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Thread: Half a mandolin question

  1. #1

    Default Half a mandolin question

    I just bought an antique banjo-mandolin just to experiment with sound, and to have a familiar scale length. Maybe 1900-1920. In fairly untouched condition. The neck has moved upward over the years, so I started to remedy it, so far not well, but the issue concerns a dowel joint with the neck that isnít solid enough. The neck is pulled tight agains t the rim by wedges on the long stick that spans the rim, and the loose mortise in the rim allows for a little alignment.
    So far, I used heating tape and softened the tenon on the stick and repositioned, but after putting a little string tension on, the neck moved. The hind end of the stick is a fixed position, not an adjustment.
    So, should I go after the round neck tenon with epoxy or hardware, shim the neck heel/rim connection, or do something else?
    Although itís pristine and antique, Iím not adverse to making mods.

    A second issue is the Waverly tailpiece, but I can probably deal with it.

  2. #2
    Registered User sunburst's Avatar
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    Default Re: Half a mandolin question

    From what you've said, the dowel is mortised into the neck heel. That joint needs to fit, not a loose fit with glue in gaps, but a good fit with shims in gaps. As for the neck angle, a good fit there probably won't improve anything because the neck angle is determined by the fit of the heel to the rim. The heel can be cut/shaved/sanded for a better fit, and as long as the neck is reasonable straight, that should do it.

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  4. #3
    Teacher, luthier
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    Default Re: Half a mandolin question

    The instrument needs a neck re-set. Just like a neck set on a guitar or mandolin, this involves reshaping the fit of the heel to join the rim of the instrument at the angle necessary to provide good string action and alignment with the centerline of the instrument.

    The dowel stick performs the same function as the dovetail or mortise and tenon joint on a guitar or mandolin. It must fit snugly into the heel cavity, and exit the heel at the angle necessary to secure it snugly to the rim and pull the neck securely into position without forcing the rim out of shape. In shops that are set up for banjo work, this is often accomplished by plugging or shimming the heel cavity and recutting the cavity by using a sled and a horizontal drill on a Shopsmith. The correct off-set for the cavity is approximately 3 degrees from horizontal on a five string banjo. I don't know what the off-set would be for a banjo-mandolin. You're going to have to figure that out.

    If you are not tooled up to do the work on the cavity with a good horizontal boring set up, you are going to have to find a way to do the work by hand. This might involve using shim[s] in the heel cavity and finding an accurate way to re-cut the cavity with hand tools.

    If you go to Banjo Hangout, you will find numerous archived threads describing some of the techniques involved in resetting a banjo neck. If you read carefully, you will be able to separate the luthiers' techniques from the hackers' bad ideas.

    The two most important things are that the neck angles are correct and that the dowel holds the heel securely in position without stressing the banjo rim. Everything on a banjo should fit snugly and securely, but nothing have to be forced into position.

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  6. #4

    Default Re: Half a mandolin question

    Thanks. I’ll get at it in a day or two. Just might mention that a medium size lathe is also a horizontal boring machine, best a metal-cutting version, where the ‘sled’ only has to be a clamp on the compound. I’ve seen floor-standing drill presses modified to do the same thing, but it looks clumsy and complicated. One thing I’ve noticed so far is that since the rim has only slight curvature, the fretboard can easily be misaligned in the other (roll?) direction, and needs a setup gauge when it goes back together. No accurate V or recess for alignment.

  7. #5
    Registered User Greg Mirken's Avatar
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    Default Re: Half a mandolin question

    I'm very familiar with the banjo neck attachment you describe. It's not the best, but it works. Taking material off the heel won't get you anywhere; you'll just end up with a gap. Does the end of the dowel stick mortise into the rim? If not, it's easy to plug the hole in the dowel for the tailpiece bolt and redrill a little lower [ that raises the end of the dowel and allows the neck to angle down]. Then use a shim between the rim and the fingerboard to force the neck back as you tighten up the tailpiece bolt and tap the wedges.
    The dowel has to be solid in the heel cavity of course. It will have been glued with hide glue, and if it's loose you will need to reglue it. The two reasons the action raises on these old things is the neck itself bowing [unrelated to the joint, obviously] and the rim deforming ["potato chipping"].
    The tolerances are always loose, and when reassembling it you grab the neck and twist it one way or the other to get the fingerboard in line with the head.
    Shade Tree Fretted Instrument Repair
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  9. #6

