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Work Coordinate System
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Obviously it would be difficult to write a CNC program in relation to Machine Coordinates. The home position is far away from the table, so values in the CNC program would be large and have no easily recognized relation to the part model. To make programming and setting up the CNC easier, a Work Coordinate System (WCS) is established for each CNC program.

The WCS is a point selected by the CNC programmer on the part, stock or fixture. While the WCS can be the same as the part origin in CAD, it does not have to be. While it can be located anywhere in the machine envelope, its selection requires careful consideration.

The WCS location must be able to be found by mechanical means such as an edge finder, coaxial indicator or part probe.

It must be located with high precision: typically plus or minus .001 inches or less.

It must be repeatable: parts must be placed in exactly the same position every time.

It should take into account how the part will be rotated and moved as different sides of the part are machined.

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Figure : Work Coordinate System (WCS)

For example, Figure shows a part gripped in a vise. The outside dimensions of the part have already been milled to size on a manual machine before being set on the CNC machine.

The CNC is used to make the holes, pockets, and slot in this part. The WCS is located in the upper-¬‐left corner of the block. This corner is easily found using an Edge Finder or Probe.

The term, Job, means a unique machining setup on the machine. For example, a part that requires the part to be moved or rotated three times on the CNC is said to be composed of three jobs; one for each setup.

The outside shape of this part was machined to size on a manual mill before being set on the CNC machine. The CNC is used to create the holes, pockets, and slot on this block. Before clamping the part, the vise is aligned and bolted to the machine table. This assures the part WCS X-¬‐axis is aligned with the machine X-¬‐axis.

Parallels (precision ground rails) are used to support the part. These ensure the XY-¬‐Plane of the part is parallel to the machine table XY-¬‐Plane.

The left face of the part rests against a Vise Stop. The Vise Stop establishes the X-¬‐axis origin. As new parts are loaded into the vise, the operator slides them against the vise stop. This ensures all parts are loaded into the exact same position each time.

The vise has two jaws; a fixed back jaw a front jaw that can close and open to grip or release the part. Because the location of the moving jaw varies depending how much force the operator uses, it is best to locate the WCS in reference to the fixed jaw. The fixed jaw position is not significantly affected by clamping force.

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