Profile of the Dome and bridge.

Construction and repair of Classical Guitar and related instruments
Jasonm
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Profile of the Dome and bridge.

Post by Jasonm » Mon Apr 16, 2018 1:53 am

Im designing the dome profile on my Solera, and I'm wondering how I will shape the underside of the bridge if the dome tapers not just side to side, but also slopes from 2mm at the lower bout to flat just in front of the bridge. The lower harmonic bar would be flat, the top would be pretty flat with the dome mostly behind the bridge. Kind of like Romanillios

I know it's common to have the bridge centered over the lowest portion of the dome, and then shape the underside of the bridge to match. But if there is a slope from back to front as well then I would think that this also would have to be compensated - so that the bridge would not lean forward. Or... maybe I don't need to worry about this?

Stephen Faulk
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Re: Profile of the Dome and bridge.

Post by Stephen Faulk » Mon Apr 16, 2018 3:58 am

Dude...

You can probably figure it out when you fit the bridge to the arch of the top. Might be a good idea to make two or three bridges so if one does not seem satisfactory you try to fit another one.

Turn the bridge upside down in a vice and use a scraper to scrape a concave arch under the bridge. If you want to get help with the fit tape a piece of glassine paper to the top of the arch and rub a #4 B pencil ( remember my neurosis about soft pencils? LOL ) on it and get it covered with graphite. Then place the bridge where is supposed to go and move it back and forth about a mm. lift it up and the graphite on the bridge will indicate where it fits. Scrape that off carefully and do it again.

You can get glassine at the art store. It's smooth, thin and won't allow the graphite to penetrate it. Or you can use chalk. If you try this, you really have to mind the graphite. Wash your hands and remove the glassine very carefully and wash your hands again before you touch the top. Take an eraser and remove all the chalk or graphite from the bridge.

Probably a harrowing experience if you do this on an unfinished top, but I'm ok with doing this on a top that is French polished. I use low tack masking tape to mask off where the bridge will go before I French polish, then feather the edge of the finish after the tape gets pulled. A 3" wide strip of glassine can be taped taut across the top over the bridge and carefully powdered with chalk or graphite. If you are mindful the work will be neat and precise.

If you want further descriptions on 'chalk fitting' the violin making sites and books go into it.
Patience at the bending iron pays in rounded dividends!

Jasonm
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Re: Profile of the Dome and bridge.

Post by Jasonm » Mon Apr 16, 2018 7:42 pm

Maestro Faulk,

I posted here to save you the trouble of answering another question! But, always appreciate your advice. Building 3 bridges will be expected simply because I have done every step of this build at least twice.

I'll check out the glassline paper, never heard of it. I probably would have loaded the top with graphite dust. Last time at the art store I bought HB pencils and now I need a #4B! But hey I'm liking the HB. Hard like diamond.

One more thing. if the dome profile extends past the bridge, tapering to zero near the sound hole, does this then result in the strings not being parallel to the top? Or, if there is not much doming in this area maybe it just gets pulled flat by the string tension?

Thanks for feeding this little bird

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Steve Ganz
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Re: Profile of the Dome and bridge.

Post by Steve Ganz » Mon Apr 16, 2018 8:09 pm

This worked for me.
Step 1. Make the solera.
Step 2 Get a 3" x 9" (play with the dimensions) block of wood
Step 3 Cover Solera with a piece of coarse sandpaper, and sand the block until it mates with the solera
Step 4 Attach medium sandpaper to the convex surface created in step 3
Result: There: a surface against which you can rub the bridge bottom for a final fit of the bridge to the top of the guitar that was formed in the solera.
That surface, combined with scraping the bottom of the bridge, will make a perfect fit of the bridge to the domed top.
Steve

Jasonm
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Re: Profile of the Dome and bridge.

