Every chainsaw chain is a set of tiny chisels riding on a track. Sharpening is simply restoring the geometry of those chisels. Three numbers describe that geometry, and once you understand them, filing stops being a mysterious skill and becomes a repeatable procedure.
The top plate angle (usually 30°, sometimes 25° or 10°)
Looking down at the chain from above, the cutting edge is not square to the bar — it is swept back at an angle. For general crosscutting in most chains this is 30°. Chains intended for cold, frozen or dirty wood are often filed at 25° because the shallower angle leaves more steel behind the edge, so the edge survives abuse better at the cost of some cutting speed. Ripping chains used for milling go far shallower, around 10°, because they cut along the grain rather than across it.
There is no universally correct number: there is only the number the chain manufacturer specifies, and the requirement that every cutter on the chain shares it. A chain filed at a perfect 30° on one side and 22° on the other will always cut in a curve, no matter how sharp each individual tooth feels.
The side plate angle (the file's tilt)
The second angle is the tilt of the file relative to horizontal, typically between 0° and 10° depending on the chain. This controls the hook of the cutting edge. Too much hook and the cutter grabs, chatters and dulls fast. Too little and the cutter has no bite and needs pressure to feed. Hand filing gets this wrong more often than it gets the top plate wrong, because you can see the top plate angle marked on the chain but you cannot see the file's tilt while you are working.
The depth gauge (raker) height
In front of each cutter sits a small hump called the depth gauge or raker. Its height above the cutter's leading edge determines how deep a bite each tooth takes — typically around 0.025" for general purpose chain. Here is the part most beginners miss: every time you sharpen a cutter, you remove metal and the cutter gets shorter, which means the effective depth gauge height increases. Sharpen five or six times without ever touching the rakers and you will have a razor-sharp chain that refuses to cut, because the gauges are now standing proud of the cutters and physically preventing them from reaching the wood.
This is why so many people report that sharpening "made it worse." It did not — the top plate is fine and the rakers are now too tall.
Why consistency beats precision
If every cutter on your chain is filed at 28° instead of the specified 30°, the chain will still cut beautifully and pull straight. If half the cutters are at 30° and half at 25°, the chain will pull hard to one side, load the bar rails unevenly, and feel dull long before it is. This is the whole reason guided sharpening exists: a guide that mechanically fixes the angle produces identical cutters even in the hands of someone who has never filed before.
How different tools handle the geometry
Free-hand filing with a round file relies entirely on your eye and your wrist. It is fast for a touch-up and it is the cheapest method, but consistency drifts as you get tired — and you always get tired around tooth thirty.
A file guide sitting on the chain sets the top plate angle and the file height, which fixes most of the problem, but you still supply the stroke and the pressure.
A bar-mounted guide clamps to the bar itself and indexes each tooth, so the angle is repeated mechanically for every cutter. Traditional bar-mounted guides do this with a hand file. Crank-driven bar-mounted sharpeners do it with a rotating burr, which removes the stroke pressure variable as well.
A bench grinder is the most precise of all, but it requires removing the chain and returning to a workshop — which is fine for a shop and useless in the field.
A practical routine
Count your strokes. Whatever you do to the first cutter, do exactly the same to every other cutter. Work one side of the chain all the way round, then turn the saw and do the other side. Check the depth gauges with a gauge tool every third sharpening. Mark your starting tooth with a marker so you know when you have gone all the way round — this one habit prevents more crooked chains than any tool.