repair or replace neck block

[Martian]

I use mortise and tenon neck joints. and something went real wrong. The rim is done and top on and trimmed tight . Putting in the neck into siminoffs style neck clamp, I over clamped and broke the top part of the block. I reglued it, and thought I could even pin it from the bottom with a dowel. but I really need to consider is a block not something you want to repair. then I realized to replace it, I doubt if I could get it out without taking the top off . Then getting the top back on ( with glue creep), and keep it where it needs to be. I have even considered a pin in both points, and blocks. How would you go about doing this repair? i have already cut another block but it sits beside the rim on the bench

[Bill McCall]

Pictures are worth a 1000 words.

[rcc56]

Pictures are worth a 1000 words.

Agreed.

If you decide that you must replace the block and the body is already assembled, you will be probably be better off removing the back rather than the top.

[sunburst]

If it is broken where I think it is, I think a replacement is the best repair.
(If pics show up and I'm wrong about where it is broken I may retract that statement.)

[Martian]

the back is not on at this time

[rcc56]

If the back is off, you can carve out all or part of the block bit by bit with a drill, router, chisels, gouges, or whatever else seems handy.

If possible, put the instrument back in the mold while you're carving.

Since we can't see what's going on, I'll suggest the possibility of cutting out plenty of wood around the damaged area, produce a regular gluing surface, and graft in a good sized chunk of new wood that extends outwards past the mortise. Think about physics-- in which direction will mechanical force be pulling when the instrument is under tension? Can you graft in such a manner that force will not be pulling the joint apart?

[HoGo]

If it is broken where I think it is, I think a replacement is the best repair.
(If pics show up and I'm wrong about where it is broken I may retract that statement.)

If I think the same I believe you could save it. Few thin long dowels wisely placed and gluedwith good structural epoxy would do it. Most of the strength of the joint is in the contact of neck with back and the upper part of heel against the solid part of block. Side forces are minimal. But I may be wrong if it is not the area...

[rcc56]

Whatever you do, there should be a way to execute the work without removing the top.

Do you know somebody who has a smart phone who can shoot us a picture or two??

[Martian]

I have posted pictures on here before, and we have spent the last 20 minutes trying to. I have the picture loaded, and cannot find a " post quick reply" button

[Martian]

[sunburst]

That's what I thought.
That is a weak place in a head block with the grain running that direction, and the most common mandolin neck joint failure that I see in repair work. Because there is short grain there, it is not strong before it's broken, and once broken and glued I have little faith in the joint holding up long term.
This is also a reason that I don't like the Simminof-style mortise and tenon joint. It requires a wide mortise and makes the grain even shorter in the head block.
I would replace the block, and think about a different neck joint, but that's just me...

BTW, I don't think of the job of removing the block as being particularly difficult, but that might have to do with years of experience. Furthermore, at the risk of appearing too critical, it looks like the block to rim joint could be improved anyway.

[belbein]

I give complete deference to John and the other real luthiers here, who all have a lot more experience and ability and smarts than I do. I'm only chiming in because I just replaced a disintegrated neck block ... only I had to replace about 75% of the block, i.e. the area all the way around the mortise. So a few things I learned: You need to use form to secure the whole body not only to keep everything steady, but to be sure that (1) the ribs don't flex when you remove part of that block and (2) when you go to put it back together, you can get the peghead-to-neck-to-tailpiece all aligned.

On #1: I found that the ribs, which should have been at 90 degrees relative to the table, were at 89 to 91 or so when I went to put everything back together. That meant dry fitting the back (which I had to remove) didn't fit and will have to be molding-ed some way. It's possible that I screwed it up when I built it--there is some empirical evidence for this conclusion. However--and remember I'm replacing a lot more than you are--remember that the neck block is one major cross-body structural support, so when it's less stable, there may be some torque on the ribs. I thought about this and that I probably should secure the whole thing in braces of some sort before I sawed it out, but I didn't take action.

On #2: I did smartly take counsel of my fears of losing the peghead-neckblock-tailpiece line. My fix was rather gross and sort of embarrassing and I won't confess to it, but a finer fix would be what others suggest. That's what I'll do if there's ever a next time. I kinda felt like a surgeon doing experimental surgery: I was successful in my main experimental effort, so who cares if the patient died?

I don't think this is a particularly difficult repair, but then that's because I tried it and so far it's working. I'm willing to try anything and learn from it. I figure (and this is a piece of advice, by the way): once there's a major problem, the instrument is throw-away-able. And throw-away-able instruments are great opportunities to push the envelope and learn some skills. Because, hey, it's already consigned to the dust heap. If the repair doesn't work, it's the only failure in life where you won't make things worse. (Of course the other side of the coin is that if it works, it will be invisible, and we're the only people you can brag to. But your band of brothers will appreciate your efforts.)

Go for it. Post pictures as you do it. Inquiring minds ...

[Martian]

thanks all, I am going to replace it

[Martian]

curious John, I think I used grain orientation according to Siminoffs book. With a Mortise joint would I be better quarter turning the grain. an you were not to critical on the rim block fit. It was the only place on the hole thing that was like that. I tried a dovetail and I was terrible at it. thanks again

[rcc56]

There's no need to pull off that top. Just carve away everything that's brown with the tools of your choice, and watch your fingers.
Follow John's guidance about the grain if you use the same kind of joint.

You could look at this as an opportunity to improve your dovetail skills. Or not.

Bear in mind that the Larsons worked without electricity for at least the first 20 years they were in business. They cut hundreds of dovetails by hand.

[CarlM]

I think I used grain orientation according to Siminoffs book. With a Mortise joint would I be better quarter turning the grain

Siminoffs book, if I recall correctly, had the grain at an angle through the block roughly 45 degrees to the axis of the mandolin.

[sunburst]

Siminoffs book, if I recall correctly, had the grain at an angle through the block roughly 45 degrees to the axis of the mandolin.

Not quite 45 degrees, but angled. That makes the grain even shorter on one side of the mortise.

[HoGo]

I would suggest different thing...
I think replacing the block would be big job even for seasoned builder and Martian (I guess) is novice. I believe it would not be too hard to remove the block considering the gaps, but for novice doing that without splitting the ribs would be nearly impossible and fitting new block at least half decently woud be even harder...
Since the area is near back and accessible my suggestion would be chisel away 1/8-1/4" of wood from damaged area all the way from tip to the inside of the block (in the direction of the arrow) and add new layer of cross grain wood that will bridge the crack, something like half of underside of the block. If you fit and glue it well it will restore much of the original srtength of the short grain where it is needed.
Next suggestion would be not use the dowels a'la Siminoff but rather one good woodscrew into the heel. I would fit the neck in place at correct angle and pre-drill hole 1/2" from back through the block into the heel and then relieve the hole in block and use one long screw (no threads in the block, just in neck) that will pull the neck against the block. You could use barrel nut in the heel and standard bolt but that can be tricky to install precisely.
And make sure he back is glued tightly to neck heel when that time comes.