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Minimum heel thickness for butt joint
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Author:  James Orr [ Fri Sep 23, 2016 7:48 pm ]
Post subject:  Minimum heel thickness for butt joint
When designing a heel for a butt joint, what tends to be a realistic minimum thickness? The heel I've drawn up is a working draft. Not finalized, but close to what appeals to me aesthetically. I'll likely fine tune the curve (lower left of the drawing) to flow with the body as well as I can.

As it is now, the maximum thickness down the center is 3/4", and the narrowest is about 11/16" (just a hair under 3/4"). My threaded inserts are 3/8" in diameter by 1/2" long. I plan recess them about 1/16" into the heel, which doesn't leave a lot of real estate behind them.

I'd love to hear your thoughts about whether this should be structurally sound, or if I need to make some adjustments.
Author:  Ken Franklin [ Sat Sep 24, 2016 4:07 am ]
Post subject:  Re: Minimum heel thickness for butt joint
I use butt joints a lot but I do an MT joint for that type of heel. But it might work. You can always grind your inserts down. The thing to do is make a test joint and try to break it. Let us know your results. Whatever you decide put a dowel in the heel so the insert threads don't just tear out the end grain.
Author:  doncaparker [ Sat Sep 24, 2016 6:47 am ]
Post subject:  Re: Minimum heel thickness for butt joint
I am using a butt joint with no tenon on the two I am building now, and I like it enough to make it my new normal. I have a thicker and rounder heel than you do; about twice as thick at the top of the curve. I did it that way because I worry about all of that stress at a spot with a short grain orientation. Experiment with a mock-up, obviously, and see how hard or easy it is to break the heel using the leverage of the neck. I think the dowel is good for more than just giving the inserts more to grab; I think it strengthens the heel and provides some protection against a break there.

If you add a tenon, you can get away with a lot less heel. If you go without a tenon, I worry about having as thin a heel as you want to use. No real life bad breaks to report; just concern.
Author:  kencierp [ Sat Sep 24, 2016 9:05 am ]
Post subject:  Re: Minimum heel thickness for butt joint
Looks precarious to me -- instead of inserts I'd implant a brass rod the length of the heel, bore the vertical hole so the circumference is proud of the heel end. Grind a flat on the protrusion (epoxy the rod in place) drill and tap to accommodate fasteners. McPherson does something similar albeit much more complex.
Author:  DannyV [ Sat Sep 24, 2016 10:05 am ]
Post subject:  Re: Minimum heel thickness for butt joint
That looks to me to have the potential for problems simply due to the lack of material. That heel looks like a good candidate for a M & T. But then that's all I have used and all the heels I have done have been more traditional. If you do go with a butt some sort of X grain reinforcement would be a good idea.
Author:  Alex Kleon [ Sat Sep 24, 2016 5:52 pm ]
Post subject:  Re: Minimum heel thickness for butt joint
This is the only picture that I took of the butt joint that I did. Depth is 15/16" at the heel cap. I drilled a 1/2" vertical hole in the heel block, glued in a piece of dowel, and then drilled an off centre vertical hole through the dowel for the two barrel nuts. There's a spacer dowel between the two barrels for alignment. There's about 3/16" of dowel for the barrels to bear against.
Image

Hopefully my narrative is understandable. A hand drawing might be better.

Alex


Sent from my iPad using Tapatalk
Author:  James Orr [ Sat Sep 24, 2016 10:28 pm ]
Post subject:  Re: Minimum heel thickness for butt joint
Alex, thanks. I think I understand. re you describing a way that you used barrel nuts vertically rather than horizontally?

Thanks for the feedback, everyone. Much appreciated. [:Y:] Ken, Trevor advises on a similar setup in Build.
Author:  Alex Kleon [ Sat Sep 24, 2016 10:36 pm ]
Post subject:  Re: Minimum heel thickness for butt joint
James Orr wrote:
Alex, thanks. I think I understand. re you describing a way that you used barrel nuts vertically rather than horizontally?

Thanks for the feedback, everyone. Much appreciated. [:Y:] Ken, Trevor advises on a similar setup in Build.


Correct, James. Seems to work well, so far.

Alex
Author:  murrmac [ Sun Sep 25, 2016 4:53 am ]
Post subject:  Re: Minimum heel thickness for butt joint
When I first came across the tapped brass vertical dowel process it made a whole lot of sense, but I have since been thinking that an aluminum dowel would serve the same purpose, and would also weigh significantly less than brass.

Any reason not to use aluminum here?
Author:  Woodie G [ Sun Sep 25, 2016 7:15 am ]
Post subject:  Re: Minimum heel thickness for butt joint
When we use butted neck joints (we have three under construction which use that design and one student guitar), we use a 1/2" hardwood dowel run vertically on the center line from heel to underside of fretboard, and the Italian-made Hillman 1/4-20 threaded inserts - the 1/2" lowest in the neck and a deeper 3/4" for the upper insert. The Hillman inserts require a 5/16" pilot, versus the 3/8" for other steel inserts, so there is more beef. The Hillman part number for the smaller diameter Italian-made inserts is 57109-G.

