Issues When Using Existing ASME Standards (Y14.3, Y14.4, Y14.5) for Dimensioning and Tolerancing of Digital Drawings
( A White Paper ) Version 1.0, by Alex Krulikowski, 21OC98
The contents of this
paper are solely the opinions of the author. They do not represent any company,
organization, or committee position.
Table of Contents
Abstract
Eliminating 2D-drawings has been a goal in industry for many years. It
is estimated that by the year 2000, 70% of new designs in the U.S. use
a CAD model to communicate a portion of the design requirements. Dimensioning
and tolerancing using digital drawings1 based on solid models2 is one
of the keys to eliminating 2D-drawings. This paper examines the dimensioning
and tolerancing practices described in ASME Y14.5M – 1994 and points out
where they may not be directly applicable to dimensioning and tolerancing
of digital drawings based on solid models. In these cases revisions to
the Y14.5 standard or resolution in other standards (like Y14.41) is needed.
It is not the intention of this paper to indicate where each item should
be addressed, or what the solution should be in each case. The main purpose
of this paper is to bring dimensioning and tolerancing of digital drawing
issues to the surface for discussion and resolution.
Introduction
The dimensioning and tolerancing of digital drawings based solid models
is becoming of increasing interest to firms all around the world. It appears
that it will finally break the chains of having to depend on a 2D-drawing
for communicating product requirements. Industry has been trying to eliminate
2D-drawings ever since the use of CAD systems became widespread. The topic
of dimensioning and tolerancing has been developed over the last 160 years,
into a sophisticated, comprehensive language called GD&T. GD&T
although not perfect, does work well for traditional 2D-drawings with
orthographic views and with some modifications and additions can be used
to tolerance digital drawings based on solid models.
Dimensioning and tolerancing
of digital drawings based on solid models creates a number of interesting
and sometimes problematic situations. If we want to replace 2D-drawings
with digital drawings based on solid models, these issues need to be addressed.
The ASME Y14.5M – 1994 standard is used for dimensioning and tolerancing
2D-drawings in the US and in many other countries around the world. This
standard is the premier standard for dimensioning and tolerancing 2D-drawings.
It also includes a few practices that apply to digital drawings based
on solid models. But to a large extent, the dimensioning and tolerancing
is based on specifying the dimensions and tolerances on orthographic views
of a drawing. Another ASME standard is in development, Y14.41, it will
address many of issues involved with tolerancing of digital drawings based
on solid models.
Currently each company
using the Y14.5 standard has the ability to, and often does, add or customize
the specification of tolerances to suit their local needs. This is legitimized
in the Y14.5 standard by paragraph 1.1.4. When the tolerancing digital
drawings based on solid models becomes popular, the CAD software will
also check the legality of each tolerance specification. This will be
a good news/bad news event. It will be good news because the CAD software
will improve the dimensioning and tolerancing of parts and in some cases
even be tutorial to the designer. Downstream activities like analysis,
manufacturing, and inspection will all benefit from the improved quality
of tolerance specifications. The bad news is the software will not recognize
a lot of the local tolerancing practices used by many companies. In some
cases companies will have great difficulty getting the CAD software to
accept their tolerancing information.
The scope of this
paper is limited to issues on specifying part tolerances and the views
required to accomplish the tolerancing on digital drawings.
Currently there are
a variety of methods used by companies are assigning tolerances to solid
models. In order to analyze the needs of tolerancing digital drawings
based on solid models, a single set of conditions needs to be used. The
list of conditions below is the basis for the comments in this paper:
1. A digital drawing is required.
2. Measured distances of the model are used as “CAD
system specified basic dimensions” and geometric controls are used
to tolerance the CAD system specified basic dimensions.
3. If a toleranced dimension is specified, the part
feature(s) involved are no longer CAD system specified basic to each other.
4. The display of the tolerances must be interpretable
by a human without interrogating the solid model (database).
5. The dimensions and tolerances shown are associative
to the model and can be interpreted for analysis, manufacturing, and inspection.
1Digital drawing
– An electronic file that uses a digital representation of the part geometry
along with dimensions and tolerances and other annotations to communicate
design requirements.
2Solid Model –
In this paper, this term is used as a generic term to refer to part geometry
defined in a digital database. Currently companies use solid models, wireframe
models, and sheet solids.
