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Bridge too short after taking top off

 
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KenN
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Joined: 27 Mar 2007
Posts: 89
Location: Goodrich, MI

PostPosted: Sun Feb 24, 2008 12:41 pm    Post subject: Bridge too short after taking top off Reply with quote

Last fall I took the top off the only intrument I've finished so far to make the bass bar smaller. The plan called for a huge bass bar and I decided to trim it back some to see the effects. The other day I finally got around to glueing it back on. I was surprised to find the strings don't clear the fingerboard now. I checked some of the measurements, the nut is still about even with the bottom of the plate, the elevation at the fingerboard is about 22mm and the bridge is about 31mm. The back and belly both seem to be about the same height, as they were before. What do you think happened?
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Leif Luscombe
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Joined: 19 Mar 2007
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Location: Mount Elgin, Ontario, Canada

PostPosted: Sun Feb 24, 2008 1:25 pm    Post subject: Reply with quote

22 at the fingerboard is a bit high, IMO. What was the height before? This is the usual place that would account for the difference; one must be careful when re-gluing the top plate not to affect the neck projection.
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Andres Sender
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Joined: 23 Mar 2007
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PostPosted: Sun Feb 24, 2008 4:45 pm    Post subject: Reply with quote

Sounds like you may have inadvertently executed a "neck pull-up".
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KenN
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Joined: 27 Mar 2007
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Location: Goodrich, MI

PostPosted: Sun Feb 24, 2008 6:22 pm    Post subject: Reply with quote

I honestly don't remember what it was. It isn't a dimension that is given that much, mostly elevation at the bridge. I checked on the Bergonzi model I have glued up and it's elevation at the fb is 22mm too. But it will work fine with a 30-31mm bridge because it has an entirely different long arch than the Stainer model, and the fb is only 159mm long as well.
Andres, what's a neck pull up?
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violinarius
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Joined: 14 Dec 2007
Posts: 171

PostPosted: Sun Feb 24, 2008 10:45 pm    Post subject: Reply with quote

Do you know what the projection was at the bridge before the top was removed, and what it is now.

The projection at the end of the fingerboard, 22 mm now, can change a very small amount, and due to the angles become a very big amount at the bridge, where it matters most.


Last edited by violinarius on Mon Feb 25, 2008 11:24 pm; edited 1 time in total
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KenN
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Joined: 27 Mar 2007
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Location: Goodrich, MI

PostPosted: Mon Feb 25, 2008 7:41 pm    Post subject: Reply with quote

I didn't really check that. I had the bridge at about 30-31 and the string angle at about 158 degrees. What everything else was I don't know.
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violinarius
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PostPosted: Mon Feb 25, 2008 11:25 pm    Post subject: Reply with quote

What is your projection at the bridge now?
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KenN
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Location: Goodrich, MI

PostPosted: Tue Feb 26, 2008 7:39 pm    Post subject: Reply with quote

Now it is 29mm. The g string has maybe .5mm clearance, but the e string rubs the fingerboard. The projection was probably about 25-26mm so it moved 3-4 mm so there goes the clearance.
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violinarius
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PostPosted: Tue Feb 26, 2008 10:28 pm    Post subject: Reply with quote

"Last fall I took the top off the only intrument I've finished so far to make the bass bar smaller." - KenN
---------------------
"Sounds like you may have inadvertently executed a "neck pull-up"." - Anders Sender
---------------------
During your re-gluing of the top back on you accidentally did a "neck pull-up".

The projection should be around 27 mm to 26.5 mm, and not 29 mm.
Remove the top and re-glue it again, but check that you have 27 mm before gluing.
Now someone with experience will be able to say how much of the top needs to be unglued, as it maybe possible that all of the top does not need to come off.

When the top or back is off an instrument, the rib cage is open to flexing, and your neck has changed positions. This is why when gluing the plates on, 'they' recommend gluing the stiffer back on first, to gain some initial stability.
Usually the neck goes in after the top is on, which is a very stable box, but when the top came off and the neck was on, you had some bending of the rib garland/frame in the area of your button.
This should make your original bridge much closer to working, but if things are off, some adjustment may still be needed to get things right.

It's always a good idea to measure things before you undo them.
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