This is something I posted on the MyLesPaul forum recently and thought I’d share it here a I find it quite interesting…
Some of my fret slots were off on a couple of boards I slotted and I wanted to see how much it would affect the sound and if they were still usable (some off by as much as ~0.65-0.7 mm from where they should be). I had read somewhere (more than one place actually) that off by more than 0.5 mm is unacceptable.
I came across this write-up online where a comparison was done among some instruments and I was surprised by what was found – on a 1975 Larrivee classical guitar some of the frets were off by as much as 0.9 mm from their ideal theoretical placement. Even moreso, some frets off by 0.80 mm on a German Vasquez Rubio classical guitar (a $5000-7000 handmade classical guitar). This one had an [I]average[/I] fret placement error of 0.53 mm!
I read elsewhere, after much research, that the fret spacing has to be quite far off before it becomes a sound issue (1.0 mm or greater). At 1 mm slot spacing error, supposedly the pitch will be off by 3 cents at the first fret and 6 cents at the twelfth. It goes up more toward the bridge. A difference of 6 cents is the ‘just barely detectable’ threshold for human perception of sound apparently (according to Wikipedia).
I concluded that it probably wouldn’t have a huge effect on my instruments so long as it’s less than 0.8 mm of inaccuracy and not hugely noticeable so it isn’t a visual problem – if it’s enough for Larrivee and G.V. Rubio then it’s enough for me until I get a decent slotting jig set up for more accurate cuts. So I decided to use the fretboards rather than chucking them.
However I thought it was quite interesting – you can click here to see the full write-up.