Wednesday, October 20, 2010

A New Jig

 One of the tasks in guitar-building that is both critical and daunting is establishing the neck angle when making the tenon that joins the neck to the body.  The neck of the guitar is angled back from the top a scant 1-2 degrees.  That's not much, and that's why it can be a pain to make.  Imagine drawing a line that starts at the very edge of the fretboard side of the heel and then angles back 1 degree to the heel cap--heck, it's hard just to hold the ruler straight when drawing the line.  Then sanding the angle while keeping the shoulder of the heel perfectly level.

I did it last time, but not without some trial which led to error which led to me making a whole new neck.  Mahogany neck blanks cost around $35, so I was interested in finding a better, more efficient way.

I found a website that offered a jig that would set the angle straight from the guitar top and then allow you to rout the tenon at the precise angle, but it's made of metal and obviously for professional luthiers who are going to do this a lot.  It cost $899.  (But today it's on sale for an amazing $799 while supplies last!)

Not going to happen.

But then the owner of the guitar-building forum that I subscribe to, Robbie O'Brien, offered to the forum members plans for building a very similar jig for just $20, with the cost refunded to the first person to build the jig, use it on an actual build, and post the results on the forum.

Well, I sent the $20 in right away, and not because I cared about the refund.  I have two necks to build and I want them--and every one after them--done right, and as efficiently as possible.

I had to purchase about $75 worth of hardware, and a half-sheet of cabinet-grade plywood, and it took me parts of two weekends to build the jig.  

Before ever using a new jig on an actual guitar, it's imperative to test it on scrap.  I spent the better part of Saturday morning testing the mortise side on a piece of 2 x 4.  

Test cuts in 2 x 4

Once I was sure I had everything dialed in and was getting consistent results, I was ready to move on to the guitars.

Tim's guitar clamped on the mortise side of the jig
I started with Tim's guitar.  I put masking tape on the neck end of the sides and connected the center line of the top with that of the back (it's easier to see a line drawn on masking tape than on the dark brown rosewood), and lined up all the centerlines with those of the template.  

Centerlines lined up (ignore the parallax)
 The bearing at the top of the router bit rides against the clear acrylic template and gives the shape to the mortise.  Since I'm doing a full-depth cut, I work very slowly, routing out about 1/8" at a time, but in just a couple of minutes I got a mortise cut into the guitar.

Mortise Routed

Five minutes later and the mortise is cut in Austin's guitar.  (This is why doing two guitars at once is faster than doing two separately: there's only half the setup and cleanup time involved.  Set it up, do two guitars, then clean it up.  I could do more guitars at once except I can't afford to have that much money tied up in materials.)
Fine looking mortise
Now onto the neck tenons, which is why I built this jig in the first place.  Here's how it works:

To achieve the neck angle I want a straight-edge that is flush with the 14th fret (where the neck joins the body) to have a gap of about 3.5 mm at the bridge area of the top.  The jig has an aluminum angle bar that extends above the top of the jig that is attached to a pivot board underneath it.  This pivot board is attached to the top of the jig with hinges, and the neck is clamped to the pivot board.

The guitar body is stood upside down on the top of the jig, with the guitar soundboard flush against the aluminum angle bar at the centerline.  

Top flush against bar
 With the bridge location marked on the soundboard, I turn a knob on the pivot board that pushes it away from the vertical axis, thus moving the bar away from the soundboard.  I fiddle with the knob until the gap at the bridge location is 3.5 mm, then lock it down.

Gap at bridge (which is several inches below the end of the bar)
Note: if a millimeter is pretty small; a half millimeter is even smaller (d'uh).  So measuring 3.5 mm is basically, "Well, it's more than 3 mm but it's not 4 mm.  There's a limit to the precision needed.  I could use a dial indicator, but this is one of those times when pretty close is close enough.  I can adjust the height of the bridge for each guitar to compensate for any deviance in neck angel from guitar to guitar.

Neck clamped to jig
I have a neck that I had screwed up by making too thin, and this is why I don't throw away my mistakes--there eventually will come a time when they can be used.  I glue an endblock onto this neck, and then test the jig on this neck.  On the first pass I notice that the tenon fits pretty loose in the mortise, and I want a tighter fit, so I put some masking tape on the template tenon, which will result in a wider tenon.  I keep adding masking tape and cutting tenons until I get a nice fit in the mortise.

Neck clamped to pivot board, which is slightly off vertical
Then it's just a matter of routing the tenon.  I don't do a full cut because I want to sneak up on the 14th fret line.  I want the tenon on the fretboard side to be exactly at the 14th fret line, while it will be a little deeper at the heel cap.

Boom!  Done, and perfectly!  The tenon fits snugly in the mortise, and the neck fits flush onto the top of the guitar sides.  

I put the other neck in, put the other body next to the aluminum bar, and make a small adjustment (because the body lengths are different, the bridge location is different as well), rout the tenon, and, once again, a perfect fit.

Snug fit (the gap at the bottom is irrelevant; it's the sides that need to be tight)
On both guitars I put the neck on the guitar body, hold a straight-edge on the neck from around  the nut area, and measure the gap at the bridge.  In theory it should be 3.5 mm.

In reality?  3.5 mm.

I love it when things work the way they should.

A task that has been filled with apprehension and frustration I can now face with confidence.  The nice thing about this jig is that setup is pretty simple: clamp it to the workbench, put the right bit in the router, adjust the templates for the size mortise and tenon needed, and it's ready to go.
And it didn't cost me $799.

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