Unformatted text preview: Chapter 2 Contents
CNC Machine Axes CNC Coordinate System Absolute Programming Incremental Programming Word Address Programming G Code M Code Other Code Developing Part Program
Turning Center Cutters
Drilling Calculating CNC Machine Axes
CNC equipment executes machining operations by
CNC performing a linear and rotary motion.
The machines automatically knows how to move in
response to an axis command, therefore the
programmer need not be concerned whether it is the
spindle or table that moves.
The machine axis will be defined in term of spindle
CNC machine can perform simultaneously along their
axis CNC Milling vs. CNC Lathe
CNC CNC Coordinate System
CNC Coordinate system
Cartesian Polar Coordinate System Absolute Programming
Absolute programming specifies a position or an end
Absolute point from the workpiece coordinate zero (datum)
point Incremental Programming
Incremental programming specifies the movement or
Incremental distance from the point where currently located.
Incremental specify distance and direction of the
machine to move. Word Address Programming
The word address format for numerical control
The programming precisely controls machine movement and
function through the use of short sentence. i.e.
A series of command blocks that execute motions and
machine functions in order to manufacture a part.
These commands consists of addresses, words and
characters. Letter Address Commands
N Line number or sequence number G Preparatory functions, which set up the mode in
which the rest of the operations are to be
executed F Feed rate of controlled axis S Spindle speed setting T Latter address for a tool call M Miscellaneous function H and D Auxiliary letter address code used for tool offset
storage N0010 G00 X-1.00 Y-1.00; (point 1)
N0020 G01 X-.25 Y-.25 F10.0; (point 2)
N0030 G01 Y3.25; (point 3)
N0040 G01 X4.25; (point 4)
N0050 G01 Y-.25; (point 5)
N0060 G01 X-.50; (point 6) Part Datum Location – feature of the part from which
Part the majority of the dimensions of the part are located.
Sequence Number (Nxxxx) – Identifying blocks of
information within the program.
Preparatory Function (G-Codes) – Used to set the
control for various machine movements.
Spindle Control Functions (S) – Spindle speeds are
controlled with an S followed by digits.
Miscellaneous Function (M-Codes) – Perform
miscellaneous machine function such as tool
changes, coolant control and spindle operations.
Tool Calls – Start with a T and then the tool number.
Tool G Code
Non-Linear Rapid moves
Plane of Interpolation
Plane of Interpolation
Plane of Interpolation
Return to Machine zero
Tool length comp. +
Tool length comp. Tool Length comp. Cancel G44
G99 Tool length comp. Tool Length comp. Cancel
Set Program Zero
Set Local Coordinate Systems
Rough turning canned cycle
Finishing turning canned cycle
Canned Cycle cancel
Spot Drilling Cycle
Peck Drilling Cycle
Set Program Zero
Direct Spindle Speed Programming
Return to initial level
Return to R level M Code
Code Meaning M0/M00 Program stop M1/M01 Optional stop M2/M02 Program stop M3/M03 Spindle normal rotation M4/M04 Spindle reverse rotation M5/M05 Spindle off M6/M06 Tool change M8/M08 Coolant on M9/M09 Coolant off M30 Program reset/tape rewind M98 Sub-Program call M99 Return to previous program Other Codes
Auxiliary Axis, *A supported D
F Offset CRCEIPR Feedrate
IPM/IPR Feedrate H
I,J,K Tool length offset call
Arc center location N
O Sequence Numbers
Program Numbers P
Q Dwell time
Peck Depth R
R Rapid level
Arc center radius S
T Spindle speed
Tool number U,V,W
X,Y,Z Incremental Move in X, Y, Z
Absolute Move in X, Y, Z Part Program Manuscript
Part Developing Part Program
Steps involved in developing part program:
Cutting process parameters planning
Job and tool setup planning
Machining path planning
Part program writing
Part program validation
Documentation for NC Programming Procedures
7. Startup or preliminary procedure
Workpiece location block
Spindle speed control
Tool motion block
Program end procedure Process Planning
Machine tool used Fixtures required Sequence of operation Requirement for each operation in form of cutting tools
and process parameters
and Axes Selection
CNC machine tools rely on the axes system to
CNC describe the tool/axes motion
To have all positive coordinate value, use the lower
left corner as datum
Z-axis datum is normally set at top surface of the part.
