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Preparing The Tenon For Cumpiano Style Neck Attachment
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Author:  Terence Kennedy [ Tue Sep 21, 2010 11:01 am ]
Post subject:  Preparing The Tenon For Cumpiano Style Neck Attachment
There was a recent thread on drilling the tenon for a barrel nut type attachment. I just did one so I thought I'd show how I've been doing it. I've done it pretty much the same way on the last 30 guitars.

Stick a transfer punch of proper diameter through the head block hole and mark the tenon with the neck in proper position. I inlay Maple strips in my tenons for extra strength.

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I use this jig for drilling and use another transfer punch to align with the marks on the tenon. You can reposition it for each set of holes if the spacing is different than on the jig. I stick strips of tape on either side of the tenon to be sure the jig fits snugly. That also helps with tear out when the drill bit goes through the other side of the tenon.

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I get the barrel nuts from Ace. They are a little wider than my tenons so I have to file them a bit

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They are 10mm and I couldn't find a 10mm drill bushing for my jig so I drill 3/8" and carefully widen with a reamer and 10mm bit in reverse.

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I usually flood the tenon area with thin CA and install the nuts.

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If you can't get a bolt the right length here is a handy jig to cut them off the right length every time.

Terry

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Author:  Arnt Rian [ Tue Sep 21, 2010 3:19 pm ]
Post subject:  Re: Preparing The Tenon For Cumpiano Style Neck Attachment
That's a well though out process, Terrence.

I notice the holes for the bolts (not the barrels) in the tenon are quite large, is this to make alignment of the bolts easier? It looks to me like the holes are larger than needed for this...?
Author:  Terence Kennedy [ Tue Sep 21, 2010 4:18 pm ]
Post subject:  Re: Preparing The Tenon For Cumpiano Style Neck Attachment
Arnt Rian wrote:
That's a well though out process, Terrence.

I notice the holes for the bolts (not the barrels) in the tenon are quite large, is this to make alignment of the bolts easier? It looks to me like the holes are larger than needed for this...?


Yes. The tenon holes are 5/16" and the headblock holes are 3/8". The thing I like best is the way the barrel nuts can self-align to the neck angle. Probably not that big a deal in steel string flat tops but I've found it really nice with the higher neck angles in resonators and archtops. You need a little bigger hole in the tenon for that to happen.
Author:  ChuckB [ Tue Sep 21, 2010 6:24 pm ]
Post subject:  Re: Preparing The Tenon For Cumpiano Style Neck Attachment
Terry, Those drill bushings look a lot like the ones I use on my jigs (from Proffesional Hardware & Supply).... the two piece set where the bushing guide screws into a mounted insert. I have been able to cut through the bushing insert with a hack saw blade for a thin jig. I wonder if you couldn't drill out a 3/8" bushing to 10 mm too?

Chuck
Author:  Terence Kennedy [ Wed Sep 22, 2010 8:01 am ]
Post subject:  Re: Preparing The Tenon For Cumpiano Style Neck Attachment
That's a good idea Chuck. I've got some extra 3/8" inserts, I'll give it a try and report back. Thanks!
Author:  Barry Daniels [ Wed Sep 22, 2010 9:12 am ]
Post subject:  Re: Preparing The Tenon For Cumpiano Style Neck Attachment
The large holes for the attachment bolts actually removes quite a bit of the wood that is holding the barrel nuts in place. You could achieve the same angle adjustment capability by cutting slots in place of the round holes. Leaving more wood in this area would provide a larger margin of safety.
Author:  Terence Kennedy [ Thu Sep 23, 2010 11:40 am ]
Post subject:  Re: Preparing The Tenon For Cumpiano Style Neck Attachment
Barry Daniels wrote:
The large holes for the attachment bolts actually removes quite a bit of the wood that is holding the barrel nuts in place. You could achieve the same angle adjustment capability by cutting slots in place of the round holes. Leaving more wood in this area would provide a larger margin of safety.


A while back I posted some testing I did on the Cumpiano system using a dummy mortise and tenon with a torque wrench. With carbon bar or maple reinforcement there shouldn't be a problem. The plain tenon without any reinforcement failed in tension at a fairly low pressure.

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