mikemcnerney wrote: OK Hesh how do I determine the runout.
And on some new construction I am getting close to fitting the bridges and I need a tutrorial on how to match the curve to the top.
I watched Parkers but of course it is for an archtop and he hollows out the bridge.
Hi Mike, hope you are doing great and Happy New year to you.
For reading runout: First we read runout when regluing a bridge for the bridge removal process. We are going to use pallet knives and such, thin, flat, metal tools with rounded edges to get under the bridge and release the remaining glue bond from the top.
So the goal here is to insert the tools in a direction that helps to lay top wood fibers back down not get under them and lift them and damage the top. This is why we want to read runout when removing a bridge. It's rarely mentioned (here) but a key part of doing-no-harm, quality repair work. The better the condition of the top when the bridge is off the less work has to be done to the mating gluing surface when we reglue the bridge.
So if you've ever looked straight on at a guitar and one side was darker than the other with a clear line down the middle what you are seeing is how the light in the room refracts differently because of the top run out. This is very much like when you mow your lawn and go one way for a row and the other way for the next row. The light refracts differently for the two rows when viewed because the grass is leaning in opposite directions because of how the mower cut it.
I tip the guitar up at the head stock as if to look at the neck straightness and view the top. The lighter color side has the runout going toward the tail end of the instrument or away from me.
This is just me but the next thing I do is take a piece of masking tape, tape it to my shirt, remove it (this is to make it less tacky) and then stick it on my clean bench. Next I take a marker and make a large arrow on it. I place the tape (when the marker is sure to be dry...) on the side of the guitar with the runout running one way or the other with the arrow corresponding to the direction of the runout. This step is optional but I do it because it informs me which direction to insert the pallet knives to loosen the bridge. With this determined I do at it using what I've learned to help me get the bridge off cleanly with less or no top damage.
Removal is a heat process we use tightly fitting cardboard shields lined in foil with the shiny side facing out. I use HVAC reflective tape to span any gaps that might expose the guitar top and finish to the heat I am going to apply.
Now we do this professionally so our methods not only work great they work fast but you have to be vigilant and present in the moment the entire time to avoid a fire or damage.
We use a heat lamp after the shields are in place to heat up the bridge that is poking through the cardboard lined in foil shield. I heat for maybe a minute with the lamp moving left and right over the bridge and very nearly touching it or just 1/2" an inch away.
We've always used heat lamps including for neck removal of the extension. A few years ago one of the big producers of guitars took our advice and now uses heat lamps too for extension removal and such. You just have to be present in the moment because of the fire risk.
With BRW bridges you will see some smoke at times and the oils start to run and smell them too. So I heat for maybe a minute and then place the heat lamp safely somewhere where it will not catch anything on fire and turn it off. With the pallet knives I then insert being informed as to the direction by having read and marked the top for runout.
If you used a Titebond original like glue or HHG I am looking for the feel in the knives to be gooey and softer as the glue starts to soften. If the glue does not feel soft or I hear or feel cracking I replace the shield and heat some more until things feel soft.
When the glue is soft I carefully and slowly make progress with the knives going slowly not wanting to damage the top. At some point the bridge pops off.
The the fun of expanding the bridge patch, cleaning up the surfaces, scraping the bridge bottom to better fit the top begins.
Be sure before you start any of this to understand why the bridge lifted so this does not happen again. There are two primary reasons whey a bridge will lift.
1). The guitar dried out
2). Internal damage or structure failure such as a loose brace permitting the guitar top to seek a different shape or radius.
If any of these things or conditions exist fix them first before removing and replacing the bridge.
Hope this helps it's a comprehensive approach to a professional bridge reglue and evaluation of anything related.
We spend the extra time doing all of this and as such our reglues show zero tell tale that the bridge ever lifted. We do not clear finish to the peremeter because that can look both sloppy and not OEMish AND it takes choices away from the next luthier who has to reattach the bridge if the steward lets the thing dry out again. We remove finish very close to the perimeter but not all the way to it.
Mike let me know if you have any questions?
Thanks
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