Form Datum - Calypso

Below is an excerpt from the Calypso Handbook

What the Form Datum does is establish a FUNCTIONAL Datum Reference Frame coordinate system, similar to if you were setting up the primary, secondary, and tertiary datums on a functional checking gage. This is ONLY for a datum reference frame and does not modify how the base and secondary alignments evaluate their respective coordinate system. In the screen shot below, you can notice the different origin points are for a base alignment and datum reference frame.

To set the Form Datum correctly click on Extras/ Settings/ Measurement/ Form Datum, check both the Outer Tangential Element and the Re. Calculation as per ISO 5459 boxes.

What is happening?

When we create our base alignment the alignment uses the intersection point of your features as below.

Base Alignment

This is based on the measurement of the actual features of the part.

Form Datum uses the highest contact points that would touch a Datum Feature Simulator, in order of primary, secondary, and tertiary features, establish a real coordinate system. Calypso will establish the proper location through the form datum reference calculation per ISO 5459. Again this is ONLY used for a Datum Reference Frame and does not affect the Base Alignment.

Form Datum

Understanding the 6 Degrees of Freedom

The following is an excerpt from The CMM Handbook

Using The Correct CMM Alignment Principles

Understanding the principle of the 6 degrees of freedom is essential to aligning your part correctly on the Coordinate Measuring Machine (CMM). When a part is placed on the CMM the location of the part is not known.  It must be defined by using several features known as datums.

These datums are defined on the blueprint and must be measured to define the part’s location to the CMM’s home position. If a program were written without a complete datum alignment all the linear locations in your program would come from the machines home position and not from the locations given on the blueprint. The subsequent parts would have to be placed in the exact location and alignment for your program to work. Writing a datum alignment routine at the beginning of your program will allow you to place the part anywhere on the CMM surface plate and will insure that your part will be aligned properly and your program will run correctly each time.

There will be three alignment functions used to define the datums (1) Orientation – this will use a 3d element to align the part to a spatial alignment. (2) Alignment – an element is used to align the part square to a CMM axis. (3) Origin – this will determine where the zero point is set. CMM softwares use alignment dialog boxes that allow you to set all the required datum elements at one time.

Words To Be Familiar With

Alignment – A part must be aligned to the CMM axes before you can begin to fully measure your part. The alignment is based on the datums specified on the blueprint. This alignment routine will square the part to the CMM and set the reference point (Origin) for all other dimensions to be referenced to.

Datum – A feature on the blueprint that is designated with a Datum letter. This feature is used to establish an origin that will be used to reference all other dimensions from.

Origin – The part’s zero point. The reference point on a part where the all the datums converge. The origin is where all other features are dimensioned from.

Coordinate System – After your alignment is complete and your origin is set this is known as a coordinate system.

Rotational – This degree of freedom allows the part to rotate about a given axis.

Translational – This degree of freedom allows the part to move transitional along an axis.

These principles constrain the 6 degrees of freedom.

6 Degrees of Freedom 

When a part is held up in space it has “six degrees of freedom”, 3 Rotational and 3 Translational. The part can rotate about the X, Y, Z axes (rotational) and move along each of the three axes (translational). As shown below.

The part can rotate about the X, Y, Z axes and move along each of the three axes

In order to align a part to the CMM all degrees of freedom must be constrained, there are exceptions but, in our example, all must be constrained. For this example, we will place the part in a DRF, a datum reference frame. This will represent how we will constrain the degrees of freedom when we probe the part.

Placing the part on the surface plate or on a fixture constrains 3 degrees of freedom. The part can longer move up and down in Z (1 translation) or rotate about the X axis or about the Y axis (2 rotational).

Constraining 3 degrees of freedom

As you can see, we can still rotate about Z and move the part back and forth in the X and Y axes.

Constraining one side of the block will limit 2 more degrees of freedom. Removing the ability to rotate in the Z axis (1 rotational) and constrains the ability to move in the X axis (1 translational). The part can still move back and forth in the Y axis.

Constraining one side of the block will limit 2 more degrees of freedom.

 Constraining the last degree of freedom requires restricting the back-and-forth motion of the last axis.

Restricting the back-and-forth motion of the last axis.

Now all 6 degrees of freedom have been constrained. You can see how this is accomplished on an open surface plate setup by merely pushing the part against a knee block or some sort of fixture. With CMM software the constraining of features is done with probing the part.

Calypso Handbook

The Calypso Handbook serves as a comprehensive reference manual for Calypso programming. It includes embedded videos to facilitate understanding of various topics covered in the manual. Elevate your programming skills with these expert tips, each of which has been applied in real-world programming and verified to be effective.

Cartesian Distance Video

Learn how Cartesian Distance works in Calypso

Cartesian Distance

Click on Size / Distance or Form and Location / Cartesian Distance

In this example, the “Primary Datum” is the same as the “Spatial Rotation” and “Secondary Datum” is the same as the “Planar Rotation” inside an alignment. Selecting “Parallel To” or “Perpendicular” specifies the axis direction. i.e. the “X”, “Y” or “Z” axis

NOTE: In Calypso whenever two features are recalled, the first feature defines the feature origin and the second feature defines the orientation of the feature.

Example:

If the Circle is the first input and Plane1 the second input. The initial distance orientation will be defined by Plane1. Further control is offered by defining the Primary and Secondary Datum

Simple Distance

Select Form and Location/ Distance/ Simple Distance

The holes are checked to each along a Datum Reference Frame. In this case the Base Alignment. That means the X and Y axes of this Characteristic are normal (parallel) to the Base Alignment X and Y.

Alignment with Offset Plane

The offset plane can be used in an alignment construction. A good example of an offset plane is cast datum points, where they are rarely on the same plane.

Offset Plane

The first step is to probe 3 Space Points on the model at different heights. If you have a CAD model change the feature type selection to Define a Space Point and place 3 points on the model at different heights, i.e. the cast datum points.

Click on Features/ Additional Features/ Offset Plane      

Click on Evaluation

Click on the Edit button at Point Modification

Point A1 in our example is at 0

·        Enter 0 in Point 1

·        Enter -10.00 in Point 2

·        Enter -8.00 in Point 3

This is the method to create an offset plane. It can now be used in a Base Alignment, or as you would any feature.

CAD TRANSFORMATION

Selecting Features From The CAD Model

Please load the CMM Quarterly model by clicking on CAD/ Load Model and navigating to the model.

You may use your own model and part if you wish. Make sure your first part is a verily prismatic part. Something square without a lot of curved surfaces to begin with. Just apply the same principles to your part program.

Move the CAD Origin

Our first step after importing the CAD model is to make sure the alignment is in the proper location. The CAD alignment on the model must match the blueprint. In addition, the CAD model must be rotated as the part sits on the CMM table.

In this example the origin must be moved to the corner of the part.

We will need to extract some features off the model to obtain the nominals from the model in order to move the origin to the correct location.

Click on the lower toolbar and select Extract Features.

Pick all three of the planes on the example and create an Intersection (Construction/ Intersection) of the three planes

Open the Intersection

Click on CAD/ Modification/ CAD Model Transformation