Question for Luthiers: thinning top on a finished guitar

Construction and repair of Classical Guitar and related instruments
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attila57
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Re: Question for Luthiers: thinning top on a finished guitar

Post by attila57 » Mon Jul 31, 2017 6:56 pm

SteveL123 wrote:
Thu Jul 27, 2017 5:42 pm
OK, point taken in not using a plane. Wouldn't tape on the corners of the scraper make the scraper sit higher, plus it will wear through with use (if I don't keep an eye on it) then the corners will dig into the top? How about rounding the corners with a grinder instead of tape?
Hi Steve,

I'm not a great expert, but I like tinkering with guitars, too. A lot of good advice has been given, so I just say do what you like, but don't overdo it. A guitar with a slightly thicker top is definitely better than a ruined guitar... If I were you I wouldn't remove the bridge, it sounds to be good as it is with the curve underneath, I wouldn't go under 2.4 mms at the edges, and I'd use a scraper and sandpaper.
You cannot change the tone character with these operations though. A guitar with an unpleasant tone will stay that, although it may become louder. And one with a pleasant tone doesn't really need scraping...
Music to hear, why hear'st thou music sadly?
Sweets with sweets war not, joy delights in joy...

William Shakespeare, Sonnet 8

Paul Micheletti
Amateur luthier
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Location: San Diego, CA

Re: Question for Luthiers: thinning top on a finished guitar

Post by Paul Micheletti » Tue Aug 01, 2017 12:11 am

Hi Steve,

Confessional: The first guitar I made was a pig. I made a classical guitar in a junior college woodworking class and the instructor knew only steel string guitars. He instructed me to make the top thickness the same as other students which resulted in a guitar with a very muted voice. Like yours, it had an overly thick cedar top. I think it was initially well over 3mm thick and not profiled so the entire top was the same thickness. But this build was a long time ago and cannot recall the exact number.

I did not have a hacklinger gauge, so I could not exactly do before/after measurements. I just knew that my guitar was a pig and I wanted to put it on a diet asap. I steamed off the bridge and then took a freshly sharpened block plane to the perimiter of the lower bout on the top, taking down both the top and the binding. The top was stiff out there and the sides supported enough pressure to operate a plane. I'm guessing I took it down almost 1mm around the periphery of the lower bout (leaving the upper bout untouched). I tried a bit of planing in the bridge area, but the top was flexing under the plane since I had already taken down the perimeter. That flexing was not at all comfortable so I switched to a sanding block with 150g paper for thinning the center of the top. I probably only went down 0.5mm in the center.

After refinishing the top and gluing the bridge back on, it sounded much better. Not great, but definitely better. It got me through a couple years of beginning level playing lessons until I built myself a proper guitar.

SteveL123
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Re: Question for Luthiers: thinning top on a finished guitar

Post by SteveL123 » Tue Aug 01, 2017 2:20 am

simonm wrote:
Thu Jul 27, 2017 7:12 pm
When you get to the sanding stage, you may find that the dust is unbearable. Red cedar dust is nasty stuff. I strongly recommend a mask and/or working out of doors. Cedar is also very prone to damage. Just looking at it seems to put dings in it.
Thanks for the warning about cedar. I'll work outdoors and wear a mask with a fan blowing the dust away from me.

SteveL123
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Re: Question for Luthiers: thinning top on a finished guitar

Post by SteveL123 » Tue Aug 01, 2017 2:32 am

attila57 wrote:
Mon Jul 31, 2017 6:56 pm
SteveL123 wrote:
Thu Jul 27, 2017 5:42 pm
OK, point taken in not using a plane. Wouldn't tape on the corners of the scraper make the scraper sit higher, plus it will wear through with use (if I don't keep an eye on it) then the corners will dig into the top? How about rounding the corners with a grinder instead of tape?
Hi Steve,

I'm not a great expert, but I like tinkering with guitars, too. A lot of good advice has been given, so I just say do what you like, but don't overdo it. A guitar with a slightly thicker top is definitely better than a ruined guitar... If I were you I wouldn't remove the bridge, it sounds to be good as it is with the curve underneath, I wouldn't go under 2.4 mms at the edges, and I'd use a scraper and sandpaper.
You cannot change the tone character with these operations though. A guitar with an unpleasant tone will stay that, although it may become louder. And one with a pleasant tone doesn't really need scraping...
Thanks for the advice atilla. I will leave the bridge on so that I can string it up as I go and observe the changes in sound.

You say tone character will not change by thinning the top. What is the explanation behind that? What has to change if I want deeper base and sweeter trebles. Anyway it should be a fun experiment. I paid $100 for this guitar in 1985 from a couple who bought it in Spain on vacation there the previous year. They didn't play and I was lucky to buy it from them. That's how I got into classical guitar (I played electric before that).

SteveL123
Posts: 486
Joined: Thu Feb 09, 2017 5:05 pm

Re: Question for Luthiers: thinning top on a finished guitar

Post by SteveL123 » Tue Aug 01, 2017 2:36 am

Paul Micheletti wrote:
Tue Aug 01, 2017 12:11 am
Hi Steve,

Confessional: The first guitar I made was a pig. I made a classical guitar in a junior college woodworking class and the instructor knew only steel string guitars. He instructed me to make the top thickness the same as other students which resulted in a guitar with a very muted voice. Like yours, it had an overly thick cedar top. I think it was initially well over 3mm thick and not profiled so the entire top was the same thickness. But this build was a long time ago and cannot recall the exact number.