The issues resulting
from using Y14.5 to dimensioning and tolerance digital drawings based
on solid models are categorized into six areas: CAD system specified basic
dimensions, use of toleranced dimensions, fundamental rules, view dependent
tolerances, datum specification issues, and display issues.
The tables shown below
are a result of an initial comparison between ASME Y14.5 and the practices
used on tolerancing of solid models. Additional items will be found as
attention is brought on to this topic.
A.
Issues when using measured distances of the model as CAD system specified
basic dimensions of the part
Using measured distances of the model as CAD system specified basic dimensions.
This is simply the practice of using the model distances as basic dimensions
to define part features and then using geometric controls to tolerance
the features. Many companies have been using this practice for a number
of years now. CAD system specified basic dimensions can save significant
design time. However, a number of interesting issues arise.
(Note – When human specified basic dimensions are used, the rules and
conventions for display, use and tolerancing are well documented in the
ASME Y14.5 standard.)
Currently, the ASME
Y14.5 standard does not specifically address these conditions. The list
below is a collection of items that need to be documented on how
specify and interpret basic relationships on digital drawings based on
solid models.
Issue
No.
|
Description
|
Y14.5
Ref
|
Issues
|
Comments
|
A1
|
Specifying Basic
Dimensions |
Para 1.1.10 |
If basic dimensions
are automatically specified in the CAD system, respecifying basic
dimensions is not necessary and will add confusion for the drawing
user. The values may be different between the two basic dimensions. |
Should basic
dimensions be allowed to be specified when dimensioning solid models? |
A2
|
User specified
basic dimensions |
Para 1.3.10 |
hen a basic dimension
is shown should it be a reference dimension? |
|
A3
|
Use of a note
to specify basic dimensions |
Fig 6-12 |
Use of a note
to specify basic dimensions Fig 6-12 Y14.5 allows the use of
the note “UNTOLERANCED DIMENSIONS ARE BASIC” to denote basic dimensions.
Can this practice be used on solid models? |
|
A4
|
Use of a general
tolerance note to tolerance basic dimensions |
Para 1.3.9 &
5.2.1.1 |
Currently, basic
dimensions are toleranced by geometric controls. Is it acceptable
to use a general note to apply tolerance to a basic dimension? |
Rules need to
be established on the use of general notes to tolerance basic dimensions. |
A5
|
Use of CAD system
specified basic for size of a feature of size with only the
tolerance shown on the drawing |
Para 1.3.9 &
5.2.1.1 |
Currently, basic
dimensions are toleranced by geometric controls. Is it acceptable
to apply a tolerance to a basic dimension? |
This affects
the application of Rule #1. |
A6
|
Dimensioning
of 90° angles |
|
It is impossible
to see the difference between a 90° CAD system specified angle
and an 89° angle in many cases. Should angles that are near 90
be dimensioned instead of CAD system specified? |
Not dimensioning
the angles would require the user to interrogate the database to determine
each angle. The danger is the drawing user may think an angle is a
right angle and not realize the need to check further. Would a user
have to check every (near 90°) angle to know what value it is? |
A7
|
Use of CAD system
specified basic dimensions for datum target, location, size, and orientation |
|
In solid model
dimensioning the dimensions that describe datum targets are CAD system
specified. Is it desirable or acceptable to re-specify the basic dimensions
as well? |
Also see issues
A1, A2, A3, A4. |
B.
Issues when using toleranced dimensions on a solid model.
The practice of using toleranced dimensions to define part features has
been around even longer that geometric tolerancing. Most of the standards
community would agree on the shortcomings and limited usefulness of toleranced
dimensions. Limiting or prohibiting the use of toleranced dimensions has
been discussed in National and International standards organizations.
Toleranced dimensions, as they are currently defined, present many problems
for use with solid models.
