Profile of a surface 1

[pratyu]

Hi all... I'm a GD&T newbie & here I have a simple yet silly question...

Suppose I have a block & let's say one of its sides says that it has a Profile of Surface tolerance as "1 mm"

In another case, having the datums conventionally defined (in both cases), it says it has a Profile of Surface tolerance as "1 mm" w.r.t (let's say) A|B|C

I wanna know this : 1. What is the difference in the profile tol. with & without DATUMS?
2. When to give datum reference & when not to?

 

[pmarc]

1. What is the difference in the profile tol. with & without DATUMS?

Have a look at attached picture:

http://files.engineering.com/getfil...-c9b6-4104-88b4-1feb363aba2a&file=profile.png

This should help you to grasp the key difference. In general, profile without datum feature references does not control any relationship between controlled surface and datum(s). For simple surfaces, like shown in case #2 in the attachment, this means the profile is capable to control form of the surface only (flatness error in this particular case). The controlled surface can be at any angle relative to datum plane A and at any distance from A. For irregular features of size, like irregular holes or pins, if the true contour of the feature is defined with basic dimensions, profile will control size of the feature in addition to its form.

If profile feature control frame contains at least single datum feature reference, it controls more than just size and/or form. In case #1 in the attachment it controls location of the surface relative to datum plane A and in the same time parallelism of the surface to A (the profile tolerance zone is located from and oriented to datum plane A).

2. When to give datum reference & when not to?

If you are interested in controlling form and/or size of the feature without controlling its relationship to any datum(s), profile without datum feature references can be used. If more than that is needed, that is orientation and location control, datum feature references shall be specified.

 

[Belanger]

In that attachment, pmarc, I think you meant to remove the basic dim box around the height of 10

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[pmarc]

Well, actually I did not, J-P. I rather wanted to show that the basic dimension would have certain meaning in case #1, and would be meaningless in case #2.

 

[pratyu]

pmarc,

Why don't we require basic dimension in the second case?

 

[pmarc]

Basic dimension is meaningless in case #2 because profile feature control frame is not controlling any relation between controlled surface and datum plane A (due to lack of datum feature A reference).

 

[pratyu]

Okay!
In case 1, suppose I remove the basic dimension, and give the tolerance 10 +/- 1 mm, and the datum is referenced, & the actual height value is 10.05 mm, we have a profile tolerance zone of 1 mm at the height of 10.05 mm. Am I correct?

 

[pmarc]

In case 1, suppose I remove the basic dimension, and give the tolerance 10 +/- 1 mm, and the datum is referenced, & the actual height value is 10.05 mm, we have a profile tolerance zone of 1 mm at the height of 10.05 mm. Am I correct?

Well, not exactly. If dimension is directly toleranced, say 10 +/- 1 mm, profile tolerance 1 wrt A is free to float within 9 - 11 bandwidth, but it is not centered/located at any value.
If the actual height value (measured from datum plane A) is 10.05 everywhere, that means the actual profile of surface error is 0.