I discussed the topic of minimal heel neck designs in the shop yesterday and the concerns were that - while the inserts could likely be made to work with some cross-grain reinforcement, the tight turn at the heel/neck shaft transition had the potential to be a significant stress riser, and without a tenon to transfer the bending loads to , there is not much structure there to keep things together.

We've had a number of Somogyi guitars in over the past year, which have similar heel contours, but that builder uses a tenon design that is locked into the neck block with two dowels so that the loads at the heel are transferred to the tenon and then into the neck. While there are some doubt in the minds of the builders here as to just how difficult a neck reset would be with this design, there is agreement that the joint allows a minimalist/vestigial heel design to work well.

One of the local builders has come up with a full bolt-on heel-less design for his acoustics (the bolts run from back to front, versus the face-to-heel fasteners used on something like a Taylor Baby. The logical progression from long, structurally beefy heels like the Breedlove and Taylor designs, through Martin's moderate heel profiles, and then to more extreme reductionist/vestigial heel designs like Somogyi's suggests doing away with the heel entirely in favor of a bolt-up design.

The final concern noted was heel geometry after a neck reset or two. While the face of the insert could be left proud of the face and the body hole chamfered, with relief for fitting and removal of material for the reset, that short heel gets even shorter...it may be worth seeing what 1/8" of material off the heel might leave after 2-3 resets.
Author:  Clay S. [ Sun Sep 25, 2016 9:01 am ]
Post subject:  Re: Minimum heel thickness for butt joint
Another way to visually reduce the size of the heel is to recess it into the body as was done on many old romantic guitars. For the Gibson style heel this is relatively easy to do, and I have done it for "Spanish" style heels also. It also allows you to adjust the angle of the neck using shims or removing material (as is usual for resets). Aside from cutting a tight neck pocket no special considerations are necessary for the construction. Cracked heels are common on many conventionally built dovetail necks. As others have mentioned the grain orientation makes this a weak point
Author:  James Orr [ Sun Sep 25, 2016 6:25 pm ]
Post subject:  Re: Minimum heel thickness for butt joint
That's a lot to think about, Woodie. Thank you! I have the Hillman inserts in my Amazon cart to hold them for now.

It seems like some reinforcement is necessary no matter what. Aluminum does sound like a good idea if I go that route as opposed to hardwood. If I do that, I wonder how much material is recommended I should leave between the anchor and the face of the heel. 1/8"

Another option I thought of this morning was to do the hardwood down down the center and carbon fiber dowels flanking its sides.

I prefer the butt joint at this point. I can do the mortis and tenon, but I'm not setup for it. I'd need to develop some templates.
Author:  murrmac [ Mon Sep 26, 2016 11:50 am ]
Post subject:  Re: Minimum heel thickness for butt joint
James Orr wrote:
It seems like some reinforcement is necessary no matter what. Aluminum does sound like a good idea if I go that route as opposed to hardwood.


If you were to use an aluminum dowel, the inserts would be superfluous ... the bolts will go straight into tapped holes in the aluminum dowel, bypassing the need for any inserts.
Author:  James Orr [ Mon Sep 26, 2016 2:06 pm ]
Post subject:  Re: Minimum heel thickness for butt joint
murrmac wrote:
James Orr wrote:
It seems like some reinforcement is necessary no matter what. Aluminum does sound like a good idea if I go that route as opposed to hardwood.


If you were to use an aluminum dowel, the inserts would be superfluous ... the bolts will go straight into tapped holes in the aluminum dowel, bypassing the need for any inserts.


Yep. We're on the same page.
Author:  rlrhett [ Mon Sep 26, 2016 2:38 pm ]
Post subject:  Re: Minimum heel thickness for butt joint
murrmac wrote:
James Orr wrote:
It seems like some reinforcement is necessary no matter what. Aluminum does sound like a good idea if I go that route as opposed to hardwood.


If you were to use an aluminum dowel, the inserts would be superfluous ... the bolts will go straight into tapped holes in the aluminum dowel, bypassing the need for any inserts.



Brilliant!

