Part Setup and Datum Qualifying using a CMM

[@ringlord13]

YES! More of this please. I REALLY HOPE Mitutoyo is working on their version of this showing how to accomplish this on the software side. It would be excellent for students.

[@ginotacobell3098]

Excellent video! I rarely see Mitutoyo doing this kind of training online. Even this video doesn’t show much about MCosmos. Hopefully there are more videos showing more into MCosmos. Thanks for sharing!

[@Gdandtbasics]

Glad you enjoyed it!

[@JOEPASHLEY17]

Hello, thanks for the video, where can I find the rest in the CMM series?

[@Zakariae92]

Hello sir, thanks for the video and your great explanation. is it possible to find the cad model somewhere please ? thank you in advance

[@aysegulkuru4316]

Could you please explain this orange stick which is installed into the inspection part. As I see this orange stick is a kind of datum simulator. As far as I know, we do not use datum while measuring flatness. Why did you choose it, how did you choose the place of intallation, how can you make sure that it won't affect the measure? Could you please show the installation video for more clarification. Thanks.

[@Gdandtbasics]

The orange stick in this video is not a datum simulator and it does not affect how the part is measured using a CMM. In this case it is simply elevating the part so the probe can access all the surfaces at once. The CMM will act indirectly as the "datum simulator"

[@philipcalhoun8556]

How is the inspection report turned on?

[@pmungliful]

how to do alignment with 321 when three planes are not perpendicular to each other. For that datum target points defined 3 points for primary, 2 points for secondary and 1 point for tertiary. ? thank you

[@Gdandtbasics]

If you consider 3 mutually orthogonal planes creating the datum reference frames on a drawing. The drawing will have "implied" 90 degree basic dimensions for these planes that are not shown. However now if you picture the third plane being at an angle ( lets say 30 degrees ) from the second plane. The only addition that needs to be included to complete this is to add the basic dimension of 30 degrees locating that surface. In cases like this, if this is the secondary datum feature, angularity will often be used to qualify the datum feature in question back to the primary datum.

[@edivaldobulba4508]

This video is very enlightening and I marked that I liked it. However, the C datum has a free degree of freedom and it seems to me that according to the CAD drawing, other features do not allow this. Wouldn't it be more logical to adopt a third datum before defining datum C for later inspection of other features?

[@edivaldobulba4508]

My apologies. All the other features I referred to can be referenced by the C datum

[@alanchang0118]

🙋‍♂️ Does it mean the datum in CMM is always RMB? If the part is restrained, when I probe datum feature to setup datum, does the datum also RMB?

[@ZakisHereNow]

I’ll save some people time. This is not a software tutorial for programming a CMM. It’s only using a CMM for measurements.

[@christophergrim9925]

I am confused about how datum axis C is established. Is the axis established wherever the hole ends up being -whether the hole is in tolerance or out of tolerance? Or is the axis established based off of the basic dimensions and the axis is perfectly positioned? If the axis is always perfectly positioned based off of the basic dimensions, this means that the hole will never be perfectly coaxial/concentric with the axis since there will always be some inherent error in the hole. Can you help clarify this for me? Thanks

[@Gdandtbasics]

Thanks for the question! The Datum Axis C is established where the imperfect hole is, BUT the imperfect axis MUST be verified to be with in tolerance with respect to A and B first ( Location and Parallel to A and Perpendicular to B ). Remember without Datum C involved we have not constrained all 6 degrees of freedom yet. So we can locate and orientate the tolerance zone for Datum Feature C back to Datum A and B using the basic dimensions but that final degree of translation that is missing will be what Datum Axis C locks in.

[@sem7207]

@@Gdandtbasics I'm afraid that your answer is incorrect. According to the ASME Y14.5 standard the datums are derived from the datum feature simulators and not the datum features. Datum feature simulators have fixed location and orientation relative to each other according to the basic dimensions. Therefore the datum will be not where the imperfect hole is, but at the basic distance from datum A. When establishing the constraint of the part to datum feature simulator C the Mitotoyo program should have simulated the largest cylinder the center of which is located in the basic true position of datum feature C, touching the actual hole surface on the high points where it is able to reach. I wonder if the CMM program complies to the standard or is it based on a similar error?

[@omerbexiibihii5682]

Goldmine

[@Gdandtbasics]

Thanks!

[@denniskingsland2827]

Every CMM software I have used defaults to best fit planes. (Simplified version; the points are averaged to create the plane.) You won't get the three highest points when creating the plane unless the software supports tangent planes. Comment is based on the four different CMM software packages that I have used. I realize that you need to "dumb it down" but this has real world consequences especially as flatness tolerances get bigger.

[@tomgeiss5777]

Keep in mind this is not our opinion - this is a requirement of the ASME GD&T standards. Using best fit planes for establishing datums - whether it’s the default in CMM software or not - violates the ASME Y14.5 standards as it does not mimic the functional intent (physical gage blocks use high points after all - not an arbitrary average plane) Datum setup is independent of flatness as any flatness requirement on a datum feature is a separate reportable measurement vs the concept of setting up datums for inspecting other part features.

[@denniskingsland2827]

@@tomgeiss5777 I've always said that default CMM feature construction assumed perfectly formed parts.

[@tomgeiss5777]

@@denniskingsland2827 In many ways yes - however the parts themselves are never assumed perfect - but the datums derived from them are. The datums are the perfect plane derived from the imperfect features on the part. Not to get technically wordy but the primary datum is derived from the unrelated actual mating envelope (UAME). Actual Mating Envelope means how a perfect mating feature would contact the part. This can only be the highest points. With your experience I’m sure you have measured parts using gage blocks in the 3-2-1 rule of contact. This numbers represent the first contact points made with a (semi) perfect set of gage blocks to restrain all 6 degrees of freedom. A CMM is intended to replicate a physical datum simulation like gage blocks would. That’s why you have to use the high points. Honestly it’s a surprising topic at every place we train, due to some CMM software being a bit behind the curve on the standards. A few software programs view a datum plane (high points), and establishing the location, orientation or form of a plane (all points within a given tolerance zone) as being the same, when they require different approaches.

[@denniskingsland2827]

@@tomgeiss5777 Absolutely agree that there are no perfectly formed parts and that derived datums are perfect. My comments have been about how the derived datums are actually derived is important. A primary derived datum feature can orient the part differently depending on if was created using the typically default best fit plane or if was created using a tangent plane. "Rocker" Primary datum features are more common than what one might think. The CMM training I've received over the years rarely if ever discussed using anything but the default best fit plane.

[@mcdoogle274]

@@denniskingsland2827 It’s even more complicated in the standard. ASME and ISO had to find a way to overcome the "rocker" planes problem, and as you mentioned, it’s a very common problem. ASME's solution until the newest standard was to use a procedurally defined tangent plane, which was totally impractical for use on a CMM, and ISO used a tangent plane parallel to the Chebycheff plane. Both standards are expected to eventually choose as a default method the so-called "L2"-method, which basically is the tangent plane to a filtered Gaussian plane, which can be easily be done in a CMM software, provided you take enough points. ASME will bring this change with the next mathematical standards update, ISO is expected to follow, so that this matter will be better standardized among the different standards systems.

[@jerryschwarzbach3253]

Just about as dry as it gets.