Metal Cutting

Speed and Feed for the Mill 311

Speed and Feed for the Mill provides a thorough explanation of cutting variables for mill operations, including how these variables are measured, selected, and set. Many variables affect speed and feed selection, primarily the type of cutting operation, tool material, and workpiece material. This class covers speed and feed selection for both manual and CNC machines.

The proper selection of speed and feed is necessary to maximize tool life, productivity, and surface finish quality. Without an understanding of cutting variables, tools will wear prematurely, machine components will sustain increased wear and tear, and the number of scrap parts produced will increase.

  • Difficulty Advanced

  • Format Online

  • Number of Lessons 19

  • Language English

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Course Outline
  • Cutting Variables for the Mill
  • Mill Cutting Variables in Action
  • Workpiece Machinability
  • Factors Affecting Machinability
  • Tool Properties and Tool Wear
  • Materials and Machinability Review
  • Balancing Cutting Variables
  • Manual and CNC Mills
  • Machine Tool and Cutting Variable Review
  • Speed and Feed Selection
  • Speed Selection for the Mill
  • Speed Conversion Formulas
  • Conversion Formulas: In Action
  • Speed Measurement Review
  • Feed Selection for the Mill
  • Feed Conversion Formulas
  • Feed Measurement Review
  • Optimizing Speeds and Feeds
  • Final Review
Objectives
  • Describe cutting variables for the mill.
  • Describe cutting variables for the mill.
  • Describe machinability.
  • Describe factors that affect machinability.
  • Describe factors that affect tool wear.
  • Describe other factors that affect cutting variable selection.
  • Explain the difference in cutting variable selection for CNC and manual mills.
  • Describe how to select cutting variables using reference materials.
  • Describe the speed measurements rpm and sfm.
  • Describe how to convert between rpm and sfm.
  • Describe how to convert between rpm and sfm.
  • Describe ipm, fpt, and fpr.
  • Describe how to find ipm using other measurements.
  • Identify methods and reasons for adjusting cutting variables.
Glossary
vocabulary term
Definition

additives

A substance that has been intentionally added to a metal to improve its properties. Additives often improve machinability.

aluminum alloys

A metal made from combining aluminum with one or more other metals to enhance its properties. Aluminum alloys contain a blend of metals and have high machinability ratings.

bismuth

A white, brittle metal with a pinkish tinge. Bismuth may be added to a metal to improve machinability.

carbide

A common cutting tool material used to make both indexable inserts and solid cutting tools. Carbide tools are hard and wear resistant when used under optimal milling conditions.

carbon

A non-metallic chemical element that combines readily with metallic elements. Carbon is found in steel, carbide, diamond, and other materials.

case hardening

The heating of a metal within a carbon-rich environment to increase carbon levels on the surface of the metal. Case hardening creates a hardened exterior shell.

chip load

The linear distance traveled by the cutter during the engagement of a single cutting tooth. Chip load, which is also known as feed per tooth (fpt), is the feed measurement for multiple cutting edge tools.

chips

An unwanted piece of material that is removed from a workpiece. Chips are formed when a tool cuts or grinds a workpiece.

clamping force

The compressive force that a holds a workpiece in place. Clamping forces resist secondary tool forces.

CNC machines

Computer numerical control machine. A sophisticated, precise machine tool run by a computer that requires programmed speed and feed rate values. CNC machines are very rigid and are capable of fast cutting speeds.

cold working

The shaping of metal at temperatures much lower than the metal's molten state. Cold working occurs at room temperature.

column-and-knee mill

The original and most basic type of manual mill. Column-and-knee mills are still used for small or specialized part runs.

computer numerical control machines

CNC machines. A sophisticated, precise machine tool run by a computer that requires programmed speed and feed rate values. Computer numerical control machines are very rigid and are capable of fast cutting speeds.

constant

A variable or number that does not change value. The constant 3.82 (318.3) is used in the formula machinists use to convert mill speed from surface feet (meters) per minute to revolutions per minute.

continuous chips

A chip that does not break apart and instead forms a long, curled string. Continuous chips tend to be created by ductile metals.

cutting

The use of single- or multi-point tools to separate metal from a workpiece in the form of chips. Cutting processes vary based on the requirements of a finished part.

cutting edge

The edge of a cutting tool that engages the workpiece material and removes material in the form of chips. A tool may have a single cutting edge or multiple cutting edges.

cutting edge strength

The ability of an insert edge to resist deformation or breakage. Cutting edge strength is affected by rake and relief angles.

