BMFS 3373
CNC TECHNOLOGY
Lecture 10
Lecture Objectives
 At the end of the lecture, you will be able to:
 Explain the different types of positioning modes for CNC lathe
operation
 Know the important locations for programming and setup
which includes reference point, machining origin and program
origin
 Know the important preparaotory (G) codes and miscelleneous
(M) codes used in programming lathe operations
 State the codes for specifying values of the spindle speed and
tool feed.
 Explain the tool nose radius compensation
Cartesian Coordinates in CNC Lathe
 CNC lathe program can be written to move the tool in the following
modes:
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Absolute Positioning
Incremental Positioning
Mixed (Absolute & Incremental) Positioning
Cartesian Coordinates in CNC Lathe
 Absolute Positioning:
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The new position of the tool is given by its X and Z distances from a fixed
home or origin (0,0)
the X position of the tool is specified in terms of diameter or twice the
distance from the spindle centerline.
 Incremental Positioning:
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The new position of the tool is specified by inputting its direction and
distance from the last position achieved.
the U is entered as the directed change in diameter from last positioned
achieved.
Cartesian Coordinates in CNC Lathe
Absolute Positioning
Incremental Positioning
Feature Location – Machine Setup
Feature Location – Machine Setup
 Reference Point (Machine Zero)
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The position of the turret when the machine’s axes are zeroed out
It is set once by the manufacturer
Also known as Machine Home
 Machining Origin (Tool Change Position)
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It is determined at setup and is input at the beginning of the word address program
before the first tool change by using a zero-offset command.
The machine indexes the tools at this position.
It is position when the turret is set with its longest tool at least 1.0 from the face of the
part and 1.0 from the diameter of the stock. Then the control is zeroed out at this
location
The turret is returned to the reference point with the relative distance of the two points
(reference & machining) is recorded
Machining Origin
Word Address Program:
N0020 G50 X10.Z5.
N0030 T0101
Set Machining Origin at X10.Z5.
before tool change position
(T0101)
Feature Location – Machine Setup
 Program Origin(Program Zero)
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The point at which all dimensions are defined in the part program.
The setup uses tool offsets as a mean of locating the program zero with respect to the
machining origin. The controller will compute the corresponding X and Z move relative
to the machining origin. It will then execute the move relative to the machining origin.
Program Origin
 Manual Touch Off Method
Program Origin
 Q-Setter / Tool Setter Method
Setup Procedure
 The setup operation normally begins with the setup person securing the
required OD and ID cutting tools in the turret as specified by the
operation sheet.
 The setup person must measure and enter the value of the variety tool
length offsets or geometry offsets of each tool into the system memory.
The system will compensate these variation by analyzing the initial
distance between the tip of the tool and the program zero.
 There are two methods where tool offsets are recorded:
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
Manual Touch Off
Q-Setter
 Another concern during machining is the tool wear. To overcome this,
the operator opens the wear offset page and enters the tool wear offset
to compensate the differences. This is done during production
machining in order to meet tolerances
Setup Procedure
 Manual Touch Off Method
Setup Procedure
 Q-Setter / Tool Setter Method
Important Lathes Preparatory Functions
Tool Edge Programming
 It is in effect when offsets are measured to the edge of the tool.
 With this programming, the part geometry can be input directly without
the consideration of the cutting tool nose radius.
 Thus it can only applied to part geometries consisting of horizontal or
vertical lines. Other geometries will require additional compensation
program known as tool nose radius (TNR) compensation.
Tool Nose Radius Compensation
Tool Nose Radius Compensation
Tool Nose Radius Compensation
To setup Tool Nose Radius (TNR) Compensation, the control is instructed
to open an offset file in memory to key-in the following information:
 X and Z tool offsets to the tool edge
 Size of the tool nose radius
 Tool nose vector
Tool Nose Radius Compensation
Tool Nose Vector
 The tool tip (imaginary tool point) of a single point tool has a specific
location from the center of the tool nose radius.
 The tool nose vector indicates this location to the controller.
 The controller will use this information to properly determine the tool
movement in response to a compensation command
Tool Nose Vector
Tool Nose Radius Compensation
Some restriction when handling a tool nose radius compensation:
 A G code activating TNR compensation is entered as a separate block
in the program. It must be commanded before the tool starts to cut.
 The first and second block following a TNR compensation code must
contain XZ linear motion command that signals the initiation or
termination of the compensation. Failure to do so, will result in over or
undercutting
 Motion in the G40, G41 or G42 block must be greater than twice the
value of the tool nose radius that will ensure no over or undercutting to
happen
Tool Nose Radius Compensation
Tool Nose Radius Compensation
 A G41 or G42 indicates TNR compensation is start up on the next
linear move.
 The MCU transfers the tool positioning point from the imaginary point A
as shown in Fig 19-21 and 19-22 to the center of the tool nose radius.
 The controller will offset the values with respect to the inputs stored in
the memory placed during setup.
 A G40 will cancel out the compensation, and the MCU will shift back
the tool position point back to the imaginary point A.
End
Chapter 10