How deep can a milled pocket be?

Depth is one of the quiet cost drivers in milling. Deep, narrow pockets need long tools that must run slowly, so time and risk grow quickly with every extra millimetre.

Depth versus tool diameter

Standard end mills cut comfortably to about 3× their diameter. Up to 4× is routine with care. Beyond that, the shop needs long-reach or necked tools, slower feeds and more passes. Above about 6× diameter, the walls start to taper and the finish suffers.

The narrowest part of the pocket decides the tool diameter. A 10 mm wide slot can use at most an 8–10 mm tool, so it should stay under about 30–40 mm deep.

Ways to keep pockets shallow

• Open the pocket to one side so the tool can come in horizontally.
• Machine from both sides if the part is a frame or housing.
• Split a deep box into a frame and a bolted or bonded bottom plate.
• Widen narrow slots so a stiffer tool fits.

Blind holes are different

Drilled holes can go deeper than milled pockets because drills are designed for it. Standard drills manage 4–6× diameter easily; peck drilling extends that to about 10×. Deeper than that needs gun drills, which most job shops do not have.