Machining a critical hole

To manufacturing engineers, a quick question:
When making a critical hole, why do we still “bore” it instead of just milling it on a modern CNC?

Here’s the short answer (even in 2025):
1️⃣ Perfect roundness
A boring head traces a true circle by design. Interpolated milling? Tiny servo lag or backlash = micro-polygons or ovality.
2️⃣ Mirror-like surface finish
One light finishing pass with a boring bar → Ra 0.4–1.6 µm without extra spring passes.
3️⃣ 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.
4️⃣ Dead-straight deep holes
L/D > 5:1? Good luck keeping an end mill from walking. A guided boring bar laughs at deflection.
5️⃣ Lower cutting forces & chatter
Boring = mostly radial load. Side-milling a hole = interrupted cuts in every direction → vibration city.

Yes, modern 5-axis machines with helical boring and crazy-rigid setups can get close… but when the print says ±0.005 mm, perfect circularity, and Ra 0.8 on a 100 mm deep bore in 4140?
You’re still reaching for the boring head.

Milling is fast for roughing and complex features.
Boring is still king when “good enough” isn’t.

Who else finishes every critical bore the old-school way — even on brand-new machines? Drop your favorite boring trick or horror story below!