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Tenon theory

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stuckinthemud

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I am building a small lap harp. The pillar and neck are hawthorn the sound box black poplar. I am not a joiner I am cutting mortice and tenons, 3 of, forming a triangle, no glue, held together by the string tension of about 300lb, there will be a twisting force as the strings on the neck are offset by a half inch from the centreline. What are the correct proportions of a tenon? I think I read there is no hard and fast rule but the cheeks and tenon should feel 'balanced', sort of 1/2 for the tenon, 1/4 for each cheek?
 

Sandyn

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There was a thread on here have a look while you wait for some replies. Hopefully it will be interesting.
 

Doug B

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I am building a small lap harp. The pillar and neck are hawthorn the sound box black poplar. I am not a joiner I am cutting mortice and tenons, 3 of, forming a triangle, no glue, held together by the string tension of about 300lb, there will be a twisting force as the strings on the neck are offset by a half inch from the centreline. What are the correct proportions of a tenon? I think I read there is no hard and fast rule but the cheeks and tenon should feel 'balanced', sort of 1/2 for the tenon, 1/4 for each cheek?
Please post up some photos of the build it sounds very interesting.
 

TheTiddles

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Thirds is the usual “rule” and it’s not a bad one.

I can’t think of why your configuration would drive anything different, there will be some torsion on the joint, mostly compression drawing the shoulders together (presuming you’re doing it that way as the alternative will be to shear off the tennon which probably won’t happen, but you’d ideally not rely on just that to take the load as it’s inherently less large than the shoulder faying surfaces)

Aidan
 

Jacob

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Good idea to look at a measured drawing of a harp or two?
 

stuckinthemud

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It would, but harps are not often made like this anymore and I haven't found a drawing for this form of construction . Plenty of exterior drawings, none of the joints, although, I do have the papers by Karen Loomis...forgot those, just a sec.

Yes, the joints are in compression
 
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stuckinthemud

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I checked my files, and after your comments (thankyou TheTiddles) its clear the 14th century maker used thirds. Thank you Jacob for the reminder/nudge.
Screenshot_20201219-174209_Word.jpg
 

Phil Pascoe

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What are the correct proportions of a tenon? I think I read there is no hard and fast rule but the cheeks and tenon should feel 'balanced', sort of 1/2 for the tenon, 1/4 for each cheek?
Usually 1/3 - 1/3 - 1/3. With those dimensions there'd be a danger of a shoulder breaking out on small work.
 

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