Issue
No. |
Description |
Y14.5
Ref |
Issues |
Comments |
B1
|
Use of toleranced
dimensions with no nominal specified. |
Para 1.3.2.8 |
On toleranced
dimensions where the nominal is not stated, it results in confusion
on the value to model the feature. |
Rules and conventions
need to be established on the use of toleranced dimensions on solid
models. Perhaps all toleranced dimensions on solid models should have
a nominal stated? (Some believe that when using a model, the model
size should be defined as the nominal.) |
B2
|
Use of max, and
min dimensions |
Para 2.5 |
When a max or
min dimension is specified, no nominal exists. The value the model
is made to is not defined clearly. |
Rules and conventions
need to be established on the use of max and min dimensions on solid
models. Perhaps a max dimension could be presented as 2`+0 / -2`? |
B3
|
Use of
a toleranced dimension (which is not a feature of size) with no datum
sequence specified? |
|
A toleranced
dimension has no datum sequence specified, therefore assumptions must
be made on how to interpret the dimension for analysis or measurement. |
If a toleranced
dimension is to be used for analysis or inspection, a datum sequence
is needed. Rules and conventions need to be established for the use
and interpretation of toleranced dimensions on solid models. Perhaps
it is time to limit the use of toleranced dimensions. |
B4
|
Specifying both
a basic and a toleranced dimension to a part feature |
Fig 4-33 |
Y14.5 allows
the use of both a basic and a toleranced dimension shown to a part
surface. With solid models, a CAD system specified basic dimension
already exists to every part surface and when a toleranced dimension
is added it overrides the CAD system specified basic dimension. |
Rules and conventions
need to be established for the use of toleranced dimensions and CAD
system specified basic dimensions |
B5
|
Use of overall
dimensions when CAD system specified basic dimensions are present |
Para 1.7.7
Fig 1-17, Fig 1-18 |
In Y14.5
when an overall dimension is specified, one of the sub-dimensions
must be a reference dimension. With solid models, the sub dimensions
would often be CAD system specified basic dimensions. How can a CAD
system specified basic dimension be designated as a reference dimension? |
Rules and conventions
need to be established for the use of overall toleranced dimensions
and CAD system specified basic dimensions |
B6
|
Use of the word
“true” when dimensioning a feature not in true projection |
Para 1.8.2.3 |
In Y14.5 it is
required to use the word “true” when dimensioning a part feature that
is not in the plane of the paper. In a solid model drawing many of
the dimensions are shown in views where the feature is not in true
projection. This requirement to use “true” may need to be relaxed
for solid models. |
|
B7
|
Rules for spacing
of dimension lines |
Para 1.7.1 |
When dimensioning
a non-orthographic view should the rules for spacing of dimension
lines shown in Y14.5 fig 1-6 be used or should new rules be created? |
|
B8
|
Extension lines |
Para 1.7.2 |
Extension lines
applied to a solid model do not always communicate the design intent,
should new rules for extension lines be written? |
Perhaps extension
planes should be used? |
C.
Fundamental Rules
In paragraph 1.4 of Y14.5 there are 14 fundamental rules. The rules
are written to a large extent to cover 2D drawings. When applying the
rules to solid models, several issues need to be addressed.
Issue
No.
|
Description |
Y14.5
Ref |
Issues |
Comment |
C1
|
Each surface
of the model is at a basic relationship to every other surface
on the model. |
Para 1.4 Fundamental
rule (a) |
Is it necessary
to specify a tolerance for all the basic relationships? Currently,
Y14.5 requires each dimension to have a tolerance. Does each
CAD system specified basic dimension need a tolerance? |
Perhaps each
feature should require a tolerance not each dimension. |
C2
|
CAD system specified
basic dimensions applying at assembly levels. |
Para 1.4 Fundamental
rule (n) |
A measured model
distance as a basic dimension appears at each level the model appears.
Does a CAD system specified basic a dimension apply at all levels
where the model appears automatically? In certain cases the intent
may be to have the CAD system specified basic dimension from the detail
apply at the assembly. How do you invoke certain CAD system specified
basic dimensions and disallow the rest. |
Since a measured
model distance as a basic dimension appears at each level the model
appears, there needs to be additional rules describing the levels
where a CAD system specified basic dimension does apply. |
C3
|
Implied toleranced
90° angles and CAD system specified basic 90° angles. |
Para 1.4 Fundamental
rules (i) and (j) |
Does the implied
90° angles only apply to orthographic views?
Is there a need for 90° implied toleranced angles? |
The rules for
basic and toleranced 90° angles need to be revised. |
D.
View dependent tolerances
When dimensioning and tolerancing digital drawings, non-orthographic views
are often used. The use of non-orthographic views requires additional
documentation of rules and conventions for display of dimensions and tolerances.
(Note – When orthographic views are used the rules and conventions for
display of dimensions and tolerances are shown in the ASME standards.)