cutting forces

A stress and pressure generated by the motion of the cutting tool and the resistance of a workpiece. Cutting forces are affected by rake and relief angles.

depth of cut

The thickness of material removed by one pass of the cutting tool. Depth of cut measures how far the cutting tool penetrates the surface of the workpiece during a cut.

diameter

d. The distance from one edge of a circle to the opposite edge that passes through the center. Diameter measurements are required with round or cylindrical features.

ductility

A material's ability to be drawn, stretched, or formed without breaking. Ductility is generally present in the absence of strength.

feed

The rate at which the cutting tool travels along the length of the workpiece. Feed measures linear movement.

feed per revolution

fpr. The linear distance that a tool advances during one rotation of the workpiece or cutting tool. On the mill, feed per revolution may be used to convert feed per tooth (fpt) to either inches per minute (ipm) or millimeters per minute (mm/min).

feed per tooth

fpt. The linear distance traveled by the cutter during the engagement of a single cutting tooth. Feed per tooth, which is also known as chip load, is the feed measurement for multiple cutting edge tools.

fpr

Feed per revolution. The linear distance that a tool advances during one rotation of the workpiece or cutting tool. On the mill, fpr may be used to convert feed per tooth (fpt) to either inches per minute (ipm) or millimeters per minute (mm/min).

fpt

Feed per tooth. The linear distance traveled by the cutter during the engagement of a single cutting tooth. Feed per tooth, which is also known as chip load, is the feed measurement for multiple cutting edge tools.

free-machining steels

A particular grade of steel that has small amounts of additional alloying elements to improve machinability. Free-machining steels may include added bismuth or sulfur.

hardness

A material's ability to resist scratching, indentation, or penetration. Increased hardness of workpiece materials makes them more difficult to machine, and they generate more heat during the machining process.

high-speed steel

HSS. An inexpensive cutting tool material that has high toughness. High-speed steel is tougher than carbide but offers less hardness and wear resistance.

horsepower

hp. A unit of power used to describe machine strength. Increased horsepower allows a machine's spindle to exert a greater amount of force.

in./tooth

Inches per tooth. A unit of measurement for feed that indicates the amount of material removed by each tooth of a cutting tool. Inches per tooth is the English unit of measurement for feed per tooth.

inches per minute

ipm. The distance in inches that the entire tool advances in one minute. Inches per minute is an English measurement of feed for a rotating cutting tool on the mill.

inches per tooth

in./tooth. A unit of measurement for feed that indicates the amount of material removed by each tooth of a cutting tool. Inches per tooth is the English unit of measurement for feed per tooth.

inclusions

A substance that is unintentionally present in a metal and that often has undesirable properties. Inclusions tend to be harder than the metal itself and reduce machinability.

Inconel

A superalloy made of nickel and chromium that is designed to perform well in extreme environments. Inconel is very difficult to machine and resists both oxidation and corrosion.

ipm

Inches per minute. The distance in inches that the entire tool advances in one minute. Inches per minute is an English measurement of feed for a rotating cutting tool on the mill.

lead

A bluish white metal that is very soft and ductile. Though lead can be added to a metal to improve machinability, it is less desirable because of the risk of lead poisoning.

lubricant

A substance that reduces or prevents friction, resistance, heat, and wear during a variety of manufacturing processes. Lubricants include oil, grease, and graphite.

m/min

Surface meters per minute. The distance in meters that the cutting surface travels in one minute. Surface meters per minute, which is a metric measurement of speed, depends on both cutting tool size and rpm.

machinability

The ability of a metal to be cut and shaped by machine processes such as sawing, grinding, turning, or drilling. Machinability indicates the relative ease with which a metal can be machined.

machinability rating

The ease with which a cutting process is able to machine a material as rated on a scale with 1.0 as the standard. A machinability rating that falls below 1.0 indicates a material is more difficult to machine, and a rating above 1.0 indicates a material is more easily machined.

meters per minute

m/min. The distance in meters that the cutting surface travels in one minute. Surface meters per minute, which is a metric measurement for speed, depends on both cutting tool size and rpm.

mill

A machine that uses a rotating multi-point tool to remove metal from the surface of a workpiece. Mills may be operated either manually or by computer numerical control (CNC).

millimeters per minute

mm/min. The distance in millimeters that the entire tool advances in one minute. Millimeters per minute is a metric measurement for feed of a rotating cutting tool on the mill.

