Can't find nut slot files for seven string guitars

[voyage]

On my 7 string guitars I might use anywhere from a .070 gauge to .074 gauge. But all of the nut file makers skip from .065 to .075 to .085. How does one cut nut slots if one uses a .070 or .072?

Thanks,
Jim

[Jimmy blue note]

I have bass nut files I got from StewMac a while ago. They have long paid for themselves and allow me to cut any width nut slot. If the slot is wider than the exact fit, the V shape slot holds the string just fine. There are also nut seating files and contour files that will shape a slot to what ever I need. This has worked for me. I cut my nuts so I don't need deep slots; I don't have a lot of nut above the mid point of the string but that's just the way I like to make a string nut. This way of seating a string in a wide V has been good for me and I'm a 7 string player. I also like to polish and use teflon in the nut groove so the strings don't bind on the windings.
Hope you find something that works for you.

[garybaldy]

I made my own.

[GuyBoden]

I made my own.

Excellent, do they work?

[garybaldy]

Excellent, do they work?

I wouldn't cut a new nut with them but they're perfect for making adjustments. You can double them up for virtually any width.

[nevershouldhavesoldit]

If the slot is wider than the exact fit, the V shape slot holds the string just fine.

Be very careful with V shaped nut slots. The string puts outward pressure on the walls that tries to push them apart. This puts a lot of stress on the material at the bottom that tends to crack it.

At the very least, you should use a small round file to radius the junction of the walls, if the file you used to cut the slot didn’t do that. The reason rounded slot walls are important (apart from better contact between string and nut, which I think takes a tiny bit of brilliance off the tone and makes it a bit smoother) is that complete contact between the bottom half of the string and the nut diffuses the direction of the force on the nut walls so that most of it is directed downward against the nut and doesn’t try to push the walls further apart.

If you prefer a V shaped slot, filing a small radius at the depth and polishing it smooth reduces the tendency to crack the nut. The slightest surface break becomes what’s called a stress riser, which is a tiny focus at which the forces are opposing each other and pushing the irregularity apart. Cutting or filing a V into the nut leaves this kind of weakness where the two walls meet at the bottom.

Even on a perfectly straight smooth surface, a tiny scratch can act as a stress riser that then extends and propagates into a crack under pressure. Worse, a V shaped slot puts the most pressure on the nut, while a slot that’s perfectly concentric with the string puts the least amount of pressure on it. Remember that pressure is force per unit area. Even though the force is the same regardless of the shape of the slot, applying the same force to a much smaller contact area is potentially much more damaging.

This kind of nut breakage also happens when the string is larger than the top of the slot and sits on the edges, which is often the result of fitting heavier strings without addressing the slots. A slot edge can crumble or crack off under the string where it’s hard to see, causing buzzing, intonation issues, tuning instability etc.

The 7th string is the most vulnerable to these issues because of its size and because it’s at the end of the nut. A crack through the bottom leaves the outer end free. I had the ends of 2 nuts break off on 7s I got more than 25 years ago during the first few years after I switched to 7s. I experimented with different sizes and styles of 7th strings and obviously wasn’t as meticulous about nut setup as I should have been. So I’ve taken this very seriously ever since.

[voyage]

Be very careful with V shaped nut slots. The string puts outward pressure on the walls that tries to push them apart. This puts a lot of stress on the material at the bottom that tends to crack it.

At the very least, you should use a small round file to radius the junction of the walls, if the file you used to cut the slot didn’t do that. The reason rounded slot walls are important (apart from better contact between string and nut, which I think takes a tiny bit of brilliance off the tone and makes it a bit smoother) is that complete contact between the bottom half of the string and the nut diffuses the direction of the force on the nut walls so that most of it is directed downward against the nut and doesn’t try to push the walls further apart.

If you prefer a V shaped slot, filing a small radius at the depth and polishing it smooth reduces the tendency to crack the nut. The slightest surface break becomes what’s called a stress riser, which is a tiny focus at which the forces are opposing each other and pushing the irregularity apart. Cutting or filing a V into the nut leaves this kind of weakness where the two walls meet at the bottom.

Even on a perfectly straight smooth surface, a tiny scratch can act as a stress riser that then extends and propagates into a crack under pressure. Worse, a V shaped slot puts the most pressure on the nut, while a slot that’s perfectly concentric with the string puts the least amount of pressure on it. Remember that pressure is force per unit area. Even though the force is the same regardless of the shape of the slot, applying the same force to a much smaller contact area is potentially much more damaging.

This kind of nut breakage also happens when the string is larger than the top of the slot and sits on the edges, which is often the result of fitting heavier strings without addressing the slots. A slot edge can crumble or crack off under the string where it’s hard to see, causing buzzing, intonation issues, tuning instability etc.

The 7th string is the most vulnerable to these issues because of its size and because it’s at the end of the nut. A crack through the bottom leaves the outer end free. I had the ends of 2 nuts break off on 7s I got more than 25 years ago during the first few years after I switched to 7s. I experimented with different sizes and styles of 7th strings and obviously wasn’t as meticulous about nut setup as I should have been. So I’ve taken this very seriously ever since.

Thanks very much. I wonder where I could find a round file as small as .070? What do you use? I have a set of fine old Jewelers files but there's nothing that small in there. Maybe the tool that garybaldy described would be good for rounding the bottom.

[nevershouldhavesoldit]

Thanks very much. I wonder where I could find a round file as small as .070? What do you use? I have a set of fine old Jewelers files but there's nothing that small in there. Maybe the tool that garybaldy described would be good for rounding the bottom.

I use carburetor jet files to round the bottom of the slots - the 1.8 mm is 0.071". I cut the slots just a hair short of the desired depth and polish the slot bottom gently with 1000 grit paper folded into a U around a 6 to 10" piece of a string. I haven't cut a slot in a few years and don't remember what size string I use for the 0.075" slot. The string with the paper around it should slip into the slot with little to no pressure. It only takes a few 2" strokes through it with very little downward pressure on the string to remove enough material. Stop after each pull and check width and depth before removing more.

There are as many shapes for slots as thre are luthiers. I like to gently curve the slot bottom downward toward the headstock and flare the end a bit so that the strings have a gentle path. This webpage from luthier.com explains it very well and has great illustrations. I use root canal files to shape the slot because they're flexible and tapered, and they work very easily without gouging if you're careful. They're also cheap and readly available.

[voyage]

Holy cow! Root canal files? Are you a dentist? Where do you get those?

Thanks for your advice much appreciated.

[nevershouldhavesoldit]

Holy cow! Root canal files? Are you a dentist? Where do you get those?

My father was a dentist, and I grew up playing with the tools in his office lab. A lot of dental tools are very useful in guitar repair and maintenance. I still have his little files, chisels, scrapers, tweezers, etc. But you can get pretty much anything on the internet. When in doubt, check Amazon - the ones in the middle that say “previously viewed” are the ones I have right now. But I think they’re all pretty much the same for our purposes.

[voyage]

Thanks very much for that.

[nevershouldhavesoldit]

Thanks very much for that.

It’s my pleasure. There are many wonderful tools out there for watchmaking, jewelry repair, medicine, surgery, art, auto maintenance and repair, plumbing, electronics, computer repair, electrical work, etc etc that we can use to great advantage. I’m always on the lookout for it.