Post by Jasonm » Mon Apr 16, 2018 10:11 pm

Steve Ganz wrote:
Mon Apr 16, 2018 8:09 pm
This worked for me.
Step 1. Make the solera.
Step 2 Get a 3" x 9" (play with the dimensions) block of wood
Step 3 Cover Solera with a piece of coarse sandpaper, and sand the block until it mates with the solera
Step 4 Attach medium sandpaper to the convex surface created in step 3
Result: There: a surface against which you can rub the bridge bottom for a final fit of the bridge to the top of the guitar that was formed in the solera.
That surface, combined with scraping the bottom of the bridge, will make a perfect fit of the bridge to the domed top.
Brilliant

Stephen Faulk
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Re: Profile of the Dome and bridge.

Post by Stephen Faulk » Tue Apr 17, 2018 2:13 am

I have made the arched sanding board too. I seldom use it. One problem with teaching it is that unless it's carefully used it creates a rounded surface under the bridge that has to be doctored up with a scraper. Almost all back and forth sanding operation rock the work. For example if you fit bass or cello bridges there's a jig to hold them vertical while you grind the feet over sandpaper to get a rough fit, but after that you have to go back an do careful fitting by knife work. For me anything sanded needs to be checked with a straight edge and probably fit with a few passes of scraper. Theoretically I think it's good thing to teach.

Not all tops come off the solera and stay and perfect fit to the solera either, they move a tiny bit. I like the arched sanding block to get the basic shape of the top, but move on to fine tune the fit with a scraper, or at least check the underside of the bridge with a straight edge to make sure it its not even slightly convex. I use hide glue on bridges and it's important to to have a smooth gluing surface.
Patience at the bending iron pays in rounded dividends!

Stephen Faulk
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Re: Profile of the Dome and bridge.

Post by Stephen Faulk » Tue Apr 17, 2018 9:15 am

I was being overbearing..sorry, I know what both of you meant. I was having a coffee fingers/brain morning.
Patience at the bending iron pays in rounded dividends!

Jasonm
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Re: Profile of the Dome and bridge.

Post by Jasonm » Tue Apr 17, 2018 5:25 pm

Stephen Faulk wrote:
Tue Apr 17, 2018 2:13 am
I have made the arched sanding board too. I seldom use it. One problem with teaching it is that unless it's carefully used it creates a rounded surface under the bridge that has to be doctored up with a scraper. Almost all back and forth sanding operation rock the work. For example if you fit bass or cello bridges there's a jig to hold them vertical while you grind the feet over sandpaper to get a rough fit, but after that you have to go back an do careful fitting by knife work. For me anything sanded needs to be checked with a straight edge and probably fit with a few passes of scraper. Theoretically I think it's good thing to teach.

Not all tops come off the solera and stay and perfect fit to the solera either, they move a tiny bit. I like the arched sanding block to get the basic shape of the top, but move on to fine tune the fit with a scraper, or at least check the underside of the bridge with a straight edge to make sure it its not even slightly convex. I use hide glue on bridges and it's important to to have a smooth gluing surface.
that is a good point. I have a hard time sanding and applying even pressure and taking too much from the ends.

Jon Gillard
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Re: Profile of the Dome and bridge.

Post by Jon Gillard » Tue Apr 17, 2018 5:32 pm

If you have a drum sander, you can mount your bridge blank to a piece of plywood domed to match your solera, then pass through sander and a matching curve will be sanded into the base of your bridge. Tip courtesy of Robbie O'Brien.
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Steve Ganz
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Re: Profile of the Dome and bridge.

Post by Steve Ganz » Tue Apr 17, 2018 9:14 pm

Jon Gillard wrote:
Tue Apr 17, 2018 5:32 pm
If you have a drum sander, you can mount your bridge blank to a piece of plywood domed to match your solera, then pass through sander and a matching curve will be sanded into the base of your bridge. Tip courtesy of Robbie O'Brien.
In one of his videos? I'm having trouble visualizing how that would work.

He says at the end of this video that he scrapes the bottom.

Steve

Jasonm
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Re: Profile of the Dome and bridge.

Post by Jasonm » Tue Apr 17, 2018 10:30 pm

Steve Ganz wrote:
Tue Apr 17, 2018 9:14 pm
Jon Gillard wrote:
Tue Apr 17, 2018 5:32 pm
If you have a drum sander, you can mount your bridge blank to a piece of plywood domed to match your solera, then pass through sander and a matching curve will be sanded into the base of your bridge. Tip courtesy of Robbie O'Brien.
In one of his videos? I'm having trouble visualizing how that would work.

