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I finished up this octave mandolin two weeks ago. It’s the first one of a new design. The previous ones I’ve built have a small body about the size of a baritone ukulele with a 10.5” lower bout and 14” body length. For this one, the body is parlor guitar size with a 13” lower bout and an 18” body length. I was looking to get a fuller sound with more low end in particular and it does both.

Adirondack spruce top
Curly birdseye maple back, sides, rosette, and headstock plate
Curly big leaf maple neck
Cocobolo binding on body, fretboard, and sound hole
Indian rosewood fretboard and bridge

Tuned G D A E one full octave lower than on a standard mandolin. The open G for the lowest string course is the same as the third fret G on the low E string on a guitar in standard tuning. The open E course is the same as the open high E string on a guitar. The total string tension is about 170 lbs.




Left to right:
Standard F-style mandolin, small body octave mandolin, the new octave mandolin, dreadnought.



I’ve settled on using a 21” scale length. Octave mandolin scale lengths I’ve seen and played range from 19” to 23” (which is a huge range compared to acoustic guitars). The shorter lengths are easier to play but the low string tension results in the low strings rattling against each other within courses with anything more than moderately hard picking. The longer lengths make it hard to impossible, for me at least, to play fast single note stuff particularly in first position because of the long stretches caused by the tuning in fifths. A 21” scale length is a good compromise between playability and enough string tension to minimize string rattle in the low string course.

I’m going to post some photos of the building process followed by some photos of the finished instrument. Thanks for looking.


Joined top and back plates.


Routing the rosette ring channel.


Ready to install sound hole binding and rosette purfling.


Completed rosette


Ready to install braces


Vacuum clamping the top braces

_________________
Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

Carved top braces. I went with tapered lower x-brace arms.



Top fitted to the sides


Back fitted to the sides


Closing up the box


Routing binding/purfling channels


I bend the bindings and side purflings on a hot pipe with the side purflings sandwiched tightly between the bindings. That keeps them from twisting while being bent.


Ready to install binding and purfling

_________________
Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

Shop dog supervising work


Rough cut curly maple neck blank


Logo inlay cut out of pearl


Inlay recess done


Head plate glued to headstock


I rough cut the headstock shape on the bandsaw. The blocks double stick taped underneath raise the neck to give clearance for the neck shaft above the bandsaw table and keep the headstock level.


Final heastock shape done on the Robo-Sander using a template.


Gluing the binding onto the fretboard. I use this setup on my router table. The fretboard and binding are sandwiched between two pieces of wood with clamping pressure provided by the featherboards pushing everything against the fence. I wick thin CA into the joint between the binding and the fretboard. I made this setup years ago out of three pieces of scrap plywood and it works so I've never fancied it up.

_________________
Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

Fretboard glued to neck


The heels on my necks have to flat faces above the curve. I cut them with a pull saw.


I use Dragon rasps to do most of the shaping of the heel curve.


I remove most of the wood from the neck shaft using a spokeshave. I've settled on this after using various rasps and chisels for that task. This works best for me.


After most of the material has been removed with the spokeshave, I switch to Dragon rasps for the rest of the shaping.


On to the finish application. I'm using Endurovar and I brush it on. To make the application faster and easier, I came up with a handle for the body that works as a rotisserie for easy access to all sides of the body. It's made from two pieces of scrap wood and an 8" lag bolt that I had sitting around. It gets bolted to the body using the same hardware for attaching the neck. The wood block furthest from the body gets clamped in the vice and then the body can be rotated to whatever position is convenient. The eye at the end of the lag bolt comes in handy for hanging the body while the finish cures.

_________________
Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

This is what the finish looks like after the brushing is done.



And after level sanding and buffing



Final neck attachment to the body


Installing frets


This is the device I built about a month ago for removing the tangs from under the fret ends. It uses a cutting wheel in my Dremel and it's a huge improvement over the fret nipper I've been using all along. It never damages a fret end, it can be adjusted to give perfect removal of the tang, and it's way faster to do all the frets with this than with the nipper.


Jig for slotting the bridge. This is another example of a jig I made from scrap wood as a prototype and it worked so well that I never saw any reason to make it fancier.

_________________
Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

Routing the saddle slot


You may notice that this slot looks unusually wide and it's not angled. That's because the big differences in compensation points between string courses and having double string courses makes for a zig zag shape that's hard to fit onto a narrow angled saddle like we typically use for a guitar.


Finished bridge. I use two strings per bridge pin hole to maximize the gluing surface under the bridge.


Ready to scribe the bridge outline in the finish


Ready to remove the finish from the bridge footprint


Ready to glue the bridge on


Locating the compensation points on the saddle


Strings on for the first time

_________________
Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

_________________
Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

_________________
Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

That's a really beautiful instrument, Jay. Some questions:

- How do you like the vacuum jig? Is that the LuthierTool one?
- Is that a 13th fret attach? Is that common for these instruments?
- With the tension being 170 did you have to make any special considerations with the bracing / bridge?

Would love to hear it if you get a sample.

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Thanks, Brad.

I built the vacuum frame based on the info on the Joe Woodworker website. https://www.joewoodworker.com/veneering/vacuum-frame-press.htm I like it a lot. It's straight forward to use and works fine. The version I built takes advantage of the fact that the top on my island bench is an old benchtop from a biology lab. The surface is smooth and I use it as the base for the vacuum frame when gluing braces. The frame seals well enough against that benchtop. My shop is small so I didn't want to have to have a dedicated vacuum clamping table taking up room. With this setup, I can store the frame out of the way against the wall on top of a cabinet when not in use. I use a small GAST electric vacuum pump I got off ebay to draw a vacuum. A minor disadvantage of my setup is that the seal against the benchtop isn't quite good enough to be able to turn the pump off once it has reached max vacuum. Almost, but not quite. I have to leave the pump on, but it's pretty quiet. I leave the braces under vacuum for one hour. The guy I learned to build guitars from had a dedicated vacuum clamping table with a hinged vacuum frame in his shop and using it was what sold me on the idea. His table had a good enough seal to be able to turn the pump off after reaching max vacuum.

