Machining
Metal Cutting Process
INT 201
Eastern Kentucky University
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, 9th edition. McGraw-Hill
3)
Groover, M. (2004). Fundamentals of Modern Manufacturing: Materials, Processes, and Systems, 2 nd 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
Machining Fundamentals
• Milling Machines
SPINDLE
TRANSVERSELY
TABLE
LONGITUDINALLY
COLUMN
KNEE
VERTICALLY
BASE
Basic Machining Parameters
Basic Machining Parameters
• Cutting Speed
• Spindle Speed (RPM)
• Feed Rate
• Metal Removal Rate
• Chip Load
• Machine Horse Power
Basic Machining Parameters
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
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)
• Calculated at the outside diameter of the cutter
Cutting Speed (SFPM)
• 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)
• 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)
Material
Low-Carbon Steel
Stainless Steel
Aluminum (and its alloys)
Brass
Suggested
Cutting Speed Range
SFPM
80-100
60-80
400-1000
200-300
Cutting Speed Formula
SFPM = (π * D * N)/12
where
SFPM = Surface Feet Per Minute
π = Circumference constant per inch of Diameter
D = Diameter of the cutter, measured in inches
N = 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)?
SFPM  (  D  N)/12
SFPM  ((3.14)  (2 inches)  (100 RPM))/ 12 in/ft
SFPM  (628)/12  52.33 surface ft/min
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)
SFPM * 12
N
*D
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?
N
SFPM *12 250 SFPM *12 3,000


 318.5  320 RPM
π*D
3.14 * 3 in
9.42
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 * T
where
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 * F
where
MRR = Cubic inches removed per minute
W = Width of Cut
D = Depth of Cut
F = Feed rate
MRR Graphic
Volume =
In3 / min
Depth
Feed
Rate
Width
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”
Feed Rate =
4”/min
.125”
MRR Example
3”
Feed Rate =
4”/min
.125”
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 * MRR
where
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?