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I've been using an M&T bolt on neck joint but hadn't done it with Honduran mahogany yet (HM). I drill the hole through the tenon and about 2/3 into the heel, lightly cut some threads for the threaded inserts, install the inserts and usually wick in some CA to harden the wood "threads" and lock in the insert. Since HM readily soaks up liquid via end grain I thought some experiments were in order to see if the glue would run through the heel to the show side. GLAD I did.

So I cut several sections from some neck cut-offs of different thicknesses. Applied a variety of glues to the end grain to see what happened. WOW, kind of like a sieve. Here are visuals, glue type, thickness of wood... Even epoxy capillaried its way through. Only thick CA didn't migrate through but wouldn't serve my purposes because it wouldn't wick enough. All the water based glues were fine even when thinned out.

From the application side


From the back side


I applied CA to one piece and inverted it so the glue was on the bottom and it shot right through. Well that's how the leaves get water! Good thing I'm OCD about guitar parts I have a lot of work into.

I also cut a dowel and drill a hole down the middle and glue it in the insert hole so the insert never comes out.

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Brian R, Wood Mechanic
N8ZED

I built one guitar where the thin CA wicked through the mahogany heel and was visible on the outside of the heel. I wasn't sure exactly what I'd end up doing--building another neck was my first thought. But, I went ahead and tried putting a finish on the neck with the CA, and fortunately the spots disappeared under the finish. But it's the last time I used super thin CA on mahogany end grain.

Dave

Good point Dave. One of my test samples was shellac instead of glue and the look compared to the CA is very similar and would likely disappear as it did for you. Wouldn't have eliminated the OH "Heck" moment if I witnessed CA coming out of the wrong side of the neck. I try and avoid those moments of panic.

I just did another test with an insert in the bottom of the hole on a mocked up heel. I tried medium CA and it did what I'd hoped. Followed the threads along the insert and when applied carefully, it didn't go through to the show side of the heel. So, I'll try it on the neck of lesser importance first.

Thanks for the reply Dave. With your experience, I'm sure you've heard of folks jamming a dowel stick into a banjo heel without a relief path for the hydraulic pressure. Thus my concerns here too.

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Brian R, Wood Mechanic
N8ZED

I wish I had tested like you did first. It was definitely an Oh Heck moment! I had used thin CA on bunches of banjo necks, but the heels are thicker than on a guitar and I was typically using curly maple, walnut or some other less porous wood than mahogany. never had a problem with those. Live and learn! I'm using medium CA now without a problem.

Dave

With fast drying stuff like shellac and CA, wicking depth is highly dependent on how much you apply. If you wipe with a relatively dry shellac cloth, let it dry, and repeat a few times, you can seal the endgrain with almost no penetration. Wiping with a wet cloth will darken the grain, but dry before it gets in too deep. Pouring a puddle will get your result.

Unfortunately, the thin CA makes it hard not to get a puddle--just a few drops from a pipette is too much at times...

Dave

Most of the test glue was ~2 drops. Thin CA is even hard to control with a micro pipette. I sometimes take the delivery end of a disposable pipette and stretch it out with pliers and cut it off. Making a nano tip. Works pretty good. Just like making an eye dropper from glass tubing.

Was most surprised by the penetration of the different epoxies with much higher viscosity.

I expected the CA to penetrate which is what led to the test to determine how deep in hog in the first place, and curious about other glues not known for penetration properties.

I wipe shellac in binding channels when I use CA for shell purflings, but its hard to put several light coats in a 3/8" hole that's almost 1 1/2" deep.

This was a tough prospect. You don't want to run a screw in and out of an end grain hole. And while allowing some penetration to harden up the wood to hold the threads of the insert, I didn't want it to run out the show side of the heel.

But I learned a few things that would get the job done and some that wouldn't. I know others are using threaded inserts for bolt on necks but hadn't seen much in the way of details.

Appreciate the comments.

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Brian R, Wood Mechanic
N8ZED

I never use CA on Mahogany for this reason
I also won't use it on Spruce. It tends to discolor over time.

It is best to test any wood you have not used before , glue can let a signature under a finish that looks terrible.

The more mistakes one makes the more on learns. The Japanese say that the difference between the expert and the amateur is the expert made all the mistakes.

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

Good judgment comes from experience, and experience comes from bad judgment.

I have earned my share, and somebody else's, of good judgment (unfortunately).

CA and epoxy are indistinguishable in the way they wet out mahogany, so even if one selects the wrong viscosity of CA for something like cross-grain tenon reinforcement installation (because thin CA may wick all the way from tenon to heel), no permanent harm done. Not the case for paste wood fillers, which require a consistent surface treatment for best results (for example, a wash coat of shellac or clean/bare wood for tinted filler).

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For the times they are a changin'

- Bob Dylan

In my opinion, Honduran mahogany is a fairly soft wood and is marginal for holding bolt inserts. Obviously your adding the CA to help avoid this "pull-out" problem. Even though my necks are Honduran mahogany my inserts are installed into hard maple. This avoids the problem you're experiencing and provides a MUCH more secure anchoring of the insert.

Also, you are putting the insert into end grain which compounds the problem of a weak anchor regardless of the wood. When I put a small block of maple in the heel it presents side grain to the insert which by itself provides a stronger anchor.

Harder wood and side grain makes for a much superior anchor and there are no CD bleed through concerns.

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I've never had problems with inserts staying put in mahogany, but I'm using hanger bolts and hex head cap nuts nowadays. Seems a whole lot more secure. I've had those same doubts, even though I've not had any failures. The hanger bolts have been standard issue on bluegrass banjos for decades and rarely fail.

