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Let's assume that you are using a finished neck. The heel should be shaped in such a way that when you hold it up to the rim, you get an angle of about three degrees. NOTE: Three degrees is a nominal measure. Other factors make other measures acceptable. Such factors include:
- Bridge height
- Action (string height over the fingerboard) you want
- Presence of a frailing scoop.
Step 1: Locating holes for the coordinator rods.
The first challenge to locating coordinator rod holes is to find points on the circumference of the rim that are exactly opposite one another along the axis of the diameter. Machinists use a combination square, but most combination squares are too small for an 11" diameter rim. I made a similar tool out of wood.
TIP: Examine the rim for blemishes or, if your rim has purfling, the point at which the ends of the purfling strips meet. This is a good area to make the neck connection because the neck witll cover that point on the rim.
Place the combination square or your home made tool on the top of the rim. Mark tines at opposite sides of the rim. Use a square to mark vertical lines on the surface of the rim.
TIP: if you made your own tool, draw the lines on the top of the rim. Change the positio of the tool so that it is 180 degrees opposite. If the tool does not lie exactly across the lines you just drew, then it is not square. However you can still use the tool. Align the tool with one of the lines you drew and draw a line at the point of the discrepancy. The spot between the two lines can be used to locate the coordinator rod holes.
Here is an exaggerated illustration.
Step 2: Drill holes for the coordinator rods.
After locating the locations at both ends of the diameter you need two sets of holes at either side of the rim. These holes need to be at the same height to make the coordinator rods parallel.
Depending on the type of coordinator rods you are using, one of the holes in not a through hole. It is very difficult to locate that hole from inside the rim, so I sometimes drill a pilot hole from outside the rim all the way through and then redrill the proper dimension hole from the inside of the rim.
TIP: If you not making a bluegrass pot with a bluegrass tonering, you are free to place the coordinator rod holes where you like. I did a rough measurement from a Gibson Mastertone blueprint and it appears that the top coordinator rod is one inch (approximately 25 mm) from the top of the tonering and about 1.3 inches (approximately 33 mm) between the rods. USE THESE MEASURES AS A GUIDLINE ONLY.
Step 3: Mark the hole position on the neck heel.
Once these holes are drilled, you can use the holes at the neck end to locate and mark the holes to be drilled in the neck. Make a rod with a pointed end the same diameter as your rim holes. Hold the neck up to the rim and poke it with the rod to locate the holes for the neck heel. It is ideal if you put the rim together with all the hardware and head to see exactly where to hold the neck when you locate the holes for the neck heel. The fingerboard should be level with or just slightly higher than the notch in the tension hoop.
One mistake I made on my first banjos was that my neck angle as seen from the top was off and the strings were not equidistant from the edge of the fret board. In fact the first string was unplayable because it was so far off. Not only should you put the rim together with the hardware, you should put pegs and a slotted nut on the neck and string it up. In other words, you have to complete all functional aspects of the neck. And you need a banjo bridge and tailpiece. You don't have to use banjo strings. I have thought of using fishing line, for instance. But you need to see the final position of the strings in relation to everything else. You don't need to tighten the strings much, just enough so they keep a straight line.
Step 4: Drill the holes in the neck heel.
Do the best you can to drill the holes in the neck heel according to the backset angle of the neck in relation to the rim. This is around three degrees. You can also lay a small square on the end of the neck. The drill should be 90 degrees in relation to the concave portion of the heel.
To be honest, I use a drill press and made a very complex jig for holding the neck just right. If you are making just one banjo, it might not be worth making a jig. Also not everyone has a drill press. If you are using a hand drill, have someone watch the drilling process and tell you if your drill is not aligned properly.
Start with a small drill bit so that you can get it started exactly where you made a small indentation in the previous step. Go to a medium size drill bit and then to the final size drill. If you start drilling with a large drill bit, your drill bit may wander off the mark. This is something to avoid!
Step 5: Assembly.
Most coordinator rods are made for 3/4 inch thick rims. If your rims walls are thinner than you may need a spacer inside the rim. You can by nylon and spacers at most hardware stores.
Put the banjo together and see the neck is in proper relation to the rim.
IF YOUR NECK IS NOT PROPERLY ALIGNED
If the neck is not properly aligned, don't despair.
One trick I used on early banjos was to drill the holes in the neck large and glue the lag bolts in the holes while the neck is held in the proper position. Use epoxy and put a layer of plastic wrap between the neck and the rim so you don't glue the neck to the rim. To make this trick work, you must have the neck and banjo completely assembled WITH STRINGS and bridge so you can see exactly where the strings lie. Hold the rim in a vise and position the neck perfectly using long pieces of tape attached to the headstock while the glue cures.
In this drawing you can see the tape holding the neck to assure proper alignment left and right. You will also need tape holding the neck backwards in the proper front to back orientation.