So I got some cheap chisels and they're all ground to different angles and I want to correct this on the bench grinder. I am going to do this free hand, I might build a jig. But I want to know what is a good angle to shoot for. Which I know is 25°. But I want to know a little better why it is not a good idea to go steeper. I say it is not a good idea because no website seems to explain why it is just 'bad' or something. I also want to know what each angle is used for. Like how might 20° benefit over 30°. All in all I want to know what angle will give a super cut. Like a hot knife through butter.
The sharper the angle, the sharper the chisel and the better it will slice through the wood fibers. However, this comes at the cost of durability. The shallower the angle, the more durable is the edge (since you have more metal behind the edge). As you can imagine, the angle is a matter of compromise - sharper or more durable. 25 degrees is a good middle point, so that's why it's typically used from the factory.
You should choose the compromise point based on your needs and the task at hand. End grain paring will require a sharper edge and it's typically done with hand pressure only, so a sharper angle (20 degrees or even sharper) will work well. For mortise cutting in side grain, where the cut doesn't have to be super clean, and you pound at the chisel with a mallet you might prefer 30 degrees or even shallower. For general work chisels the 25 degrees should work well.
If you can afford to have multiple chisels, each set sharpened to a different angle (20 degrees for paring, 30 for heavy work, 25 for in-between), you can use the right tool for the job. If you have only a single set the default 25 degrees bevel should typically be used.
In my opinion, the biggest factor is fragility of the edge. A sharper angle will dull faster and is more likely to chip. So, it really depends on what your use is. For heavy gouges I use with a mallet, I may use angles over 30°. Chisels for mortising hinges on a door, mid 20s. For hand-held chasing of purfling ledges on a guitar build, the angle may be closer to 20°. In situations like this, the chisel is acting more as a knife. All these are approximations, my anality stops [just] short of actually measuring.
My typical sharpening scheme begins with grinding on a wet-wheel to produce a somewhat sharper angle than I want when done. For the final honing on a stone, I use a guide with the blade set to a slightly heavier angle
I want to know what is a good angle to shoot for.
25° is the standard (almost universal) grind angle for a bevel, on plane irons as well as chisels, but few people sharpen either at that angle. Or more accurately few people hone or whet their edges at that angle1.
You can hone at this same angle as well if the usage won't punish that thinner edge (e.g. with chisels, not regularly chopping into hard woods) but it's the norm in the West to hone most tools at a slightly steeper angle than the grind angle2, i.e. 30°. It isn't that important if this target is not met precisely. Most people when honing freehand end up with an edge anywhere in the region of 28-32° depending on the day, the size and shape of the chisel and other factors and a small difference in angle like this makes no real difference, you can't see it or feel it in use.
why it is not a good idea to go steeper
Steeper cutting edges can act like they're less sharp (they're not truly less sharp but there is more resistance from the wood so it feels like it). This effect doesn't matter on most plane irons3 but it is of vital importance when it comes to most chisels because of the way they're used.
However, sometimes it is a good idea to go steeper. It's quite normal for example to hone mortising chisel steeper than bench chisels because the former are driven hard into wood using a mallet or hammer and there may also be some levering out of chips, which as you can imagine could easily damage a thinner edge.
Like how might 20* benefit over 30*.
Almost no chisels for general use are honed shallower than 25°, the primary reason being the resulting fragility of the cutting edge.
You can however deliberately do this on chisels for specific applications, for example paring chisels may be sharpened at 20° (and very occasionally even shallower). However, be aware that you may only get away with this on chisels that are slightly better than average and/or when used mainly on softer woods.
Since you mention cheaper chisels specifically, as a rule with lower-cost chisels you need to sharpen conservatively so as not to end up with edges that are too fragile, i.e. that blunt too easily and/or are prone to rolling over or chipping.
You can test this out as you go and adjust accordingly. Begin honing at 25° and see how the edges hold up in use on the wood you're using, if they blunt or get damaged too easily it's simple the next time you hone to raise the handle slightly higher. If approximately 30° is still too fragile raise it a bit more to about 35° which will rarely not be strong enough.
1 Another way of referring to these is the primary bevel and the secondary bevel (in older texts the "ground bevel" and the "whetted bevel"). These days you'll often see reference to a microbevel but this isn't truly a distinct thing, it's basically just a particularly narrow secondary bevel regardless of what angle it's at.
2 Different in Asia where there's usually one flat bevel on edged tools which is maintained by the user.
3 Most planes mount the iron bevel-down, in bevel-up planes however the bevel angle is directly tied to performance, in short the shallower the angle the lower the resistance, and the steeper you go the more resistance increases. But the quality of the planed surface also goes up as you go steeper so there's a tradeoff there.