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Octave mandotar just completed
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Author:  J De Rocher [ Sat Apr 18, 2020 5:02 pm ]
Post subject:  Octave mandotar just completed
I finished up this hybrid between an octave mandolin and a guitar a couple days ago. It's the first instrument like this I've built and I'm pretty happy with how it turned out for a first go at it. I've had a semi-crappy Michael Kelly F-style octave mandolin for a few years but never liked much it because the size and shape of the body makes it really awkward to play unless you are willing to always play standing up. It's also a string rattler which doesn't encourage playing it. So I decided to build one designed from scratch.

Settling on dimensional specs was something of a challenge because there's nothing at all like a standard for octave mandolins. From reading books and lots of information on the web, they are all over the map in terms of designs and dimensions. For example, scale lengths range from 19" to 24". That 5" difference is huge when you consider that standard Martin and Gibson guitar scale lengths differ by less than one inch. Nut widths also vary over a wide range. And there is no standard at all for how to distribute the string courses at the nut and the saddle.

I started out aiming for a body size about half way between a mandolin and a parlor guitar, but I could find only one off-the-shelf case that would fit and it cost about $350. I ended up going a little smaller with body dimensions similar to baritone ukuleles.

The Michael Kelly octave mandolin I have has a 19" scale which is unusually short. I always suspected that the short scale contributed some to the string rattle problem particularly on the G course strings because it leaves the strings slightly floppy. I switched out the original strings to heavier ones and it reduced the rattle a bit so I decided to go with a longer scale for the new design. At the same time, I wanted to keep the scale short enough that I could keep using the same fingerings for the songs I know on mandolin. With the 19" scale, I was forced to learn to stretch to keep the fingerings so I didn't want the scale to be very much longer and settled on a 20" scale.

Upper bout width = 8.0""
Lower bout width = 10.5"
Body length = 14.5"
Body depth at tail = 3.5"

Top: Lutz spruce
Back and sides: Walnut
Neck: Khaya
Binding, fretboard, & bridge: Indian rosewood
Rosette & head plate: Walnut

Since the design isn't based on an existing instrument, the bracing I settled on for the top was somewhat of a guess. It's x-braced with one tonebar and there's one finger brace on each side. I was concerned that it might be overbraced, but it turned out to be surprisingly loud and it has a lot of sustain. Chords have a nice full sound. The 20" scale is working out for playability. I have to stretch just a little bit more when playing in the first five frets than on the 19" scale, but I'm making the adjustment. For my hands, I wouldn't go with anything longer because I would have to learn new fingerings for the mandolin songs I know.

I went with a two-strings per bridge pin design. Thanks to John Parchem for tips in another OLF thread on how to go about slotting the bridge pin holes for this to work properly.

This was my first time using SilverTip epoxy for pore filling (thanks Michael Colbert for the recommendation). I plan to keep using it. It was also my first time using EnduroVar finish and I like it a lot. Big thanks go to Woodie G for her brush-on finishing schedule for EnduroVar and to Michael Colbert for final sanding and polishing tips specific for EnduroVar.

I got quite a bit of helpful general information from The Bouzouki Book by Graham McDonald and The Mandolin Manual by John Troughton. They are good reference books.

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Author:  bcombs510 [ Sat Apr 18, 2020 7:49 pm ]
Post subject:  Re: Octave mandotar just completed
Fantastic. It’s really beautiful! Thanks for putting it beside the others so we can see the size.


Sent from my iPhone using Tapatalk Pro
Author:  Chris Pile [ Sat Apr 18, 2020 8:24 pm ]
Post subject:  Re: Octave mandotar just completed
Thumbs up from me!
Author:  J De Rocher [ Mon Apr 20, 2020 12:46 pm ]
Post subject:  Re: Octave mandotar just completed
I'll follow up here with some photos of how this went together.

I designed the body shape using the program G Thang 0.7 written by RM Motolla. After converting the plan file to pdf format, I got it printed out full scale at Kinko's and use it to make a plan.

