Why not exactly 90 degrees is the angle of the corner rounding knives?

Paperfox S-3 corner rounder

Corner rounding is a very sensitive operation, because even the smallest inaccuracy causes a visible error.

Corner rounding knife for S-3 corner rounder

Ideally the corners could be rounded with a 90 degrees knife, or we could round the corners even with a simple quarter-circle profile.

Corner rounding tools for MP-1 press

Unfortunately, in practice, this profile does not result in the best rounding because:

• The corners of the sheets to be rounded are not exactly right-angled, and the angles of the corners are not exactly the same.
• The adjustment of the guides of the corner rounders are not perfect, so the rounding does not fit exactly to the corner of the sheets. 

The drawing below shows how the corner is rounded when its opening angle is equal to smaller or greater than the angle of the corner rounding knife.

The first column shows how a perfectly right-angled knife would behave when rounding a perfectly right-angled corner with perfectly accurate adjustment. (Perfectly)

The second column shows a knife with an opening angle (95 degrees) slightly larger than the corner angle of the paper. Although the result is not perfect, but even with some misalignment, the error is not significant.

In the third and fourth columns there is a knife with an opening angle (85 degrees) slightly smaller than the corner of the paper. The knife in this case not cuts the corner completely or the blade edge will cut into the paper. Even the slightest inaccuracy results a visible error.

Therefore, it is advisable to choose the opening angle of the rounding blades slightly higher than 90 degrees.

The situation is similar with the side guides in the corner rounders. If the angle of the guides is slightly larger than the corner angle of the paper, it will result in a much less visible error than if it would be smaller.