Issue
No. |
Description |
Y14.5
Ref |
Issues |
Comments |
D1
|
Use of the “between”
symbol |
Para 3.3.11 |
Which surface
does the profile control apply to, the front or the top? |
Should the use
of the “between” symbol be restricted to orthographic views? |
D2
|
Use of the “Straightness”
symbol, applied to a surface. |
Para 6.4.1.5
and fig 6-6 |
In which direction
does the tolerance zone apply? |
Should the use
of the “Straightness” symbol, applied to a surface, should be restricted
to orthographic views or should ASME define new rules for its use
and interpretation |
D3
|
Use of the “Profile
of a line” symbol |
Para 6.5.3 and
fig 6-18 |
In which direction
does the tolerance zone apply? |
Should the use
of the “profile of a line” symbol should be restricted to orthographic
views or should ASME define new rules for use and interpretation |
D4
|
Use of the “All
around” symbol |
Para 6.5.1 and
fig 6-19 |
In which direction
does the all around symbol apply? |
Should the use
of the “all around” symbol should be restricted to orthographic views
or should ASME define new rules for its use and interpretation? |
D5
|
Use of “Unilateral”
tolerance zones with profile |
Para 6.5.3 and
fig 6-11 |
It is difficult
to determine which surface is toleranced.The “Unilateral” tolerance
zone specification can become obscured when the model is rotated. |
Perhaps a new
modifier or revised rules would eliminate the need for view dependency
or should unilateral profile be limited to orthographic views? |
D6
|
Use of “Unequal
bi-lateral” tolerance zones with profile |
Para 6.5.1 and
para 6.5.3 |
Is a CAD system
specified basic dimension OK for the offset? When the model is rotated,
the specification can become difficult (or incorrect) to interpret. |
Perhaps a new
modifier or revised rules would eliminate the need for view dependency
or should the use of “Unequal bi-lateral” tolerance zones be
restricted to orthographic view |
D7
|
Bi-directional
positional tolerancing rectangular coordinate method |
Para 5.9.1 and
fig 5-41 |
What determines
the direction of the tolerance zones? |
Should the use
bi-directional positional tolerancing be limited to orthographic views
or should ASME define new rules for its use and interpretation? |
D8
|
Dimensioning
to a true profile |
Para 6.5.1 |
A profile control
must be applied to a true profile. In a non-orthographic view a true
profile doesn’t exist. Should we limit the use of profile to an orthographic
view or should ASME define new rules for the use of profile? |
|
D9
|
Limited length
or area indication |
Para 1.7.3 and
fig 1-11 |
The chain line
to could be out of site when the model is rotated. How far away from
the view can the line be shown? |
|
D10
|
Use of a chain
line to specify a partial datum |
Para 4.5.10 and
fig 4-23 |
Does the chain
line apply to the top or side of the part?. How far away from the
view can the line be shown? When the view is rotated, the chain line
may be hidden behind the view. |
|
E.
Datum specification issues
When specifying datums on digital drawings, in many cases, the current
Y14.5 conventions apply. However, there are a few areas where the Y14.5
conventions do not directly apply. In these cases new conventions need
to be established.
Issue
No.
|
Description |
Y14.5
Ref |
Issues |
Comments |
E1
|
Datum feature
symbol location |
|
Can the datum
feature symbol be shown on the edge between two surfaces to designate
one of the surfaces as a datum feature? |
|
E2
|
Datum feature
symbols on extension lines |
|
When the datum
feature symbol is shown on extension lines in a non-orthographic view
it is not clear that the entire surface is the datum feature. |
|
E3
|
Specifying equalizing
datum targets. |
Para 4.6.6
fig 4-38 |
The absence of
a basic dimension denotes the ability for the targets to adjust to
center the part. In solid model dimensioning, datum targets are located
by basic dimensions that are CAD system specified. This creates difficulty
when specifying an equalizing datum target. |
Perhaps a new
symbol for movable datum targets should be considered along with revising
the current convention on the absence of a basic dimension indicating
a movable target. |
E4
|
Dashed leader
lines |
Para 4.6.1 |
As the model
is rotated datum targets that appear on the far side of the part can
move to the near side of the part.can If dashed leader lines are used,
they become incorrect |
Eliminate the
use of dashed leader lines |
E5
|
Datum symbol
termination |
Para 1.7.4 |
When directed
to a surface, should the datum feature symbol triangle be attached
to the surface or should a leader terminating with a dot be used? |
|
F.
