BMFS 3373 CNC TECHNOLOGY Lecture 10

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BMFS 3373CNC TECHNOLOGYLecture 10
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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

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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

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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.

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Cartesian Coordinates in CNC Lathe
Absolute Positioning
Incremental Positioning
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Feature Location Machine Setup
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Feature Location Machine Setup

• Reference Point (Machine Zero)
• The position of the turret when the machines
  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

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Machining Origin
Word Address Program N0020 G50 X10.Z5. N0030
T0101 Set Machining Origin at X10.Z5. before
tool change position (T0101)
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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.

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Program Origin

• Manual Touch Off Method

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Program Origin

• Q-Setter / Tool Setter Method

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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

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Setup Procedure

• Manual Touch Off Method

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Setup Procedure

• Q-Setter / Tool Setter Method

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Important Lathes Preparatory Functions
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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.

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Tool Nose Radius Compensation
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Tool Nose Radius Compensation
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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

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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

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Tool Nose Vector
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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

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Tool Nose Radius Compensation
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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.

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End Chapter 10