gd&t - perpendicularity

[Eins73]

Hi All,
I've got problem of measuring the required spec. here is the sketch (see attachment). How should the dimension been measured.
Thank you

 

[CheckerHater]

More like this:

 

[pmarc]

Actual perpendicularity error is the minimum possible distance between two parallel planes perfectly perpendicular to datum plane A within which all points of measured surface lie. The minimum distance must be smaller than the specified value of perpendicularity tolerance (in your case 0.05).

That means you do not search for any tangent plane - you check how far apart the two parallel planes, as described above, are.

 

[Eins73]

hi All,
I still can't agree with the point selected for the measurement. How do we determine the point to measure (as at the bottom radius, sit on a datum plane)? please see attachment.

 

[pmarc]

Like I already said, forget about tangent plane (green line on the sketch).
1. Probe the surface (you will get a cloud of points).
2. Take two imaginary planes that are parallel to each other and perpendicular to datum plane A and try to close them as much as possible (so that all inspected points are in between the planes).
3. Measure distance between the imaginary planes.
4. If the distance is smaller than or equal to 0.05, geometry meets the print. Otherwise it does not.

 

[CheckerHater]

"Measure distance between the imaginary planes" - brilliant!

 

[pmarc]

Imaginary means "not existing on real part" in this case. Just like the green tangent plane does not exist on real part too.

 

[Eins73]

the green line refering to the part plane (X1), and the plane cross the datum plane at which another plane (Y1) of the perpendicular to datum is set. the distance of the plane Y1 to the maximum slant of the part along the datum is measured as the perpendiculrity ?

 

[pmarc]

Eins73,
Allow me to doubt that the green plane is the part surface. Perfect surfaces (in this case perfectly flat) do not exist, so the green perfect plane cannot represent actual surface. It is an approximation at best.

On your initial sketch MEASUREMENT B is the correct method of perpendicularity error verification. Two horizontal lines define extent of measured surface and two vertical extension lines spaced 0.03 apart exactly represent two planes I have been talking about in my previous posts. Since the distance between the two vertical lines is not greater than 0.05, actual measured surface meets print specification.

 

[CheckerHater]

Eins73,
Please try to imagine the following:
Perpendicularity is NOT relation between two planes.
Perpendicularity is a requirement that all points of one surface lay within the tolerance zone, perpendicular to the plane (called “datum plane”) derived from another surface.

 

[dingy2]

Perpendicularity of a plane to a plane can be either all angle or all flatness or a combination of both. I have attached a example on how one should measure this requirement.

Dave D.
www.qmsi.ca

 

[pmarc]

CH,
Perpendicularity is not relation between two planes - that is absolutely correct.
But actual perpendicularity error is the distance between two planes.
Please try to spend more than a moment on reading my posts. Thank you.

 

[powerhound]

It looks like measurement B in your OP is how you should be checking perpendicularity.

John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[CheckerHater]

pmarc,
I was not trying to correct your post, I was trying to give OP different prspective, different view at the problem.
In fact, I was so confident that your post is correct, I didn't even look at it.