Guitar: Why Thompson Belly reducer works?

Subject: Why Thompson Belly reducer works?
From: Richard Bridge
Date: 23 Dec 13 - 09:50 AM

I don't see why this tool works to reduce guitar belly bulge. Who can explain it to me? I know HOW it allegedly works.

You heat two big alumin(i)um blocks, put one on the outside of the bridge and one in the inner bridge plate and clamp them on TIGHT. This softens the hide glue that holds the outer and inner bridge plates. Leave to cool. Take off tool. Bulge is reduced.

Here's a link to the Stew-mac site with it on

http://www.stewmac.com/shop/Tools/Clamps,_support_tools/Thompson_Belly_Reducer.html?actn=100101&xst=3&xsr=20914


But bulge (and collapse in front of the bridge) is caused by the bridge twisting towards the neck. So why does this work?   If you replaced the outer and inner bridge plates, the outer having a slight lump under the middle, and the inner being slightly concave, I could see it pulling the middle of the top (and the whole bridge assembly) towards the back of the guitar - but it seems to do that without the re-shaping.

Baffle-a-Bridge...

Subject: RE: Why Thompson Belly reducer works?
From: JohnInKansas
Date: 23 Dec 13 - 11:05 AM

It looks like the same principle used for bending the wood for a number of other instrument pieces. With a little moisture and heat to around 150F, the wood becomes "flexible" and can be reshaped fairly drastically, and will retain the new shape and become rigid again if held in the desired shape until it cools.

The "belly" that develops is because string tension makes the bridge twist on the top plate, and the "soundboard" develops, over time, an "S" shape with a depression on the string side and a rise on the other side. The correction is to moisten and heat and make the belly (locally) flat like it was originally.

Fairly thin wood bends pretty easily. My homemade "bender" worked well enough to make nice side boards for a couple of dulcimers. The same principle "sort of works" for particle board or even cardboard, although it's a little more difficult to work the bends in without cracking the cheap stuff.

It looks like a fairly straight forward way of getting to the local area that needs to be "bent back to original" to make the bridge stand up straight, with minimal damage to the original parts.

John

Subject: RE: Why Thompson Belly reducer works?
From: Richard Bridge
Date: 23 Dec 13 - 01:08 PM

I have previously been told (by Graham Noden, the well kown luthier in Denmark Street) that steam bending of wood does not result in the re-formed shape being strong enough to stay re-formed. I wanted him to do it with a Hagstrom J-45 (the oldest one in my collection).

Secondly, it would seem to follow that if the above is the principle, it would be clever to depress the top a bit so as to get the saddle fully down to the same level as the sides AND twist it a little bit too far back - and neither of those functions seems to be present in the belly reducer.

If I get energetic a bit later I may produce the Youtube links for a geezer using wither this belly reducer or a very similar one (he also puts a Bridge Doctor in, and my experience of them is very good for adjusting angle - but does not include them getting the bridge down to the same level as the sides.

Subject: RE: Why Thompson Belly reducer works?
From: Jack Campin
Date: 23 Dec 13 - 02:10 PM

Why aren't guitars made like Munir-Bashir-style Iraqi ouds, with the strings fixed to a tailpiece and the bridge floating? No twist.

http://www.lutes-strings.de/english/Aoud_baghdad-1.php

Subject: RE: Why Thompson Belly reducer works?
From: Richard Bridge
Date: 23 Dec 13 - 02:43 PM

You get a different sound, less complex. Compare tailpiece guitars with pegbridge ones.

Subject: RE: Why Thompson Belly reducer works?
From: JohnInKansas
Date: 24 Dec 13 - 08:26 AM

The "art" of steam bending wood, or any of the several other methods of wood bending, isn't completely trivial, and it's reasonable for an individual luthier to base an opinion on personal results. If "I" can't do it with the results "I" want, "I" wouldn't want to recommend the method, and would do something else(?).

For bending under conditions likely in a repair shop, the moisture content of the wood is somewhat critical. Insufficient moisture won't allow the wood fibers to "slip" and the bent wood may lose some strength. Some woods may have sufficient moisture to make mild bends without additional moisture, while some may need only a "wipe" with a damp cloth, and a few may need a "slather" and time for the moisture to penetrate before bending.

Some accomodation for the range of moisture at the time of bending can be achieved with slight adjustments of the temperature. A sufficient temperature to allow the wood to "move" is necessary, but too high a temperature can "explode" the fibers, separating them so the bent part will lose some strength. An extremely high temperature can, of course, char the wood, which is never a disirable effect in a bending process. An insufficent temperature may "tear" the wood fibers which also could weaken the resulting part.

Failure to hold the wood in the bent shape until it cools and until any moisture added for the bending can escape can tend to let the wood "relax" to a shape other than intended, and may leave uneven stresses in the wood, with fibers "sprung to shape" (stretched like a spring) fighting with those actually "reformed to shape," giving the impression of weakness when everything settles out.

Steam bending has been used for centuries with good results, but it does take some practice to get good results for complex bends. A simple "flattening" to restore the shape, as intended with the fixture in question, shouldn't be too difficult. If you're not confident of what results you're likely to get, you can always practice on something other than your best guitar, before making the repair you want.

