Nut slotting files for bone bridges

[Rabbit]

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.

[Simon Barden]

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

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.