Applies to: All levels of Geomet
Last updated:
Tuesday December 08, 2009.
Datum Bonus Tolerance when applied to a complying feature,
allows movement of the origin based on the MMC / LMC value of the reference
feature.
Summary of the Datum Bonus
Tolerance (DBT) Process
Not all features qualify for DBT. The feature must have a
size characteristic such as a circle. Geomet supports DBT only on
2D Circle
features in version 6.66 and below. Circles are a 2D feature that is
projected into a PCS base plane. Therefore, the DBT is constrained to the
same base plane as the circle.
DBT is an automatic calculation in Geomet. By pressing the
Origin key <l> after measuring and applying tolerance to the circle, Geomet will build the DBT value and
flag the projected PCS base plane if the circle does not have a
tolerance applied on its size, then DBT will not be available.
Review of DBT in action
DBT can be best understood in the assembly environment.
Using the example of two bores that have two posts inserted that will be
inserted can be used to explain the benefits of DBT.
The figure below illustrates the origin which is not
taking advantage of DBT as it was designated RFS, or Regardless of Feature
Size.
The assembly of these two components results in a
interference caused by the normal manufacturing process that resulted of
both the mating bores, and even the posts, not at the expected normal
position values.
The axis of the Origin is defined as the XY location of
the axis of the bore. Even if the mating post is undersized, but in the
tolerance zone, it is not allowed to move for a best fit consideration.
In the figure below, the Origin is tagged as encoded with
DBT and has the designator
in it's Datum
Frame Reference. The attachment of the MMC symbol on the Origin label instructs the tolerance
tool to add in the DBT value when determining the final tolerance zone.
As you can see, the post that fits into the bore marked as
the Origin can move to its MMC limit allowing the mating post to fit within
the bore for a proper assembly.
Important Reminder
Geomet calculates the available DBT value only when there
the origin feature, the circle, has its size tolerance applied to it.
Applying DBT in the Tolerance Tool
Applying
DBT is a simple process of placing a check in the Datum Bonus Tolerance
group as shown in the figure at right.
The Total Tolerance Zone will update with the new total
zone calculated.
