The 25-or-30 debate runs in every sharpening thread, and it can go forty replies without landing. Both sides are usually right. Neither side realizes it's answering a different question than the other.
The primary bevel angle means two completely different things depending on whether your iron sits bevel-down or bevel-up. Once you see that, the argument ends.
Bevel-down: the bevel doesn't set the cutting angle
On a standard bench plane, the iron rests bevel-down on the frog. The bed is 45 degrees. That's your cutting angle, full stop. Grind at 25 degrees, grind at 30, grind at 35 — the plane cuts at 45 every time, because the bevel faces down and away from the work.
What the bevel angle actually controls is clearance — the gap between the back of the bevel and the wood surface. Lower bevel, more clearance, longer runway before the edge starts to drag. Higher bevel, less clearance, the edge holds up longer but feels a little less keen when it's fresh.
So the bevel-down argument is a durability argument. Both sides are right about their own trade-off; they're just talking past each other because neither named what the bevel actually controls. Twenty-five degrees has been the standard for a long time because it gives enough durability without wasting steel. Grind bench planes at 25 and don't give it much thought.
Bevel-up: now the bevel is the steering wheel
Flip the iron over. On a bevel-up plane — a low-angle block plane, a bevel-up jack — the bevel faces up into the cut. The bed is usually around 12 degrees. The cutting angle is bed plus bevel. Set a 25-degree bevel and you're cutting at 37 degrees: slicey, useful for end grain, won't hold up against difficult figure. Set a 50-degree bevel and you're cutting at 62 degrees, up in the scraperlike range, best tearout resistance, more drag.
That flexibility is the whole reason these planes exist. Every degree you put on the grind changes how the plane behaves. This is why bevel-up users argue about bevel angles more than anybody else. They're not obsessing for nothing — they're actually adjusting the tool.
Why 45 is a good number and not a guarantee
The standard bench plane bed sits at 45 degrees because that puts it near the bottom of the zone where chips shear cleanly. Below about 40 degrees, the cut tends to fail in tension ahead of the tool — what you see as tearout in difficult grain. Through the mid-40s to the low 50s, the chip shears at the cut line and lifts away. Higher than that, you're scraping more than cutting: best grain control, most friction.
At 45 degrees, a bench plane sits in the clean-shear zone by design. Good general purpose. Not immune to tearout. When you're planing across a flame figure and it tears anyway, the fix is not grinding at 30 instead of 25.
Before you set the grinder
Answer one question first: bevel-up or bevel-down?
For bevel-down, 25 degrees is a solid default. Grind to 30 if you're working harder steels or need more from an edge before it needs attention. The plane cuts the same either way.
For bevel-up, the bevel angle is a real decision. Know the application, set the angle accordingly, and keep a second iron ground for the other mode if you use the plane for more than one job.
The argument wasn't wrong. Both sides just needed to name the plane first.