# How to do a profile on a surface with one datum

B

#### brossbach

I'm learning somewhat on how to measure a profile of a surface. I need help on how to do this correctly and to do a correct cpk on the profile. Here is example of what I'm dealing with. On the design print I'm dealing with a ignition coil. On this print it calls out for this profile. Here is an example.Profile symbol/2.0/datum A. Sorry it wouldn't paste my picture to show better example. This is really new to me. I doing this on CMM / With a MM3 program. If someone could help step by step I would really appreciate it. Also, if I would a cpk study on this what would be the tolerance zone for this +/-.

#### Marc

##### Fully vaccinated are you?
Ayone have any ideas for this one? (Bump)

#### Paul F. Jackson

##### Quite Involved in Discussions
If the measured feature and the datum feature are parallel to one another, then measure the displacement and compare it to the basic. Chart the point to establish process control. When control is established, determine the size of the profile zone required to contain each measured point and compare the mean and associated standard deviation of the resultant zones to specified profile tolerance. Report Cpk from (Cpu or Cpku) in other words compare the encroachment of the distribution on the upper limit and discard the Cpl.

If Datum A is an axis and the measured feature is displaced from it then the measured points can be compared from their basics, charted and controlled, reported according to the size of the zone necessary to contain each one, and predicted for capability. Just like above.

If the measured feature is defined at any other angle than 180 degrees to the datum feature plane or axis then only orientation to A can be determined. For instance a surface or axis can be measured for perpendicularity to a datum surface or axis. Note: The measured feature is free to rotate perpendicular to the datum surface A. Chart the oriented feature form deviation which is the size of the profile zone needed to contain all measured points on the feature. Establish control and then use the same resultant zones to predict capability.

If the datum feature A is a point (center of a sphere) then the displacement can be measured as well but it is probably easier to orient the measured feature and measure the displacement of the datum feature to it.

Good luck, Paul

#### Proud Liberal

##### Quite Involved in Discussions
There isn't enough information in your post to help you. If you'd like an opinion, email a section of the blueprint to [email protected]. I've taught GD&T classes and have more than 10 years experience running a CMM. Profiles can be tricky. The most common error is to report the maximum deviation from nominal as the profile when in reality it is twice the max deviation. Unless your problem deals with a planar surface parallel to datum -A-, the solution will require some best fit algorythm to be performed on the data.

C

#### Coleman Donnelly

I have a similar problem - I have profile of a trim edge back to Datum A|B|C but only Datum A is identified on the print - Datum A consists of two target zones on the same plane. My trim edge would best be described as a plane though not much of one (2mm x 24mm) i could report angularity or perpendicularity to datum A but i want to make sure that this is a legal and technicaly correct way of measuring the part. Would i then report the perpendiculariy value of my trim edge back to Datum A since there is no control fram to define 2 point orientation/rotation or 1 point origin location?

TIA

#### Stijloor

Super Moderator
I have a similar problem - I have profile of a trim edge back to Datum A|B|C but only Datum A is identified on the print - Datum A consists of two target zones on the same plane. My trim edge would best be described as a plane though not much of one (2mm x 24mm) i could report angularity or perpendicularity to datum A but i want to make sure that this is a legal and technicaly correct way of measuring the part. Would i then report the perpendiculariy value of my trim edge back to Datum A since there is no control frame to define 2 point orientation/rotation or 1 point origin location?

TIA

It would help us if you could provide a sketch. Can you attach one?

Stijloor.

C

#### Coleman Donnelly

My mistake... Drawing does not reference a datum B or C...

#### Attachments

• sketch.xls
133 KB · Views: 317

#### Stijloor

Super Moderator
My mistake... Drawing does not reference a datum B or C...

Coleman,

I am still confused....

The only call out visible on the sketch is the "flatness within 0.5" callout.
It is not clear to me if this applies to the entire top surface or just to the "25 Zones." I can not discern another callout....

Can you further clarify?

Stijloor.

C

#### Coleman Donnelly

the call out is spelled out in a general note on a different page of the drawing... It calls out all trim edges to be 1.0 back to datum A. Datum A is the 2 target zones identified on the sketch.

#### Stijloor

Super Moderator
the call out is spelled out in a general note on a different page of the drawing... It calls out all trim edges to be 1.0 back to datum A. Datum A is the 2 target zones identified on the sketch.

1.0 what to datum A?

The datum targets are incorrectly specified. (Do you have Y14.5 handy?)
They should be drawn in the view where it makes most sense using the correct symbology and basic dimensions.

Don't give up..we're getting there....

Stijloor.