Acoustic Guitar Construction

Here is something that has me puzzled. Now I have not been learning luthiery for very long but I have questions about scale lengths on guitars. I am fairly clear about small bodied 12 fret guitars with 12.75 and 12.9 scales with a compensation factor of .15” added in to get the final string length. But I have 2 sets of plans for 2 very similar OM guitars- one has a scale length of 24.9” has 14 frets to body (13.80”) and overall string length of 25.5”. The other is 25.4” also 14 fr to body (14.08”)with overall length of 25.9”. The only thing that makes sense it the 12th fret is correct for the scale lengths involed. What am I missing about this ? Can I make a 24.9 scale guitar with a string length of what ever I choose as long as the 12th fret 12.45? Please help me figure this out.

Steve

Hi Steve,

The scale of a guitar is measured from the Nut to the 12th fret, and then the same distance again to the saddle. So if you measure 12.9 to the 12th fret, you need to measure 12.9 from the 12th fret to the saddle.

Scale length is the larger of the 2 numbers e.g. either the 24.9 scale or or the 25.4 scale length, theoretically measured from the leading edge of the nut to the middle of the saddle where the string sits and breaks over the edge. The 12th fret is one half of the scale length e.g. 12.45 for a 24.9 or 12.7 for a 25.4. Compensation is the additional length needed to account (or if you will "compensate") for the change in string length when a string is fretted (remember when you push a string to the fret it lengthens it, in essence the distance from A (the nut) to B (the saddle) is shorter than the distance from A (the nut) to B (the fret) to C (the saddle)), that number is usually anywhere from 0.10 to 0.125 depending on who the builder is and how they make their saddles.

Also to answer one of your questions - No you can't just take any length if the 12th fret is the right number.

I understand that aspect of the math and I have been adding .15" on the guitars that I've done and doubling the 12th fret gives me on the 24.9" a total length of 25.05" which works great for these. But when I look at these plans there seems to be well over .55" of compensation. Is that in the ball park? I built my first two guitars from Cumpiano's book and have used the design layout that is suggested there ie. doubling the 12th and adding .15" and you should in theory be able to build any scale length you desire. These plans that I am talking about are Scott Antes-APG-7 and 14. I just don't get the extra length.

Steve

stevem wrote:But when I look at these plans there seems to be well over .55" of compensation. Is that in the ball park?Steve

No, that can't be right. Have you ever seen the Stew-Mac saddlematic? Check it out and look at the instructions. They can give you a good idea how the compensation is added to the total scale length.

http://www.stewmac.com/shop/Tools/Speci ... ns#details

This conversation has been on going since the first guitars were made -- its why I invented the KMG Bridge Setter tool. Not to mention that I once glue the bridge on a commissioned guitar exactly 1" off location!! I see errors in the conversation above and even in Cumpiano's book. Somewhere I do have a very good comprehensive explanation and procedure, when I locate it I'll post it.

thanks Tim for the S-M info and thanks in advance Ken, I look for the post

Steve