millimeters per tooth

mm/tooth. A unit of measurement for feed that indicates the amount of material removed by each tooth of a cutting tool. Millimeters per tooth is the metric unit of measurement for feed per tooth.

milling

A cutting operation during which a rotating multi-point cutting tool is fed along a part's surface to remove material. Milling operations are very versatile and generally produce flat surfaces.

mm/min

Millimeters per minute. The distance in millimeters that the entire tool advances in one minute. Millimeters per minute is a metric measurement for feed of a rotating cutting tool on the mill.

mm/tooth

Millimeters per tooth. A unit of measurement for feed that indicates the amount of material removed by each tooth of a cutting tool. Millimeters per tooth is the metric unit of measurement for feed per tooth.

multi-point tool

A cutting tool that uses two or more cutting edges to remove material. Multi-point tools are used in milling, sawing, and drilling.

part program

A series of digitized instructions used by a CNC machine. Part programs guide the machine tool to perform the necessary sequence of operations to machine a specific workpiece.

pi

A special constant value that relates the diameter of a circle to its circumference. Pi is roughly 3.14 and is used to find the circumference and area of a circle.

rake angles

An angle formed by the face of the tool and a line parallel to the floor when viewed from the side facing the end of the workpiece. Rake angles affect cutting forces and contact between the tool and workpiece.

relief angles

An angle formed by the surface of the workpiece and the bottom end of the cutting tool. Relief angles influence tool wear rates and cutting edge strength.

revolutions per minute

rpm. The number of revolutions that a spindle or cutting tool completes in one minute. Revolutions per minute is a measurement of speed in both the English and metric systems.

rigidity

The quality of a workpiece, machine, or machine setup characterized by being stiff and inflexible. Improved rigidity reduces vibration.

rpm

Revolutions per minute. The number of revolutions that a spindle or cutting tool completes in one minute. Revolutions per minute is a measurement of speed in both the English and metric systems.

sfm

Surface feet per minute. The distance in feet that the cutting surface travels in one minute. Surface feet per minute, which is an English measurement of speed, depends on both cutting tool size and rpm.

shear forces

A force that attempts to cause the internal structure of a material to slide against itself. Enough shear force will cause the material to separate.

shear region

The area on a workpiece where a cutting tool cuts and separates the workpiece into smaller sections. Shear regions are sometimes created during work hardening processes.

softness

A material's ability to be scratched, indented, or penetrated by another material. Softness is the absence of hardness.

speed

The rate at which the workpiece surface and cutting tool pass each other at the point of contact. Speed for a mill refers to the rotation of the cutting tool on the spindle.

spindle

The part of the machine tool that spins. On the mill, the spindle holds the cutting tool.

standard

A unit of measurement to which other units are compared. The standard for a material's machinability rating is 1.0, which corresponds with the rating for steel grade B-1112.

strength

A material's ability to resist forces that attempt to break or deform it. Increased strength of workpiece material makes the material more difficult to machine.

sulfur

A pale yellow, brittle element. Sulfur is often added to a metal to improve machinability.

superalloy

An alloy consisting of numerous alloying elements that is very expensive and designed to perform well at elevated temperatures. Superalloys have very high hardness and low machinability ratings.

surface feet per minute

sfm. The distance in feet that the cutting surface travels in one minute. Surface feet per minute, which is an English measurement for speed, depends on both cutting tool size and rpm.

surface finish

The degree of smoothness of a part's surface after it has been manufactured. Surface finish is the result of the surface roughness, waviness, and flaws remaining on the part.

tolerances

An unwanted but acceptable deviation from a given dimension. Tolerances indicate the allowable difference between a physical feature and its intended design.

tool life

The length of time that a cutting tool can function properly before it begins to fail. Tool life is affected by several different cutting variables.

tool wear

The rate at which the cutting edge of a tool degrades during machining. Tool wear greatly affects manufacturing costs.

T-slots

A T-shaped opening that runs the length of a machine table. T-slots enable machinists to clamp vises and other workholding components onto the table.

variables

The changeable aspects of a given operation. Variables for cutting operations include speed and feed.

vise

A workholding device with two jaws that grip and hold a workpiece in place. Vises are the most common workholding devices used in milling.

work hardening

Increasing the hardness of a metal by bending or shaping it. Work hardening processes include cold working and case hardening.

workholding device

A device used to support, locate, and hold a workpiece during machining. Workholding devices accurately reference the tool performing the operation on the part being held.

worktable

The part of a machine tool that supports the workpiece and any workholding devices. Worktables are an important component of mills.