Hosco Ultimate Fret Crowning Optimizer (UFO) H-FF-UFO2 For crowning frets after leveling Radius - 1.5, 1.6, 1.7, 1.8mm Nickel-plated steel 600 grit diamond coating 1 15/16" (50mm) diameter 1/4" (7mm) thickness The Ultimate Fret crown Optimizer (UFO) can help you restore the crown as close as the original shape with minimal strokes. For those aiming for a more accurate crown shape, UFO would be the first choice with 4 different crowning radii.  Conventional ”stick” type crown file seemingly contacts frets lengthwise which makes it very hard to determine the exact point the file is in contact with the fret. With the UFOs round shape, you can easily see exactly where you are filing. The UFO is a palm-size disk, with the point of effort and load aligned, which will transmit more effort at the point of contact where the sanding is occurring.  To widen the field of work, having both UFO 1(1.1, 1.2, 1.3, 1.4mm radii) & UFO 2(1.5, 1.6, 1.7, 1.8mm radii) is highly recommended so that you could handle a broad range of frets.

The Ultimate Fret crown Optimizer (UFO) can help you restore the crown as close as the original shape with minimal strokes.

For those aiming for a more accurate crown shape, UFO would be the first choice with 4 different crowning radii. 

Conventional ”stick” type crown file seemingly contacts frets lengthwise which makes it very hard to determine the exact point the file is in contact with the fret. With the UFOs round shape, you can easily see exactly where you are filing. The UFO is a palm-size disk, with the point of effort and load aligned, which will transmit more effort at the point of contact where the sanding is occurring. 

To widen the field of work, having both UFO 1(1.1, 1.2, 1.3, 1.4mm radii) & UFO 2(1.5, 1.6, 1.7, 1.8mm radii) is highly recommended so that you could handle a broad range of frets.