Let's explore what happens when we apply Form Datum. Have you used Form Datum? Do you know what it is used for? In the Extra/ Settings/ Measurement flyout you will see the Form Datum tab.
When applying true position to a feature of size with form datum selected Calypso can change the Datum evaluation calculation algorithm. Let’s explore why.
In our example we will use a simple part with three holes.
We have measured the part and setup the datum alignment.
Each of these features has been measured with the default evaluation algorithm, LSQ (Least Squares). We will explore how Calypso will change the Datum features from LSQ to Outer Tangential to simulate a gage check.
Now let’s apply true position per the blueprint. Select Circle1 as the feature we will evaluate. Notice that Circle1 evaluation remains LSQ, as it was measured.
It is important to apply the datum structure as shown in the True Position dialog box. Although the feature was measured in the base alignment and is dimensioned back to the base alignment it would seem that it would be acceptable to leave the datum structure as the base alignment but it is important to rebuild the alignment within the true position dialog. This allows Calypso to apply any datum mobility and this is the only way to allow MMC on any of the datum features.
The Feature – Circle1
The measured feature maintains its measured evaluation method. In this case Least Squares ,LSQ. To verify this look toward the bottom of the dialog box under Evaluation settings and you will see LSQ Feature.
Calypso allows you to modify this but without modification it remains as measured.
When Datum B, a cylinder, is selected the Evaluation Settings is changed to an Outer Tangential Element.
The same is true with Datum C, a circle. The Evaluation Settings are changed to evaluate the circle as an Outer Tangential Element.
When Calypso changes the evaluation setting to Outer Tangential Element it is applying a gage simulation. It will try to duplicate an assembly mating or assembly gage simulation to match what happens when the part is assembled. If you think about a surface, any surface, only the three highest points will contact the mating surface. Lay a flat part on the surface plate, regardless of what it looks like only the three highest points are contacting the surface plate. The same when you, in this example, slide this part over pins it will contact the pins with the highest points.
What to watch out for
Be aware that Outer Tangential is very susceptible to outers so filtering and outer elimination should be activated. Also depending on your machining techniques, drilling, honing, end mill, ... using Outer Tangential will move the actual center of the feature based on measurement and outlier elimination. Now it may only be microns but be aware it may move it. Have you ever accepted a part only to have it fit in assembly? This might be the reason.
Should I switch to using Form Datum with Outer Tangential?
I would recommend it. This simulates what will happen in assembly. Let's look at another example.
Here is the physical part
Here the part is measured with a LSQ line. This is not wrong because with form datum applying the correct evaluation it will report the true position correctly.
Now look at the two pictures again. The first one shows where the part origin would be in an assembly position when the part is pushed up against the left edge. The second shows the part alignment based on you measuring the edge using the default settings, LSQ. This would be true no matter what software you are using, LSQ is LSQ. So now you have reported back to the machine operator to move the hole position based on the True Position using the measured feature evaluation setting, not applying Form Datum.
Now take the part to assembly and you can see how the holes are not correct and this part will not assemble.