Attachment:
Octave mandotar build 1.jpg


I also wanted to see if I would have to modify my headstock design to accommodate eight tuners so I downloaded the Gotoh spec page for the tuners I planned to use from their website and printed it out at full scale, cut the tuners out, and place them on the headstock plan to see how they fit. I was able to use the headstock design without modifying it.

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Octave mandotar build 1a.jpg


New body mold made from three layers of 3/4" ply.

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Octave mandotar build 2.jpg


I bent the sides on my hot pipe. The walnut bent nicely.

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Octave mandotar build 3.jpg


Shooting the back plates to join them.

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Octave mandotar build 4.jpg


Joining the top plates using Bora clamps. I learned this approach from Rick Davis. It sets up fast, the pressure can be adjusted easily, and the clamp heads swivel so they work on plates that have angled edges as well as on plates with straight parallel edges.

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Octave mandotar build 4a.jpg


This is my deflection testing set up. It's made from scrap wood and two 1/2" dowels. It stores very compactly. I do both with-grain and cross-grain measurements.

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Octave mandotar build 5.jpg
Author:  Bryan Bear [ Mon Apr 20, 2020 2:39 pm ]
Post subject:  Re: Octave mandotar just completed
Beautiful and it is nice to see a bit of your process too. Nice work.
Author:  J De Rocher [ Mon Apr 20, 2020 3:06 pm ]
Post subject:  Re: Octave mandotar just completed
Here are some more pics.


Making the rosette. I rout the channels with a Bosch Colt in a Bishop-Cochran base which is super accurate. The small diameter dark ring is a piece of Indian rosewood bent on the hot pipe and will be binding for the sound hole. It gets inlaid into the inner routed channel. That channel is routed just shy of going all the way through the top. I learned this technique from Rick Davis.

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Octave mandotar build 6.jpg


Completed rosette with sound hole binding.

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Octave mandotar build 7.jpg


Gluing the braces to the top by vacuum clamping.

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Octave mandotar build 8.jpg


Top braces carved. I do most of the brace carving/shaping with a chisel. It may be hard to tell from the photo, but the lower x-brace arms taper in height from the X to the ends. I decide to go with tapered braces on this one because the top I used had very low cross-grain stiffness and I was concerned that scalloped x-braces might not provide enough support. Don't know if that was true, but that's what I went with.

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Octave mandotar build 9.jpg


Carved back braces.

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Octave mandotar build 9a.jpg


Rims complete and linings notched for brace ends.

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Octave mandotar build 10.jpg


Sides fitted to the top.

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Octave mandotar build 11.jpg


I installed a K&K mandolin pickup just for the heck of it to see how it would work.

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Octave mandotar build 12.jpg
Author:  J De Rocher [ Mon Apr 20, 2020 5:09 pm ]
Post subject:  Re: Octave mandotar just completed
Some more:

Closing up the box.

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Octave mandotar build 13.jpg


I use a pull saw to cut the channel for the end graft.

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Octave mandotar build 12a.jpg


I install the side purfling separately from the binding. It makes it easy to get nice miter joints at the end graft and at the cutaway if there is one. I picked up this method here on the OLF from John Giltzow.

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Octave mandotar build 14.jpg

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Octave mandotar build 14a.jpg


Ready to install binding and top purfling.

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Octave mandotar build 15.jpg


I do one-piece necks so I made this jig for cutting the 15 degree headstock face angle on the bandsaw. The jointed neck blank is clamped to the jig which has a track bar on its bottom side at a 15-degree angle that slides in the miter slot. The cut face only needs a bit of light sanding and it's done.

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Octave mandotar build 16.jpg

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Rough cut neck.

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Octave mandotar build 18.jpg
Author:  J De Rocher [ Tue Apr 21, 2020 8:10 pm ]
Post subject:  Re: Octave mandotar just completed
A few more:

I use dowel locator pins to keep the head plate from sliding around when gluing it to the headstock.

