The neck angle is the angle at which the plane of the neck diverges from the plane of the guitar top. This angle will determine the thickness of the bridge, and the height of the strings above the top. In the jig, therefore, the neck is not square to the top of the jig in the forward and back direction. This angle is set by adjustment of the jig as explained in the LMI video (URL below). It's about a 40 minute video and it explains most of what I am trying to describe here.
The goal of the neck angle set up is to have 3.5 mm clearance between a straight edge laid on the neck and the guitar top at the saddle position. The reasoning behind this dimension is also explained in the video. The final target is a 12 mm clearance of the strings above the top at the bridge_ (8.5 mm bridge thickness plus 3.5 mm saddle height, on my guitars).
A word of caution: The LMI video states that the neck angle bar on the jig is adjusted to 3.5 mm off the top of the guitar as pictured below. This adjustment is correct only if the top and the sides are at a perfect 90 degree angle at the neck block. This is not always the case. So, the first thing to do is measure the angle that the sides make with plane of the top at the neck block. If it is more or less than 90 degrees, more or less adjustment will be needed in the set up of the angle jig.
The critical target is to have 3.5 mm clearance between a straight edge on the neck and the guitar top at the saddle position when the neck is fully seated in the dovetail joint... AND have the center line of the guitar body and neck the same.
YouTube: Dovetail Neck Angle Jig___
The dovetail neck joint could fade from favor with luthiers because a straight mortise and tenon is much easier to execute. Of course, a straight tenon must be secured with some type of hardware to stay in place. You can see that a shop named Woody Strings will probably never go for nuts and bolts to hold a guitar together.
Next month_ Neck carving and Fretboards
Once the neck is fitted to the guitar, the center line can be checked by pulling a black thread down the center joint of the top to the center of the neck at the nut.
To say that every jig and template has its idiosyncrasies would be an understatement. Robert O'Brien explains some of this in the videos produced for LMI and the sale of the LMI dovetail jig. There is one video on how to assemble the dovetail jig and another video on how to use it. Brief notes after viewing the videos several times would include a recommendation to pay particular attention to square and/or parrallelto the center line when assembling the jig. The manufactured parts of the jig are precise (CNC cut I am sure), but the video does not emphasize enough (IMO) regarding the precision that is needed in the assembly of the jig. Also, there are a couple of non-critical but significant items that are not mentioned in the video (like the attachment of the knob on the neck angle adjusting rod).
By nature, luthiers are very precise craftsmen in the cutting and joining of wood. It is only after one starts to use the jig, however, that you may realize that one millimeter or one degree out of square will significantly impact the the cut and fit of the dovetail.
The final caution with this jig and these videos is that your guitar is probably slightly different than Robert O'Brien's guitar in the video. Robert is very good at explaining things and I think that I would like Robert quite a lot if I ever met him. So pay attention when he says "this is how I do it on my guitars". Guitars can be quite different from one to the next, even from the same maker.
Bottom Line: The LMI jig is quite good but O'Brien's advice to practice on a few trial blocks should be heeded. The jig does take some getting used to.
Neck Joint Construction and Geometry_
Before the dovetail joint is cut, it is a good idea to rout the neck stock to accept the truss rod. This will help fix, and stay true to, the center line when cutting the dovetail tenon.
There are many types and dimensions of truss rods so careful measurement is required to cut a snug slot into the neck. The slot must be exactly down the center of the neck and only slightly larger than the truss rod to accommodate the epoxy fill. It is much easier to rout this slot on a table router while the edges of the neck stock are still square and parallel to the center line of the neck.
In the photos shown here I am routing the old fashioned way by holding the neck in place with a fence to guide the router down the center of the neck. I do not recommend this method. For a number of reasons I had to rout the channel this way on this particular neck. If you don't have a table router, this method works quite well but it requires diligence and precision in measurement and set up. Whether one uses a table router or a fence-guided router, it is important to cut the truss rod channel in multiple passes which means that the set up must be solidly fixed.
In using the dovetail jig there are two critical variables when cutting the mortise and tenon that are both impacted by the set up and use of the jig. Both of these variables can be controlled when one realizes how the jig is used... but one must realize that the convenience of routing the dovetail in a jig is balanced by the reality that a modification in one variable could easily impact the other.
The two critical variables are:
* Neck angle relative to the guitar centerline (left and right)
* Neck angle relative to the top of the guitar (up and down)
Staying on the Guitar Center Line_
The center joint of the book-matched guitar top should be the center line of the guitar body. The center line of the guitar body (extended) should lay on top of the center line of the neck. The dovetail mortise is cut first and is relatively straight forward as explained in the the LMI video. Cutting the dovetail tenon will set the angles at which the neck joins the body both left and right and up and down, i.e. both of the critical variables. Proceed carefully and check your work often when cutting the tenon.
When the neck is placed in the tenon jig it should be exactly square (left and right) to the top of the jig (i.e. the surface on which the router rides). Maintaining the neck on the center line of the is monitored through an observation hole in the jig.
April was a month of construction but not so much construction of guitars as it was a month of constructing the finishing shop addition. As explained last month, the finishing process of spraying multiple coats of lacquer really needs a space that is built for that purpose... which is not on the same ventilation system as the rooms in which one eats and sleeps. So, a finishing shop is being built in the back yard, separate from the house. It will be completed in a couple of weeks.
April produced the construction of a clean, dry space of about 200 square feet. It now needs siding, soffits and trim. Currently, the only difference between the shop and a shed for a lawn mower is that the finishing shop will be bug and rodent free (hopefully). That means screened soffits, gable vents, and windows. It will be a relief to be able to spray a coat of lacquer and leave the guitar in place. More on the finishing process later.
Staying on the Centerline
Setting the Neck Angle
The Dovetail Joint _
Creating the joint that holds the neck onto the body of the guitar is probably the most critical step in building a guitar that will be playable, stay in tune and not require major repair down the road. As I have said before, the guitar is essentially a box with a stick attached. These two geometries do not naturally fit together.
Even major guitar manufacturers differ in how they join the neck to the body. For the longest time, hardware was considered a no-no even though many guitars needed to have the dovetail neck joint disassembled and modified after only a few years of ordinary wear and tear. This repair is called a neck re-set and every acoustic guitar repairman worth his salt has had to learn how to unglue a neck from a guitar, shave a millimeter or two in the right places, and rejoin the neck to the body.
Nowadays, with international travel and instruments manufactured off shore to begin with, neck resets are often required even on new instruments due to changes in climate and humidity. As one might imagine, resets on dovetail instruments are an expensive repair. A solution to that problem has been the acceptance of hardware-assisted neck joints. Much credit needs to be given to Taylor guitars in persevering in the development and acceptance of bolt-on necks.
All this being said, Lenzi guitars (and Martin, and most manufacturers) still use the traditional dovetail joint to secure the neck to the body. The jig that I use to cut the dovetail mortise and tenon was purchased from Luthier's Mercantile International (LMI) and it works quite well... up to a point.