SETTING THE NECK ANGLE
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 soundboard. This angle is set by adjustment of the jig early in the set up as explained in the first 5-6 minutes of the LMI video (URL below). In total, the video is about a 40 minutes 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 in a home-shop set up. So, the first thing to do is measure the angle that the sides make with plane of the soundboard 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 neck 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 bass/treble center line of the guitar body and neck the same.
YouTube: Dovetail Neck Angle Jig
https://www.youtube.com/watch?v=qq1ff1-cdrQ
That's all for now,
Woody
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
In the video, Robert O'Brien explains the best procedure for using the router to cut away the shoulders of the heel block leaving a dovetail tenon that should fit nicely into the mortise already created. Robbie recommends, and I agree, that the last 5-10% of the shaping of the tenon should be done by hand. 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. Slight adjustments might need to be made by hand. This is not uncommon in a home-shop set up.
LMI FIXTURE ASSEMBLY
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 parrallel to 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 well enough 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 rigidly fixed in place. The set up pictured below was a challenge to keep fixed in position.
For a simple routing fixture that works, check out this video.
https://www.youtube.com/watch?v=wnzVngO_lCY
For nore information see this LMI video on routing for and installing the truss rod.
https://www.youtube.com/watch?v=tLY3iN1Atn4&lc=UghkhJKKpmobF3gCoAEC
THE DOVETAIL TENON
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 correctly it should be exactly square (left and right) to the top of the jig (i.e. the travel surface of the router). To keep the dovetail tenon square to surface of the fixture and on the center line of the neck monitor the set up through the observation hole in the jig. Woody plugs a short length of spruce with the centerlin marked on it into the truss-rod channel. By eye, I line up this line with a mark I've made at the bottom, center of the viewing hole in the fixture. This is just a little aid in making sure that the truss-rod channel is aligned correctly.
All of this assumes that you LMI Neck Angle Jig has been assembled correctly and to tight tolerances.
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 bass/treble centerline (left and right)
* Neck angle relative to the top of the guitar (up and down)
Staying on the Guitar Center Line
THE DOVETAIL MORTISE
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 O'Brien's LMI video.
https://www.youtube.com/watch?v=qq1ff1-cdrQ
It's just a bit tricky, but important, to make sure that the centerline of the guitar body is square to the surface of the Neck Joint Fixture (i.e. the travel surface of the router). Pictured below, the square is aligned with the center of the mortise and the glue joint in the center of the soundboard.
Finishing Shop_
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.
April, 2016
Finishing Shop
Dovetail Joint
Neck Geometry
Staying on the Centerline
Setting the Neck Angle
The Dovetail Joint _