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Octave mandotar build 19.jpg


Here's the shaped headstock. I rough cut the outline on the bandsaw and then finish it using the Robo-Sander.

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Octave mandotar build 20.jpg


This is the jig (fixture?) I use to taper fretboards. That's a 1" top bearing flush trim router bit.

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Octave mandotar build 21.jpg


I came up with this set up on the spur of the moment for gluing the binding onto the fretboard the first time I needed to do it and worked fine so it's what I've been using ever since. Four pieces of scrap wood and two featherboards against the fence on the router table. I use CA glue.

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Octave mandotar build 22.jpg


The radiused sanding beam gets used as a clamping caul to glue the fretboard to the neck.

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Octave mandotar build 24.jpg


Neck ready for carving.

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Octave mandotar build 24a.jpg


I use the pull saw to cut the flat faces on my neck heels. Carving the transition from the two flat faces into the curved part of the heel is the trickiest part of my neck design. I mainly use Dragon rasps for that task.

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Octave mandotar build 24b.jpg


My favorite tool for the initial shaping of the neck shaft is an old spokeshave. Works great.

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Octave mandotar build 25.jpg
Author:  Chris Pile [ Tue Apr 21, 2020 11:05 pm ]
Post subject:  Re: Octave mandotar just completed
If I had more thumbs....
Author:  Bryan Bear [ Wed Apr 22, 2020 9:44 am ]
Post subject:  Re: Octave mandotar just completed
That end graft looks great!

At what point do you fit the heel to the body? This part has always been a bit frustrating for me. I decided to do something a little differently on the two I'm doing now and floss fit the heel while the neck is still mostly unshaped. I'm finding it easier to do with the fretboard off. Do you have any pictures of your process?
Author:  J De Rocher [ Wed Apr 22, 2020 5:20 pm ]
Post subject:  Re: Octave mandotar just completed
Here is a last set of photos. Thanks for looking.

I've been using Z-poxy to pore fill for a while, but this was my first time using SilverTip epoxy. It works fine. A big plus is that it doesn't smell like it will kill you.

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Octave mandotar build 26.jpg


Another first for me on this one was using EnduroVar finish (gloss). I've been using EM6000 up to now. I brushed it on using Woodie G's method. I like it and plan to keep using it. It looks great when done, maybe a bit better than EM6000. The biggest advantage to me of EnduroVar over EM6000 for brush application is that it goes on much faster.

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Octave mandotar build 27.jpg


After buffing:

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Octave mandotar build 28.jpg


Here's another jig made out of whatever scrap wood was around. It works so I've never tried to fancy it up. The L-shaped piece of ply on top can be clamped down however I want to set the angle of the saddle slot. The piece of on-edge ply that runs across it serves as a fence for the router in a Bishop-Cochran base.

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Octave mandotar build 29.jpg


Fitting the bridge to the top.
A note about the saddle slot here. In my research on octave mandolin specs, I couldn't find a definitive answer for what the scale compensation should be. Probably because octave mandolin scales cover such a wide range and the gauges of string sets also seem to vary quite a bit. As insurance, I went with a wide saddle blank (7/32") set straight across. It worked out fine.

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Octave mandotar build 30.jpg


Finish removed from bridge footprint and ready for bridge to be glued on.I use the StewMac Saddlematic tool to locate the bridge before scoring the finish for removal.

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Octave mandotar build 31.jpg


This is the clamp I use for gluing the bridge on. Made by George Lamb who sells them on ebay. It goes on pretty fast and leaves clear access for removing squeeze out.

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Octave mandotar build 32.jpg
Author:  J De Rocher [ Wed Apr 22, 2020 5:30 pm ]
Post subject:  Re: Octave mandotar just completed
Bryan Bear wrote:
That end graft looks great!

