Speeds and feeds are the most important considerations to achieve the best results from cutting tools. Improper speeds and feeds often cause low production, poor quality, and damage to the tool. Speeds that are too high or feeds that are too light can lead to rapid wear and dulling of the cutter, reducing tool life.

Speed is measured in peripheral feet per minute. It is often referred to as cutting speed or surface speed. Feed is usually measured and stated in inches per minute (IPM). It takes into consideration the number of cutting teeth (or flutes), the feed per tooth (or cutting edges), and the revolutions per minute. Feed recommendation tables for drills are based on 2-flute drills.

To establish operating conditions, all feeds rates should be calculated from the chipload or feed per tooth. The highest possible feed per tooth will usually provide longer tool life. However, excessive feeds may overload the tool and cause the cutting edges to chip or break.
 
Following are many of the commonly used formulas for calculating operating parameters for cutting tools:

NOTE: for all formulas, D = diameter and T = number of teeth.

SFM: surface feet per minute/cutting speed:  .262 x RPM x D
RPM: revolutions per minute/rotational speed: (3.82 x SFM) ÷ D or SFM ÷ (.262 x D)
IPM: inches per minute/machine feed rate: RPM x IPR (or) T x IPT x RPM
IPT: inches per tooth/feed per tooth: IPM ÷ (RPM x T)
IPR: inches per revolution-feed per revolution: IPT x T (or) IPM ÷ RPM
Inches to mm: inches x 25.4 or inches ÷ 0.03937
MM to inches: mm ÷ 25.4 (or) mm x 0.03937

Technical Tip #30 - Formulas for Cutting Tool Speed & Feeds

1. Common Machining Formulas

SFM (surface feet per minute) – defined as the distance in feet traveled in one minute by a point on a part surface being machined. On a lathe, it is the rate at which the surface of the stock passes the cutting tool. This takes into account the diameter of the stock. On a mill, it is the rate at which the cutter moves past the stationary stock. This takes into account the diameter of the cutter.

sfm =  π x D x rpm
                 12

RPM (revolutions per minute) – defined as the number of revolutions a part or cutter will rotate in one minute.

rpm = 12 x sfm
             π x D

IPR (inches per revolution) – defined as a feed value reporting how far an insert or cutter travels in one revolution.

ipr = ipm
        rpm

IPM (inches per minute) – defined as a feed value reporting how far an insert or cutter travels in one minute

ipm = rpm x ipr

2. Nose Radius Selection and Surface Finish   

Nose radius and feed rate have the greatest impact on surface finish. Assuming the feed rate remains the same, the larger the nose radius is means the better the surface finish will be. While improving the surface finish, a larger nose radius will also increase the cutting forces applied to the workpiece.

Technical Tip #123 - Common Machining Formulas