Luthier Tips du Jour - Bolt on Mortise and Tenon Neck Joint

Uploaded by OBrienGuitars on 14.05.2009

So, you were able to bend your sides without breaking them.
You got the heel blocks glued in, hopefully in the correct position.
You took on the task of cutting a circle in your top and installing a rosette.
You got the top and back thicknessed to the correct specs.
You braced and voiced the top and back and installed them on the rims
after first fighting the kerfing around the tight radius of the waist.
You have made the neck and even routed a slot for the truss rod.
All of these may seem like impossible tasks and you may even think,
How Have I even gotten this far? Now we face the next step, how to attach the neck to the body.
If you follow my instructions here this seemingly impossible task can be done with ease.
There are several themes and variations on neck joints.
Here I will be showing how to execute a bolt on mortise and tenon.
Start by making sure the area where the neck will go is flat.
You will have serious problems setting a neck if this is not the case.
On my guitars I want a gap between a straightedge and the top of the guitar at the saddle location of 2.5mm-3.5mm or .098 - .138
This will leave a very small gap, also known as a skoshe,
between the straightedge and the top just in front of the soundhole.
I reproduce this gap and angle with the sides using a sliding bevel.
One side of the sliding bevel is for the angle of the fretboard side of the neck.
The other side is for the heel block angle of the neck.
Transfer this line to the neck blank at the appropriate fret.
You can see that it is not a 90 degree angle.
Measure down from the top of the neck blank 1 and 1/8 inch and 2 and 1/4 inch, or 28.5mm and 57.5mm
and mark this measurement on the heel block of the neck.
To bolt on the neck I will be using a bolt like the one you see here.
I need to drill holes for the barrel nuts.
I drill the front edge of the holes about 1/32 of an inch or .8mm back from the angled line I just drew on my heel block.
The size of the hole is 3/8 or about 10mm.
You do not need to drill all the way out the other side of the neck blank.
Just make sure the hole goes about 1/4 inch or roughly 13mm past the centerline of the neck.
Knowing exactly where the angled line of my 14th fret is
I can measure and mark out the rough shape of the heel block.
I use my bandsaw to remove this material.
I can also mark the depth of the heel block.
I like for the heel block on my guitars to be flush with the binding.
Next I mark the finished width of heel block at the top and bottom and remove the excess material.
Be extremely careful while making this cut.
I am now ready to cut the mortise.
I will be using my High Tech, BR549, patent applied for mortise cutting jig.
You could also do it by hand.
I place the jig on the guitar and tighten the screws until I hear a crack.
I then back off the screws about a quarter turn.
The jig can then be clamped to the edge of your workbench.
I will use a 1/2 inch straight cut bit with a 5/8 inch guide bushing in my router.
Notice that my jig has a stop that allows you to cut the mortise to match the depth of the guitar body.
I like to stop about 1/4 inch or 6.3mm short of the overall depth of the neck, not the body.
Next I set the depth of cut for the tenon to exactly 7/8ths of an inch or 22mm.
I make the cut using a plunge router.
Do not try and do this is one pass.
I usually take about 6 passes to complete the cut.
When done, remove the guitar from the jig.
Now it is time to cut the tenon.
I will be using my version of a jig that many luthiers use to do this.
I start by placing the guitar on the top of the jig.
It has an adjustable shelf that allows me to set the angle of the neck
so that I get the correct gap I mentioned earlier at the saddle location.
This is measured using the aluminum rod that extends from the shelf.
When the gap is correct it means the angle of the neck is correct.
I then lock in the angle with the wing nuts.
The length of the tenon will be 7/8ths of an inch or 22mm.
and the end must have the same angle that you traced on the heel block at the 12th or 14th fret.
Place the neck centered on the shelf of the jig and secure it in place.
I have drilled a hole in the jig so I can see that the nut end of the neck is aligned
with the centerline of the jig.
The top of the jig has a template used to cut the tenon shape onto the heel block.
Once again I use the plunge router and set the depth of cut to 7/8ths of an inch or 22mm.
Make the cut in several passes, adjusting the plunge router a little deeper after each cut.
If you are only making one guitar it will take you longer to make the jig
then to just cut the tenon by hand.
However, a jig and a router makes it easier and perhaps more accurate.
I have set up my jig so that the tenon is just a skoshe wider than the mortise
so now I have some hand work to do to fine tune the fit.
I use a rabet plane to narrow the tenon.
You could also use a file or chisel.
Next I need to adjust the depth of the tenon so that the top of the neck sits flush with the top of the soundboard.
Mark the amount to be removed on the tenon and then use a chisel to remove it.
Take small bites with the chisel and be careful to keep the part of the neck that joins with the body of the guitar straight.
A file can be used to round the edges.
The tenon should fit snug and the top of the neck should be flush with the soundboard.
Next I remark the width of the neck at the fretboard side and heel cap.
I can then use a chisel to relieve the area inside these lines.
Only the outside edges will touch the guitar body.
Place the neck on the body and check the fit where the neck meets the body.
You can fine tune the fit by flossing the shoulders of the neck with a piece of 80 grit sandpaper.
Take your time and do it right.
There should be no gaps where the neck meets the body.
Be careful to not change the alignment or angle of the neck.
Use a straightedge to check both of these.
If necessary adjust the fit until it is aligned and you have the correct angle.
Now I mark and drill holes for the bolts in the tenon of the neck.
The hole is about one or two sizes bigger than the diameter of the bolt.
Insert the barrel nuts and make sure the bolts can thread into them.
Next, transfer the hole measurements into the mortise and drill them.
A scrap block can be held against the inside of the heel block to keep it from blowing out while drilling.
Now I can attach the neck. The bolts only need to be snug.
Do not over tighten them.
The bolt on mortise and tenon joint is a very structurally stable joint and no glue is needed.
With a little practice and the right tools this seemingly impossible mission can be executed flawlessly
and your guitar will not self destruct when the strings are installed.