting the top?
It looks like a very useful product for the shop.

Thanks
John

Howdy John,
That was some of the "detail" that got cut from the final edition. Robbie did an amazing job of editing all the footage and information and getting the gist of the process all crammed into just two videos! Amazing job, Robbie. But a lot of detail of necessity got left out.

The Pink and plain 4" wide rolls are 3-M film products. I got them as samples to test after talking to the film division rep., the trick is to get the film with some "Post-It" level sticky on it so it can be pulled up later w/o ripping up the wood. The two numbers are
396 392 6062
335 4243 1311
You can see photos of the labels on my web site at http://dunwellguitar.com and click on the Luthier Pages button. Then go to the Bouzouki building link and Page #1 about 1/2 way down.

All that being said, I don't use that any more. (heh, heh) All this stuff is a moving target and I'm constantly making changes. While we were shooting the video Robbie mentioned a similar sheet of stuff used for protecting carpets for workmen. Turns out Home Despot has it in 24" and 36" wide rolls, 50' long for ~$15, which works better for the palette and the top widths. Similar/same level of sticky on it. I tested some onto some top off-cuts and left it on for a month. It peeled up just fine without leaving behind any goo or ripping up the wood. I have put the photos and data for the carpet protector film into the same page just below the discussion of the 3-M films. If you have been to this page before you may need to do a re-load from your browser to get the new page and not your cached page.

Another benefit of this film is that I leave it on right up until the finishing schedule. When I'm binding I just cut it with the router along with the top wood. Then when taping in the binding while gluing the tape really holds onto it and it doesn't let the tape pull up any slivers when you remove it. Very nice.

Alan D.

That's a great tip for all of us. It would be particularly useful making Cedar topped guitars, for some protection against dings.

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Waddy

Photobucket Build Album Library

Sound Clips of most of my guitars

On No!!
Now I have to do another tips du jour on using carpet protecting film in lutherie.

Alan-
Great info- thanks a lot. Hopefully the HD store here (Canada) will have the same stuff. If it will hold binding tape, that could eliminate a lot of problems as well..

Cheers
John

I bought some yesterday at Home Despot. Reasonably inexpensive.

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Waddy

Photobucket Build Album Library

Sound Clips of most of my guitars