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4.0 - Coordinate Systems

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Lesson contents

• 4.1 Cartesian Coordinate System• 4.2 Vertical Milling Center (VMC) motion• 4.3 Work Coordinate System (WCS)• 4.4 Machine & Tool Offsets

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objectives

• Identify element of Cartesian coordinate system.• List the axes and motion of the movable parts of a Vertical

Machining Center (VMC).• List the major elements of a closed-loop servo control mechanism.• Explain Home Position and how it is found.• Explain the purpose of the Work Coordinate System (WCS) and

considerations for its selection.

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objectives

• Explain the purpose and Fixture and Tool Length Offsets (TLO).• Describe how and why a part should be properly rotated in a vise

to machine different surfaces.• Describe how to use a 1-2-3 block to set the TLO.• List the two systems of units used by a CNC.

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4.1 – Cartesian coordinate system

• Number Line is basic element.• A Line marked at equal intervals.• One point on line marked as Origin (Datum).• Numbers one side (+) and other side (-).

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3D Cartesian coordinate system

• Three numbers lines at right angles.

• Labeled and oriented as shown on VMC.

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quadrants

• Shown looking down from machine spindle.

• Part often in Q4 for vise work-holding.

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Units

• In United States:• Most tools in Inch units.• Most machinists familiar with Inch units.• Before programming:

• Convert model to Inch units (Units setting in SolidWorks).

• Program using Inch tools.• Except for Metric-specific features (Taps, tap drills, etc).

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Units

• Inch.• Metric (mm).• HSMWorks uses the system of units SolidWorks is set to.

Units and Maximum CNC PrecisionData Type Inch Units Metric Units

Coordinate inches .0001 mm .001Speed rev/min 1. rev/min 1.Feed in/min 1. mm/min 1.

Tap Feed in/min .001 mm/min .01Table 1: Units and Precision

Table 1: Units and Precision

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4.2 - vmc Machine motion

• Column (Z).• Table (XY).

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Closed loop servo control

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CNC Machine coordinates

• Machine at Home Position.

• Always think in terms of Tool Motion not Table Motion.

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4.3 - Work coordinate system (wcs)

• Contains shift values (deltaX, deltaY, deltaZ) from Machine Home to CNC Program Datum.

• Allows program to be written before machine/part are “set up”.• Before part location on machine is precisely known.

• Allows CNC program coordinate values to be smaller and in terms of part, not machine.

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WCS selection

• A point on part, stock, or fixture.• Can be same as CAD datum (or not).• Able to be found precisely by mechanical means.

• Edge finder • Finds edges.

• Coaxial indicator• Finds centers.

• Part probe• Finds edges or centers.

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WCS selection

• Locate with high precision (+/- .001in).• Repeatable:

• Does not move.• New parts loaded must be placed the same.

• Take into account how part is rotated for machining multiple sides.

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WCS Example

• Job: A unique machining setup.• Each time the part is moved or rotated on the machine is a new Job.• Each Job can have its own Fixture Offset.• Assign as follows:

• Job 1 = G54 (most common)• Job 2 = G55• Job 3 = G56• Etc….

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WCS Example: Job 1

• Job 1 usually sets G54 off back-left corner of stock.• It is easy to locate.• It is repeatable: against fixed jaw and vise stop.

• Make sure in CAD/CAM enough stock is left all around part so tool finish passes remove material.• Saw cut stock is usually +/-.05in tolerance or more.

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WCS Examples: Job 1

• G54.• Datum: Back-Left Stock Corner.

• Fixed jaw sets Y0.• Vise Stop sets X0.

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Wcs example: Job 2

• G54 (Datum did not shift, so can use same offset).

• Datum: Back-Left Stock Corner.• Rotate about Y-axis.• Keeps Y0 (X-Z Plane) against fixed

vise jaw.

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Wcs example: job 3

• G55.• Datum: Back-Left Stock

Corner• Rotate about Y-Axis

again.

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4.4 - Machine & Tool offsets

• Tool Offsets (H1, H2, H3, etc).• Compensate for varying tool lengths.

• Fixture Offsets (G54, G55, G56, etc).• Compensate for distance from machine home to CNC program

datum.• Diameter Offsets (D1, D2, D3, etc).

• Account for tool diameter wear.• Covered in Lesson 5.

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Fixture Offset XY

• Delta-X | Delta-Y• Machine Home to CNC

Program Datum.

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Fixture offset xy (plan view)

• G54 XY Shown.• G54-G59 Available.• Shift values stored in memory

registers on CNC control.

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Fixture offset z

• May or may not be used, depending on how machine is set up and used.

• See Lesson 6 and Appendix E for more detailed information.

*There are many ways to use Fixture Offset Z and TLO. This works well and is consistent with machines with Tool Probes. Please see Appendix E-Alternate Tool Setting Strategies for other methods.

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Set tool length offset (TLO)

• Tool Length Offset is added to Fixture Offset Z to determine distance of tool to WCS Z-zero.

• Three ways to use TLO:• Distance from home to part datum (easiest).• Distance from home to known position, such as top of 1-2-3 block

resting on table (better).• Distance from home to top of Tool probe (best).

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3 Ways to set tool Length offset (Tlo)

• 1-2-3 Block better than Part Datum.• Tool probe is fastest/easiest method.

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Tool setting probe

• Used to automatically set TLO.• Simpler.• Faster.• Safer (no hands in machine).• No 1-2-3 block or dial indicator process.

• An option on many machines.• Please refer to CNC user manual on how to

use.

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Summary

• CNC based on Cartesian Coordinate System.• Most parts set in Quadrant IV for vise work.• CNC Column controls Z.• CNC Table controls XY.• Think in terms of Tool Tip Motion, not machine motion.• CNC uses closed loop servo control.

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summary

• Home position: machine position after start and Home sequence.• WCS: The CNC program coordinate System.• Machine offsets:

• Shift Machine Coordinates to Part Coordinates.• Account for varying tool lengths and diameters.

• Fixture Offset XY:• Moves spindle centerline over part datum.

• Fixture Offset Z:• Distance from tool setting position to part datum.• Works with TLO.

• TLO:• Compensates for different tool lengths.

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Summary

• WCS: Must be found by mechanical means, precise, and repeatable.

• WCS-1 (G54): Usually back-left corner of stock.• WCS-2 (G55), WCS-3 (G56), etc.• Rotate part to maintain datum against fixed vise jaw.• CNC Units: Inch or Metric

• Usually Inch in U.S.