Cutting Fret-slots

Before choosing a tool for cutting fret-slots in the billet, read the articles on Measuring Fretwire Thickness and Selecting Tool Types. Because the diameters of these cutters are so small, they tend to be quite sensitive to the damaging effects of runout (TIR). Assuming that it has not been measured in a few weeks, take the time to measure spindle TIR.

If the fretboard has just been engraved with a 60° v-tip cutter and both the v-tip bit and the fret-slot cutter have depth setting rings mounted, there is no need to re-zero the Z-axis. Just swap tools, load the fret slotting code and you are ready to go.

EXAMPLE:
For this example, assume that the fret-slots will be be cut to accommodate the same 0.023" (0.584 mm) fretwire used in the article on fretboard engraving. If the billet is made of ebony, or any other very hard and/or silica entrained hardwood, a 0.023" diameter 3-flute mother-of-pearl (MOP) cutter will be required.

Referring to the Feeds and Speeds Calculator:

• Select the Feed Calculator button
• Enter:
• • RPM = 24000
  • Flutes = 3
  • Tool Diameter = 0.023
  • Chipload = 0.00069
  • Percent of tool Diameter = 3 (the chipload 0.00069, will automatically appear)
  • Runout = 0.00 (never really true but OK for this example)
• Click on the green Feed button
• The calculated feed-rate, 49.68 IPM (1,262 mm/min) will appear in the Feed field in the upper right.

The calculated feed-rate assumes a "pass depth" (aka "depth per pass")is equal to ONE BIT DIAMETER, in this case 0.023" (0.584 mm), meaning that the fret-slot will be cut in multiple passes.

It should be noted that, although 50 IPM may seem pretty fast for such a small bit, the tool will probably be happier, and give a better cut, at a feed-rate in the 75 to 90 IPM (1,448 to 2,286 mm/min) range. This is true for all but the hardest of materials used to fabricate fretboards. However, it is a good idea to start out with a feed-rate based on a chipload equal to 3% of the tool diameter and increase from there. (take a look at the Rules of Thumb in our "First Steps" article)

Runout is a BAD thing!

This is a good point to examine the effect that TIR has on tool performance. As explained in the article on measuring runout, TIR is basically a technical term for bit wobble or eccentricity. If a cutter is wobbling as it spins, the first thing you might notice is that the kerf (slot) width is larger than the cutting diameter of the tool. If large enough, this might result in loose, poorly anchored fretwires.

Less noticeable, but possibly more important, is the effect it has on on the quality of the cut and the life of the cutter. In technical terms, when a bit wobbles, the flutes experience variable chiploads. One flute may have too much to cut, while another might have nothing at all. To keep the tool from breaking, it may even be necessary to lower the feed-rate, which in turn, lowers the shearing forces and will increase the amount of heat that builds up on the tool. In very hard materials, it will probably also be necessary to reduce the pass depth to 1/2 of the tool diameter.

To put this into perspective, consider the example above with various amounts of TIR.

• RPM = 24000
• Flutes = 3
• Tool Diameter = 0.023
• Chipload = 0.00069
• Percent of tool Diameter = 3

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Points to consider when cutting fret-slots in a rough-cut, engraved (scribed) billet,

• the first pass should be made in the OPPOSITE direction to that of the engraving cut
• The depth of the first cut should be equal to the depth of the engraving cut meaning that only the very tip of the bit will actually be cutting anything. The V-slot will serve to "guide" the cutting direction of this intial pass which, will in turn, serve to guide all subsequent passes.

This combination will act to significantly reduce tool deflection and result im more accurate placement of the fret-slot. It will also eliminate one slot-cutting pass at every fret position, speeding up production of the fretboard.

Cutting Fret-slots video courtesy of D. Wieland, Dark Art Guitars

Fret-slot Cut Through "V" Channel

Finished Chamfered Fret-slots video courtesy of D. Wieland, Dark Art Guitars