Have a repair I was going to use walnut dowel on. Hadn't sourced the dowel yet, but have plenty of aluminum 1/2" dowels. Aluminum it is then!
Author:  Colin North [ Mon Sep 26, 2016 3:18 pm ]
Post subject:  Re: Minimum heel thickness for butt joint
Square section will be better than round as the bolt tension will tend to split the wood with it vertical in the heel.
Likewise a tenon as suggested would be a must rather than a butt joint, which will leave little wood between the face and the aluminium bar.
I use this joint type for all my builds now (from Trevor Gore's Book) but with a 3/8" brass bar, so far anyway.
Author:  Pmaj7 [ Mon Sep 26, 2016 3:18 pm ]
Post subject:  Re: Minimum heel thickness for butt joint
Hmmm..... all this talk of aluminum dowels got me thinking. It would actually serve two purposes. It would be a little extra work to tap. However, I wonder if those aluminum threads would hold as well as Steel? I guess as long as they hold better than the wood! Lol
Author:  Alex Kleon [ Mon Sep 26, 2016 3:54 pm ]
Post subject:  Re: Minimum heel thickness for butt joint
pat macaluso wrote:
Hmmm..... all this talk of aluminum dowels got me thinking. It would actually serve two purposes. It would be a little extra work to tap. However, I wonder if those aluminum threads would hold as well as Steel? I guess as long as they hold better than the wood! Lol


I like the idea of tapping aluminum rod instead of two barrel bolts that I used. Unless you tighten the bolts to wood crushing tension, the aluminum threads should work quite nicely! Love this Forum! [:Y:]

Alex
Author:  Jeff Highland [ Mon Sep 26, 2016 4:42 pm ]
Post subject:  Re: Minimum heel thickness for butt joint
IMHO that heel thickness for a butt joint is inadequate, even with a dowel or rod though it.
Where you see minimal heel thicknesses like that is on some archtops, but they have the extra strength from a 5/8" dovetail tenon on the inside, and heel cracks are still common.
It's not just the string tension, it's the accidental knock on the headstock.....
Author:  James Orr [ Mon Sep 26, 2016 7:10 pm ]
Post subject:  Re: Minimum heel thickness for butt joint
This has been a great discussion. [:Y:] Thanks everybody.

I've added an 1/8" to the neck-body joint side of the heel, everything else remaining the same. If I do the inserts as opposed to tapped rod, they'll obviously come forward that much as well.

The heel is now 13/16" where it transitions to the neck shaft, and 7/8" at the bottom. Do you think that's thick enough, or would it be best to enter mortis land? I'm hoping to avoid the mortis/tenon simply because the butt is so elegantly simple.
Author:  Clay S. [ Mon Sep 26, 2016 8:53 pm ]
Post subject:  Re: Minimum heel thickness for butt joint
The extra width of the semi- Gibson style heel you have drawn out should give a bit more strength than a Spanish style heel as Martin uses, so my guess would be that at 13/16 ths you may be O.K. But that's just a guess.
Author:  Jonny [ Mon Sep 26, 2016 9:09 pm ]
Post subject:  Re: Minimum heel thickness for butt joint
Woodie G wrote:
When we use butted neck joints (we have three under construction which use that design and one student guitar), we use a 1/2" hardwood dowel run vertically on the center line from heel to underside of fretboard, and the Italian-made Hillman 1/4-20 threaded inserts - the 1/2" lowest in the neck and a deeper 3/4" for the upper insert. The Hillman inserts require a 5/16" pilot, versus the 3/8" for other steel inserts, so there is more beef. The Hillman part number for the smaller diameter Italian-made inserts is 57109-G.



Any link for these?

Jonny
Author:  James Orr [ Tue Sep 27, 2016 12:44 am ]
Post subject:  Re: Minimum heel thickness for butt joint
Jonny wrote:

Any link for these?

Jonny


Here they are on Amazon.
Author:  murrmac [ Tue Sep 27, 2016 4:40 am ]
Post subject:  Re: Minimum heel thickness for butt joint
Colin North wrote:
Square section will be better than round as the bolt tension will tend to split the wood with it vertical in the heel.


Hi Colin, I see where you're coming from, but if the hole is made a nice sliding fit, and the rod well glued in with epoxy, wouldn't that negate any wedging effect ?

The square section would require a laminated neck, as well, would it not? The round dowel could be fitted into a one piece neck.

( I feel guilty about taking issue with Colin, as his beautiful guitars were the first I ever saw using this mode of neck attachment ...)
Author:  Colin North [ Tue Sep 27, 2016 5:36 am ]
Post subject:  Re: Minimum heel thickness for butt joint
murrmac wrote:
Colin North wrote:
Square section will be better than round as the bolt tension will tend to split the wood with it vertical in the heel.


Hi Colin, I see where you're coming from, but if the hole is made a nice sliding fit, and the rod well glued in with epoxy, wouldn't that negate any wedging effect ?

The square section would require a laminated neck, as well, would it not? The round dowel could be fitted into a one piece neck.

( I feel guilty about taking issue with Colin, as his beautiful guitars were the first I ever saw using this mode of neck attachment ...)

No worries and thank you.
Yes, that method would minimise wedging effect, but I wouldn't like to service it if the thread did strip -
Square section does not require laminated neck, just a tenon as I mentioned - with brass rod you cannot tighten it enough by hand to strip thread or pull though the wood (18mm wide tenon, 16 mm long)
Trevor Gore said this, and I've verified it, (as someone known to be very heavy handed) - one piece mahogany neck, normal allen key.
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