Steve Stevens

At this stage we have done the rough cuts and the neck should begin to slide into the mortise.  It doesn't need to go very far.   In fact we really do not want it to slide all the way in.   We need to be able to fine tune the fit which is why we made the saw cuts outside the layout lines on the neck and inside the layout lines on the body mortise.   With the neck fitted into the mortise as far as it will comfortably go, begin to look at the alignment.  Use pieces of tape on the body and neck to make notes for your evaluation of the fit.

These notes are consistent with a slight twist in the current neck orientation.  They tell us to trim the tenon to allow the heel to move in the direction of the arrow.
We are not ready to remove any wood yet.  Now look at the following picture that shows how we observed the twist.

Two pine sticks have been placed on the guitar.  Find a flat spot on the body (in this case it is behind the sound hole near the bridge location) and another on the neck.  Then sight from this position to verify the neck is either flat to the body or not.  Note the tape on the body that is marked with the center line.  This should also confirm the neck twist.

The remaining alignment is down the center line of the neck and sound board.   The straight edge used here is a piece of soft maple I keep around for marking straight lines.  From time to time I need to clean up the edges with a sharp plane to keep it straight.  This doesn't need to be dead machinist flat, just straight enough to help keep the neck lined up with the body.  Wood is nice for the job here because it doesn't weigh much and won't mark up the soft spruce.

So, these are the checks needed as trim work progresses.   Put the neck and body together, evaluate where to trim to correct any alignment problems, mark the pieces for trimming, take it apart, do the trimming, and put it together again.   Keep doing this until the fit is right.

Since the dove tail sides of the mortise are used to hold the neck tight to the body, the final fit will have a small clearance between the bottom of the mortise and the end grain of the dovetail on the neck.  This will allow the dovetailed sides to pull the neck tight to the body sides when the fitting is completed.  
Check the clearance at the mortise bottom frequently to verify it isn't touching the neck.  Here a narrow piece of sandpaper is being used to check the clearance.

In the example guitar, the neck heel design is narrowed toward the heel cap.   This may change the fit of the neck to the guitar body.  Note the sides of the neck have been trimmed to roughly the same angle as the dovetail.

The neck sides need to match the fingerboard width at the 14th fret and here the finger board is being used to establish this width for the neck.

Here the sides of the neck are trimmed with a spokeshave to bring the neck more closely to it's finished size near the heel area.

Keep checking the alignment and make sure the bottom of the dovetail mortise is still not touching the neck.  As the fit comes closer to completion, the sides of the dovetail are flattened with a file.   One of the most common mistakes in filing these flat surfaces is the rounding of the near and far sides of the surface.  To guard against this tendency, use the end to the file to remove only the center surface.  

Once in a while, hold the file tightly against the surface and flatten the whole surface carefully to avoid rounding the edges.

This shows the heel fit has opened slightly.  Checking with the slip of sandpaper showed the neck end had begun to bottom out in the mortise.  A light trimming of the heel area on the neck dovetail corrected this gap.

As the final fit is quite near, do some fine sanding of the guitar sides where the neck butts to the sides.  This area tends to become dented from the repeated fitting of the neck during the process.

Fitting these joints is a thoughtful process.  Normally, small shaves of wood in just the right places will coax the most ill fitting neck into place.

As a final note:  If the joint becomes sloppy from removal of too much wood from either the neck or the body, try this;  Cut veneer thickness pieces of neck wood and glue them tightly fitted to the over trimmed dovetail neck tenon.  This will build the neck half of the joint back up to the point you can try again.  Take your time, keep those chisels sharp and tap lightly.

Steve