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Double Bass sound post patch

kurt ratering

kurt ratering

Dec 2, 2008
241
0
waltham, mass.
www.johnsonstring.com
Disclosures
bass luthier, johnson string inst.
#1
2hrk47a.jpg

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#5
the first pic is the top covered with clear wrap and a dam built around the area i wanted to cast.
then i poured plaster in there with a little chicken wire re-bar and the three wood legs to keep it steady when i used it.
the third pic is the result, my custom fit plaster bed.
forth is the top clamped to the cast and my bench. i made the patch bed with some round bottom fingerplanes and my scraper. the straight edge and light let me see how smooth the bed is by looking at the shadow.
the last shot is the trimmed patch.
i guess i should have taken some pics of the patch before i trimmed it down. it will be a while before i get to hear her, as a new fingerboard is in order as well, along with getting the rest of that muslin out of there!
 
#13
wow! i wasnt expecting this much intrest, i will try to answer guestions in the order they were posted.
-the sides of the dam are made from heavy tag board which i covered with packing tape. one thing i didnt photograph was that i used modeling clay where the dam met the top to seal it closed so plaster wouldnt leak everywhere, i also mixed a fair amount titebond in with the plaster as i have heard that i can help prevent cracking if it dries too fast.
-this was the first cast i have done where i have had the forsight to put in those feet. although next time i think i will turn them 90 degrees to make clamping a little easier.
-i think that i spent around a day and a half between carving the bed and fitting the patch...after waiting four weeks for the plaster to dry!
-i cut the patch wood from a really nice, really old fiddle top from Gewa. i caught a little heat from my fiddle shop co-workers for using such a nice piece of spruce because its "just a bass" :eyebrow:
thanks for being curious!
 
#15
i think that skewing the grain a little is a good idea, just so if it wants to open up it has to fight across the patch grain. in the pics of the s.p. crack on the back of that bass (that bassist14 linked to) i think i have to agree with Arnold, that the patch was insufficient, either not deep enough or not well seasoned wood. alot of the older basses i see have sound post cracks which dont scare me as much as bass bar cracks. i dont think that either are necessarily turn and run types of damage, depending on how well they were repaired. either way, the patch will do a much better job than the muslin that was there before (if you look at the last pic i put up you can see the outline of where the muslin was over the s.p. cracks).
 
#18
I'd go so far as to propose that a patched belly will even be structurally better than an un-patched one if the wood and grain is well chosen and executed. Whether or not there's a significant detriment to tone is another question. I doubt whether there is much of a difference.

I have been patching the FF tab corners on my new tops from the get go, and I believe they are better for it. The amount of possible shrinkage on a small piece of wood is extremely small, and the potential for problems with the patch are insignificant compared to the potential for the corner snapping off and needing a fiddly repair.
 
#19
Anyone use "Friendly Plastic" for partial molds? With several of the 28 oz. pails you get enough material for a cast this size. The great thing about this stuff is that you can reuse it again and again instead of throwing the single use plaster cast into a landfill. At first I worried about the heat damaging the varnish but if you let it cool enough so that you can hold the softened plastic in your hand it seems to be fine. Works for odd shaped gluing cauls as well.

Check it out.
 

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