He says at the end of this video that he scrapes the bottom.

I think he was referring to this video

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geoff-bristol
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Re: Profile of the Dome and bridge.

Post by geoff-bristol » Tue Apr 17, 2018 11:41 pm

On a fan braced top - pressed into a salero dome - the top invariably needs scraping to fair the curve at the bridge position. That leaves it being a pretty unique shape. Make sure - with a light and flexi straight edge you have a smooth even dome there.You will have devil of a job to fit a bridge to a curve with a lump in the middle !

Chalk fitting is easy with rosewood - as white chalk shows so well. See where its touching - and where not.
I use a combiantion on small curved sole thumb planes, scraper and the occasional 'flatting' of the base with a large stiff scraper. Tape the top - masked off for gluing etc - and chalk the top spruce.

Before starting I check with temp strings what saddle height I'm going to need. That determines the bridge thickness I want at the slot. So I have to work the bridge down to nearly the final required thickness, so when it gets there its close but rough. Then chalk fit it pretty close.
I then reduce the bridge wings to their final thickness. When that's done - I do a final fit and scrape.

Dry clamp it with the four bridge clamps and check if I have any gaps developing when clamped up. I often use a small pad under the outer two clamp feet above the bridge - which has two bearing points. One at the bitter end of the wing - one further in. This helps any tendancy for the wing tips to lift when the middle is clamped - and spreads the load of the outer clamp right to the wing tip.

I pin the bridge with two temp 2mm brass pins in the gulley slot - for gluing ( I use hot glue )

Lot to be said for a flat top !

James Frieson
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Re: Profile of the Dome and bridge.

Post by James Frieson » Wed Apr 18, 2018 1:13 pm

The first way I prepared bridge bottoms was , cut the length of the bridge arms , but leave the material on to be crosscut later . Then since there is some spring in the wood , plane the bottom of the bridge hollow . Standard traditional operating procedure . Made a lot like that and never had a bridge come off .
Long time ago I fashioned a bar of aluminium to replicate the shape of the top arch surface , with just a tad extra curve . Drilled a hole in it to receive a heating rod ( electric ) , and just clamped the bridge on it at both ends and turned it on . Worked like 90 and I never had a problem with it , it did a perfect job .
Now I have multiple hand planes of my own fashioning that can plane the recess of the solera or that for the back , matching planes that can plane the braces to fit perfectly , and for the bridge hollow when the wood is too figured for a plane , a scraper plane that also does it .
Mine usually fit just so and I don't have a problem with them but If the bridge is made so that it touches at both ends and makes contact at center with a light push down , that works well .
Sanding blocks shaped well can work quite efficiently and precisely .

Jon Gillard
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Re: Profile of the Dome and bridge.

Post by Jon Gillard » Wed Apr 18, 2018 2:01 pm

Jasonm wrote:
Tue Apr 17, 2018 10:30 pm
[quote="Steve Ganz" post_id=<a href="tel:1272943">1272943</a> time=<a href="tel:1523999673">1523999673</a> user_id=13734]
[quote="Jon Gillard" post_id=<a href="tel:1272877">1272877</a> time=<a href="tel:1523986337">1523986337</a> user_id=87484]
If you have a drum sander, you can mount your bridge blank to a piece of plywood domed to match your solera, then pass through sander and a matching curve will be sanded into the base of your bridge. Tip courtesy of Robbie O'Brien.
In one of his videos? I'm having trouble visualizing how that would work.

He says at the end of this video that he scrapes the bottom.


[/quote]

I think he was referring to this video

[/quote]
Yes, thanks for finding this, at about5.35 you can see this demonstrated, I’ve done a few like this and been pleased with the results, and speed.
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Steve Ganz
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Re: Profile of the Dome and bridge.

Post by Steve Ganz » Wed Apr 18, 2018 7:13 pm

Interesting jig in that second video. Thanks.
Steve

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