Why Does Surface Finish Vary on the Same Part With the Same Tool and Speeds?

Because "same tool and speeds" doesn't mean the same cutting conditions everywhere on a part. Wall angle, feature depth, tool engagement, and rigidity all change as the cutter moves, and any one of them shifts the finish even when the spindle speed and feed rate in the program never change.

Effective chip load isn't constant

Programmed feed rate is a target for the whole path, but the actual chip load a flute sees depends on radial engagement, which changes on curved walls, corners, and ramping moves. A tool cutting a straight wall at 40% stepover sees a different load than the same tool cutting a tight inside corner where engagement briefly spikes.

That spike shows up as a visibly rougher band right at the corner even though nothing in the program changed there.

Rigidity changes with reach and support

A feature near the top of a part, close to the vise, gets machined with a short, stiff toolpath and stays rigid. The same tool cutting a feature deep in a pocket, or near the top of a tall thin wall, now flexes more because there's more unsupported length, either more tool stickout or less material behind the wall to resist deflection.

More deflection means more rubbing and tearing instead of clean shearing. That reads as a rougher surface at identical spindle speed and feed.

Grain and hardness aren't perfectly uniform

Real stock isn't a perfectly homogeneous block. Extruded or rolled aluminum has grain direction, and cutting with the grain versus across it can show up as a finish difference on the same part. Forgings and castings can have local hardness variation near the surface skin versus the core. Neither shows up on a speeds-and-feeds calculator, but both show up in the actual part.

Climb vs conventional on the same wall

If a toolpath switches cutting direction between passes, common on pocket strategies that reverse to save air moves, one pass is climb milling and the next is conventional on the same wall. Climb tends to shear cleanly and pull chips away. Conventional tends to rub before it bites, especially in gummier materials, and leaves a visibly different finish on the same surface a moment later.

What to actually check

Map where the rough finish shows up relative to the part geometry before blaming the machine. Rough finish that follows corners points to engagement spikes. Rough finish that follows depth or wall height points to deflection. Rough finish that follows grain direction or a hardness map points to the material, not the program. Each one gets fixed a different way: slowing the feed at corners, adding support to a thin wall, or accepting that a casting's skin will never finish like its core.