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Scarf joint and headstock strength?
http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=43632		 Page 1 of 1
Author:  awaldrop [ Wed Jun 11, 2014 9:20 pm ]
Post subject:  Scarf joint and headstock strength?
Hi, first time builder question here.

I'm building a steel string acoustic from a pre-made mahogany neck and headstock with a scarf joint.

The current thickness of the headstock is .61 inches. I'd like to sandwich 2 layers of veneer to the bottom and one veneer and headstock plate to the top, in doing so I'm much to thick for my tuning machines to fit, so I would have to remove some material from the headstock to keep the thickness under a max of .65 for my turning machines.

The sandwich breaks down like this

Rosewood headstock plate: .12
Maple veneer: .2
headstock: .61
Black veneer: .2
Maple veneer: .2

To get it to work I'd need to reduce the thickness of the headstock down to .47. I'm afraid this will reduce the strength of the scarf joint to failure with the tension of the strings.

Would something like this work? Design and aesthetics aside, is this advisable? I know I have options to reduce the amount of veneer I'm adding, but I'm more curious as to how strong that joint is and if the added material would account for the material lost?

Any suggestions and expertise would be very helpful, I'm including an image to better visualize my project.
http://i.imgur.com/uwzBwrQ.jpg

Thanks
-Andy W
Author:  Glen H [ Wed Jun 11, 2014 10:25 pm ]
Post subject:  Re: Scarf joint and headstock strength?
You'll be fine in my opinion. Scarf joints are typically stronger than 1 piece necks. I have gone down to .5 without scarf joint and without back veneers.
Author:  DennisK [ Wed Jun 11, 2014 10:37 pm ]
Post subject:  Re: Scarf joint and headstock strength?
Pretty sure the long-grain headplate bridging the joint will actually make it stronger than it was to begin with. The back veneer may strengthen, weaken, or have no effect on it depending on how it connects to the shaft piece. But being .04" thick, even the maximum weakening effect isn't enough to worry about.
Author:  Anthony Z [ Wed Jun 11, 2014 10:54 pm ]
Post subject:  Re: Scarf joint and headstock strength?
Dennis, the veneer on the back of the headstock I think will strengthen (as will the headplate veneer). A really nice look on backstrapping the headstock (without a volute) is to extend the veneer past the nut and blend it into the neck ending in a "V" shape.
Author:  Jeff Highland [ Wed Jun 11, 2014 11:49 pm ]
Post subject:  Re: Scarf joint and headstock strength?
You are getting your decimal point in the wrong place for starters.
Veneers are generally 0.023" thick not the 0.2 you are stating.
Your headstock plate at 0.12 is excessively thick especially if used with the layers of veneer listed
I don't care for the proposed notching in of the headplate and veneer into the top surface, it is creating a weak area at the nut.

I would suggest thinning the headplate to 0.050 or less, and gluing it and the maple veneer to the existing front of the headstock
Then remove any necessary thickness from the back of the headstock before gluing on the back veneers.
Author:  Ken Franklin [ Thu Jun 12, 2014 1:45 am ]
Post subject:  Re: Scarf joint and headstock strength?
Jeff Highland wrote:
You are getting your decimal point in the wrong place for starters.
Veneers are generally 0.023" thick not the 0.2 you are stating.
Your headstock plate at 0.12 is excessively thick especially if used with the layers of veneer listed
I don't care for the proposed notching in of the headplate and veneer into the top surface, it is creating a weak area at the nut.

I would suggest thinning the headplate to 0.050 or less, and gluing it and the maple veneer to the existing front of the headstock
Then remove any necessary thickness from the back of the headstock before gluing on the back veneers.


What Jeff said.
Author:  Tim Mullin [ Thu Jun 12, 2014 4:00 am ]
Post subject:  Re: Scarf joint and headstock strength?
First, your headplate and back veneers will increase the breaking strength of the headstock, so no worries there.

Second, there's no issue with the 0.12 headplate itself. The strength of your overall 0.65" headstock laminate will not suffer. (I do this quite regularly, rather than binding a headstock to get the same "look" of extending the binding and side purfling of the fingerboard).

However, I don't like the idea of "notching" to receive the headplate in the nut area. I would simply apply the thick headplate to the top surface, and you then have options for how to treat the transition of the backstrap in the neck area, as others have suggested. The angle of the pic below is not directly from the side, but you can see the effect. In this case the backstrap transition uses a "smiley" volute.
Attachment:
EIR-Englemann Florentine cutaway 220 small.jpg
Author:  Michiyuki Kubo [ Thu Jun 12, 2014 4:20 am ]
Post subject:  Re: Scarf joint and headstock strength?
This is what I get for reading forums when it's too early. Thought he was talking about a slotted head the whole time till Tim posted that pic hahaha.
Author:  Tim Mullin [ Thu Jun 12, 2014 4:45 am ]
Post subject:  Re: Scarf joint and headstock strength?
Different guitar, same idea -- angled from the front. You see how the fingerboard binding lines up with the thick headplate and the side purfling of the fingerboard is aligned with the veneers.
Attachment:
Andrew's OM 607 small.jpg
Author:  awaldrop [ Thu Jun 12, 2014 8:38 am ]
Post subject:  Re: Scarf joint and headstock strength?
Yes, you're right, I got my decimals messed up. Shouldn't post to forums before I go to bed.

Appreciate you understanding what I meant in spite of what I wrote.

Great feedback, appreciate the help!
Author:  Haans [ Thu Jun 12, 2014 9:07 am ]
Post subject:  Re: Scarf joint and headstock strength?
I have been using a reverse scarf on a few instruments that have a slotted headstock.

Image
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