CNC Work Offsets and Gauge Line

• The work offset is the distance from the machine zero to the part zero
• Each axis has an offset value stored in the machine controller's offset registry
• The machine controller uses these values to move the tool to the correct position for machining

Work Offsets

• Work offsets allow for specification of new coordinate systems based on the part zero's relative distance from the machine's home position
• Work offset commands are modal, remaining active until canceled by another work offset command
• Work offsets are programmed using G-codes G54 through G59
• Most machines select G54 on startup
• X and Y axis offsets indicate the distance from the Machine Home to the part zero
• The Z-Offset value shows the distance from the part zero to the Machine Home in the Z direction
• The Machine Home position in the Z direction is the spindle nose
• The part zero in the Z direction is the top of the workpiece

Gauge Line

• When mounting a tool holder with a cutting tool in a CNC milling machine spindle, the holder's taper fits against the spindle's inside taper and is secured by a pull bar
• Manufacturing precision ensures the tool holder sits in a constant position in the spindle
• The position is the gauge line, determined by the machine manufacturer
• Used for tool length measurement and any tool motion along the Z-axis
• The distance is fixed from the gauge line and the table top face

Tool Length Offset

• Since tools vary in length, the spindle's travel distance from Machine Home to the part's top face also varies
• Tool length offsets (H01 to H99) allow the machine control to account for different tool lengths
• The H-address character selects the tool length offset entry from the offsets memory

Tool Length Offsets

• When applied, the machine adds the tool height to the Work Z-Axis Offset
• Represents the distance from the spindle nose to the cutter tip
• Application ensures the programmed cut depth matches the actual depth of machine movement into the workpiece
• Tool length offsets can be programmed positive with G43 or negative with G44, followed by 'H' and the length offset address from the Machine' Tool Length offset Table
• G49 cancels tool length compensation
• Positive tool offsets represents the tool's length measured from the Gage Line of the spindle (typically spindle nose) to its tip
• The longer the tool, the larger the Tool Length offset
• Z Work Offset will represent the distance between the same Gage Line to the top of the part
• G43 is commonly used
• Negative tool offset represents the distance between the tip of the tool to the top of the part
• Application ensures programmed cut depth matches actual machine movement into the workpiece
• The machine control unit subtracts the positive tool length offsets values when G44 is programmed
• G44 is hardly used

G81 - Standard Drilling Cycle

• Most basic hole-making canned cycle for drilling holes in a single drill stroke, and is used for drilling a series of holes when a dwell or peck drilling is not needed
• The cutting tool traverses to the top of the hole before moving to the R-plane
• The tool feeds to the specified Z position at the programmed feed rate from the R-plane
• After drilling to the specified depth is completed, the tool retracts in rapid mode to either the intial Z position, using G98
• The tool retracts in rapid mode to either the R plane, using G99
• G98 returns the tool to the initial Z position where G81 is called
• G99 returns the tool to the R-plane specified in the canned cycle

G82 - Counter Boring Cycle

• Normal drilling cycle where the cutting tool dwells for specified time at the bottom of the hole at each Z-movement's end
• Normally used when center drilling, spot drilling, or when machining a flat bottom hole where accurate drilling depth is needed
• The 'P' Word defines the dwell time and is written in milliseconds
• The cutting tool traverse to the top of the hole before moving to the R-plane
• The tool feeds to the specified Z position at the programmed feed rate from R-plane position
• At the bottom of the hole, the cutting tool dwells for a specified time
• After the dwell time, the tool retracts in rapid mode
• G98 returns the tool to the initial Z position
• G99 returns the tool to the R-plane

G83 - Peck Drilling Cycle

• Similar to G81 except that the tool is backed out of the hole at certain intervals throughout drilling to clear chips and cool the tool
• The Q-word is the only new code that specifies the peck depth (the depth of cut for each cutting feed)
• The tool drills to the peck depth and then back out to the R-plane, then rapidly returns to the bottom of the hole and plunges another peck depth in the next peck
• Continues until the hole's Z-depth is reached
• G98 returns the tool to the initial Z position
• G99 returns the tool to the R-plane

G84 - Right Hand Tapping Cycle

• Used to machine threads into pre-drilled holes in a component using a tap held in a standard tool holder
• Used for peck tapping, where a portion of the thread is cut and the spindle reverses to relieve swarf from the tap and is then returned to machine more of the thread
• The 'P' Word defines the dwell time and is written in milliseconds
• In, the cutting tool traverse to the top of the pre-drilled hole before moving to the R-plane
• The tap feeds to the specified Z position, in clockwise direction, at the programmed feed rate from R-plane position
• At the bottom of the hole, the tap dwells for a specified time
• After the dwell time, the spindle is rotated counter-clockwise to allow the tap to retract at the programmed feed rate
• If G98 is programmed, tap retracts to the R-plane where the spindle's direction changes to clockwise before the tap returns to the initial Z position
• If G99 is programmed, the tap retracts to R-plane, changes direction to clockwise

G53 - Machine Coordinate System

• Temporarily cancels work coordinate offsets and uses the machine coordinate system
• Non-modal, effective only in the block it is commanded
• Uses the machine home position coordinates
• Each axis's zero point is the position where the machine goes during a Zero Return
• When programmed, G53 reverts to the machine's default system for that block
• This can establish a fixed position for tool changes, regardless of the part position
• The tool change position is determined by the tool's actual distance from the machine zero position, not the program zero

This lesson explains work offsets in CNC machining. It covers how work offsets define coordinate systems based on the part zero's distance from the machine's home position. It also describes the role of G-codes G54 through G59 in programming work offsets.