Display issues
Dimensioning and tolerancing of digital drawings is complicated by the
use of non-orthographic part views. The trend seems to be to tolerance
a non-orthographic view. Problems begin to occur when the rules for tolerancing
orthographic views are being used on non-orthographic views. The list
below demonstrates a number of issues that need to be resolved when using
Y14.5 to tolerance digital drawings.
Issue
No. |
Description |
Y14.5
Ref |
Issues |
Comments |
F1
|
The display of
cutting planes for sections on a non-orthographic views
|
Y14.3
Para 3.2.1
|
On orthographic
views cutting planes are shown by using a cutting plane line which
represents the edge view of a cutting plane. On non-orthographic views,
a convention for the direction the cutting plane cuts through the
part needs to be established. |
Perhaps a full
plane should be shown instead of the edge view of the cutting plane
as currently used in orthographic views. |
F2
|
Display of a
section cut from a non-orthographic view .
|
|
Can a section
be cut from a non-orthographic view be displayed as an orthographic
view? |
|
F3
|
Direction for
reading notes
|
Para 1.7.5.1,
Y14.4 para 3.7.2
|
The direction
for reading notes is from the bottom of the drawing. Do we apply this
convention to digital drawings or are new conventions to be used? |
|
F4
|
Application of
Dimensions
|
Para 1.7
Y14.4 Para 2.7
|
In orthographic
views there are rules for dimensioning which cover the following.
- Spacing of dimension lines
- Alignment of dimension lines
- Leader lines.
When tolerancing non-orthographic views, these rules may not be applicable. |
Rules are needed
for which dimensions used on non-orthographic views. |
F5
|
Terminating leader
lines
|
Para 1.7.4
|
When a leader
terminates on a surface, is it permissible to use a leader terminating
with an arrow, or must a dot be used? |
Rules are needed
for terminating a leader line on a surface of a non-orthographic view. |
F6
|
Text not in plane
of screen
|
Y14.4
Para 3.7
|
In many CAD systems
the tolerances and notes are placed in the plane of the work coordinate
system. When the model is rotated, the tolerances and notes also rotate.
The tolerances and notes can become unreadable or their meaning can
change. Rules and conventions for the readability and legality of
rotated tolerances need to be established |
|
G.
Conclusions
Y14.5 works well for
2D drawings and a large portion of the concepts can be carried over into
digital drawings. This paper points out many of the areas where revisions
or additions to Y14.5 are needed. If these items are not addressed, companies
will go in various directions and digital drawings will take much longer
to become the primary drawing type used in industry. The items discussed
in this paper should be documented in a national standard or each software
supplier may end up with their own version of how these items are addressed.
Display issues are
the largest problem in moving from tolerancing 2D-drawings to tolerancing
digital drawings. It is here where the decision will be made on
whether tolerancing digital drawings will be a natural step in the evolution
of tolerancing or if a new system of tolerancing is being born. The decision
can be reduced to the following questions. Do we want to dimension and
tolerance solid models, or do we want to make digital drawings using solid
models. Do we want to build on the existing rules used on 2D-drawings
or do we want to create a new dimensioning and tolerancing system, which
is non-human interpretable?
Display
Method
|
Amount
of current ASME standards that could be used.
|
Comments
|
The display is
interpretable by a human without interrogating the database.
|
75-90%
|
- May
require the use of stored model views for tolerancing.
- Easier transition for the workforce
- Can work with electronic data or printout
- More difficult for software manufacturers |
The database
contains a portion of the information needed to interpret the display.
|
20-40%
|
- Porcupine views
or charted tolerances would exist
- Difficult transition for the workforce
- May need to make human interpretable drawings as well
- CAD system dependent until translators are perfected
- Easier for software manufacturers |
I believe that
industry would be best served by a system that would allow as much
of the current language of dimensioning and tolerancing as possible
to be used and to preserve the ability for human interpretation
of digital drawings.
2005
Update
The
ASME Y14.41-2003 Standard
on Digital Product Definition Data Practices
defines the
exceptions and additional requirements to existing ASME standards
for using product definition data or drawings in 3D digital format.
The new standard advances the capabilities of Y14.5, Dimensioning
and Tolerancing, the widely used standard pertaining to 2D engineering
drawings.
The Y14.41 Standard establishes requirements
for preparing, organizing and interpreting 3D digital product images.ETI
has developed a Solid Model
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|