×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

No Datum for Form tolerances

No Datum for Form tolerances

No Datum for Form tolerances

(OP)
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

RE: No Datum for Form tolerances

vellingiri,

Quote (From ASME Y14.5-2009):

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

RE: No Datum for Form tolerances

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

RE: No Datum for Form tolerances

(OP)
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

RE: No Datum for Form tolerances

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

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources