LINES and VECTORS (a curious note)
The difference between lines and vectors is that all of the 'lines' that you draw in a CNC prorgam are actually vectors until they are located relative to a Datum Position (the 0,0 point in the X,Y planes).
To do this in the Vectric software you make a choice of the X,Y zero location. The default location is in the center of job but the software allows you to can change that if you want. It's almost like multiple choice. Do you want the 0,0,0 to be here? There? Some other place? You tell it. Then all of the tool paths will be located relative to your selection of the zero point (datum position).
Thus, one creates a vector drawing and the software converts to lines (which you then select as toolpaths for various operations_ pocket, profile, inlay, engraving, drilling, etc.). What has changed is that the vectors, in addition to having direction and magnitude, now have location__ per your instruction. So, they become, by definition, lines.
A vector is like a teenager...lots of stamina and brains but no fixed point of application. Once the teenager joins the soccer team (or gets a job) they become a line. That is, they have a fixed location to apply their skills. Vectors don't have location. They exist in the theoretical. Vectors (and teenagers) become lines when their location becomes fixed relative to the rest of the universe. Sometimes, it's much easier in the software than it is in real life.
All this has (almost) nothing to do with carving a guitar bridge.
CNC Routing the Bridge
Routing the bridge with a CNC begins with the same process of design, but, instead of drawing with a pencil, one draws the bridge in the CNC software.
First layout the point where the e-string will cross the lead edge of the saddle. For steel string guitars with 25.4" scale, this point is 5mm from the lead edge of the bridge on the e-string line. Then layout the point where the E-string crosses the lead edge of the saddle at a point 8.5mm from the front edge of the bridge. Connecting these two points with a straight line will mark the lead edge of the saddle. As the saddle is to be ~76 mm long, this line will need to be extended about 10mm beyond the string marks on each side.
The saddle will be centered on the bridge but on a slight angle for reasons explained at the end of last month's journal entry (string compensation).
The plan drawing shown below is a screen shot of the bridge drawn in the Vectric CNC software. After inputting the job size and choosing the X,Y zero point in the Job Set Up, vectors from the drawing are selected and the parameters of the tool path are input for a tool selected to cut that toolpath. This job takes 8 different tool paths to complete.
Carving a guitar bridge is not the most challenging part of guitar making. That's not to say that it's easy, just that the procedures are common wood-working procedures_design, layout, cut, rout, drill, scrape and sand. The CNC puts the fun back into it with more thought and less hand work required. This month Woody will explain both procedures.
Cutting the Saddle Slot
The only thing that might need explanation is how to cut the saddle slot with a router. Once the slot is laid out on the bridge blank, the first challenge is to keep the blank from moving when cutting it with a tool that is at least ten times it's weight.
A solution is to attach the bridge blank to a spoil board with contact cement and/or screws or brads in the excess surface area of the bridge blank. Before I invested in a CNC I used a fixture with rails to guide the router (as pictured below).
As you can see, I used the fixture to cut the saddle slot after gluing the bridge to the soundboard. To do this with confidence, I recommend that you first practice with the bridge (or a faux bridge) secured to a spoil board.
Draw a line parallel to the line of the saddle that we just talked about but 1/8" behind it. Finally, apply a length of masking tape to both lines of the saddle layout and parallel so that there is a 1/8" space between them for the full length of the saddle (see photo above). This space between the tape will be the track of your 1/8" router bit a little later on.
Note 1: The masking tape could be applied before layout marks are made as this makes them easier to see. Then the saddle slot location (parallel lines) could be cut out with an X-acto knife.
Note 2: The "Raindrop" MOP inlays on each side of the bridge are an aesthetic complement and will not be discussed here.
Making the Bridge by Hand
It's easy enough to say: Buy a rosewood or ebony bridge blank (about $10), set it up on your work bench and remove all of the wood that doesn't look like a guitar bridge. Here are some tricks that will help.
Woody's procedure (as always) is to begin with the end in mind. That is, design your bridge first. The bridge blank that works with Woody’s design is 175mm long and about 50mm wide and 10mm thick (after running it through the thickness sander). If your bridge blank is larger, no problem. The excess surface area will make it easier to hold down while working on it.
Woody's finished bridge will be 154mm long and 37mm at the widest point. The final thickness of the bridge will probably be a bit less than 10mm but this thickness leaves plenty of bridge to work with if the neck angle is a bit steeper than expected. If not, one can carve the bridge down later.
Questions should be directed to:
Now, the CNC version.
As you can see, Woody doesn't put a lot of time into re-inventing the bridge design. The bridge is primarily a functional component of the guitar’s acoustic sound system. One could say that, with the bridge, form follows function. Simpler is better.
The design of the bridge must allow the energy in the strings to be transferred into the soundboard efficiently. Another obvious requirement is that the bridge must stay attached to the soundboard when under a load of 160-180 pounds of torque when the guitar is strung up.
Unlike bridges of the viol family, guitar bridges pull on the sound board where violin bridges push on the soundboard. Thus, guitar soundboards are stressed in tension while violin soundbards are stressed in compression. Acoustically, this makes a difference but we won't go into that here. What is important is that, in the end, the glue joint holding the guitar bridge in place must be able to withstand a very significant force. The design shown here puts the most glue surface where it is needed most_aft of the saddle. (a little boat builder lingo there)
That's it for this month kids and boys. Hope to see everyone back here next month.
Play safe... Keep your neighbors healthy.
Carving a Guitar Bridge
The fixture has two steel rails screwed to a 10" x 16" piece of counter top stock. The distance between the rails is the diameter of the router base. The exact distance from the router bit to the edge of the router base is 71mm. In the photo above the spacer marked "71mm" tells you where to place the fixture in relation to the saddle slot location on the bridge.
The fixture is not hard to make and, when used correctly, works perfectly. A more complete description of this unique process is the final discussion of Woody's August, 2018 Luthier's Journal.
When drawing the guitar bridge plan on the bridge blank, start from the center and work outward with the layout of each of the 6 strings. Remember the string spacing impacts (or is impacted by) the width of the fretboard. As a rule of thumb, the string spacing should be approximately the same width as the width of the fret board where it joins with the body (i.e. at the 12th or 14th fret).
We're talking here about the distance between the center of the e-string and the center of the E-string (1st and 6th). On dreadnoughts, for instance, this dimension will vary between 55.5mm and 58mm between different manufacturers.
String layouts vary a little on different guitars according to the preference of the guitar maker and the style of the guitar player. Finger style players need a little more room between the strings while flat-pickers like less.
So, first, lay out lines on the bridge blank representing the strings, evenly and symmetrically spaced around the centerline. The 3/16" holes for the string pins are laid out similarly. In the Woody design, the bridge pin holes are arranged in an arc.Straight line layouts work just as well. It’s an aesthetic choice more than anything else. Use an awl to mark the centers of the 6 bridge pin holes.