about slotting nuts with string scraps and pumice

[Bob Hammond]

Hi, the specialized files for slotting nuts do work well, but this alternative method might work for you. It is slow.

It employs the abrasive properties of pumice to cut the slot, and is driven by the mechanical action of stroking a guitar string that is held in a jeweler's saw frame.

I'm going to post the pics and commentary 'one at a time' because it's easier for me to describe it all:

In this picture, you can see the necessary items. There are scraps of guitar strings, a razor blade (serrated by Al Dodson's method), a jeweler's saw frame (Grobet), a needle file, and pumice, (and the nut blank held in the vise).

[Bob Hammond]

In this picture, I've chucked a scrap of bare steel string into the saw frame, then clamped the saw frame in the vise, and I am stroking the bare string wire with the needle file. This nicks the wire to make 'saw teeth and gullets' that pick up the abrasive pumice. It's not necessary to do this with a wire-wound string because the pumice will collect in the interstices of the windings.

[Bob Hammond]

In this pic, you can see that I'm starting the groove with a razor blade, but a fine saw or file will do too

[Bob Hammond]

In this picture, I've sprinkled some pumice on the paper, and then I will pick it up with a fingertip and pack it into the groove that was started with the razor blade.

[Bob Hammond]

Here you can see the string working the pumice to form the slot (I've reversed the nut in the vise to make it easier to photo it with my only two hands). I dab on more pumice as needed while cutting.

[Bob Hammond]

When using the bronze wire-wound strings, as you saw away there will be a black deposit that forms in the slots. This will wash away with soap and water. Also, be careful about spreading pumice dust around an indoor shop. It abrades many things (e.g. the screw of the vise), and it's probably not a good idea to breathe it either.

Since the human hand may vary in stroking the path of the slot, then one may cut the slot too large. It might be a good idea to start cutting a slot with a smaller gauge of wire, and then do a final touchup with the string of the desired gauge.

[Darren Galloway]

Awesome! Great tutorial Bob. I use strings glued to a Popsicle stick right now to finesse a slot. This is a whole new level of string usage.

[Michael Lewis]

I admire the tenacity it takes to make this system function well. I don't have the patience and I do have nut files, so that is what i use. As an alternative to the nut files one can use a triangular file to cut the slots. The string will rest against two surfaces so it can not rattle unless the slots are badly cut, and the 60 degree notch will not bind the strings. It works well enough that I used the triangular file for several years before I found the gauged nut files. You need a triangular file with sharp edges, not rounded.

[Bob Hammond]

Hmm, I don't know why I've never tried more aggressive grits. Are there any suggestions? Maybe emery dust?

[Alan Carruth]

That might work faster if you cut the slot almost to size and depth first with a saw, and then use the string and pumice to clean it up. A nice trick, in any case: it's always good to have more than one way to do things.

Alan Carruth / Luthier

[Bob Hammond]

Speed was very significantly improved with the use of Permatex Valve Grinding Compound (Item# 80036 (34A)). It is available at your local auto parts store for about $4. One 1.5oz tube will last a long long time.

It is a more aggressive paste of silicon carbide grit that is suspended in ethylene glycol, and it is mixable with water. I used a small dab or two straight out of the tube (" a little dab will do ya!"). I tried both a 0.011" dulcimer string (stroked with a file), and a bronze-wound 026" G string. With the fine string, I soon saw wisps of water vapor and then saw and smelled a bit of smoke(!) as the bone was cut. Also, with the fine wire, I did find it helpful to use and old hex screwdriver bit as a caul, to press down a bit on the wire. The Permatex paste is also much neater, in that it small amounts can be dispensed, and it doesn't disperse so quickly everywhere as pumice dust does.

I like the process better now. It cuts fast enough so that it's not so tedious, but slow enough that it's not so easy to overshoot the mark. I think it may be a good idea to go back to the pumice for a final surface finish to the slot, but maybe not. I think it's possible to get a near-perfectly matching slot every time in a reasonable time.

[Bob Hammond]

Ah, I've never used nut files so I shouldn't comment on their performance. But if you haven't bought files yet, then I suggest that you try this technique with the valve compound and then touch up with pumice if needed. I think it's possible to cut slots perfectly for any size of string.

Last night I didn't have much time to test. I tried cutting slots with the valve compound using a bare steel string and a bronze-wound G string (Martin medium) on a scrap of bone. I was a bit timid in the stroke length and pressure. Today, I cut a full set of slots and found that the valve compound cuts so fast that I overshot the marked depth with not very many strokes at all!

Darren, I think that the rigidity of your string-on-a-stick tool might work better than chucking the wire in the jeweler's saw. I'll give it a try, because the string in the saw frame bows a bit and so it tends to cut on the edges first. I like to start the cut from the shallow-angled backslope (the peg edge) first and then approach the fret-side edge, because if I make a mistake in starting the slot then I can grind the backslope to erase the mistake. If I get it right the first time, then I just grind the backslope to suit. But this not a serious issue.