I don't know if you can tell from the photo, but the top is sitting on a radius dish that fits under the frame.

There's no standard for which fret the neck joint happens at. For this OM, with the 21" scale, the body length/shape, and where I wanted the bridge to be, the 13th fret join was just how it worked out.

170 lbs string tension is in the range of string tensions for six string acoustic guitars so the bracing is based on what I've used for my parlor guitars.

I'm working toward doing a sound sample. The scale length on this one is a bit longer than I've used before and it's enough of a difference that I'm having to adjust my technique a bit to play cleanly. Once I get there, I'll record something.

_________________
Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

Jay,

A lovely build.

I wiill probably "borrow" the rotisserie idea. I have a bicycle mechanic's stand that will hold a guitar for finishing just fine. The rotisserie will make it easier. Thanks!

Could you elaborate on your neck joint? How did you match the curve of the neck heal to the body?

Thanks,

Steve

There are two parts to it. One is cutting a radius in the end of the neck and the other is putting a radius on the heel cap.

For the radius on the end of the neck, I use a jig to cut the radius on my bandsaw. In addition to cutting the radius, it also allows me to cut the correct angles to align the neck along the centerline and to give the correct neck angle for the desired clearance over the top at the saddle location. Here are some photos that show the basic steps.

Here's a neck blank clamped to the jig. The jig is clamped to the fence and pivots around a vertical bolt. Sweeping the jig with the neck through the saw blade produces the radius cut. I originally made this jig from some random scrap pieces that were lying around as a prototype. It works so I've never made a prettied up version of it.




The pivot bolt (yellow arrow) is centered on a line that runs perpendicular to the leading edge of the bandsaw blade. The neck blank is in position here for a first cut. I start with enough extra length on the neck end to be able to do up to four test cuts to get the neck angles correct. I've done this enough times now that it takes two or three cuts to get a final cut that puts the neck alignment right on the mark. The second line from the end of the neck marks the neck/body joint position and the final radius cut will go there.




This next photo shows after the first cut. The distance from the center of the pivot bolt to the blade is the radius of the cut. The jig is clamped to the fence so the radius of the cut is adjustable. I cut the radius to be slightly smaller than the radius of the end of the body so that the neck will rest on the outside edges and not be "high centered" in the middle. After this first cut, I test the fit to the body for skew on the centerline and clearance above the top at the saddle. Any correction to skew is made by adjusting the side-to-side angle of the neck on the jig. The neck angle for clearance at the saddle is adjusted by moving a shim under the neck. I clamp the neck back into the jig and make a thin second cut and check the fit again. A third cut usually gets both angles right on the mark.

Now is where the first line from the end of the neck comes in. I use a bolt-on neck with a mortise and tenon. I'm sure this neck joint could be done as a butt joint, but I like the idea of having the tenon in a mortise more. So, the problem was how to cut a radius in the neck end and have a tenon. I solved this by using a floating tenon that goes in after cutting the radius. After getting the neck alignment settings on the jig dialed in for both neck angles, I cut the neck end off square on that first line so I can rout a mortise for a tenon.




Here's the neck after cutting the end square and routing the mortise for the tenon and routing the truss rod channel.




The neck then goes back on the jig for the final radius cut at the neck/body joint position. The tenon is a three-layer laminate with the grain alternating by 90 degrees in each layer.




This is what the neck end looks like after the final radius cut and the tenon glued in. One unexpected benefit of using this jig is that I can get the final radius exactly on target for neck skew along the centerline and neck angle for clearance at the saddle. I don't have to spend time flossing the neck joint to get the fit correct.




Here is what the fit to the body looks like.




For the heel cap, I draw the radius of the end of the body on the piece I'm going to use for the heel cap and cut it as accurately as I can on the bandsaw. I then sand out band saw imperfections with adhesive backed sandpaper on a dowel to get the fit pretty close. I fine tune the fit using the same approach we use to fit the bridge to the top. I bolt the neck onto the body with a piece of sandpaper in between the neck and the body with the sandpaper sticking up where the heel cap goes. I place the wood piece on the bottom end of the heel and slide it side to side against the sandpaper on the body to get a tight fit.




I hope this all makes sense. If something's not clear, let me know.

_________________
Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

Jay,

THanks! This is a very clever system. And very adapable, too. My interest was sparked both by your build as well as a video Isaw of Marshall Brune doing a classical build with Rob Scallon [url]https://www.youtube.com/watch?v=qmDAIlEGO_Q/url]. Brune uses a splined joint which he claims is superior to the Spanish heal. He makes it using a too-large-for-my-shop sanding machine that very quickly and accurately mates up the neck with the body.

I saw your build and had to know. Your system appears like it could also do a 'negative' angle for a classical guitar. I'll try it out on some 'ukes that are next on grandpa's list of things to build.

Thanks for posting,

Steve

That sanding machine in the video for making the neck/body joint is pretty slick. You'd definitely need a big shop for that though.

What is a negative angle for a classical guitar? Is that the forward tilt of the neck relative to the body?

_________________
Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

I believe so.

Jay,

yes, the 'negative angel' is the forward tilt of the neck. The nylon strings are floppier and need more room at the 12 fret.

THanks again,

Steve