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Thread: OM Rib Height

  1. #1
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    Default OM Rib Height

    Hi everyone.
    Having built an A Style mandolin I am now embarking on an Octave Mando fool that I am! I just love the sound of them.
    I am working on increasing the measurements from my A Style by approximately 50%. I like the A Style shape. I have also been reading Graham McDonald's Bouzouki book getting ideas from there.
    He suggests there that the ribs should be 80mm high. This of course is for a Bouzouki.
    My question is - does anyone have thoughts on rib heights for an octave mando. From my rough drawings so far the body will be about 14" wide, 17" long in the classic A Style shape.

    Many thanks in anticipation.
    Jeremy

  2. #2
    Mandolin & Mandola maker
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    Default Re: OM Rib Height

    My octave mandolins are 300mm wide and 390mm long in the body. Body depth is 65mm internal. A style shape. Scale length is 21 inches, nut width 34mm. There are no standard sizes for octave mandolins, you can build them however you like. Mine are smaller than most because I wanted to maximize comfort and playability, hence the shorter scale length. Some might say they are too small, or the scale length is too short, but they work, and work beautifully both in terms of playability and sound.

    See - http://petercoombe.com/octave-mandolin.htm
    Peter Coombe - mandolins, mandolas and guitars
    http://www.petercoombe.com

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  4. #3
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    Default Re: OM Rib Height

    Thanks for that Peter, very helpful. I am probably goinng for a 21.5 inch scale length using a Pre slotted fingerboard that I already have. I prefer a carved front - what are your thoughts on carved vs flat?

    I like the look of yours..... I also have seen your construction notes which were very useful when I built my first mando.

    Jeremy

  5. #4
    Mandolin & Mandola maker
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    Default Re: OM Rib Height

    Carved will give you a different sound and they are a heck of a lot more work and are more expensive to make. I was going to make a carved OM after making the first flat top OM, but I liked the sound of the flat top so much I changed my mind and started work on 2 more. Now probably will never make a carved top.
    Peter Coombe - mandolins, mandolas and guitars
    http://www.petercoombe.com

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  7. #5
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    Default Re: OM Rib Height

    Thanks Peter. I appreciate that there will be a difference in sound. I have already made an A style mandolin and have spent 25 years pottering around with the violin family (both making and restoring) and totally agree that carving is more work, but I do like the feel and look of a carved top. They are, as you say, more expensive especially when you consider the amount of wood that ends up on the shop floor!

    I suppose that I am put off a flat top by my own ignorance and inexperience. I have read both of Graham McDonald's books and that of John Troughton but cannot quite see how the neck geometry works to get sufficient height for the bridge. I also cannot see how the fingerboard extension fits to the curved front. Neither book seems to mention it. What height of bridge should I be aiming for? I am assuming that I should use a floating bridge and separate tailpiece.

    Whilst I want to make an Octave Mando does it have to be flat top or carved top? I quite fancy making it in the Celtic style with carved top and flat back. Whichever style I plan to use a glued mortice & tenon neck joint.

    I appear to have gone well off topic, but any (polite)comments would be helpful.

    Jeremy

  8. #6
    Hands of Pot Metal
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    Default Re: OM Rib Height

    Quote Originally Posted by Jeremy A View Post
    ....I have read both of Graham McDonald's books and that of John Troughton but cannot quite see how the neck geometry works to get sufficient height for the bridge. I also cannot see how the fingerboard extension fits to the curved front. Neither book seems to mention it. What height of bridge should I be aiming for? I am assuming that I should use a floating bridge and separate tailpiece.
    Did you make an elevation as part of the drawings? You should easily see the dimensions needed for the neck geometry. And you'd need a detailed drawing of the top to show how the fingerboard extension will fit, unless the top is maintained as flat in that area.

    Nice project.
    Play it like you mean it

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  10. #7
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    Default Re: OM Rib Height

    Mine have a dovetail neck joint, neck angle is 1.5deg. The top and back have an induced arch (domed liked a guitar) of 15 inches. The bridge height I aim for is 22mm. What I do is to fit the dovetail and measure the bridge height as I go and stop when a straight edge gives me 12mm off the soundboard at the bridge position. That assumes 6mm for the fingerboard, 1mm for the frets and 3mm for the action on the E string to give 22mm bridge height. The fingerboard is likely to be off the soundboard so I made an extension that fits under the end of the fingerboard that is glued to the soundboard. The raised fingerboard gives better access to the higher frets so is a good thing to have.
    Peter Coombe - mandolins, mandolas and guitars
    http://www.petercoombe.com

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  12. #8
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    Default Re: OM Rib Height

    I am just an amateur, but I've built about a dozen OM's in the past few years. Mine have been about the same size as Peter's with the exceptions that I have used a 3" (~75mm) body depth & 22" scale length. I made one with carved top & back but have recently become more interested in flat tops (15' radius induced arch) again. I built a test set-up as inspired by James Condino & have greatly enjoyed the ability to experiment with bracing patterns. I use a mortise & tenon bolt-on neck pretty much like Graham McDonald suggests. I cantilever the fretboard over the top using a similar piece of wood underneath to reinforce the fb. I've been making my own bridges, but I think you need to allow at least 18mm if you want to use an adjustable one.

    I don't know if my experience helps any; at least it's another data-set. Good luck with your build.

  13. #9
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    Default Re: OM Rib Height

    I forgot to mention that if you are not already familiar with Andrew Mowry's Neck Geometry Calculator, it can be quite helpful:

    https://mowrystrings.com/neck%20geom...alculator.html

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  15. #10
    Registered User tonydxn's Avatar
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    Default Re: OM Rib Height

    Hi Jeremy
    I'm in the middle of my first OM build too. I'm basing my design on this mandola made by Louis Patenotte in the 60's (I think).
    Click image for larger version. 

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    The body will be 400mm long, 310mm wide and 85mm deep at the tailpiece end. Flat top, without the cant, X-braced, 510mm scale. Bridge will be about 13mm high, giving a break angle of about 10 degrees.

    As we are in roughly the same part of the world (I'm in Mansfield, Notts) we can get together and compare instruments if you like, when they're done.
    Mandolins: Bandolim by Antonio Pereira Cabral
    German flatback mandola by unknown maker converted from a descant Waldzither

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