All +ve Z coordinate value keep the tool away from
the part (to avoid collision).
The zero Z coordinate can be easily set by touching
the tool tip to the top surface during the tool setup.
Set the datum geometric center of the part if all
geometry is symmetrical.
geometry Tools Selection
The largest size of the end mill should be chosen for
The better surface finish and high mrr. The max. tool
radius of the curvature being generated.
End mill always approach the work piece from the
side. Slot drill can approach from the side or the
top of the work piece.
top Tooling for Machining Centers
Tooling Cutting Tools
Most machining centers
Most use some form of HSS
or carbide insert
endmill as the basic
Insert endmills cut
many times faster than
HSS, but the HSS
endmills leave a better
finish when side cutting.
finish Cutting Tools
Facemills flatten large
Facemills surfaces quickly and
with an excellent finish.
Notice the engine block
being finished in one
pass with a large cutter.
pass Cutting Tools
Ball endmills (both HSS
Ball and insert) are used for a
variety of profiling
operations such as the mold
shown in the picture.
shown Slitting and side cutters
Slitting are used when deep,
narrow slots must be cut.
narrow Calculations for Speed and Feed Turning Center Cutters
What types of cutters are used on CNC turning Centers?
Carbide (and other hard materials) insert turning and boring
High Speed Steel (HSS) drills and taps
Where to find information for calculating RPM and feed
rates? Cutting tool manufacturer (first choice)
Machining Data Handbook
Machinery’s Handbook Standard Insert Shapes
Standard V – used for profiling, weakest insert,
used 2 edges per side.
D – somewhat stronger, used for
profiling when the angle allows it, 2
edges per side.
T – commonly used for turning
because it has 3 edges per side.
C – popular insert because the same
holder can be used for turning and
facing. 2 edges per side.
W – newest shape. Can turn and face
like the C, but 3 edges per side.
S – Very strong, but mostly used for
chamfering because it won’t cut a
square shoulder. 4 edges per side.
R – strongest insert but least
commonly Typical Turning,
Threading, and Parting Tools Tool Holder Hand
For most CNC turning
For centers, the cutter is on the
back side of the part and is
Right Hand tool then turns
towards the chuck.
Left Hand tool then turns
towards the tailstock.
If the cutter is symmetrical
with the shank, it is called
NEUTRAL Tooling Considerations
Tooling choices depend on the type of workpiece, the
Tooling machine, and the desired surface finish.
Harder workpieces require harder cutters.
Modern cutters require the turning center to have high
spindle speeds and powerful motors.
spindle Cutting process parameters planning
Recommended procedure for machining are as follows
Face mill top surface
Rough machine the profile of the part
Drill and tap
Finish profile surfaces
Finish reaming Cutting Speed
What is cutting speed? Not RPM
Relative speed of the work and cutter
Units in feet/minute (fpm) or
Units meter/minute (mpm)
Usually designated as V, cs, or S
Tabulated in the book based on material,
cutter type, and type of cut (roughing or
Needed to calculate RPM Calculating Turning RPM
The formula for calculating RPM is : 12 × v
N = RPM =
π ×D Where
V = cutting speed to be looked up in the handbook
D = diameter being cut
do this: 12 × V ÷ π ÷ D = Note the difference between this and the actual
formula. Types of Cuts
Roughing – primary considerations: Just removing metal, surface finish does
Just not matter.
Requires a strong cutter.
Generally have deep depth of cuts and fast
The cutting speed is generally adjusted
slower to keep heat down.
slower Types of Cuts
Finishing – primary considerations:
Must meet required surface finish and size
Requires a hard cutter to hold its shape well.
Generally have small depth of cuts and slow
The cutting speed is generally adjusted
upward to give a better surface finish.
upward Surface Finish Requirements
She surface finish depends on
the feed rate and on the cutter
Generally, a large nose radius
and a slow feed rate coupled
with high cutting speed gives
the best finish.
However, too large of a nose
radius induces chatter ruining
the finish and the size.
