CNC Speeds, Feeds & Machining Parameters

Question 1

What formula is used to calculate spindle RPM when surface footage (SFM) and tool diameter are known?

Accepted Answer

RPM = (SFM × 3.82) / Diameter

Answer

The standard formula is RPM = (SFM × 3.82) / Diameter, where 3.82 is derived from 12/π.

Question 2

When machining 6061 aluminum with a 1/2-inch HSS end mill, which SFM range is most appropriate?

Accepted Answer

400–600 SFM

Answer

Aluminum is a soft, free-machining material and HSS tools can run 400–600 SFM without excessive heat buildup.

Question 3

What effect does increasing feed rate have on chip thickness in CNC milling?

Accepted Answer

Chip thickness increases

Answer

Higher feed rate means the tool advances farther per tooth engagement, producing thicker chips.

Question 4

The recommended starting chip load for a 1/2-inch carbide end mill in mild steel is closest to:

Accepted Answer

0.004 inch per tooth

Answer

A 0.004 inch-per-tooth chip load is a typical starting point for a 1/2-inch carbide end mill in mild steel.

Question 5

What is the primary reason for reducing spindle speed when drilling deep holes?

Accepted Answer

To allow adequate chip evacuation and prevent drill breakage

Answer

Deep holes trap chips, and reduced speed combined with peck drilling cycles allows chips to clear before the drill binds or breaks.

(CNC) Computer Numerical Control Machinist Technical Practice Test