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Photo essay on one approach for layout and milling the purfling channel around the fretboard extension. This approach is built around having an semi-finished neck, as minor errors in layout or fitting is much more evident on 12 fret guitars versus 14 fret 'shorty' extensions. The decision to be made for the Style 42 extension work is whether to do the inlay work and build the neck and fretboard to those constraints, or pattern the purfling off a semi-completed neck. For a small shop doing one-off instruments, working from a completed neck seems like the easier way to go.

Note that the channel is wider than just the shell and the two black-white-black purfling lines - there is an inner border that extends just a bit beyond the edge of the extension, and allows a bit of adjustment for alignment or neck reset.

A few other notes:

- The MDF templates are all fabricated from 1/4" material, which is more consistent in thickness than most plywood

- The trim router used MUST have provisions for centering the bit in the bushing. Whiteside sells a matched bushing and centering post kit, but the base must still allow adjustment to allow the bushing and bit to be adjusted to be concentric.

- A new 1/4" down-cut bit is a must...the burning seen in the final shots was the sign that the bit was on it's last legs...the subsequent routs on the other dreadnaught and 000 were free of issues after the bit used for the essay was repurposed as a scraper burnishing tool.

- A quick review of the VintageMartin site shows that there is a lot of variation on the details of the 42, 44, and 45 purfling, including distance from the final fret to the fretboard binding, the visible width for the inner boarder, where the corners of the extension land on the middle ring purfling and where the edge of the extension purfling meets the inner ring.

- Again - the longer length of the 12 fret extension makes that purfling work MUCH pickier re: good layout and accurate milling than 14 fret work...and that was exactly the comment made to the boss by the Custom Shop worker that he and Mr. Morelli spent time with a few weeks ago.

Process:

1. With the neck fitted/aligned, a trim-width template is used to determine where the routed edges will fall, to include the extension purfling corners and meeting with the inner ring. The width of the inner border can be adjusted here a bit, but too narrow a border risks an uneven reveal on the extension purfling, and too wide adds a darker, bolder trim line than might be desired in what is a delicate-looking detail. Future repair people will thank you for a wider border, but the owner will look for orthodoxy first and ease of reset not at all.



2. Using a custom-width guide with width equal to the sum of purfling width + border reveal + bushing offset, lay out where the tabs of double-stick tape will go



3. Using the guides, place the two template pieces for the sides of the extension





4. Add the template piece for the base of the extension

_________________
For the times they are a changin'

- Bob Dylan

Process Steps (Continued)

5. Add the filler strips, carefully aligning them to avoid the bushing dropping into the gaps between templates





6. Mill the channel, taking care to move through the varying density materials of the rosette







7. Carefully trim the corners of the extension - they are not quite square, and the soft spruce may tear if the chisel is not extremely sharp



That's it. There are other methods, and the boys in the shop have tried a few of them, but this approach seemed to work well for the three Style 42's which the shop produced in this calendar year.

And to repeat: use a fresh downcut bit!

_________________
For the times they are a changin'

- Bob Dylan

Woody

Thanks for that - makes it look easy. Can you document cutting the miters at the soundhole?

Ed

This is what I love to learn! You must've read my mind as I was just thinking about a way to cut this in an easy to repeat manner.

Thanks so much for this! I am a few guitars away from going full 45 on one and needed a tutorial.

I'll check if the boss or Mr. Morelli have any shots...these three guitars were joint builds between the two, but I had the opportunity to work on two of the three extension purfling jobs. If I recall, I started with the end-of-extension purfs as a dry fit (these were all done in CA and shop-made UHMW plastic - commercial strips seem to be a bit undersized), then tacked down before working the binding to extension miter joint.

I believe that if the purfling channels are well sealed with shellac, CA is the easiest way to go for these more complex binding jobs. Although Martin still uses Elmer's white glue for setting shell on the Authentics, we use CA, as the BSI Thin CA (blue label) does not shrink or yellow after cure, and scrapes and sands beautifully. Using CA allows you to perfect the dry fits before committing to glue-up.

The miters on the shell were trivial once the channels were formed - an 8" mill bastard file and a few 0.080" thick spruce scraps tacked at 90 and 43.5 deg to one of the bench hooks worked...safe-edging the file or working with a cutting board scrap under the bench hook keep the bench from getting too beat up. All the extension work is done with straight shell, so the toughest part is getting the in-between miters filler piece trimmed to exact fit.

_________________
For the times they are a changin'

- Bob Dylan

Great tutorial!

One thing I notice is that there is almost no fretboard left after the 20th fret.....

It looks like once the fret is put in that the fret would just about touch the binding.

Is this how Martin does it?

All three guitars were planned as Style 40, but were built as Style 42. The boss made the decision that he had enough to come close to vintage practice, so went for it. Had they been planned as Style 42/45 from the beginning, we'd have pushed the soundhole towards the bridge by 1/8". As it is, the length beyond the fret is about 1/3 of the distance between the 19th and 20th, versus the 40%-50% seen in Vintagemartin.com. There are some shots of vintage 42 and 45 style instruments which are essentially as we built, but def older ones...with the current CAD/CAM system at martin, all of the tough layout work can be done before anything is cut, so soundhole position an be fixed to 0.001" without issue.

The other thing that shifting the soundhole centroid a bit towards the bridge does is lands the corners of the extension purflng on the outside purfling line of the middle ring, which to my eyes is ideal. I think the guitars came out nicely, even with the very slightly shorter length beyond the 20th fret, and while the red shell Mr. Hall mentioned is a bit more authentic, end-grain ivoroid and the blue-green-violet solid shell we prefer looks quite attractive.





The link to the Vintagemartin.com page shows that Martin was a bit stingy with extension real estate when it came time to make up a 12 fret Style 42 or 45, but with a little prior planning (e.g., knowing the guitar will be trimmed out as a traditional 42), your preferred treatment can be achieved.

http://vintagemartin.com/style45.html

_________________
For the times they are a changin'

- Bob Dylan