Acoustic Nut File Analysis

This article focuses on acoustic guitar nut file sets.

I have long wondered about the variety of nut file slot cutting widths when compared to some sort of best fit for each string. I install a lot of light gauge string sets, especially the Martin MA540 Light Gauge set. So, that is my focus here. In this analysis I have determined that .004-inch should be added to each string slot. So, for a .054 gauge string, the ideal nut file should cut a slot .058 wide. Maybe .060 is also acceptable and in some materials, preferable.

The goal is to eliminate string binding in the nut. And I want my nut slots to look like they came from the factory.

Stewmac suggests that the nut slot file should match the size of the string. So, according to them, to cut a slot for a .054 string, the file width should be .054 inch. I hope I am quoting them accurately. Of course, they realize the string won’t slide (tune) in a slot that matches the string diameter. They suggest further that you rock the file in the slot to produce the desired slot width. This seems very imprecise, and in my opinion, not acceptable. I would rather have a file that cuts the slot correctly in the first place. Unfortunately, all of the nut file sets I have seen make the Stewmac cut width assumption.

Thus, the following table:

light
gauge
set
Theoretical
Ideal *
Slot
Width
Uo-
Chikyu
2502
Stewmac
Gauged
0884
H/P Hosco Stewmac
Diamond
2328
Stewmac
Double
Edge
4547
.010** .010 .010
.012
.013 .013 .013
.012 .016 .016 .016 .016 .016 .016 NONE
.016 .020 NONE NONE NONE NONE NONE .020
.024 .028 .026 .024 .024 .026
.028
.025 .029 .032 NONE NONE .032 .032 .032
.032 .036 .036 .035 .036 NONE .036 .036
.042 .042 .042 .042
.042 .046 NONE .046 NONE .046 NONE NONE
.050 .050 .050 .050
.054 .058/.060 NONE NONE NONE .058 NONE .060
.054 .055 .056

* Assuming plus .004 to reduce string sticking or friction against nut sides. This may be arbitrary, but it does work for all conventional nut materials: bone, TUSQ, Corian, plastic.

**All inches

Comments on the file sets

All rat tail/needle file sets are useless for cutting nut slots. I can’t imagine anyone using these files for such a precision operation. Might as will use a hack saw. And yet, Amazon and many other sites sell these file sets for nut slotting. Big DUH! there. If I am wrong about this please send comments.

Uo-Chicyu - I own and like this set because of the rounded ends and because the teeth run the full length of the file thereby increasing functional life. However, only three file sizes fit my criteria for proper file width.

Stewmac Gauge 0884 - These have two flaws. If you use them much the gauge number becomes unreadable. I dislike the square ends. When cutting nut slots, I don’t want to take any chance that the file will dig into the peg head surface. These go right through the three layers of tape used to protect that area.

N/P - Teeth are only on the edge, unlike the Stewmac 4347 set which has teeth up the side of the taper. Not a great design.

Hosco - This Japanese set is nicely made, and I own one. I like the extensive variety of sizes. But again, these have very sharp corners on the ends. Much worse is the sharp gripping end, which comes almost to a point. I tried several handle designs made from dowel, but nothing works. They should supply a handle.

Stewmac 2328 - Whoa Nelly, $450 and climbing for this set. I guess I am making fine jewelry now. The problem. with this set is the cutting surfaces pack with dust. They quit cutting unless you wash off the dust. The .046 is nice. They don’t show that size in the catalog presently.

Stewmac 4547 - This is my favorite set, although, only three sizes fit according to my analysis. The description text on the site is confusing. It states, "cuts on the edge only" but that is not true. They also cut part way up the tapered edge. If it included the "correct" sizes, this set would be the only set to buy. It does include the valuable .060 width file, which is most valuable.