Dave

Same concerns with smaller diameter brass inserts on mahogany...common to see these start to pull out after the M&T joint perviously used on Style 16 and lower Martins fails in shear. To be fair, that insert was never intended to take a load (used to hold the neck during finishing and as the glue dried), but we see a dozen or so of those guitars a year with separating necks.

For our own projects. we use the Cumpiano barrel nut and KD bolt joint on most mortise and tenon necks, dowel reinforcement and hanger bolts for butted neck joints, and nothing at all for dovetails. The reinforcement on the M&T joints is a cross-grain-oriented piece of 3/8" x 1/16" x tenon width unidirectional carbon fiber/epoxy composite sheet stock used to increase the resistance to bearing failure (i.e., the barrel nut being pulled towards the body and shear failure of what is at that point just a small amount of long grain resisting the bearing load). The CF is set dead center between the outer surface of the tenon and the edge of the two 5/16" holes milled for the 1/-20 bolt. We used to use CA to glue in the CF, but found that West 105/206 does a much better job of minimizing any wicking, as well as being a little more shock-resistant and less of a mess.

For butted neck joints, we use a dowel running from fretboard surface to just above the heel surface, setting the hanger bolts into face grain. Taylor uses an aluminum insert for the same job, but the dowel is a better bet for this small shop and the few butt-jointed necks we produce per year.

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For the times they are a changin'

- Bob Dylan

I was afraid this would happen. Thread drift from glue shooting through end grain to a very good discussion on the mechanics of bolt on necks. I was thinking about starting another thread just on that.

Anyone know of a way to change/edit the title of this thread to to include that?? I just tried to do it here, see if it works. That didn't work, only changed the heading of the post.

I hate it when excellent info gets hidden under a title that wouldn't lead folks to the good stuff.

I have more to add to the bolt on discussion but am away from pictures till Monday.

Thanks to all the participants.

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Brian R, Wood Mechanic
N8ZED

You can edit the title by editing the first post, at least you used to be able to do it that way.


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Steve Smith
"Music is what feelings sound like"

Thanks much Steve, worked!

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Brian R, Wood Mechanic
N8ZED

Me? I've moved over completely to the vertical bar mortised into the neck heel, à la Gore, and tapped for bolts.
Brass or aluminium, both work, and I think much stronger than most methods.

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

[quote="rbuddy"]I was afraid this would happen. Thread drift from glue shooting through end grain to a very good discussion on the mechanics of bolt on necks. [quote]

You have gotten several different suggestions. It appears a large number of suggestions focus on changing you neck connection design instead of solving the CA glue symptom in your current design.

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Yup, the OPost stemmed from locking in threaded inserts but the main objective was to share the simple glue penetration results in different glues and thicknesses of H Mahogany end grain.

Thinking a good future thread would be bolt on methods. I fully appreciate all the varied suggestions and shared options. I don't mind constructive thread drift at all.

I'm currently working on 3 bolt on necks. I've done 2 (on guitars) previously and they've been in service for over 5 years w/o problems.

Did several banjos with hanger bolts. Hanger bolts in banjos fail most often because access to them is handy and tinkerer's tend to over tighten the nuts and pull out the lag end and the wood threads. Owner abuse not method failure.

So many aspects of instrument construction violate rules accepted in furniture construction, it seems half of what we do requires some level of compromise. Cross grain wood glue joints, or combinations of metal or carbon spanning wood grain, etc, etc and the list is long.

We each choose our own poison out of necessity. Wood moves, pretty much no changing that.

I mocked up a mahogany heel block and set my insert the way I normally do, threaded in a bolt and locked it in a vise and couldn't pull it out. I use a course thread insert because finer thread just seems to destroy any "threading" going on. Then ran a bead of med CA around the threads as a "thread locker" after a test it didn't come out the show side. The CA would have had to go an inch to get there and a test proved that sufficient as long as I didn't flood the insert hole.

I take it one step further by taking a hardwood dowel the size of the insert hole and drill a hole down the middle slightly larger than the furniture bolt. Glue that in on top of the insert and saw it off flush with the tenon. Since the strongest wood glue joint is side grain to side grain and the insert would have to pull out the dowel to fail I have a high level of confidence in it holding up over time. I think it maintains a lot of the natural heel integrity and doesn't induce a lot of additional stress.

And a picture of a couple in progress:



Thanks for all the input and following along!

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Brian R, Wood Mechanic
N8ZED

Facts.

I'm just about desensitized to all this "rule breaking" after going from solid-wood furniture to guitars... but man, it was hard at first

I controlled CA glue penetration in end grain by building necks with floating tenons, tenons for bolt-on necks that are inset into a routed slot in the neck. I get easy-to-cit neck angles, anticracking reinforcement on the heel just like a dowel and no wicking of CA past the floating tenon itself. One more part and one more routed slot.

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Peter Havriluk

Sounds interesting Peter. Got any pictures handy? Grain orientation of the double ended tenon?

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Brian R, Wood Mechanic
N8ZED

I regret no pictures. And current project isn't at that stage yet. Grain orientation was the long dimension of the floating tenon, i.e., top to bottom. That generated my neck reinforcement. My floating tenons are 3/4" wide, 1" deep, half in the neck and the other half sticking out of the neck.

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Peter Havriluk

I do it this way. No glue needed.

What goes in the lower hole Dave? I like the cross dowel nut installed that way.

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Brian R, Wood Mechanic
N8ZED