If you are ordering pre-slotted fingerboards from Luthiers Mercantile or using one of our fret slotting systems, then you are wisely leaning on super-accurate CNC technology for the proper placement of your fret slots. Still, many are inclined to check to see if the slots and scale are accurate. With our pre-slotted boards, we know that errors practically never happen, but it can’t hurt to check. But measuring fret slots is tricky business. Here is how to do it!
Note: In the rare case of a dispute, Luthiers Mercantile will not pay the return shipping costs of slotted boards unless they have been checked using the following method!
First, here are a few important guidelines:
1. Fret positions are measured in thousandths of an inch, or 1/10 of a millimeter. NEVER use a template, another fingerboard, or a ruler to measure your fingerboards. Your tool of choice is calipers.
2. Our pre-slotted boards use a fret slot to mark the nut position. Most people cut the board at this position. Do not confuse the first cut as the first fret! See photo.
3. Always measure along the centerline of the board. Use a machinist’s square to accurately position the centerline perpendicular to the fret slots. Do NOT rely on the markings on the end of the boards we slot to position your centerline.
4. For those checking boards pre-slotted by LMI, please note that the first cut at the nut position is placed .012 (half the width of the .024 slot) back, or away from the bridge end of the instrument. This is so that if the builder cuts the board there, the leading edge of the nut falls at precisely the zero-point of the scale. Because of this, any measurements you take that include the nut slot should factor this in. If you have ordered “zero-fret” placement of your nut slot (very rare) then you do not need to make this adjustment.
To check the overall scale length, it’s best to check measurements between individual frets. If there are any minor inaccuracies in your calipers they will be exasperated when measuring longer distances, such as the distance of the nut to the 12th fret. You want to consistently position the very tip of the caliper at either the backward or the leading edge of the slots. But doing this accurately (positioning the tip) is extremely hard to do. For this reason, we recommend you use feeler gauges that fit snugly in the fret slots (available at Luthiers Mercantile). Place two gauges in two fret slots and then pinch them gently from the outside with calipers at the surface of the fingerboard right at the centerline. See photo. Jot down the measurement and then subtract the width of the feeler gauge. Normally this is .024 as this is the width of the slots we cut.
Once you have several such measurements at different positions on the fingerboard, compare them to the fret-to-fret distances on a spreadsheet that corresponds to your fret scale. You can use this calculator here or we can email us at firstname.lastname@example.org to send you an interactive spreadsheet.
The CNC machines we use to cut slots and to make slotting templates, are dead accurate but any wood is an imperfect medium -it’s always moving, at least a very tiny amount. Still, you should see measurements on your fingerboard within a few thousandths of an inch of those found on the chart. Slotting a board to the wrong scale is very unlikely in the LMI shop. Our machines produce the sticker (with the scale length and other specs) you find on your fingerboard at the very same time the board is slotted. The prevents the wrong sticker from going on the wrong board!
Here are the reasons some customers find problems with perfectly slotted fingerboards:
- * They confuse the nut slot with the first fret.
- * They check the board visually with an inaccurate ruler or template.
- * They place their ruler or calipers improperly when measuring.
- * They compare one slotted board with another by holding them edge-to-edge.
- * They assume the slots on a pre-existing instrument were slotted accurately, when they are not, and then they compare our slotted board to the one on the instrument. Sometimes they have ordered the wrong scale length assuming the scale length on the pre-existing instrument they are copying to be this or that length without actually checking. Some manufacturers were notorious for rounding scale length figures up and down in their catalogs etc.
- * They measure slots on the edge of the board. Many boards are very slightly tapered and so the slots that we cut are referenced off the centerline. Always measure out (using squares and calipers) the centerline yourself before tapering your fingerboard. The markings at the end of the board are only used by us for a rough reference.
- * The scale length is twice the distance from the nut to the 12th Some mistakenly check the scale length by measuring from the nut to the 11th or 13th fret!
For more information on intonation, scale length, nut placement, and more, please read this article: