I live in morbid fear of boring the reader, so please forgive the sea of words. The failure shown in the photo posted appears similar to one of the failure modes noted in the figure below. From the builder's guide I am using (with permission):
Attachment: Fig16-7rev3.jpg
Quote: 16-3 Cumpiano Hardware Store M&T Joint:
This joint is simple to make and fit, quick to reset or adjust (though not as quick to reset as a double tenon neck), uses common Knock-Down (KD) furniture hardware, and tolerates a surprising degree of error in bolt layout, but is slightly heavier than a butted hanger bolt or dovetail joint. This joint requires careful layout and reinforcement of the tenon for reliable service – it is a good choice for both new builders and those that appreciate the ease of fitting and reduce maintenance cost of bolted neck joints.
Common failure modes for the joint are shown in Figure 16-7, and suggest the following layout guidelines:
- The tenon should be 0.750” thick, 0.967” long, and as wide as possible - a tenon 0.032” narrower than the mortise avoids fitting issues, and if the mortise is milled just shy of the back binding, tenon width is maximized for best leverage for lower barrel nut/neck bolt.
- The lower barrel nut centerline must be placed at least .967” up from the bottom of the tenon and as far toward the cheek as possible to gain enough cross-sectional area to resist tensile failure below the nut or shear failure in line with the barrel nut.
- The upper barrel not is not load bearing unless the lower bolt is loose or a failure occurs; however, it is the secondary load path for neck loads and should be placed a minimum of 1.25” from the lower barrel nut centerline to prevent tensile failure.
Reinforcement: To prevent bearing failure in shear from overtightened bolts or abuse, the tenon should be reinforced with .080” cross-grain wood (adequate) or .070” (sanded to .063”) carbon fiber unidirection sheet stock (better) beyond the barrel nut holes and inset into the tenon. This joint is easily milled with a table saw using 12mm ply or MDF offset fence as shown in Figure 16-8 and 16-9.
Hardware Considerations: Standard 3/8” x 3/4” x ¼-20 barrel nuts give adequate bearing surface and are of sufficient diameter to avoid excessive bearing loads. Standard 50mm x ¼-20 KD hex head bolts are used for the bolt, and a 5/16” bolt hole gives adequate adjustability and tolerance for misalignment. The loss of tenon material from the oversized bolt holes is of no consequence, provided the tenon is reinforced as noted above. A tension (Belleville) washer may be used under the head of the bolt to provide a tension preload on the bolt with seasonal relative humidity changes, but a thin flat washer should be used between the Belleville washer and the neck block, suggesting the depth of the neck block counterbore be adjusted for the added hardware.
Note: In service, I have not seen the requirement for this precaution, provided the neck bolts are adequately torqued…a guitar seeing extremes of RH swing sufficient to compress the neck block material and allow neck movement will likely have a host of other issues (top/back cracks, viable setup & playability, etc.) before neck bolt issues become evident. Luthiers should consider suggesting more frequent inspection of neck bolts for instruments using this and other bolted joints and subjected to repeated wide swings in RH.
The carbon fiber reinforcement I used for the tenon on my parlor guitar is inserted into a 1/16" slot...I will try to find a photo that shows the reinforcement and placement of the barrel nut holes (I left my camera in the shop! Dust!!!).
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