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How do you deal with wood movement using a bolt on M&T? When the wood changes with seasonal humidity, how does the joint stay tight?

With so many hardware choices including bolt diameter, bolt head type, thread pitch and insert thread style (Robbie O and others even use a barrel nut) which one works best for you?

EDIT: This thread was re-titled to reflect the methods of work discussed in it.

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Measure Twice,

Karl Borum

I use a bolt and barrel connection and I've never had a neck joint not stay tight. I don't see a reason why it wouldn't stay tight with the two bolts and having the fretboard extension glued down.

Bolts: 10-24 button socket cap with washer
Barrels: 3/8" diameter x 1/2"
Tenon: 1/2" thick, three-layer cross-grain laminate

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Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

Folks smarter than I am have recommended using Belleville washers.

I use hanger bolts and furniture cap nuts (Allen wrench) for mine and have never had to re-tighten one yet in almost 20 years. That could certainly change if a guitar is moved to a different climate than my relatively dry one. Belleville washers sound like a good solution for big variances.

How is your 3-ply tenon attached to the slot in the heel?

It's glued in.

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Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

That’s a cool way to create a reinforced tenon. If you use the Cumpiano barrel nut system reinforcing the tenon in some way is mandatory. I cut slots and glue in maple reinforcing strips.

IMG_2240 by Terence Kennedy, on Flickr

I made a pull out strength testing jig many years ago and while the plain tenon broke fairly easily the reinforced one held even as the jig was coming apart.

I have been using this system since about 2005 (+/- 85 guitars) and can only recall seeing one really dried out guitar with loose bolts. I do include instructions for checking tightness with the instruments so owners may be doing that without me knowing. I use it on archtops too with a long Allen wrench through the end pin hole.

I think it is pretty rare with well cared for instruments.

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It's not what you don't know that hurts you, it's what you do know that's wrong.

And this style of tenon makes neck construction dead easy using a router and a table saw/bandsaw. It's just too easy, and the heel is reinforced for free. I've used this on all seven (and counting) of my scratch built guitars. I'm amazed it's not in common use.

Early on in the process I match-drill the bolt holes in the tenon and neck block on the drill press. That's simple and accurate, too.

And I've been using 10/24 cap screws and barrel nuts....

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Peter Havriluk

The bolts do come loose I have to snug them up frequently when doing set-ups in our shop. I see this on small builder, Seagulls, some Martins and others.

The only remedy is regular maintenance where the bolts are snugged up in the process. Even my Heshtones with Bellville washers can need a bit of a snugging every several years.

None of this is a concern with a dovetail joint of course.

J this is beautifully done, nice work.

That is a nice looking neck blank Jay.

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jw ( o)===:::

And another M&T - Looking like a bunch of very serviceable options and I like the variety.

I like brass threaded inserts. I don't even try to get full cut thread depth in the insert hole as splitting the neck is a clear possibility and in my testing they still held well with less. Having the threads "cut" their way in makes them likely to crumble the wood threads anyway. Once seated straight in the bottom of the hole I flow med CA with a pipette around the outer insert threads to lock them in. Common furniture bolts - 1/4-20. Nice big heads, no washers needed.

I place the inserts well into the heel past the tenon.

Then I drill out a hardwood dowel to the size of the bolt and glue it in the insert hole. That sucker is not coming out. I made a jig with steel drill guide inserts to drill the insert holes in the tenon so my method is fairly quick and uncomplicated.



My neck blocks are BIG, as my fingerboard extensions are elevated off the top so I want extra strength there anyway and it supports the bolt head well.

Even with a neck block 1 1/4 thick, there is less than 1/2" of wood where the bolt head rests. You are only talking about a couple thou of wood movement over RH extremes so I don't worry about that loosening up the connection.

I tried some bellville washers and even thin ones wanted to cut in to the neck block wood and I wasn't sure I was getting any measurable spring back benefit so have not used them. I considered curved washers with less spring tension but just have not found them necessary.

Wood exhibits some spring back tension on it's own.

I tighten the bolts with the short end of an allen wrench between thumb and finger. I haven't found they need any more. I have not had them loosen up but a quick check during a string change is easy enough. There really isn't a lot of mechanical force trying to pull neck bolts. That's why even a dovetail in short end grain works without shearing off the tails -- . Just kidding of course!




Cheers guys, This thread is doing a great job of showing options, I love that! So will folks considering bolt on necks in the future.

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Brian R, Wood Mechanic
N8ZED

Jay, great idea with the glued in tenon! Did you come up with that? I suppose you can fit the neck most of the way before you put the tenon in?

Sent from my SM-A526B using Tapatalk

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Pat

Yeah, you have to use a flat washer to provide a second hard surface for the bellville (the bolt head is the other) or it will just sink into the wood rather than compressing and doing its job.

I like that technique of gluing a wood plug to prevent the insert from pulling out.

Terrance, are your slots for the reinforcing strips 1/8" kerf?
Beautiful contour on the heal sides, by hand?

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Measure Twice,

Karl Borum

Yes 1/8”. I have a router jig to make them. The heel contour is via a Robosander jig. The template is double stick taped to the neck, rough cut with the scroll saw and finished with the robosander with the bearing moved to the top.
My first template was wood but i had one made in aluminum by a machinist friend.

http://www.kennedyguitars.com/heel-prof ... empla.html

IMG_5205 by Terence Kennedy, on Flickr

IMG_5200 by Terence Kennedy, on Flickr

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It's not what you don't know that hurts you, it's what you do know that's wrong.

THats brilliant! I have the Luthier's Friend sander with that same drum and "carrier" bearing. I've never seen it used like that!

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Measure Twice,

Karl Borum

Threaded inserts use a wood screw thread. If you can find the right screw you can make a tap. File the threads down to the level of the body of the screw at the end to make a 'lead', and then file a notch in the threads along the length of the screw down to the body or a little more. Make the leading edge of the notch radial. This will cut thread in the wood and avoid adding a lot of compression stress. Iirc, the screw I used was a lag screw with a hex head, so it was easy to get a grip on it. I only used it a few times years ago, but it worked fine.

I use Belleville Washers. They are conical springs so they can deal with some of the wood movement.

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http://www.Harvestmoonguitars.com

I did come up with it on my own and it was to solve a problem with the way I wanted to do my neck/body joint. I certainly didn't invent it though. I just hadn't heard of it before. In fact, Peter (phavriluk) clued me into the name "floating tenon" in another thread a few years ago.

Yes, it allows me to fit the neck before I put the tenon in. This came about because my guitar bodies are radiused through the neck joint and I wanted a way to put a corresponding radius in the end of the neck. The jig I came up with allows me to cut the radius on the bandsaw and has the added bonus that I can also cut exactly the right pitch and skew angles of the neck at the same time. They are so accurate that flossing is not needed. If you are interested, I described how it works in another thread here: https://luthiersforum.com/forum/viewtopic.php?f=10128&t=54781&p=726809&hilit=sweeping#p726809

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Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

Yea, it's a beauty. I wish I could find more curly maple like that for making one-piece necks. I only have enough left to make four more necks. Here's what it looks like on the finished octave mandolin:

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Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

And this idea came to me on my own (ignorance can cause us to think we've invented something) and when I bounced my ideas off Kevin LaDue, a wise and thoughtful luthier, he said what I was talking about was what he called a 'floating tenon', the technique's been around for years, and here we are!

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Peter Havriluk