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Thread: Semi-accoustic Mandolin build + various questions

  1. #1

    Default Semi-accoustic Mandolin build + various questions

    I think I'm going to have a bunch of questions so I thought I had better start a topic.
    The first mistake, of course was to decide to go my own way rather than do the sensible thing and buy a set of well founded plans, which would provide all the dimensions and things that are making me nervous, and the second is to go very unconventional on build method, rendering it difficult for folk to comment usefully anyway. The trouble is, although I have built a couple of instrument bodies, I have never constructed a neck, and that's what I'm getting nervous about.

    Anyway this is where we stand, headstock, neck/centre block assy, ribs and what will become a tail block.
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    The wood is mostly what I had so it's old Sapele, unfortunately not truly quarter sawn, but reversed so the grain is mirrored, and a band of maple in between. The plan is that what looks like a truss rod slot will contain laminated in place carbon which will run right from the headstock across the neck splice and neck/centreblock join and the tailpiece.
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    The first thing that is worrying me is that when I measure up the neck on my vintage Washburn and on guitars I am getting 15mm thickness (+ fingerboard to come on top of that). Is this reasonably conventional, I'm having a confidence crisis and thinking it looks awfully thin... The carbon should make strength no issue, so not sure why this confidence problem!

  2. #2

    Default Re: Semi-accoustic Mandolin build + various questions

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    Well, it's vaguely beginning to take shape... Piece of trivia. The wooden French curve visible belonged to my grandfather when he was a premium apprentice with the Metropolitan Railway before WW1, and were used laying out the instrument. I won't say designed..

  3. #3

    Default Re: Semi-accoustic Mandolin build - Soundboard grain orientation

    So, I have two pieces of bookmarked spruce for the top. I only ever recall seeing instruments with the grain running parallel to the centreline. For a different look it occurs to me that an essentially teardrop shaped mandolin could be put together with the grain at an angle to the centreline, maybe 25 degrees or so. It would be nearly impossible to get an exact bookmatch though unless you were truly astonishingly accurate. Is this something that could be done, or are there good reasons not to? Anyone seen it done and could point to a picture of what it looks like?

  4. #4
    Hands of Pot Metal
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    Default Re: Semi-accoustic Mandolin build + various questions

    Glue joints are strongest when the grain runs parallel in the joined piece. Mandolins have high tension, so you'd need to reinforce the top which would tend to dampen the sound. Not typically done, there's a reason for that. The commonly seen instruments are testament to the most common solution to the physical and acoustic goals.

    You say 'semi acoustic'. I have no idea what that means.
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  5. #5

    Default Re: Semi-accoustic Mandolin build + various questions

    Semi accoustic is a guitar term for an instrument which is partway between a full solid body and and a proper acoustic instrument. Typically its constructed with a solid piece about the width of the neck going the full length of the body. The idea, I suppose, was an amplified instrument that fed back much less than an amplified acoustic, but still retained some of the sonic character. Cynics might suggest they combine the worst features of both. The centre block means that the volume produced is so reduced (and tone compromised) that it can only be used amplified in public.

    So why would anyone want to build a semi acoustic mandolin? Well, it has two advantages in my special circumstances.
    The first is domestic - hopefully it will be loud enough unamplified for me to play to myself to learn tunes, but not loud enough to receive complaints from the household authority.
    The second is rooted in my limited experience and confidence as a 'luthier'. A proper mandolin requires a decent neck joint and a strong enough body to handle all the string forces. A fair degree of engineering and competent construction is required or the end result will be at best an instrument that refuses to stay in tune . With the centreblock the string loads are confined to the centreblock which is, with the neck, a single beam running the length of the instrument, and the neck to block joint is simple and very strong.

    The reason this came about is that I had always fancied a mountain/Appalachian dulcimer and bought a kit for one. I looked at the kit and thought ****, this is beyond my skill set, so purely for practice a built a very simplistic semi acoustic guitar body to go with a neck I had, and the end result has proved to be very useful in my circumstances.

  6. #6

    Default Re: Semi-accoustic Mandolin build + various questions

    @JimCh. Interesting ideas, so not the usual acoustic-electric layout. Well, unless the bridge is sitting on that center beam, you still have to have a top that is thin, supporting the string downforce and free to vibrate. So the conventional design parameters still apply if you want some sort of a pleasant sound. Bill McCall is correct: in any kind of woodworking, the worst glue joint is a butt, the best, aligned grain, so a 45 degree would be somewhat weak, but more importantly, wood swells and shrinks almost completely cross grain so the stresses on the body from a diagonal pair should be very much different. Plus, acoustically, a change in elasticity and vibrational modes. But go for it, and report back!

