Recommended cutting speed for counterboring versus drilling When finishing a previously drilled hole by counterboring (using a multi-lip counterbore), how should the cutting speed compare with that used for the prior drilling operation in the same material?

Introduction / Context:Counterboring enlarges the mouth of a hole to a precise diameter and flat-bottom seat for bolt heads or capscrews. Compared with drilling, the tool engagement area is larger and the cutting lips see heavier contact at the same peripheral speed.

Given Data / Assumptions:

• Same work material and similar tool material for drilling and counterboring.
• Counterbore has multiple cutting edges and a pilot to guide alignment.
• Goal is dimensional accuracy with good finish.

Concept / Approach:Because counterbores engage more material simultaneously at the periphery and must maintain concentricity and surface finish, recommended practice is to use a lower cutting speed than drilling. Lower speed reduces chatter and edge chipping, and helps control heat. Feed is also moderated due to increased lip contact area.

Step-by-Step Solution:Compare operations: drilling (point and lips) vs. counterboring (full peripheral lips).Recognize higher load and need for finish in counterboring.Select a reduced cutting speed relative to drilling in the same material.

Verification / Alternative check:Machining handbooks list lower surface speeds for counterboring and counter-sinking than for initial drilling to achieve clean, flat seats.

Why Other Options Are Wrong:Equal or greater speeds risk chatter and poor finish. “Exactly double” is arbitrary. Independence from material and size ignores standard speed selection practice.

Common Pitfalls:Forgetting to use cutting fluid; skipping a pilot can cause bell-mouthing and misalignment.

Final Answer:Less than drilling speed