A coordinate system is the ‘you are here’ indicator on a map and how far it is to the next feature is the linear dimension. Linear dimensioning is the method of locating a feature. In order to know the exact number to this feature you must have a start point and that is the coordinate system.

How Do I Establish Create A Coordinate System?

In order to create a coordinate system you must constrain the ‘6 degrees of Freedom’. This was covered in detail in a previous CMM Quarterly; however, in order to cover the coordinate system thoroughly we must discuss some of the aspects of constrains in this article.

Some CMM software use different terminology but the basics are the same. First, you must create a spatial orientation, spatial rotation, or a base plane. You must use a plane or any feature type that has a 3d axial line through it to create this base plane. This will constrain the first 3 degrees of freedom. This limits the part in its movement on the CMM and makes the part normal to CMM. You would use a plane, cylinder, cone, or a sphere. These 3d features have a theoretical line that runs through the feature and is normal to that feature; it is the axis line of the feature. Your CMM software will align that 3d line normal to a CMM axis.

In this picture a plane was probed on the top surface and used to create the spatial orientation. This forces the part orientation normal to the Z axis of the CMM. We also set the Z origin to this plane. As you can see it prohibits the part from moving up and down in Z but it can still move side to side in X and Y and rotate around the Z axis. This has constrained 3 degrees of freedom.
1) Movement up and down in Z. 
2) Rotation around X. 
3) Rotation around Y.

The next step is to use an axis alignment. This will make one edge of the part normal to a CMM axis. You may use either the X or the Y axis to align the part. The terminology in your system might be Axis Alignment, Planar Rotation, or some other term but the process is the same.
What features can I use to create the axial alignment? You can use a line, 2 circles that are aligned (offset circles can be used but we will not cover that in this article), a plane, a cylinder, etc…. You may use any feature that is a line or has a line running through it.

Here we probed a line on the Y axis and used this line for our axial alignment. This squares the part to the CMM Y axis. We also set the X origin on this line. This constrains 2 degrees of freedom. We can no longer rotate around the Z axis and can not move back and forth in the X axis. We now have only one degree of freedom left. We can still move the part back and forth in the Y axis.

You may now use a point or a line on the remaining side of the part and set the Y origin to that feature. This fully constrains the part.

We now have a coordinate system on the part. The coordinate system indicator shows the location of the origin of the part. This should match the starting location using the same features on your blueprint.

What if I use a CAD based system? The alignment routine is exactly the same. You would just use the model system to generate the features for your program.
This type of alignment is known as a 3, 2, 1 alignment, indicating the method of constraining the 6 degrees of freedom.

Myths And Truths About Establishing Coordinate Systems

I’ve been told I can set the part on the CMM anyway I want to.
True with some exceptions.
If you set your part at some skewed angle to the CMM axis’ the software will still align it mathematically to be normal to its own X, Y, Z axis’. As you have seen above the software will square the part to be normal to the CMM. So yes, this statement has some truth to it but some care must be given to how the part is to be probed. If you set your part on a compound angle and you are probing manually or with a joystick you may encounter some probing error. Each probe point should be probed normal to the feature. If a point ‘slides’ or glances off the surface at an angle that is not normal to the surface then you will have some cosine error.

I have to use this 3, 2, 1 alignment every time.
False.
This is by far the most common alignment you will come across but is not the only alignment. For parts that have free form surfaces and no defined prismatic features you will need to use an iterative alignment. This is accomplished by probing several points on the surfaces of the part and letting the software mathematically align the part to the nominal values of the points. This may take several iterations, each time getting closer and closer to the nominal value.

I can measure my alignment manually and run the program in DCC.
True.
Well, you can certainly do this but the best course of action would be adding a DCC alignment. After you measure your part manually you should include a repeat of the alignment in DCC making sure you are driving normal to the feature. This will better align the part and measure the alignment in the same measurement parameters that the remaining features will be measured with.

If I want to quickly check a feature I don’t need an alignment.
False.
Let’s take a gage ring as an example. If you wanted to check the diameter you could lay it on the CMM and probe the diameter and probably get a good reading because the top and bottom surfaces of a gage ring are typically ground. At a minimum you should probe the top surface and use this as your spatial orientation before checking the diameter. This will square the top to the CMM axis and allow you to check the diameter normal to the top plane.

I hope this has been helpful trainining. If you have any comments please contact CMM Quarterly though the Contact link