  7. #7

    Default Re: Semi-accoustic Mandolin build + various questions

    OK thanks, those seem sound arguments not to try it really, it was just a casual thought. Its going to be enough fun doing the join with only the short plane I have available here...

  8. #8

    Default Re: Semi-accoustic Mandolin build + various questions

    Sandpaper on a piece of glass Is your new long jointer plane.

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  10. #9

    Default Re: Semi-accoustic Mandolin build + various questions

    Or sandpaper on the top of any ground cast iron tool table... Table saw, band saw, etc.

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  12. #10

    Default Re: Semi-accoustic Mandolin build + various questions

    Nice thoughts, but if I had access to a tool table I'd have access to my long plane! Normally I do anything serious using my late father's workshop, but that's at my mother's house 40 miles away and she's in strict virus isolation, so I only have my basic house maintenance tools available. Still I expect I can sort something out although taking the french window out and using it for a sanding surface may meet objections from the domestic authorities!

  13. #11
    Registered User John Kelly's Avatar
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    Default Re: Semi-accoustic Mandolin build + various questions

    One caveat to think about when sanding in this way on a flat surface is that you need to ensure that you have the edge of your board kept vertical as you sand. it is very easy to end up with a rounded edge as you move the board backwards and forwards across the table, resulting in a poor mating surface on your two edges.
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  15. #12

    Default Re: Semi-accoustic Mandolin build + various questions

    John, correct, and same for the other components, and sanding a bridge in place. So, the emergency home kit needs a fence. A couple inches of a 2x4 or 2x4 or some random container, held with the fingers alongside the work. And no piece of glass? No problem. Any old board will do, even if it isn’t very flat, or the floor, or a table! I mentioned glass and sandpaper because it’s easy to clean between grits and good when wet sanding, as in tool sharpening. In Scotland, though, you may not have any substance called ‘grits’, which primarily is found only in the south of the US, and, in dire circumstances, may be eaten. Not to be confused with silicon carbide.

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  17. #13

    Default Re: Semi-accoustic Mandolin build - shaping the neck

    Tell me folks, does shaping a neck become less nerve-wracking when you've done a few of them? Its funny, I shaped one roughly with a piece of scrap pine for practice, and it seemed really straightforward, but doing it in anger is definitely stress inducing.

  18. #14

    Default Re: Semi-accoustic Mandolin build + various questions

    Bracing... The soundboard will be unsupported between neck and tail block, so the bridge won't be directly supported. I'm not concerned about the body bending per se under string tension as there's enough carbon in the centre block that I'd happily jump on it, but am I right in thinking that there ought to be a transverse brace under the bridge, tapering each side so that it doesn't meet the sides of the body? Its to be a spruce top.

  19. #15

    Default Re: Semi-accoustic Mandolin build - Soundholes

    Well. I've got to the stage where I have to consider sound holes. I know the topic has been done to death, because I've found any number of threads that say so, but I haven't really found the threads in which it has.

    I think I gather that f holes tend to sound more crisp, and oval holes more bass, while round sound holes don't seem to be a thing, which is a shame because they'd be far easier to make. Could someone point me at a useful thread? Also any comments on my previous post about bracing?

  20. #16

    Default Re: Semi-accoustic Mandolin build + various questions

    This project makes progress slowly.
    I decided on an oval soundhole which may have been an error cosmetically as the carbon on the centreblock is a lot more obtrusive than I expected. I also had a go at a busrt type finish which was a lot less than successful to my eyes. Too blotchy. I've since seen advice suggesting a coat of shellac first.
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    The first attempt at remediation was to consider a black border, for the same effect as binding. Dry wipe marker to the fore.
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    Maybe a slight improvement, but I was still unhappy. At this stage I was considering ripping the top off and building a new one, which could have had f holes.

    However one should not start a war before exhausting all diplomatic options, so out came my favourite boat building tool the random orbital sander.
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    Vicious sanding plus a good bit of scraping later. I just hope it's not too thin. I reckon it's still the high side of 2mm, but haven't measured it yet. An interesting side benefit is that the end grain on the spruce is still dyed rosewood brown and matches the sides reasonably, so there's a sharp transition between dark side and pale top which is not unpleasant and otherwise difficult to achieve at my skill level.
    I don't think I'll risk dye again, I'm sure it wouldn't take another sanding. Next time I feel like trying that I'll buy a piece of veneer faced mdf and make some practice pieces.

  21. #17
    Registered User mreidsma's Avatar
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    Default Re: Semi-accoustic Mandolin build + various questions

    Looking good! I have both an oval hole mandolin with a carved top (Gibson A) and a round hole flat top (Kalamaoo KM-11). I have had a few flat tops, and they have all had round holes, I think? Maybe the round/oval hole has to do with carved vs. flat tops...

    Following your build with interest!

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