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Stu

Posts: 1843
Joined: Jan. 30 2007
From: London (the South of it), England

Back thickness 

Hi guys,
What's the absolute minimum you'd go on a back?

I had a little mishap when glueing and the back strip lifted slightly and took one of the sides up a little with it.
  REPORT THIS POST AS INAPPROPRIATE |  Date Jul. 6 2020 14:53:29
 
RobF

Posts: 947
Joined: Aug. 24 2017
 

RE: Back thickness (in reply to Stu

I’m not sure I understand the problem from the description, could you post a picture or be a little more specific?

Reducing the back thickness is a bit of a last resort move, maybe there are other options, but hard to know without a clearer understanding of what the problem is. Some separation between the back and sides is fine, as long as the liners are well glued. The binding can cover that. But again, the liners have to be well attached.

If the liners are loose and this just happened then you might be able to soften the glue with heat and moisture and reclamp. Or heat and soften, pry apart a bit and push some fresh glue in and reclamp. Again, not sure how to answer based on the description.

Of course, it could just be that my reading comprehension sucks, lol.

But to address the original question, you might have wiggle room for some reduction, based on the wood and its current thickness. For instance, if the back is Rosewood at 2.4mm then knocking it down to 2.2mm is probably OK. Even 2.0mm, but that could be getting a little lightweight, depending on the wood. It’s going to change the sound, though.
  REPORT THIS POST AS INAPPROPRIATE |  Date Jul. 6 2020 16:49:04
 
Andy Culpepper

Posts: 2923
Joined: Mar. 30 2009
From: NY, USA

RE: Back thickness (in reply to Stu

To me it reads like you were joining the center strip in the back and the two back halves joined unevenly?
For Rosewood I generally have no problem going down to 1.8 mm and Cypress 1.9 or 2.0. You could get away with a bit thinner than that with an extra back brace.

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  REPORT THIS POST AS INAPPROPRIATE |  Date Jul. 6 2020 17:46:20
 
RobF

Posts: 947
Joined: Aug. 24 2017
 

RE: Back thickness (in reply to Andy Culpepper

Ahhh, that’s probably it.

Also, if that’s what’s happened and if there is concern about going too thin, then the back can be split and rejoined. Checking that the joint isn’t too off square and also not putting an extreme amount of lateral clamping pressure can help reduce creep.

Ditto on adding an extra brace if the back feels too floppy.
  REPORT THIS POST AS INAPPROPRIATE |  Date Jul. 6 2020 17:56:59
 
Stu

Posts: 1843
Joined: Jan. 30 2007
From: London (the South of it), England

RE: Back thickness (in reply to RobF

Hey Rob, I don't think it's you. Probably my explanation. I used the words sides. I meant the two plates of my back. Not the actual sides. My terminology is sloppy.

Yes Andy. You deciphered correctly. 😂

I'll see if I have a pic of when I actually glued. Its levelled now so can't take picture now. Could draw a diagram. I'll try.

That's actually slightly irrelevant now I suppose as it's level but might be interesting to discuss.

The back is level enough but due to obviously having to lose some thicknessing leeway..... I still have saw marks on two bits and I'm rather thin already.

It's cypress btw for a flamenco blanca.

Edit: it's too late to split. I ummed and ahhed and judged I be ok... With the Answers I've received I think I might just be.
  REPORT THIS POST AS INAPPROPRIATE |  Date Jul. 6 2020 18:14:02
 
RobF

Posts: 947
Joined: Aug. 24 2017
 

RE: Back thickness (in reply to Stu

quote:

Edit: it's too late to split. I ummed and ahhed and judged I be ok... With the Answers I've received I think I might just be.

Cool. I’m glad that’s sorted.

You’re almost certainly OK, and if it does feel too loose then a fourth brace is a completely acceptable option. I can’t remember who originally said this but it was to something to the tune of ”the difference between East Indian and Brazilian Rosewood is one brace”. Maybe tongue in cheek, but not entirely untrue.

A bit of creep can occasionally occur, so keeping things a touch thicker when joining is a good practice. For the back, joining with the book match facing up can help ensure everything is solid and flush on the good side if the inlay rises. The interior center strap can address any slight imperfections on that side. This implies that the inlay strip is thicker than the plates, of course, otherwise do the opposite, lol, but just don’t do the otherwise.
  REPORT THIS POST AS INAPPROPRIATE |  Date Jul. 6 2020 18:34:34
 
ernandez R

Posts: 351
Joined: Mar. 25 2019
From: Alaska USA

RE: Back thickness (in reply to Stu

Slightly off topic: I know I've mentioned how I built three guitars basically at the same time cutting woods in groups of three and saving the best of each top, sides, etc for the last.

The back I used on the first, my #3 build, was too thin by the time I had removed most of the saw cut marks. I used it anyway. I am also using a non standard "active back" bracing of the lower bought I made up myself mostly by copying a few ideas I had seen or read about when I first started building.

I've had two players now say the prefer the vary thin backed guitar out of the set of three. Last week this one guy says to me he likes how you can feel the guitar.

More often then not I will pick up #3 from my rack of five guitars over the others. It is so responsive but not as bright as her other two sisters. She does have a soul the other two lack.

I just recalled there are some differences in the top bracing as well.

And I didn't use a binding on her ether...

Anyway, I've attributed her unique traits to the too thin back.

HR



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I prefer my flamenco guitar spicy,
doesn't have to be fast,
should have some meat on the bones,
can be raw or well done,
as long as it doesn't sound like it's turning green on an elevator floor.

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  REPORT THIS POST AS INAPPROPRIATE |  Date Jul. 7 2020 1:26:46
 
RobF

Posts: 947
Joined: Aug. 24 2017
 

RE: Back thickness (in reply to ernandez R

I don’t think that’s off topic. I think it’s a positive and reassuring message.
  REPORT THIS POST AS INAPPROPRIATE |  Date Jul. 7 2020 1:57:20
 
estebanana

Posts: 7999
Joined: Oct. 16 2009
 

RE: Back thickness (in reply to Stu

Wonder Woman flies an invisible jet and it’s sheathing over the air frame is super thin, because it’s invisible.

So the deal is make the back thin enough to be invisible, but make the fuselage strong enough to not fold up under the stress of the Pratt-Whitney jet engines.

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https://www.stephenfaulkguitars.com
  REPORT THIS POST AS INAPPROPRIATE |  Date Jul. 7 2020 13:08:26
 
Stu

Posts: 1843
Joined: Jan. 30 2007
From: London (the South of it), England

RE: Back thickness (in reply to ernandez R

Yeah thanks ernandez. That's bang on topic! And interesting. I didn't know one could build without bindings.

Stephen, is the lockdown getting to you??
What a great answer. I think I understand

Ps.I can't have taken any pictures of the miss aligned glueing. Can't seem to find them
  REPORT THIS POST AS INAPPROPRIATE |  Date Jul. 7 2020 14:07:24
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