    Default Re: Half a mandolin question

    All true. Heated the heel and took out the tenon. Several surprises. First, it’s only 1/2” diameter, and the 3 degree angle is machined on the tenon, not the heel, which seems backwards. Then, the angle went the wrong direction, so it either bent over the years, or the stick was made upside down. Since the clamp relies on a screw hole, not an assembly error. Possible that this one left the factory wrong and was therefore living in its case all these decades! Reversed the stick after checking angles and clearances, and added a thin shim on the neck heel to add another 5 degrees to allow for the rim deforming. Made a test bridge of approximately reasonable height. Will string up to full tension in a day or so, but so far, I’m thinking that while this construction can work on a banjo, the eight steel strings and their 200 lbs of force is too much here. Plus, the heel of the neck has to step over the metal hoop, which makes its contact area even less. And remain removable for head change.

    If it works out and holds, I’ll just play it, but, being a compulsive designer, i might have to make something to lower the forces at the heel by spreading the load.
    Also, in tuning and detuning so far, it occurs to me that, absent a tuned cavity, a banjo sort of device is just a free membrane in space, sort of like a cone on a loudspeaker that’s not in a box. Within broad limits it should be tunable over a much wider range, much like an electric. So I could operate this thing as a mandola or an octave if it sounded interesting that way. Ya think?

  10. #7
    Registered User Mandobart's Avatar
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    Default Re: Half a mandolin question

    My 1930's Stahl banjolin has a square dowel set into the neck heel, with opposite end screwed in place from the outside of the rim.

    I set mine up for modern low action by both plugging and redrilling the dowel to rim connection and shimming the neck heel to rim with about 3/16" of rosewood. This was about 4 years ago and it still plays great.

  11. #8
    Registered User Greg Mirken's Avatar
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    Default Re: Half a mandolin question

    Well, you've got that short scale length to deal with, you know. Anything lower than mando tuning is likely to sound un-musical. Heavy strings tuned down are just going to flap in the breeze.
    But try it! what's to lose?

    Greg Mirken
    Shade Tree Fretted Instrument Repair
    Now located in Nevada City, California
    http://www.shadetreeguitars.com

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  13. #9
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    Default Re: Half a mandolin question

    I never put mandolin strings on a banjo/mandolin. I use two sets of tenor banjo strings with the heaviest string a 28. Sounds good and waaay better than the brash sound you get with heavier stings. These get a bad rap because of too heavy a string set. It is still a banjo after all.
    THE WORLD IS A BETTER PLACE JUST FOR YOUR SMILE!

  14. #10

    Default Re: Half a mandolin question

    Quote Originally Posted by pops1 View Post
    I never put mandolin strings on a banjo/mandolin. I use two sets of tenor banjo strings with the heaviest string a 28. Sounds good and waaay better than the brash sound you get with heavier stings. These get a bad rap because of too heavy a string set. It is still a banjo after all.
    Thanks! I was wondering about exactly that. And I assume these are bare and wrapped steel. The other option, Nylgut (awful name) would, I guess, be a possibility, attractive because of lower strain on the structure, and probably a mellower sound.
    I made another small change to the neck stick, enough to string up and play a little, but have been thinking of liberating the back of the stick and making it adjustable because it looks as if lowering the bridge for action adjustment, and reducing the break angle too much is a bad idea.

  15. #11
    Registered User Greg Mirken's Avatar
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    Default Re: Half a mandolin question

    Right. These usually have a pretty minimal break angle to begin with; if you lower it further the bridge just scoots around on the head when you play hard and it goes crazy out of tune.
    Shade Tree Fretted Instrument Repair
    Now located in Nevada City, California
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  17. #12

    Default Re: Half a mandolin question

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    Did it. Now adjustable action. Given the springy (original skin) head, seemed like a necessity. Now I can buy those tenor banjo strings and make a more or less conventional bridge. Lilac, I think.

  18. #13
    Registered User Greg Mirken's Avatar
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    Default Re: Half a mandolin question

    Good work. Not elegant, but I have done similar using a banjo resonator or tailpiece bracket.
    Shade Tree Fretted Instrument Repair
    Now located in Nevada City, California
    http://www.shadetreeguitars.com

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