I did not have a hacklinger gauge, so I could not exactly do before/after measurements. I just knew that my guitar was a pig and I wanted to put it on a diet asap. I steamed off the bridge and then took a freshly sharpened block plane to the perimiter of the lower bout on the top, taking down both the top and the binding. The top was stiff out there and the sides supported enough pressure to operate a plane. I'm guessing I took it down almost 1mm around the periphery of the lower bout (leaving the upper bout untouched). I tried a bit of planing in the bridge area, but the top was flexing under the plane since I had already taken down the perimeter. That flexing was not at all comfortable so I switched to a sanding block with 150g paper for thinning the center of the top. I probably only went down 0.5mm in the center.

After refinishing the top and gluing the bridge back on, it sounded much better. Not great, but definitely better. It got me through a couple years of beginning level playing lessons until I built myself a proper guitar.
Paul, thanks for your post from which I learned a few things. I'll be sure to thin the edges last.

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attila57
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Re: Question for Luthiers: thinning top on a finished guitar

Post by attila57 » Tue Aug 01, 2017 8:55 pm

SteveL123 wrote:
Tue Aug 01, 2017 2:32 am

You say tone character will not change by thinning the top. What is the explanation behind that? What has to change if I want deeper base and sweeter trebles. Anyway it should be a fun experiment. I paid $100 for this guitar in 1985 from a couple who bought it in Spain on vacation there the previous year. They didn't play and I was lucky to buy it from them. That's how I got into classical guitar (I played electric before that).
Hi Steve,

If I knew the correct explanation, I'd be José Ramirez V. Just I have scraped a few tops and that's what I have found. By thinning the top at the edges you may gain volume and if you are lucky, you won't lose sustain. Overall tone character, the ratio of frequencies, attack, decay, etc. won't change much. Still, your subjective judgement on the overall tone may improve, because you will hear all frequencies better, even the ones the ear is not so good at hearing.
If you really want to change tone character, you'd better buy another guitar! Tone character depends on lots of things, subtle interactions between the parts of the instrument. The biggest role, in my opinion, has the material itself. E.g. a rosewood back gives a different tone colour from what a cypress back can give. That's true to tops as well. The eventual tone colour depends on a big way on the interaction of these elements.
If you like experimenting, and if you are ready to even sacrifice your instrument, you can try to alter one, or more than one element in the sound production chain. Thus you may stumble upon a more pleasing tone than the present one.
These possible points of alteration are:
- Using different strings (make, hard/normal, etc.)
- Different tunings (some guitars sound bad at certain tunings)
- String tension (low/high)
- Saddle material, shape, etc.
- Top material (spruce/cedar)
- Possible impregnation of wood (see violin making)
- Water content of wood, especially the top
- Indeed, the top thickness and its cross section profile
- Top finish (shellac, lacquer)
- Bracing (type, material, thickness, etc.)
- Material of back and sides (even rosewood grade!)
- Quality of internal surface
- Body shape and size (air volume inside)
- Body finish (varnish, lacquer)
- Quality of external surface (gloss, matte)
- Sound hole(s) placing, size
- Use of nail/flesh for plucking
- Attack angle
- Indeed the whole plucking process
- Holding of the guitar, supporting points, etc.
- Place & distance of the listener.
- Auditorium size, shape, walls (acoustics)
- Subjective elements.
- &c.

As you can see, lots of factors influence the final tone sensation, some less, some more. All of these deserve chapters in textbooks or even whole books to carefully examine. Top thickness is just one of them, and as it is, not so closely linked to tone colour. Some of these elements are not so easily changed, i.e. the back & side material. Some others can be easily altered, like the way you pluck the strings. When you want to change tone character you need to consider these things and how much work you are willing to do, and indeed, how much risk you are willing to take.

Attila
Last edited by attila57 on Tue Aug 01, 2017 9:17 pm, edited 3 times in total.
Music to hear, why hear'st thou music sadly?
Sweets with sweets war not, joy delights in joy...

William Shakespeare, Sonnet 8

Dave M
Posts: 229
Joined: Tue Aug 05, 2014 1:39 pm
Location: Somerset UK

Re: Question for Luthiers: thinning top on a finished guitar

Post by Dave M » Tue Aug 01, 2017 9:03 pm

Paul I had to reply.

My first build was started on a course given by an instructor who really wasn't too energetic. It was a small body size and became even smaller with the difficulties I had bending sides.

Anyway it came out looking and sounding pretty much like a classical guitar,but really not very exciting. I decided it was seriously overbuilt. The first thing was to remove a massive ebony bridge - far too big for the size of the guitar outline. Next I thinned the top - as like you without a means to measure. And then I decided the bracing was also too big for such a small body so got inside with a little thumb plane.

My wrists and knuckles were in a serious state after a while - I have quite big hands and that classical soundhole is seriously small!

Anyway Blondie (it was a Maple back and sides) did improve a lot in volume. The tone remained a bit odd. Not a great guitar but certainly better than the original completed build.

I do think the mass of the bridge is a serious point - being influenced by Trevor Gore. Many CG designs seem to call for very wide bridges which seem too big for the lower bout they are sitting on and must add unecessary weight.

I have tried hard on all subsequent builds to avoid this overbuilding, not always successfully.

I am currently spraying a falcate braced classical with a wonderfully sounding Engelmann top and am hoping for good things
Dave

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