Bmfs 3373 cnc technology lecture 10
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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

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Bmfs 3373 cnc technology lecture 10

Lecture objectives
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
Cartesian Coordinates in CNC Lathe

  • CNC lathe program can be written to move the tool in the following modes:

    • Absolute Positioning

    • Incremental Positioning

    • Mixed (Absolute & Incremental) Positioning

Cartesian coordinates in cnc lathe1
Cartesian Coordinates in CNC Lathe

  • Absolute Positioning:

    • 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:

    • 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 lathe2
Cartesian Coordinates in CNC Lathe

Absolute Positioning

Incremental Positioning

Feature location machine setup1
Feature Location – Machine Setup

  • Reference Point (Machine Zero)

    • 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)

    • 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
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 setup2
Feature Location – Machine Setup

  • Program Origin(Program Zero)

    • 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
Program Origin

  • Manual Touch Off Method

Program origin1
Program Origin

  • Q-Setter / Tool Setter Method

Setup procedure
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:

    • 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 procedure1
Setup Procedure

  • Manual Touch Off Method

Setup procedure2
Setup Procedure

  • Q-Setter / Tool Setter Method

Tool edge programming
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 compensation2
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 compensation3
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 radius compensation4
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 compensation6
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.


Chapter 10