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curious1

Neck Set Mortice

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I’ve been meaning to post this for awhile to the 158degree neck angle discussion but it is now closed.  So, new thread.

This is the reason, to my mind, setting the bottom of the mortice deeper than the top makes the joint stronger. No doubt the engineers will correct my errors.

in the 90degree option the neck rotates around point A (the top of the neck mortice). For the neck joint to fail the glue must break in tension at the back of the mortice and the bottom must break in shear. As the neck rotates the bottom of the joint opens and encourages this type of failure. 

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in the <90degree option the neck again rotates around point A but now the bottom of the neck is forced into compression against the the bottom of the mortice. This greatly reduces the likelihood of failure in shear. Additionally the back of the mortice would need to fail in shear (C) AND in tension. 

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The small lip at the top of the ‘neck’ illustrated below indicates that the neck would need to slip upward to escape the mortice. The back of the neck heel is longer than the mortice at 90 degrees. (The hypotenuse of a 90 degree triangle is longer than its base).

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Compression under load is the principal structural feature of the dovetail joint.

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Additionally, most necks fail at the bottom of the mortice first and then the back of the mortice gives way. And while it is true that the back of the neck heel is mostly in compression that is only true only up until the bottom of the mortice fails.

Also, the geometry of the <90 degrees is strongest for lateral stability also.

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26 minutes ago, David Burgess said:

Better to not describe it as a "dovetail joint", since that has already highly pissed off a few people here. :lol:

Dovetail, dovetail, dovetail!!!!   [Runs out the back door, slamming it, and giggling.] :lol:

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IMO, take it or leave it, there shouldn't be much stress at the bottom of the heel so long as the glue is holding. If the glue is not holding, which would then bring this wedge like action into play, then the neck will give way anyways.

Plus, isn't most of the force from the strings being used to compress the neck/fb assembly linearly?

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Mmm - all to often when discussing neck joint failure hardly anyone mentions the area of glue joint on the sides of the neck. The lower third of the side areas total almost as much as the button.

cheers edi

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2 minutes ago, edi malinaric said:

Mmm - all to often when discussing neck joint failure hardly anyone mentions the area of glue joint on the sides of the neck. The lower third of the side areas total almost as much as the button.

cheers edi

Hi Edi, there’s not much to discuss. It either fits or it doesn’t. If one has trouble making it fit err to making it tighter at the bottom (at the button) and tighter at the front (ribs). 

There is not really any meaningful difference between the two neck settings styles as relates to the sides of the mortice

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3 hours ago, Violadamore said:

Dovetail, dovetail, dovetail!!!!   [Runs out the back door, slamming it, and giggling.] :lol:

better watch out.  Beteljuice will show up.  

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I agree that setting the tail of the heel in deeper is stronger.

On the other hand a well fitted neck joint simply becomes one piece of wood with the top block.

That's a big strong block of wood.

You don't need to worry about this stuff if the joint actually fits

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14 hours ago, Melvin Goldsmith said:

I agree that setting the tail of the heel in deeper is stronger.

On the other hand a well fitted neck joint simply becomes one piece of wood with the top block.

That's a big strong block of wood.

You don't need to worry about this stuff if the joint actually fits

Agreed. The joint gives it strength while the glue should just hold the joint in place.

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Hi All - I was shown that when making the joint one chalk-fits the neck so that it is about 2mm shy of the button. When gluing it home,  the clamp drives the neck down until it bottoms against the button - voila - a superbly fitted joint.

cheers edi

 

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7 minutes ago, edi malinaric said:

Hi All - I was shown that when making the joint one chalk-fits the neck so that it is about 2mm shy of the button. When gluing it home,  the clamp drives the neck down until it bottoms against the button - voila - a superbly fitted joint.

cheers edi

 

i think that's too tight

 

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12 minutes ago, edi malinaric said:

Hi All - I was shown that when making the joint one chalk-fits the neck so that it is about 2mm shy of the button. When gluing it home,  the clamp drives the neck down until it bottoms against the button - voila - a superbly fitted joint.

cheers edi

 

Hi Edi, that seems a bit dangerous :o

And also, wouldn't this prevent determining a precise fingerboard projection?

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19 hours ago, curious1 said:

Agreed. The joint gives it strength while the glue should just hold the joint in place.

I do suppose that these things would be more important if absolute strength were the paramount issue, as if nail'less wood joined beams holding up a roof load. 

That being said, I feel "we" could set the back heel angle to anything we wanted from straight 90 to any back tilt we wanted, and that, as long as what Mel points out is happening, a good fit, that any angle will be sufficient to bare the normal load,plus.

I find that a well fit junction between the block and heel is all that is needed to create the proper atmosphere for cellular unification between the two pieces.

If the intent is to create something that may be used as a club, i would suggest skipping wood and joinery all together and just go straight for the aluminium baseball bat! :lol:

But, all in all I would agree from an engineering point that, yes, it is stronger, but probably not needed., but I suppose it's good to know you could get a couple of extra whacks in there before the corpus breaks

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6 hours ago, Emilg said:

Hi Edi, that seems a bit dangerous :o

And also, wouldn't this prevent determining a precise fingerboard projection?