Industrial furniture makers might be more likely to use the "ammonia process" in which concentrated ammonia under pressure is "soaked into" the wood at a rather high temperature, making the wood "limp" so that it can be clamped to extreme bends and "hardens" as the ammonia evaporates out of it. This is NOT A PROCESS you can use at home, or in a small shop, since the ammonia can kill you.

The ammonia process, properly applied, permits bending very thick sections, but I've seen no credible evidences that for similar sections it results in a stronger end result than "steam bending," within the section thickness ranges where there's a choice between the two.

John

Subject: RE: Why Thompson Belly reducer works?
From: Richard Bridge
Date: 24 Dec 13 - 08:56 AM

Ah-hah! The Thompson belly reducer has a slight curve in each plate! http://www.strangeguitarworks.com/acoustic-guitar-belly-repair/

Subject: RE: Why Thompson Belly reducer works?
From: JohnInKansas
Date: 24 Dec 13 - 11:38 AM

Space and time are mostly curved. As long as you follow the curves flat is the same as warped. You just have to fit your space-time continuum.

If I read the description right, he belly flattener has a set of plates for flat bellies, and another set for the curvy ones. Take your pick on which to use depending which kind of belly you're trying to take the bulge out of.

The biggest problem with the kit as described is that they specify "heat the plates to 150F" and getting an accurate temperature before applying the fixture may be a problem without some "accessory equipment. For a home-shop fixer, a regular clothes iron turned upside down to lay the plates on might be accurate enough for the heating in the needed temp range, with a thermometer to check the setting; but since OSHA got in on the act most recent model irons shut themselves off if you turn them over. Some even shut off if you leave them in one spot right side up long enough to scorch your skivvies.

John

Subject: RE: Why Thompson Belly reducer works?
From: Stilly River Sage
Date: 24 Dec 13 - 11:43 AM

Whew! I thought we had a spam thread going here. :) Don't over-bake your guitars!

SRS

Subject: RE: Why Thompson Belly reducer works?
From: Richard Bridge
Date: 24 Dec 13 - 04:07 PM

Further investigations have shown that the "guitar" had every internal brace bar one cracked split or detached so a heated missive is on its way to the seller who said "in good working order"...

Clearly it was a GLO (=Guitar-like-Object)

Subject: RE: Guitar: Why Thompson Belly reducer works?
From: JohnInKansas
Date: 24 Dec 13 - 07:40 PM

Maybe he meant "it works for everything I know how to do with it." ... (?)

He might have used it to drive tent stakes, or somethin'.

John

Subject: RE: Guitar: Why Thompson Belly reducer works?
From: Stanron
Date: 24 Apr 17 - 10:48 PM

OK, anonymous guest opened a three and a half year old thread with a blank post. I'll chip in. No one so far has discussed what made the guitar deform in the first place. A 'normal' guitar should not deform in that way. However stick the guitar in front of a strong radiant heat source for an hour or so, or leave it in the boot of a car on a very hot day, and the glue may soften and the tension on the strings will cause the distortion.

It seems to me that if the Thompson method can reverse this deformation, as long as whatever caused the initial distortion is not repeated, the reversal should be stable.

Subject: RE: Guitar: Why Thompson Belly reducer works?
From: GUEST
Date: 15 Sep 17 - 04:25 PM

I found this thread because I have an inexpensive Gibson Epiphone model (PR-200) that was given to me because someone had left it near a radiator for years and the ebony bridge had nearly popped off the guitar, staying on just barely. The tension of the strings from I am guessing - the drying out of the wood - had ripped the bridge and wood up from the guitar and was shocking to look at.

However, I noticed also the bridge had been glued onto the varnished surface to begin with, not the wood, which is one strike. Strike two was that someone had previously tried to repair it with epoxy but that repair did not work. I am guessing they did nothing to hydrate the wood during the repair process.

Upon close examination I saw that not only did the ebony bridge deform (back was higher than the front) but there were tiny cracks that formed in between the holes for the strings. a sane person would have just spent $11.95 on eBay for a replacement bridge.

However, I had success using a combination of steam and pressure from a 25 lb bag of ceramic clay placed on top of a metal shim to straighten out the body.

I have avoided sanding down the bridge because on this thread someone said to drop it in boiling water, which sounds smart.

There's something about using the Thompson's belly reducer i don't like - the drilling holes int he bridge, etc. It just seems like one fix to a common problem. I can imagine other ways to heat the body and apply pressure, so I am surprised nobody else has come up with additional products for such a common problem...

Subject: RE: Guitar: Why Thompson Belly reducer works?
From: GUEST,Aaron
Date: 20 May 20 - 05:06 AM

Stanron, are you saying that if a guitar is not subject to a heat source this wont happen? Excuse me im not trying to be difficult. The last two old acoustics ive seen have this issue, my neighbours and now my "new" old guitar. I expect my neighbours was as i found it in his garage. Mine, well i bought it online from a 2nd hand dealer who didnt disclose the bulge. Ive been looking at solutions and the thomson solution seems to be the standard.

Subject: RE: Guitar: Why Thompson Belly reducer works?
From: punkfolkrocker
Date: 20 May 20 - 10:29 AM

My first ever guitar back in the 1970s was a Shaftesbury [Eko made...???] acoustic..
A Plywood top..

After I'd had it 7 or 8 years, a change to a heavier set of stings
suddenly tore the bridge off overnight..

It ripped off a chunk of splintered top layers of ply, still glued to the bottom of the bridge...