1. Cutting Speeds and Feeds
Automated Machining

2. Introduction
The tool moves through the material at a specified rotational speed, defined in revolutions per minute (RPM), and feed rate, defined in inches per minute (IPM).
Probably the most vexing problem for the beginning CNC machinist is selecting proper cutting speeds and feeds.
This selection is actually more difficult on a CNC than a manual mill because, with a manual mill, the operator can feel the cutting pressure and alter the feed based in part on the cutting force.

3. Introduction
CNC mills require calculating speeds and feeds in advance.
The best source of data about cutting speeds and feeds for a specific tool, application, and material is the tool supplier.
Another source of speeds and feeds data is CAD/CAM software.

4. CAUTION The best speed and feed data is just a starting point.
Speeds and feeds require adjustment due to many factors including:
maximum spindle speed
horsepower of the machine
rigidity of work holding
quality and condition of the machine tool itself.

5. Speed Formula
Speed - is the rotational frequency of the tool (Spindle Speed) in revolutions per minute (RPM).
SFM - (Surface Feet per Minute) is the speed at which the material moves past the cutting edge (outside diameter) of the tool in feet per minute.
SFM values depend on the tool type, tool material, and material being machined.
Circumference - is the circumference of the cutting tool in feet.

6. How Speed Formula is Derived
Because cutting tools are defined by their diameter in inches, this formula is rewritten and simplified as follows:
Dia - is the tool diameter in inches.
is a constant derived from 12/pi which converts the tool circumference in feet to diameter in inches.

7. Problem:
Calculate the cutting speed for a milling operation given the following values
Parameter
Value
Tool Diameter
.500 in
NumFlutes 4
SFM
600 ft/min
IPR
.005 in

8. Feed Formula
Feed - is the linear feed of the tool through the material in inches per minute.
Speed - is the result of the speed formula (Figure 3.15) in revolutions per minute.
CL - is the chip load, or how much material each cutting edge of the tool removes per revolution. Chip load is sometimes referred to as feed per tooth (FPT) or inches per rev (IPR).
NumFlutes - is the number of cutting flutes. (For a twist drill, this value is one.)

9. Problem:
Calculate the feed for a milling operation given the following values
Parameter
Value
Tool Diameter
.500 in
NumFlutes 4
SFM
600 ft/min
IPR
.005 in
Speed
4584 rpm

10. Tap Feed Formula
Feed is the linear feed of the tool through the material in inches per minute.
Speed is the result of the speed formula (Figure 3.15) in revolutions per minute.
TPI is the threads per inch of the tap. For example the TPI of a 1/4-20 tap is 20.