Boring: The First Choice For Precision Hole

When making a critical hole, why do we still “bore” it instead of just milling it on a modern CNC:

1. Perfect roundness:  A boring head traces a true circle by design. Interpolated milling? Tiny servo lag or backlash = micro-polygons or ovality.
2. Mirror-like surface finish: One light finishing pass with a boring bar > Ra 0.4 - 1.6 µm without extra spring passes.
3. Dial-in diameter precision: Adjust the boring head 0.001 mm at a time after a test cut. Try doing that with an end mill inventory.
4. Dead-straight deep holes
L/D > 5:1? Good luck keeping an end mill from walking. A guided boring bar laughs at deflection.
5. Lower cutting forces & chatter: Boring = mostly radial load. Side-milling a hole = interrupted cuts in every direction > vibration.

Yes, 5-axis machines with helical boring and crazy-rigid setups can get close… but when the print says ±0.005mm, perfect circularity, and Ra 0.8 on a 100 mm deep bore in 4140 alloy, You still reach for the boring head.  Milling is fast for roughing and complex features. Boring is still king when “good enough” isn’t.