Newbie blooper - rework impact on acoustics

Construction and repair of Classical Guitar and related instruments
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S.H.Tan
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Newbie blooper - rework impact on acoustics

Post by S.H.Tan » Sun Apr 15, 2018 10:08 am

Blooper is the bridge on the right, made before I really understand saddle position compensation for string stiffness. The skew is 2mm from 1st to 6th strings. I then made another with the saddle slot square to the guitar axis and verified that that is ok with the setup in the second picture. An additional 1mm increase to the scale length was noted, and I added another 0.5mm for the sound board flex when under string tension Turn out ok after installation.
IMG_20180310_145714-600x800.jpg
IMG_20180311_182051-600x800.jpg

Now being short of a bridge for my second build, I have to source for another rosewood blank.

While sourcing, I started thinking of the feasibility of reworking the bridge by recutting the slot square to the axis and necessarily wider to about 5mm, then reduce the slot width by shimming and gluing in a similar piece of rosewood. Now my question. Since the bridge is such a critical part of the tone generation, how detrimental is this acoustically, since the saddle slot area is no longer one solid piece?

A thicker saddle will add too much weight. I have also ruled out using the bridge as is, offsetting the skew either by profiling the saddle (not enough compensation especially for G string), or skewing the entire bridge (5mm skew across the entire bridge is noticeable, even with the wings reshaped).

Of course, I should be using pristine parts for any build 😀
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S.H.Tan
http://guitarkumiko.blogspot.com
Yamaha CG151S (1974) (Finish re-applied)
Hashimoto G212 (1978)
Yamaha C310
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prawnheed
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Re: Newbie blooper - rework impact on acoustics

Post by prawnheed » Sun Apr 15, 2018 12:21 pm

Use a different kind of material for the shim, call it a "tonebar" and offer it as a paid extra.

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Brian McCombs
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Re: Newbie blooper - rework impact on acoustics

Post by Brian McCombs » Sun Apr 15, 2018 5:53 pm

You could fill it with a strip, then re-cut a proper channel.
You could widen the channel, as you've surmised,and then use shim stock to fill it.

Whenever you start excavating material to fix a mistake you are taking additional risk of ruining it entirely. There is risk of trimming the front lip a bit thin and that can be real bad news if the grain direction isn't robust.

I'd probably fill it then re-cut the slot, the wood is dark - if your strip is dark it'll likely blend well.

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geoff-bristol
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Re: Newbie blooper - rework impact on acoustics

Post by geoff-bristol » Sun Apr 15, 2018 7:33 pm

I don't really get your problem ? Why 5mm skew on bridge ? The saddle has 2mm slope with the bridge square. I mm slope is fine - I cut my bridges with 1mm slope( or very close to it ) Mount the bridge skewed the other way by 1mm ? Won't see that amount.

I set bridges square @ +1mm on the front edge saddle top e - the slot is sloped about 0.8mm ( so low e is + 1.8mm ) G then notched back to mid saddle - low e back about 1/3 - all the rest leading edge. This is on a 3.2mm thick saddle.
That give a rough intonation of top e +1 - G +3 - low e +2.3 The B, D and A fall about right from the front edge.

Two of my guitars have bridges skewed by 1mm on the top and it does not notice ( they had straight slots )

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S.H.Tan
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Re: Newbie blooper - rework impact on acoustics

Post by S.H.Tan » Sun Apr 15, 2018 11:18 pm

@prawnheed: marketing, turning a bug into a feature 😉
@Brian: I agree, refill first and then cut is a better process
@Geoff: 2mm skew across the 59mm strings increase to 2.6mm across the 80mm slot, and actually to 6mm across the 190mm bridge. Now if I were to position to 1mm skew across the strings, then the 6mm skew across the bridge will half to 3mm, and by reshaping the top and the wings, I might be able to make the skew not obvious. Hmm. Thanks, worth a try. BTW, shouldn't your low e offset be 2.8 (1 offset + 0.8 slope + 1 notch= 2.8)?
S.H.Tan
http://guitarkumiko.blogspot.com
Yamaha CG151S (1974) (Finish re-applied)
Hashimoto G212 (1978)
Yamaha C310
S.H.Tan#001 (2018) SMC
S.H.Tan#002 (2018) SSC

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geoff-bristol
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Re: Newbie blooper - rework impact on acoustics

Post by geoff-bristol » Mon Apr 16, 2018 1:45 am

S.H.Tan wrote:
Sun Apr 15, 2018 11:18 pm
@prawnheed: marketing, turning a bug into a feature 😉
@Brian: I agree, refill first and then cut is a better process
@Geoff: 2mm skew across the 59mm strings increase to 2.6mm across the 80mm slot, and actually to 6mm across the 190mm bridge. Now if I were to position to 1mm skew across the strings, then the 6mm skew across the bridge will half to 3mm, and by reshaping the top and the wings, I might be able to make the skew not obvious. Hmm. Thanks, worth a try. BTW, shouldn't your low e offset be 2.8 (1 offset + 0.8 slope + 1 notch= 2.8)?
Ah - I see where you are getting your 5mm from - the entire bridge length . Yes - 2mm across the 59 is a bit extreme. I was thinking 2mm across the slot ( 80 )
My latest bridge I sloped the saddle 1mm across the slot width ( 80 ) - so when I set the bridge position, I just measure from the 12th 326 e treble - and about 326.8 bass. All I want are those two distances - and the bridge central to board.
I measure form the 12th as I intonate the nut foward by 0.5mm.
That way - most are about right off the front edge - with just the G and Low e to notch back - 'to suit'. I generally make the G middle - and the low e 1/3rd in - on the saddle crown. I have done three like that and have not felt the need to alter any of them afterwards.

As I said - a couple I did with straight slots and skewed the bridge to give me 1mm difference between the two e's ( 60mm on those two ) So - as you say - it more than 1mm on total bridge length, but it did not look odd in any way. For a new guitar though - I would be tempted to make a new one if its bugging you.

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S.H.Tan
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Re: Newbie blooper - rework impact on acoustics

Post by S.H.Tan » Sun May 13, 2018 11:27 am

My rework is to square the bridge top & bottom to the saddle slot, and reshape the wings. Lose a little bit of the wings, but also the weight (down to 17g) which is a good thing.
IMG_20180513_191526-600x800.jpg
IMG_20180513_181732-600x800.jpg
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S.H.Tan
http://guitarkumiko.blogspot.com
Yamaha CG151S (1974) (Finish re-applied)
Hashimoto G212 (1978)
Yamaha C310
S.H.Tan#001 (2018) SMC
S.H.Tan#002 (2018) SSC

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