Most inserts use a 1/32” nose
radius as a good compromise.
radius General Feed and Depth of Cut
Roughing: 0.1” to 0.25” depth of cut (radial)
0.012 inches per revolution (ipr) to 0.018 ipr feed rate Finishing: 0.03” to 0.05” depth of cut (radial)
0.006 ipr to 0.010 ipr feed rate Note 1: the depth of cut should not be less than the
tool nose radius unless special finishing inserts are
Note 2: smaller feed rates can be used if special
finishing inserts are being used.
finishing Calculating RPM for Turning
Operations with Hard Cutters
Use this procedure for carbide, ceramic, and cermet
We will adjust the cutting speed based on the desired
depth of cut and feed rate.
depth Calculating Turning RPM
Calculating Six step process:
6. Select depth of cut - as deep as possible.
Select feed - appropriate for roughing or finishing.
Find the original cutting speed in the tables.
Find the feed and depth of cut factors
Modify the original cutting speed based on step 4.
Calculate the RPM. Note: All data will be found in the “Machining”
thumb tab in the Machinery’s Handbook.
Machinery’s Calculating Turning RPM
Take 0.250 depth of cut, 0.012 feed in quenched and
tempered 8620 steel with a Brinell hardness of 300, hard
coated carbide cutter, 2.5” diameter part.
Depth of cut given at 0.25”. Step 2:
Feed rate given as 0.012 ipr. Calculating Turning RPM (continued)
Step 3: From Table 1, page 1029, locate cutting parameters
for this material Calculating Turning RPM
From Table 1, page 1029, we find
Vopt = 585 fpm ipr Vavg = 790 fpm Fopt = 0.017
Note that the table lists cutting speed as S rather than V as
used everywhere else. Note that the feed rates are given in
0.001 ipr, so the 17 listed for Fopt is actually 0.017 ipr.
0.001 Calculating Turning RPM
Once located the optimum and average cutting speeds and
the optimum feed, we finish our calculation using the data
and process described in Table 5A, page 1035.
and Calculating Turning RPM
Calculating Calculating Turning RPM
For this example following the steps in 5a:
Calculate the following ratios: And From Table 5a, page 1035, find Ff = 1.22 and Fd =
0.87 Calculating Turning RPM
As shown at the bottom of Table 5a,
V = Vopt Ff Fd
Where V = cutting speed to be used (fpm)
Vopt = optimum cutting speed from the table based on material
hardness and type of cutter
Ff = feed factor from Table 5a
Fd = depth of cut factor from Table 5a
For this example, V = (585)(1.22)(0.87) = 621 fpm Calculating Turning RPM
Finally, calculate the RPM with For this example:
RPM = 12 x 621 / p / 2.5 = 949 RPM Speed and Feed Calculations for Milling
Speed Calculating RPM for Milling
Operations with HSS Cutters
Use the same basic formula as for turning, except D is
Use now the cutter diameter:
now Tables 10 through 16 list milling data. Table 6 must be
Tables used for copper alloys.
used Example Milling RPM Calculation
Mill 4140 steel with a Brinell hardness of 200 with a ½”
Mill HSS endmill.
From Table , given V = 75 fpm, so:
RPM =12X 75 /p / 0.5 = 573
0.5 Feed Rates for Milling
Feed for milling cutters is usually tabulated as inches per
Feed tooth (ipt), but feed rates on milling machines are
programmed in inches per minute (ipm). The equation on
page 1041 in the Handbook is given as
fm = ft nt N Where fm is the feed rate in ipm we want to set the mill at.
ft is the feed rate in inches per tooth, ipt.
nt is the number of teeth on the cutter we are going to use.
N is the RPM we already calculated. Example Milling Feed Rate
Mill 4140 steel with a Brinell hardness of 200 with a ½” 4
Mill flute HSS endmill and a ¼” depth of cut.
Already calculated the RPM at 573,
From Table ft = 0.001“
fm = 0.001 x 4 x 573 = 2.3 IPM Feed Rate Concerns
Feed rates for facemill and slotting cutters vary
Slow feed rates give a better finish, but sometimes
this actually dulls the cutter faster than a more rapid
Data for carbide insert milling cutters should be
obtained from the insert manufacturer. Unlike lathe
cutters which are fairly standard, milling cutters vary
widely between manufacturers, so use your
manufacturer’s Milling Feed Direction
Milling Remember, all CNC machines are
equipped with ball screws to
minimize slop when changing feed
directions. The other advantage to
ball screws is they allow climb
milling instead of conventional
milling as done on most manual
Climb milling has many advantages
including better surface finish,
longer tool life, and the cutter
deflects away from the work rather
than into it.
