There are a couple of different ways to blot on a neck. One method uses Brass Cross Dowels and Brass Connector Bolts. Another method is by using stainless steel bolts with with brass threaded inserts imbedded in the neck tenon.
Tools & Materials:
Back Saw or Dovetail Saw
Brass Connector Bolts
Brass Cross Dowels
Mortise & Tenon Neck Joint:
Build the Mortise and Tenon Joint just as you would in our article Mortise and Tenon Neck Joint.
Brass Connector Bolts & Brass Cross Dowels
GLS Tip: Drill the barrel and bolt holes in the neck blank on the drill press while it’s square BEFORE sawing the tenon and shaping the heel. This will assure square – lined up holes that are perfectly square.
Cross Dowel Locations:
The cross dowels are located quite close to the heel side of the tenon. This is done to give as much strength to the tenon as possible by leaving wood in front of the tenon at the cross dowel locations.
Another Suggestion: to reinforce the tenon even more would be to add two strips of hardwood on either side of the tenon and glue them into place with epoxy glue. I would recommend using 1.5 – 2.0 mm stock for this. Run the grain direction of the strips 90 degrees to the neck wood.
Cut everything for the neck tenon and the head block per the plans. Next I dry fit the neck to the Head Block to make sure everything is within tolerance. I would also recommend that you oversize the holes for the bolts to allow for miss-alignment problems when it comes to attaching the neck to the guitar body.
Glue The Cross Dowels To The Tenon:
If everything aligns perfectly and you have allowed for the proper side thickness to be glued to the Head Block, go ahead and epoxy the brass cross dowels into place. This means an assembly of the neck and Head Block once again, but it assures you that the bolts and cross dowels will be in proper alignment with the assembly process begins.
I usually glue the Head Block to the sides before aligning the neck and gluing the cross dowels in place. This way you can check all the center-lines, neck alignment to the body etc. before this final step.
Gluing the Head Block to the Sides:
Now the time has come for you to glue the Head Block to the guitar sides. For detailed information of this step see the article Gluing the Head Block.
Using The Bolt-On Neck/Threaded Insert Option:
This is my option of choice for my acoustic guitars and ukuleles that I build. I find overall it is easiest and most secure to attach the necks to the head block using this method, as long as you work with accuracy. Of course this is true with most tasks by the luthier.
The process starts with proper shaping and machining of the head block. As the diagram in this post shows, the there are (2) blot locations through the head block into the neck tenon. The best way to do this is to carefully make the head block, cut the neck mortise into the block and drill the 1/4″ blot holes into the block, including the recess for the head and washer. Most of our ukulele plans show only one bolt for the connection, which is plenty adequate.
Glue the head block onto the sides and construct the body of the instrument in a normal fashion. You may want to actually fit the neck and created the bolted connection prior to gluing the back on the instrument as this will give you much better access to the neck blots. This is especially true if this is your first attempt at this type of construction.
Setting The Threaded Inserts:
Slide the neck tenon into the mortise of the head block and hold securely in place with clamps if possible. Make sure that the neck top surface (where the fretboard is glued) is level with the top plate. Insert a 1/4″ brad point drill bit in from the body side of the head block and give it a tap to mark the center of the bolt hole. Repeat for the second bolt hole. With these registration holes, you can accurately drill the insert holes with a large brad-point bit.
Place the neck into a woodworking vice with the tenon facing upward. Threaded inserts are intended to be screwed into the wood for pull-out strength. I’ve never had much success doing this in hardwood so I usually drill the closest hole diameter that the threaded insert will just slightly grip the sides of the hole. Otherwise, if it takes too much effort to thread the insert into the hardwood, it will cross-thread and be crooked – that’s something you surely don’t want to happen! I usually test drill holes in scrap wood to test the fit prior to drilling out the neck. Also, this type of fit will allow you to slightly adjust the squareness of your drilled hole to the line of the neck, which is critical.
Upon finishing the drilling, mix up some slow-set 2-part epoxy. Insert a long 1/4″ bolt into the brass insert and coat the thread thoroughly with glue. Slide the brass insert into the tenon and check for squareness to the neck with a small try square. Upon approval of squareness, coat the entire top of the insert with epoxy. Note that this means the threaded insert hole should be about 1/16″ minimum deeper than the insert itself.
Keep the long bolt in place to protect the internal threads from glue infiltration. Once the glue has partially set-up you can remove the bolt. You can judge this by trying to remove the bolt is seeing if an glue follows the bold direction.