At what point do you fit the heel to the body? This part has always been a bit frustrating for me. I decided to do something a little differently on the two I'm doing now and floss fit the heel while the neck is still mostly unshaped. I'm finding it easier to do with the fretboard off. Do you have any pictures of your process?


I fit the neck to the body when it's still the rough cut neck blank before any carving and without the fretboard.

I came up with a way to cut the set angle and the side-to-side angle at the same time on a fixture on the bandsaw. It allows me to dial in both angles so well that I haven't needed to do any flossing on the last eight guitars. I'll look and see if I have any photos that would clearly show how it works.
Author:  Victor Seal [ Thu Apr 23, 2020 6:34 am ]
Post subject:  Re: Octave mandotar just completed
Gorgeous!!! Makes me wanna build one. Does sound more like a mando or a guitar. I really like it. Beautiful work.
Author:  bcombs510 [ Thu Apr 23, 2020 9:00 am ]
Post subject:  Re: Octave mandotar just completed
Jay, do you clear finish all the way out to the edge of the bridge footprint? That's a fancy shaped bridge, was it difficult to get the edges clean?
Author:  Ruby50 [ Thu Apr 23, 2020 9:09 am ]
Post subject:  Re: Octave mandotar just completed
Very nice instrument and an excellent set of photos

Can you let us know how to get to Michael Cobert's technique for sanding and polishing Endurovar? Ia ma clost to using it.

Thanks

Ed
Author:  J De Rocher [ Thu Apr 23, 2020 1:26 pm ]
Post subject:  Re: Octave mandotar just completed
Victor Seal wrote:
Gorgeous!!! Makes me wanna build one. Does sound more like a mando or a guitar. I really like it. Beautiful work.


It definitely sounds like a cross between the two. It still has the characteristic sound of double string courses, but it has a lot more sustain than a mandolin so chords sound fuller. And being tuned an octave lower than a mandolin gives it some guitar-like qualities.
Author:  J De Rocher [ Thu Apr 23, 2020 1:50 pm ]
Post subject:  Re: Octave mandotar just completed
bcombs510 wrote:
Jay, do you clear finish all the way out to the edge of the bridge footprint? That's a fancy shaped bridge, was it difficult to get the edges clean?


The finish is cleared to just shy of the edge of the footprint. I use an Xacto #16 blade and tilt the blade out from the bridge about 10 or 15 degrees from vertical so the tip of the blade gets just under the edge of the bridge and cuts just inside the footprint. The margin is maybe 1/64". That's enough that it's hidden with the bridge in place. The finish is typically 0.004" - 0.005" thick and that tiny bit of overlap hasn't presented any problems.

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Xacto Blade #16 .jpg


I don't find it hard to get the edges clean. I use a new Xacto blade for every bridge. Before I cut around the actual bridge on the top, I do one or two practice runs using my bridge template on a piece of scrap wood with a 3M tape mask (2 layers) on it just like on the guitar top. The practice runs refresh my memory about how to hold the Xacto knife so as to not wander away from the bridge or cut into the bridge and how much pressure to apply. The practice runs also get my confidence up to do the actual cut and then I just focus on it and go at a slow steady pace. The bridge is bolted in place on the top so it can't move at all while cutting around it.

Here's what it looks like right before I cut around the bridge.

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Octave mandotar build 30a.jpg
Author:  J De Rocher [ Thu Apr 23, 2020 2:01 pm ]
Post subject:  Re: Octave mandotar just completed
Ruby50 wrote:
Can you let us know how to get to Michael Cobert's technique for sanding and polishing Endurovar? Ia ma clost to using it.

Thanks

Ed


This is what Michael does:

Dry sand with 3M Gold Fre-Cut sandpaper. 600 grit, then 800 grit.
Sand with Mirka Abralon 1000, 2000, and 4000. (Michael does these steps using a Festool ETS 125 random orbital sander. I did them by hand)
Buff with Menzerna P204 medium compound and Menzerna P175 fine compound.
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