Nut slotting files for bone bridges

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The bottom of the channel may not be completely round. I have heard that's a problem, but even the pro quality files generally don't have round bottoms.

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That's not correct. If they are proper nut files they will have a rounded cutting edge.

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Although cutting is slow, I never worry about cutting too deep. Although I have no direct experience, I have read that the good nut files are so good that it's easy to cut the slot too deep.

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That's true of any tool if you go at it like a bull at a gate. Having a good quality and sharp nut file is going to cut a much smoother slot (smoother slot = less friction and better string to nut contact).

When cutting a nut slot, there comes a point where you need to stop, check, adjust, repeat. The adjustment stage may only require a very short single direction stroke. Sometimes it only takes a bit of 1200 wet/dry sandpaper to fine tune it.
If you just go like you're sawing firewood, you'll have a problem.

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Originally Posted by McCreed

That's not correct. If they are proper nut files they will have a rounded cutting edge.

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Not really. My Hosco files have a flat bottom to then, though the very edged are rounded over with a small radius (just checked with my new digital microscope). The thinner the file, the more they approach a rounded section as there is less flat section in between the rounded ends, but they never get to a full semi-circular profile.

I have one Hosco file like yours to compliment my Hiroshima set, and whilst I've never inspected it under a microscope, it certainly cuts a definitive U-shaped slot that is visible to the naked eye. My Hosco is a .028"

Maybe I should have said "If they are proper nut files they should have a rounded cutting edge."

I've just never seen anything in forums or luthiery supply sites showing a nut file that cuts a flat or rectangular slot (other than an illustration showing an improperly shaped slot).

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I'm not saying I don't believe you. It just doesn't make sense to me.

I disagree with that drawing on several points. Mainly because you are never going to get a string to fit perfectly in a semicircular slot like that. The string will either be larger, in which case it sits higher up on two points, or smaller, when it sits on the bottom of the slot, very much like it was a square slot.

If the string did sit perfectly in a semicircular slot, then it would have far more friction than a square slot (for the same reason that wide car tyres have more grip and higher rolling resistance compared to skinny tyres).

I’d say the V slot is the worst of the lot, followed by the semi-circular slot. First one, the sides are slightly tall, but not too much if as shown, and the second one I’d like the sides to be very slightly talker as a minimum.

It’s all very well people drawing these idealised slots, but in reality the sides will almost always be a bit wider than the string, and the base is unlikely to be perfectly flat or round bur a bit rough and random.

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Originally Posted by McCreed

Having a good quality and sharp nut file is going to cut a much smoother slot (smoother slot = less friction and better string to nut contact).

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That may be. I would never suggest that my improvised tool is better than good nut files. In fact I would happily buy a set if they weren't between 13 to 26 times as expensive.

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When cutting a nut slot, there comes a point where you need to stop, check, adjust, repeat. The adjustment stage may only require a very short single direction stroke. Sometimes it only takes a bit of 1200 wet/dry sandpaper to fine tune it.
If you just go like you're sawing firewood, you'll have a problem.

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I think you have stated my problem rather nicely. I get impatient, start sawing away, and then exclaim "Oh x@#%! What have I done?!" ...and then check the string height. That has yet to be a problem with my crappy improvised tool, fortunately. I accept the assessment that it may be reaching a bit to call this an "advantage" though ;-)

I would be willing to accept that a good set of nut files would be better in *every* way than the improvised tool. My contention is more along the line of saying that if one is willing to go to the greater time and effort necessary to use the feeler gauge, you can get an adequate result.

This has been my experience thus far with the guitar nuts I have cut with feeler gauge tool. I recently did an acoustic nut this way and am pretty happy with how it sounds and plays. Decent sustain, no buzz, right string height, no complaints. But, of course, I can't A-B. I have NOT directly compared a nut cut with good nut files, and one cut with my improvised tool.

I’ll revise my statement above and say that a curved slot bottom is probably best as long as the slot is a bit wider than the string. My thinking was affected by the diagram scenarios, rather than the more real-life situations I talked about but couldn’t see clearly in my head until I got to sleep and my brain churned it over. Had to write this so I can get back to sleep (2.40 am here ATM)!

Given how much Hosco files cost , you would expect rounded bottoms. The cheap and nasty ones that came with a set of "luthier tools" are flat.If I was doing six strings I'd probably use them to get close, then switch to finish with folded wet/dry. The folded paper naturally forms a rounded surface.

Has anyone tried using the tip cleaners with a small German jeweler's file? I am thinking about giving that a try...

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Originally Posted by Rabbit

Given how much Hosco files cost , you would expect rounded bottoms.

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There seems to be an undeserved prejudice against flat bottoms here. I really don't thing there's much, if anything in it between curved and flat here (but not close-fitting round like in the diagram McCreed posted). If you had a zero fret, then the top of that won't have pits in, it will be flat (except when very worn and then then it will need flattening). The top of your frets are also flat, but I don't hear anyone say it would be a good idea if they had pits in for the strings to rest in.

Also, don't forget that Hosco nut files have been pretty much the go-to nut files for luthiers for decades.

For tuning stability, you really want to minimise friction, which means minimising the contact area between the string and the nut. There is a slightly greater contact area between the string and the nut with a rounded-bottom slot than a flat-bottomed one, maybe more with some deformation under string tension. How much difference it makes I don't really know as every nut slot is unique and even ones that look the same shape can vary a lot at the microscopic scale, where the coefficient of friction is determined.

I don't have all the answers here by a long way, and there is a lot about nuts that I have questions about that I can't find answers to, so I'm trying to do my own experiments (was just about to do one this morning until someone started up doing some major hedge-trimming with petrol trimmers and I can't hear strings change pitch). There may well be an optimum shape for a nut slot that minimises friction that sits somewhere between fully flat and fully semicircular, but I haven't come across any info on that yet.

It's probably more important that the bottom of the slot is as smooth as possible and angled correctly.

I get what you're saying about the drawing. It's not realistic and should maybe have a notation like: "for graphic illustration only" or something about not interpreting the drawing too literally.

As for "undeserved prejudice against flat bottoms" (and I'm just talking about nut slots, not arses :p) I have never seen a square edged nut file, or any luthier advocating flat bottom string slots. All the product descriptions I've come across note something like:

"Our tempered edge-cut nut files produce round-bottom slots for nuts and saddles. Ensure your strings seat properly in the nut without buzzing."

I have always worked off of the idea that rounded slots are about maximising string-to-nut-contact and creating better sound transference and resonance. But I get your point about minimising friction as well, so I guess there must be a balance.


One thing I think we certainly agree on is: "It's probably more important that the bottom of the slot is as smooth as possible and angled correctly."