Surface flatness is something of a science. There are actually gauges made to measure this, and they come in various types and with different levels of accuracy and expense.
When we left off last time, we had finished leveling and crowning the frets. At this point, the tops of the frets would all be level with each other, and the tops would also be rounded (crowned). However, you''ll recall that the initial leveling left the frets with scratches in their tops, and this would make the guitar uncomfortable to play, especially if you bend the strings. The guitar would feel "scratchy." So at this point, we need to get the scratches out, and this step is typically called polishing.
Here''s how abrasives work: as you sand something, the abrasive particles scratch the surface you''re sanding. The sanding may make the surface "flat" (a relative term, as we shall see), but it also scratches it. If you sanded with 120 grit sandpaper, you would have relatively large scratches, and if you looked at a photomicrograph of the surface, you would see that it didn''t look flat at all. The scratches would look like huge valleys.
You would then follow up with the next grit, like maybe 220, and this would also scratch the surface, but because the abrasive particles are smaller, the scratches would be smaller than the 120 grit scratches.
The idea with sanding is to obliterate the larger scratches from the previous grit with smaller scratches from the current grit. As you move up through the grits, each time completely obliterating the larger scratches from the previous grit, the scratches will get smaller and smaller with each successive grit until you can no longer see them with the naked eye.
Surface flatness is something of a science. There are actually gauges made to measure this, and they come in various types and with different levels of accuracy and expense. There are machines that will measure surface roughness in millionths of inches, so you can see that "flat" is indeed a relative term.
One simple type of surface roughness gauge is a cylinder with graduations on the side. You set it on the workpiece, and then look at its reflection in the workpiece. The flatter the workpiece, the less distortion in the reflection, and the more of the graduations you can read. The highest graduation that is readable indicates the degree of flatness. If you want to see something really flat, open an old computer hard drive (you know, one of those old 20Gb drives) and look at the platter(s). Now that is flat.
To get the frets really shiny (meaning that the scratches are really tiny), the final steps may be done with buffing compounds. Some repair people finish with 0000 steel wool, which is pretty fine, but won''t leave a finish as shiny as buffing will.
You''ll recall that at Acme, we use a surface-ground bar to level frets. One edge is coated with industrial diamond (which is fairly coarse: 220-320), and the other side has no coating. After crowning, we use various grits of sandpaper adhered to this uncoated edge (using the bar in the same way it was initially used for leveling), and finish with 600 grit paper. The final step is to mark the tops of the frets one last time, with the neck still held in the jig to prevent deflection, and use the 600 grit paper to remove the marker from the tops. This leaves the tops with very small scratches, while ensuring that everything is still level.
The final step is to buff the frets. A buffer is just a felt or muslin wheel that is "loaded" with very fine abrasive. This abrasive usually comes in a stick form (in various grits, all of which are very fine), and the stick is pressed against the rotating wheel to load it with abrasive. The guitar neck is then passed across the face of the wheel, back and forth, until the scratches from the 600 grit paper are obliterated. Acme has two buffers, each with a coarse and a fine set of buffs. One buffer is dedicated to polishing metal and the other buffer is set up for finishes (like lacquer).
The frets are first buffed using the coarse abrasive, which obliterates the 600 grit scratches, and then the fine abrasive obliterates the scratches from the coarse abrasive. And at that point, the frets look like little pieces of sterling silver jewelry. They look bee-yoo-tiful!
And the final steps are cleanup, restringing, and setup. Next month we''ll discuss setups.
Acme Guitar Works sells electronic components for electric guitars, including complete, prewired assemblies. Visit them at acmeguitarworks.com.