Always climb mill on a CNC
machining Calculating RPM for Drilling
Operations with HSS Cutters
The RPM formula for drilling is the same as for turning
The and milling, except D is now the drill diameter.
Note: deeper holes require slower cutting speeds because
the coolant cannot reach the cutting edge effectively.
Feed rates for drilling are given in a paragraph on page
1060. Note that the values are given in inches/revolution
(ipr). If you need ipm, multiply ipr by RPM.
(ipr). Drilling RPM and
Feed Calculation Example
Drill cold drawn free cutting brass, C36000, with a 1”
Drill drill. From Table, given V = 175 fpm, so:
RPM =12X175 /p /1 = 668
RPM =12X175 From page 1060, the feed would be between 0.007 and
From 0.015 ipr.
To find ipm, use ipm = Feed x RPM = 0.015 x 668 = 10
ipm Drills, Taps, and Reamers
Common HSS tools such as
Common drills, taps, and reamers are
commonly used on CNC
Note that a spot drill is used
instead of a centerdrill. Also,
spiral point or gun taps are
used for through holes and
spiral flute for blind holes.
Rarely are hand taps used on
a machining center.
machining Job and tool setup planning
Job Clamp the workpiece on the fixture firmly and adjust the cutting tool to correct position Machining path planning
To ensure the requisite manufacturing specifications are
To achieved at lowest cost. (exp. Zigzag, one way, etc
Use the canned cycles to simplify the program Part program writing
Writing program manually will require knowledge of the
Writing format and syntax of the program or
It can be generated automatically by various CAM
software (MasterCAM, Edge CAM, etc.)
software Part program validation
(verify) Part program should be verified before it can be
Part loaded into the CNC machine
Check the following:
Tool collision The dimension of the machined features
To see how the tool path is programmed
To see how the material is being removed from the
It can be done by backplotting or dynamically simulating
the actual sized tool through the work material
the Documentation for NC
Keep the component drawing, process plan sheet,
Keep programming sheet etc. as a record. It is essential for
updating the drawing or recall the previous drawing
whenever it is required
whenever Manual Part Programming Machine tool controllers are able to interpret a series of simple standard
Machine command codes in order to control the table and spindle movements and all
auxiliary functions. The prime code is the G-code, which can be used, for
example, to define fast traverse motion (G00), linear (G01) and circular
(G02 and G03) tool motion under controlled feed rates, millimeter and inch
motion (G20 and G21), tool length compensation (G43), and
preprogrammed function calls (G65). Other codes include feedrate F-codes,
cutting speed S-codes, tool selection T-codes and auxiliary functions, such
as spindle start and stop, M-codes. These machine codes, which are
outlined below, are collectively referred to as G-codes. The program for producing a part is arranged in the form of coded blocks or
lines of information. Each block contains a group of commands sufficient
for one individual machining operation. A typical block is given as follows:
N200 G01 X120.0 Y100.0 Z80.0 F150; and is interpreted that in block 200 the tool should move from its present
position in a straight line to X = 120 mm, Y = 100 mm and Z = 80 mm at a
feedrate of 150 mm/min. Some typical beginning and end sequences for CNC part programs are given in Table 2.1. Each program should have an identifying
number, in this case O006. It is good practice to start the program
with a return home (G91 G28) instruction so that all subsequent
programming is from a known datum. This home datum point is
determined by the machine tool manufacturer and is normally at
Xmax, Ymax and Zmax. For safety the Z axis is usually returned to the
home position first. The G92 X250.0 Y125.0 Z75.0 instruction
establishes the position of the workpiece in relation to the home
position. The tool defined in storage location T18 is inserted in the
spindle with the M06 instruction, and the tool in storage location T19
is positioned on standby. Absolute coordinates, millimeter
measurements and tool length compensation are selected with G90,
G21 and G43, respectively. The cutter is moved with rapid traverse
to the position Z = 100 mm and then to the position X = 0 mm, Y = 0
mm (or to any other starting position). A spindle speed of 2000 rpm
is selected with S2000 and the spindle turned on with M03. Table 2.1
Table Tool Holders
Tool All cutting tools must be held in ...
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- Spring '12
- Milling machine, Numerical control, Milling cutter, G-code