Hi Emilg - not really...

- On dangerous - i) wood is compressible and ii) the actual interference amounts to  ~ 10 thousandths of an inch - and at least half of that is probably absorbed in the non-mating portions of the chalk-fitted joint.

- on projection - the projection is your "dry-fit' projection minus the gap to the button. Seems to work out OK.

cheers edi

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57 minutes ago, edi malinaric said:

Hi Emilg - not really...

- On dangerous - i) wood is compressible and ii) the actual interference amounts to  ~ 10 thousandths of an inch - and at least half of that is probably absorbed in the non-mating portions of the chalk-fitted joint.

- on projection - the projection is your "dry-fit' projection minus the gap to the button. Seems to work out OK.

cheers edi

Hi Edi, i must admit i did this on my first 2 violins, but more out of impatience :D the gap was less that 1 mm, and i thought it would give a good fit. Now i make the neck fit to the button, and glue size the wood on both sides which makes the fit tighter again.

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5 hours ago, edi malinaric said:

 

- On dangerous - i) wood is compressible and ii) the actual interference amounts to  ~ 10 thousandths of an inch - and at least half of that is probably absorbed in the non-mating portions of the chalk-fitted joint.

 

Hi Edi, I wonder if you meant 0.2mm instead of 2mm.  2mm is more than 1/16", not a hundredth of an inch

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6 hours ago, Kevin Kelly said:

Hi Edi, I wonder if you meant 0.2mm instead of 2mm.  2mm is more than 1/16", not a hundredth of an inch

Hi Kevin - no - 2.0mm it is. It doesn't take much in the way of G-clamping to pull it home.

In  the interests of "knowing" - why not grab some scrap end block/maple bits and chisel out a top block mortise and neck heel and check it out.

cheers edi

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12 hours ago, edi malinaric said:

Hi Emilg - not really...

- On dangerous - i) wood is compressible and ii) the actual interference amounts to  ~ 10 thousandths of an inch - and at least half of that is probably absorbed in the non-mating portions of the chalk-fitted joint.

- on projection - the projection is your "dry-fit' projection minus the gap to the button. Seems to work out OK.

cheers edi

Usually I fit almost to the bottom because I am not comfortable with the pressure, since there is glue added to the joint it gets tighter anyway.

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4 hours ago, Michael Szyper said:

Usually I fit almost to the bottom because I am not comfortable with the pressure, since there is glue added to the joint it gets tighter anyway.

Hi Michael - you have a point. Hide glue is pretty wet stuff and any time you add moisture to wood it swells.

The question now is what happens down the road when the moisture migrates out of the wood? Maybe the wood shrinks and the glue  join is left under stress - just waiting for the instrument to be dropped.

How many have seen an attic-stored violins "disassembled" after being left undisturbed for a number of years? Generally there are no splinters of wood to hamper the clean and re-glue operation. Or - when removing a plate you slip in the edge of the knife and the joint obediently cracks open (sometimes luck must be on our side). These seem to indicate that the glue line is under stress that is pretty close to glue join failure - needing only a little more to fail.

We are all taught to leave a glued joint clamped and undisturbed for 24 hours. This always puzzled me a bit because there are those who do a rubbed joint! Brian taught me to rub the plates together until they "grabbed". Then immediately  smack the top plate and break the joint, re-glue, smack, re-glue and clamp. Once I was distracted from the second of the breaks, maybe by about 8 - 10 seconds. It took a almighty left hook to separate the joint! So I know just how quickly hide glue joint gains strength.

However I still glue three times and clamp for 24 hours - on the theory that the clamping introducing a compression stress that offsets the tension stress due to the gelling of the hide glue - and keeping everthing static until the excess moisture has had a chance to move out.

Anyway that's my take on it.

cheers edi

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On 6/15/2019 at 9:55 AM, edi malinaric said:

Hi All - I was shown that when making the joint one chalk-fits the neck so that it is about 2mm shy of the button. When gluing it home,  the clamp drives the neck down until it bottoms against the button - voila - a superbly fitted joint.

cheers edi

 

Edi, one of the downsides of doing it that way is that the clamp will not only pull the neck down to the button. It will also deflect the button and back up to meet the neck. This often results in the outline of the mortise being printed through to the outside of the back. I've seen this happen a number of times.

On a new unvarnished fiddle, I suppose one could scrape this deformation away, but it's something many of us don't want to happen on a repair, such as a neck reset or neck graft on a valuable instrument.

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3 hours ago, edi malinaric said:

We are all taught to leave a glued joint clamped and undisturbed for 24 hours. This always puzzled me a bit because there are those who do a rubbed joint! Brian taught me to rub the plates together until they "grabbed". Then immediately  smack the top plate and break the joint, re-glue, smack, re-glue and clamp. Once I was distracted from the second of the breaks, maybe by about 8 - 10 seconds. It took a almighty left hook to separate the joint! So I know just how quickly hide glue joint gains strength

That’s interesting stuff. How long do you wait between each ‚glueing approach‘? Do you flatten the glueing surface each time again?

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