No Datum for Form tolerances

[vellingiri]

Hi All,

Form tolerances does not require Datum refrences. but i have a doubt in this as follows.

When we measure the Form tolerance (either Flatness, Straightness etc.) obviously some part feature must be in contact with the Surface table or plate. In this case if the mating part feature with the surface plate has some irregularity right ?

How can we ensure the measured Form toleracne is perfect ?

Please clarify my understanding...

Thanks in advance.

Vellingiri
Coimbatore,India

 

[pmarc]

vellingiri,
Take a look at following link. This might help to clear your doubts.

http://tec-ease.com/gdt-tips-view.php?q=214

 

[drawoh]

vellingiri,

Flatness is the condition of a surface or derived median plane having all elements in one plane. A flatness tolerance specifies a tolerance zoone within which the surface or derived median plane must lie. ...

Flatness is a property of your surface. The datums have no effect on it. If you want to use a datum, you specify a profile tolerance.

JHG

 

[Belanger]

In your post you stated that "When we measure the Form tolerance (either Flatness, Straightness etc.) obviously some part feature must be in contact with the Surface table or plate."

There's your answer! Those points that contact the surface plate instantly form the "datum" -- a perfect plane (thus a minimum of 3 points must contact the plate) which serves as a reference for measurement. Every other point on the surface must be within a specified tolerance from that imaginary plane.

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

 

[vellingiri]

Hi John-Paul Belanger,

Thanks...

In your answer,

Those points that contact the surface plate instantly form the "datum" -- a perfect plane (thus a minimum of 3 points must contact the plate) which serves as a reference for measurement.

If the contact part feature (Perfect plane as per your answer) has more irregularities ? (say around 1 mm)
How can we measure a flatness for other feature ? (whuch is stringent requirement of say 0.2 mm) ?


Vellingiri
Coimbatore,India

 

[Belanger]

Not sure what you're asking...
The "datum" is the perfect plane formed by the 3 highest points of the surface being inspected. The surface plate may not be perfect (nothing is perfect) but that is just something we have to live with; or it could be certified to a certain flatness and then take that into account.

If you're asking about the mating part in an assembly (not the surface plate) then each part should be checked for flatness on its own. Then a simple stack-up calculation will determine the "worst case" when the two parts are assembled. The minimum gap between those 2 surfaces will be zero, and the maximum gap will be the sum of the two flatness errors.


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