Calypso Alignments Made Simple
Foundations
What is the purpose of datums and alignments?
When you measure anything, you need an accurate alignment to get accurate measurements. What do I mean by alignment?
Remember the last time you measured a room to calculate the amount flooring or paint you needed? You just measure the size of the room, without thinking too much about the alignment of your tape measure. This is a simple example of the need for datum references. A datum is a surface, line or point that is used to establish a reliable measurement. When you measure the length of the room, you automatically keep the tape measure level with the floor. The floor is the first reference surface you use to establish which way the measurement is taken. In other words, the floor is the “primary datum”. It controls the “spatial rotation”, as Calypso calls it. At the same time, you will line up the tape measure against the wall. (It wouldn’t be accurate to measure the length of the wall diagonally from another corner.) This wall is the second reference surface – the “secondary datum”. The last step is to push the end of the tape measure until it touches the far wall. To establish where the measurement originates from. That is your third reference surface – the “tertiary datum”.
When you place a part on the CMM granite surface, even if it is fixtured level and square, the part surfaces are not exactly lined up to the CMM perfectly. The machine has no idea where the part is, or whether it is sitting on a grain of sand, which would change the result slightly. Every new measurement plan (CMM program) begins as if the part is free-floating in space.
If an object is floating in space, it can move in 6 different directions (or degrees of freedom). It can move in a linear direction left/right, forward/backward, or up/down. These are called X, Y and Z directions on the CMM. The linear movement in X, Y or Z is called translation, and those are 3 degrees of freedom. The object could also twist in space left/right, forward/backward and clockwise/counterclockwise. We call this rotation around the X, Y or Z axis.
The Calypso software uses a “base alignment” to accurately find where the part is in space (XYZ origin), how much the part is tilted (levelling), and how much it is rotated (skew/clocking). In addition to that, you can construct other alignments or datum references to measure GD&T tolerances that control mating-part relationships.
Your ability to look at a part drawing, interpret the GD&T, interpret the datum references, and correctly apply alignments in Calypso will directly affect the quality of your CMM program. My goal is to help you learn this skill and apply it well.
Order of precedence
According to the GD&T standards, the datum references shown in a feature control frame are shown in order of priority (or precedence), from left to right. In other words, the first letter box determines the first datum feature that will constrain the part.
Common Misconceptions About Datums & Alignments
Datum A is always the primary datum.
Although datum A is often called out as a primary datum, it may be used as a secondary or tertiary datum, depending on the specific application. Remember, it’s not about the alphabetical sequence, but the left-to-right order of precedence.
The three datums always mean Level, Skew and Origin.
The primary datum is usually a plane or a cylinder, constraining the 3D levelling and origin in one or more directions. The secondary datum could be a plane, cylinder or point – establishing the skew and/or origin in one or more directions. The tertiary datum could also be any feature which may control skew and/or origin. The key is to remember that each datum feature will constrain the part in one or more degrees of freedom. The next datum feature will constrain the alignment using one or more degrees of freedom that are still available. Imagine going through a buffet line, where the first greedy person in line grabs as much as he can, while the second and third people get what is left over. The datum order of precedence is like that.
We always must go by the part drawing, for the datum order of precedence.
This is a big one, and it is tricky. Unfortunately, there are times when the drawing was made by someone with limited GD&T experience. This causes scenarios where following the exact datum reference sequence would result in an unrepeatable alignment. Often it can be resolved by simply swapping the order of the primary and secondary datums. You need to do this with wisdom, however. I strongly recommend you communicate with your team and customers about your decision. There are other situations where the datum references cannot be directly input into a Calypso GD&T tolerance in the same way. Again, understanding how each feature constrains the 6 degrees of freedom will help you know what makes sense. Try to understand the function of the part and its related assembly components. This will help you interpret the GD&T tolerances in a practical way.
The Base Alignment in Calypso needs to come from datum features.
Usually, it is helpful to use datum features in the base alignment, so the zero point corresponds with drawing dimensions. However, the purpose of the base alignment is to locate the part in the CMM software in the most stable way. You can use any features on the part that accomplish that. There are times when the main datum features would be less ideal, because they are less repeatable for locating the whole part, or because they are located on opposite sides of the part.
3-2-1 Alignment (Plane, Line, Point)
The 3-2-1 (Plane-Line-Point) Alignment is the most basic way of fully constraining a part.
Features you will need
- A plane with at least 3 points – the Primary datum surface
- A line with at least 2 points – the Secondary datum surface
- A single point – the Tertiary datum surface
(this article is under construction… let me know if you found it useful! steven@learncmm.com)
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