Introduction Coordinate Systems

MCS - Machine Coordinate System
PCS - Part Coordinate System
FCS - Fixture Coordinate System
ICS - Interim Coordinate System
Clearance Coordinate Systems

The MCS is the XYZ position on the CMM with respect to the Home Position. The MCS axes are mutually parallel with the movement of the CMM

Most manual CMMs do not have a home position, therefore the home position is the location of the CMM at the point of starting the system. Some manual CMMs do offer a home position that requires the operator to manually move the CMM over reference marks one axis at a time.

On a DCC system, a start-up procedure requires that you "Home" the CMM which drives to the designated home location and establishes a repeatable XYZ zero point.

A MCS is therefore not a valid coordinate system to obtain reliable measured values for your part inspection.

The PCS is the foundation that establishes the Orientation, Alignment and Origin of your part inspection. PCSs are simply the frames of reference in which part feature locations are defined.

Traditional drawings were made using 2D projections to represent 3D objects. Each 2D view shared a common reference point and common axis of alignment allowing the building of 3D features. In fact, a 3D dimensional PCS is implied.

Today, it is common to create machined parts using a 3D model created by many popular CAD tools. These models may contain all necessary information for the machinist to create the part and for the inspector to validate the final production. However, the PCS is still implied using the same procedures as if measured from a 2D drawing.

The established method for creating and using a datum frame reference can be found in the ASME Y14.5M - Dimensioning and Tolerancing manual. Geomet uses these accepted practices in the application of PCS.

To better understand this let us consider the multi-projection part shown below:

PCSPlanView.jpg (9102 bytes)

Multi-Projection 2D Drawing

In order to measure this part on a coordinate measuring machine we shall need to contact it with probes on all sides. In planning to do this it is helpful to visualize the 3D coordinate system of the part. Let us do this by making a perspective drawing from one possible direction and then overlaying a 3D PCS. The figure below is one possible view if the 3D dimensional PCS.

PCS001.jpg (21778 bytes)

Example : 3D Part Coordinate System

The way in which the PCS is overlaid on the part is determined by the datums defined in the drawing. Notice that each datum can be represented by a standard measured feature; The -A- datum is a ZX plane, datum - B - is a YZ plane and datum - C - is a XY plane. The XY, YZ and ZX planes are orthogonal to one another.

The important point to remember is that the two dimensional drawings we deal with are derived from three dimensional concepts, and in order to effectively measure a 3D part with a coordinate measuring machine we must "see" the 3D PCS with the part. This is made easier by looking at the actual part and at the same time as we study the drawing and making a habit of constructing imaginary planes in the part representing the datums defined in the drawing.

PCS Base Planes are the extracted XY, YZ or ZX datum planes of an establishes Part Coordinate System. When extracted through the Recall Component tool, they become constructed planes and given a feature number in the inspection report. They and can be used in all constructions and other feature tools including the application of tolerance nominals.

Since features of machine parts are defined in terms of their datum references, we must create Part Coordinate Systems reflecting those datums before the features can be measured. Since coordinate measuring machines have their own coordinate system, namely a system of mutually orthogonal axes of motion referred to as the Machine Coordinate System (MCS), the first problem we face is to create the PCS within the MCS. This is not trivial.

One way to create a PCS within a coordinate measuring machine is to make the PCS coincide with the MCS. The procedure consists of physically aligning the datum surfaces of the part parallel to the axes of motion of the machine and setting the X, Y and Z displays to zero with an edge finder probe in contact with datum surfaces or with a taper probe seated in a datum bore. Once the MCS and the PCS have been made to coincide, all dimensions found on the display of the coordinate measuring machine are also feature locations on the part.

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GeoWidget PCS shown in relationship to the MCS

Making the PCS and MCS coincide physically can be extremely time consuming, and sometimes impossible. We can eliminate the problem by using Geomet to create and store a skewed PCS. Once a PCS has been established, the computer converts raw data collected in the machine coordinates into measurement results in part coordinates. In the figure at the right you will see a part coordinate system with X, Y and Z axes nested inside a machine coordinate system with its own X, Y and Z axes.

The MCS and the PCS are totally skewed relative to each other. With the help of Geomet any point in the MCS can be expressed in the PCS.

As Geomet can manage the link between an MCS and a totally skewed PCS we no longer need to physically align the parts to the machine. But, we need to create the skewed PCS in Geomet before it can manage the link. The job of creating the PCS will be simple once we understand a few fundamental principles. A PCS nested within the MCS

A FCS is a previously measured coordinate system saved away for recall and used in part inspection programs. The use of an FCS allows a repeatable location in the CMM measuring cube where inspections take place. An example would be a cradle fixture that holds a production part in one orientation and fixed location, see FCS Manager.

The use of an FCS requires that the CMM have a known home position which is common on all DCC CMMs and on few manual CMMs.

The process of creating a FCS entails establishing a PCS on the fixture and then activating the FCS Manager to append the specific geometry values associated with the established PCS to the FCS database. Using an FCS requires recalling the FCS by the assigned number through the Coordinate System Manager.

The ICS is a coordinate system that has not been completed to the status of a PCS. For example, when Geomet begins a part inspection, the only coordinate system available is the MCS. Should the part to be inspected have the orient applied to it, Geomet creates an ICS based on the orient vector data and brings forward the alignment and origin data from the MCS.

During the inspection process, reported results on features are hidden until the PCS is completed. Only then will the reported data be meaningful.

A Clearance Coordinate System are used to create safe zones around the part being inspected and around any fixtures associated with the inspection. The Clearance CS contains 3 clearance planes, identified as XY, YZ and ZX base planes and an origin. These clearance planes are used as safe transition planes where the CMM can perform DCC moves without concern of accidental probe encounters. Clearance CSs are used with Offline IPs and Entry / Exit IPs in auto features.

In order to ensure that a complete PCS has been established, Geomet provides a visual PCS Setup Guide to remind you of the missing components of the current PCS under construction. In figure 1, we show the setup guide with all axis and origins still requiring definition.

PCSGuide1.jpg (7633 bytes) PCSGuide2.jpg (7847 bytes)
figure 1, PCS Setup Guide figure 2, Completed Z Axis

When you establish one of the PCS components, such as the Z primary axis derived from the Orient function on a measured or recalled qualifying feature, the setup guide will reflect the completion by "signing- off" that component with a series of "*******" as shown in figure 2. During the building of a PCS, Geomet maintains an Interim Coordinate System, ICS, which contains the resolved transformations and carries forward the missing components from the MCS, see ICS

During the inspection process, additional PCSs may be required. Should you want to create another PCS by starting with the Orient function, the PCS Setup Guide will reappear and further results will be suppressed until the ICS is resolved into a completed PCS. The setup guide will not reappear with basic transformations on a current, completed PCS that results in a new PCS. These commands are:

It should be noted that as long as a PCS is under construction, no results are displayed with each measured feature. When the PCS is completed, the PCS Guide Box will disappear from view and the suppression of results is removed.