Fret Slotting Jig

I have seen articles that describe cutting the fret slots freehand, following a scribed line with the saw. I have done it myself but I would not recommend it. Many years ago I made a fret slot cutting jig, a bodge of a job, done in a hurry when time was limited. I would have used it on the Jazz Bass project but for the fact that it was not long enough for a bass neck. So I made a bigger and better one. It features a bed of hardwood about 85 mm by 35 mm 1 metre long, an under-slung carriage that slides along the bed and carries the saw guides, a millimetre scale (steel rule) and a vernier scale. 20171215 200806

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 The bed and carriage rails were made form a single piece of hardwood, rebated each side 10 mm * 10mm with a hand held router. The table router would have been better because the bearing surface of the edge of the board is not particularly wide and there was a risk of running off, spoiling the piece but I could not be bothered to set it up. Fortunately, it went OK. A 180mm length was cut off the end and the carriage rails were cut  from this section using the band-saw. Making the rails and bed in one assured that the routed slots would align perfectly and it avoided routing fiddly little pieces. Slider strips, 19mm * 10mm were made and installed in the bed section with a few dabs of PVA. The steel rule was let into the beds surface such that it was flush with the top and the edge. The carriage was made by screwing the two slider rails to a base board of 20mm deal. The screw holes in the base board were made slightly oversize so that any slack can be taken up to give a good sliding fit on the bed. A little candle grease was applied to reduce friction and wear. I had to shave a little off the bottom of the bed to allow the carriage to slide freely, after that it slid without too much force being needed but it was tight enough not to need clamping whilst sawing the fret slots.

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Here is the carriage in place. Note the cut-out for the vernier scale.
 
Making the vernier scale was a bit of a challenge.20171218 110233 Here is the set-up I finally used. A strip of 3mm thick brass is clamped to the lathe cross slide and a steel bar is fastened in the four-jaw chuck to act as a guide for the scriber. The chuck is clamped to prevent it moving. A DTI (Dial Test Indicator) is mounted on the cross slide and measures the distance moved by the cross slide. It has to be perfectly perpendicular to the steel bar. I calculated the vernier marking positions in a spread sheet (50 divisions in 19mm) and moved the cross slide, engraving the brass as I went. I had initially tried using the cross slide lead-screw for the measurement but it was hopelessly inaccurate. Even with the chosen method, it took about five attempts before I was happy with the result. I stamped the numbers to help with reading; not too pretty but readable.

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 The finished jig in action. The 20171219 123329saw guides were made by first cutting the guide slots, then planing the bases perpendicular to the slots and finally shaping the profile.
 
The vernier scale divisions provide a measurement resolution of 0.02mm. I do not pretend that the machine is accurate to to that resolution. Indeed, the rule itself will be nothing like that accurate over its length. However the accuracy will be more than adequate for locating the fret positions.
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