Machining

INT 201INT 201

Eastern Kentucky UniversityEastern Kentucky University

Metal Cutting Process

Lecture References:

1) Degarmo E.P., Black J.T., Kosher R. (2003). Materials and Processes in Manufacturing, 9 th edition. Wiley

2) Repp, V. (1994). Metalwork: Technology and Practice, 9 th edition. McGraw-Hill

3) Groover, M. (2004). Fundamentals of Modern Manufacturing: Materials, Processes, and Systems, 2nd edition. Wiley

4) Chen, C.S. (2001) ITEC 502: Advanced Design and Manufacturing [Course]. Iowa State University

Lesson Objectives

1. REVIEW: Fundamentals of Machining

2. Identify the basic parameters associated with machining (milling)

3. Understand how these basic parameters are used to create and remove chips

4. Understand that machine parameters are directly related to type of material and the machine

Machining Fundamentals

REVIEW

•Machining is a process of removing unwanted material from a workpiece in the form of chips.

•Making and removing chips

•Importance of lubricants

•$60 billion spent annually on metal removal operations

Machining Fundamentals

Basic machine processes

• Milling Machines

Machining Fundamentals

SPINDLE

KNEE

BASE

TABLE

COLUMN

VERTICALLYVERTICALLY

LONGITUDINALLYLONGITUDINALLYTRANSVERSELYTRANSVERSELY

Basic Machining Parameters

Basic Machining Parameters

• Cutting Speed

• Spindle Speed (RPM)

• Feed Rate

• Metal Removal Rate

• Chip Load

• Machine Horse Power

Machining variables such as cutting speed, RPM, table feed rates, metal removal rates, and depth of cut all depend on:

• Work material

• Tool material

• Specific process/equipment

Basic Machining Parameters

Cutting Speed (SFPM)

• Cutting speed is the distance a point on the circumference of the milling cutter travels in one minute

• Measured in Surface Feet per Minute (SFPM)Surface Feet per Minute (SFPM)• Calculated at the outside diameter of the cutter

• Different cutting speeds should be used when machining different metals– With cutting speeds too fast, cutter will

overheat and dulls rapidly– With cutting speeds too slow, time is wasted

and production costs will increase

Cutting Speed (SFPM)

Cutting Speed (SFPM)

• Factors affecting cutting speeds in milling:– Material properties, cutting tool (HSS, carbide, cast alloy), cutting

fluids

• Cutting speed recommendations for various materials and tooling combinations can be found in sources including: Tooling Mfg. & Engineering Handbooks (Machinist Handbook, etc)

SuggestedCutting Speed Range

Material SFPM

Low-Carbon Steel 80-100Stainless Steel 60-80Aluminum (and its alloys) 400-1000Brass 200-300

Cutting Speed Formula

SFPM = (π * D * N)/12where

SFPM = Surface Feet Per Minuteπ = Circumference constant per inch of DiameterD = Diameter of the cutter, measured in inchesN = revolutions per minute of the spindle (RPM)12 = 12 inches per foot (conversion calculation to

feet)

Cutting Speed Example

If a 2 inch dia., 6 tooth milling cutter is turning at 100 revolutions per minute (RPM), what is the calculated cutting speed of the cutter (SFPM)?

ft/min surface 52.33 (628)/12 SFPM

in/ft 12RPM))/ (100 inches) (2 ((3.14) SFPM

N)/12 D ( SFPM

Spindle Speed (RPM)

The speed of the milling machine is

measured at the spindle and is measured

in REVOLUTIONS PER MINUTE (RPM)

Finding N (RPM)

D *

12 * SFPM

N

Same equation; different arrangement

SFPM = (π * D * N)/12

Determining RPM

• When milling with a 3” diameter, 8 tooth milling cutter with a recommended cutting speed of 250 SFPM tooling material combination, what is the recommended RPM?

RPM 320 5.1839.42

3,000

in 3*3.14

12*SFPM 250

D * π

12 * SFPMN

Feed Rate

• The linear distance moved along any machine axis, by the cutting tool in inches per minute.

• Feed rate in milling is determined by multiplying:– Number of teeth on the cutter– Chip load per tooth– Speed of the cutter (N)

Chip Load

Chip Load or Feed per Tooth

– The Chip Load is the amount of material removed by each tooth of the milling cutter during one revolution

–Chip load recommendations for various materials and tooling combinations can be found in machining and engineering handbooks

Milling Feed Rate Formula

F = N * Ct * Twhere

F = Feed rate in inches per minute

N = Spindle RPM

Ct = Chip Load per tooth [feed per tooth]

T = Number of teeth on cutter

Calculating Feed Rate

A 6 inch dia., 12 tooth milling cutter is turning at 250 RPM. The recommended chip per tooth is 0.004”. What is the feed rate?

F = N * Ct * T = 250 RPM * 0.004”/tooth * 12 teeth

F = 12” per minute

Metal Removal Rate

Metal Removal Rate (MRR) is the volume of material removed from the work piece in one minute.

Limited to available machine power

How much material is removed in 1 minute

MRR Formula

MRR = W * D * Fwhere

MRR = Cubic inches removed per minute

W = Width of Cut

D = Depth of Cut

F = Feed rate

MRR Graphic

Depth

Width

Feed Rate

Volume =

In3 / min

MRR Example

What is the MRR of a surface 3 inches wide that is to be milled with a 6 tooth milling cutter. Each depth of cut is 0.125 inches and the table feed rate is 4 inches per minute

3”

.125”

Feed Rate = 4”/min

MRR Example

3”

.125”

Feed Rate = 4”/min

MRR = W*D*F

MRR = (3”) * (0.125”) * (4” per minute)

MRR = 1.5 cubic inches per minute (in3/min) of material removed

Horse Power (specific) HPs

• HPs = Horse power required to remove 1 cubic inch (in3/min) of material per minute

• All machined materials have a HPs rating• HPs is used to determine the HP required for

machining.• The standard HPs = 1

– Materials with HPs > 1 require more than 1 HP/minute to remove 1 in3 of material

– Materials with HPs < 1 require less than 1 HP/minute to remove 1 in3 of material

• HPs can be found in machining and engineering handbooks

Horse Power Formula

HP = HPs * MRRwhere

HP = Horse Power required to make a desired cut

HPs = specific Horse Power of material

MRR = Material Removal Rate

HP Example

How much Horse Power is required to machine a part with a desired MRR of 3.94 in3/min and a specific Horse Power (HPs) of 1.8?

HP = HPs * MRR

HP = 1.8 * 3.94

HP = 7.1

Review

What is the maximum MRR of a material with a 1.6 HPs on a 1.2 HP machine?

How much material can be removed from the same material on a 7.1 HP machine?

Review

Machining variables such as cutting speed, RPM, table feed rates, metal removal rates, and depth of cut all depend on:

• Work material

• Tool material

• Specific process/equipment

WHY?WHY?