FINANCIAL BENEFITS
for Integrating Standardized Tolerancing Methodologies
Dr. Greg Hetland, June 1995
Even though dimensioning and tolerancing standards have been used
throughout the world for many years, much of the doubt/hesitation to
fully integrate these standards has been focused around the benefit of
utilizing one practice over another for standardization of tolerancing
methodologies. The primary difference is between the older linear
dimensioning practices and the newer geometric dimensioning and
tolerancing practices.
Paradigms
Worldwide standardization of dimensioning and tolerancing practices
has been hindered by paradigms that have been and continue to be
difficult to overcome. A few of the more common paradigms continually
mentioned by students, as well as general management, follow:
|
Older prints are easier to understand |
|
Older prints define design intent better |
|
Older prints contain all the required information |
|
GD&T is only required for critical features |
|
GD&T increases the overall cost |
|
GD&T is hard to understand |
|
Profile is hard to use |
|
Profile cannot be measured |
Current Problem
The day-to-day problem is that engineering drawings do not clearly reflect
design intent and significant differences exist between
drawing packages, which impact product standardization, as well as
individual drawing and feature interpretation due to multiple methods of
attempting to specify the same thing in different ways. Design intent is
considered a set of technical documentation defining the physical and
functional characteristics of an item. Physical Characteristics are
the geometric parameters of a configured item such as length, width,
thickness, etc., while Functional Characteristics include all
performance parameters such as range, speed, lethality, reliability,
maintainability and safety.
Industry has been faced with the primary burden of training their
employees in the engineering language of dimensioning and tolerancing
principles. In most cases, universities, technical colleges, trade
schools, etc., do not have programs built into their technical programs
to teach students the fundamental principles associated with this
international engineering language on dimensioning and tolerancing. Even
fewer teach advanced principles dealing with applications and analysis.
Additionally, seminars have existed for years, directed by consultants
with tremendous differences in technical depth of knowledge on this
subject. Based on these differences, students leave these one to five-day seminars and are expected by their employers to have the fundamental
knowledge to work proficiently on a day-to-day basis. If it were only
this easy, the problem would not exist today.
Knowledge by Job Function
Design Engineers must not only be
knowledgeable of the fundamental principles of geometric dimensioning
and tolerancing, but also be proficient in advanced applications and
analysis. These advanced capabilities are needed in order to apply the
proper and optimum symbology to the design parameter and also to apply
the correct (as well as optimum) tolerance that clearly reflects design
function.
Manufacturing Engineers also need the
fundamental knowledge to understand clearly what the designer is trying
to say by the engineering drawing. To be efficient, they also must have
a high level of competency in applications and analysis to establish
process controls that will ensure conformance to engineering
requirements, but do it at the least amount of cost. One must keep in
mind, the engineering drawing should not state how to manufacture or to
inspect the product, but should only state, in a clear engineering
language, what it should look like when it is complete.
Metrology Engineers, like the
Manufacturing Engineers, need the same broad base of knowledge, except in
this case, the Metrology Engineers need to have a clear understanding of
how to approach the dimensional metrology aspects. This is necessary to
ensure the product produced in fact conforms to the defined
requirements, or more to fact, must understand the true magnitude of
product and feature variation.
The little time devoted to teaching geometric dimensioning and
tolerancing in educational institutions makes all disciplines in the
mechanical arena today have a higher negative impact to the financial
loss within companies than is necessary. It is also the single most
needed focus area for all disciplines for reaching significant gains in
today's sub-micrometer mechanical industries.
Solution
Each organization needs a plan to implement sound product design,
drafting, manufacturing and dimensional metrology practices to ensure
optimum designs are defined clearly and consistently on the engineering
drawing. These organizations also must have the measurement capabilities
to ensure conformance to the engineering requirements. This paper states,
without hesitation, that compliance to GD&T (per the ASME Y14.5M-1994
Standard) does not mean dimensions and tolerances cannot be linear. It
simply means they need to be clear and not ambiguous.
Based on the extremely tight tolerances inherent in many drawings
that exist today, without the use of geometric symbology, the
utilization of feature and datum modifiers, and a very clear
interpretation by everyone involved, industry has very little hope of
receiving a significant return on the investment made to date or that
which is needed for future progress and Return on Investment. This
success includes the ability to establish measures that will ensure
conformance to the engineering requirements defined on the engineering
drawing.
Drawings have improved greatly in regard to reflecting design
intent, and they continue to improve based on standardization efforts
that continue to be a focus item for many design groups, as well as
increased understanding of the Y14.5 Standard. The magnitude of
standardization activity will have a tremendous impact on reducing
design costs, as well as manufacturing, inspection, and overall
procurement-related costs.
High Costs Associated to Inadequate Drawings
The cost associated to inadequate drawings is high in terms of time
lost in product ramp-up and vendor ramp-up, in-house re-engineering
time, material rework, vendor surveillance, production downtime, wasted
raw material and purchasing red tape. The following example/case
compares the typical procurement cycle resulting from a flawed drawing
with one expected from a well-researched GD&T drawing.
Case Scenario: A part is designed by G&K Industries (G&K) and will be
machined by their vendor, DLS Machine Tool (DLS) (both are fictitious
company names). Assume the following costs:
|
G&K engineering time |
$80/hour |
|
G&K purchasing time |
$50/hour |
|
DLS billing rate |
$70/hour |
|
Raw material cost |
$100/part |
Case 1: To start with, G&K Engineer A, untrained in GD&T, takes a
week to produce an engineering drawing for a critical housing. The
process consumes 20 hours at a burdened cost of $1,600 and results in a
flawed drawing. In a separate effort, G&K Engineer B, trained in GD&T,
takes two weeks to produce an adequate drawing for an almost identical
part in 37 hours, which includes a thorough tolerance analysis plus
conferences and final checks with manufacturing and quality engineering.
The total burdened cost is $3,400.
The job is to let DLS produce a first run of five parts using the
inadequate drawing, resulting in a charge of $2,300. The work is
accomplished in one week, and G&K spends $300 inspecting the lot only to
find it discrepant and unfit for rework. As a result, a meeting is
called with DLS to discuss and to resolve the problem, which G&K finally
admits was due to flaws in the drawing. The combined cost of the meeting
and of making the required changes to the drawing is $1,820, and an
order for an additional five parts is placed with DLS. The meeting and
the decision process consumed one week. As a result of the drawing
changes, the NC program and work holding fixture had to be changed,
leading to a total cost of $2,090 for the second run of five parts,
which consumed one week.
Providing grounds for high hopes, the second group of five parts passes
G&K inspection at a cost of $300 and is released to pre-production,
where they are discovered to be borderline nonfunctional as a result of
additional drawing shortcomings. Fortunately, the parts can be reworked
by DLS at a cost of $980, but at an additional in-house expense for
meetings, engineering time and production delays totaling $4,260. The
additional work consumes three weeks, and as a result of these
experiences and efforts, the inadequacies of the original drawing are
completely remedied. In review, the cost to produce the first five
usable parts, starting with the flawed drawing, was $13,650 (the ramp-up
time was seven weeks).
Case 2: The experiences are much different with the part designed by
G&K Engineer B. The unflawed drawing permits DLS to produce the first
run of five parts to specification in just one week at a total cost to
G&K of $6,040!
The ramp-up time was only three weeks. As illustrated in the
following table, there is a substantial cost difference between
investing the time and energy to create a thoroughly researched and
geometrically correct drawing, versus relying on a cycle of
experimental machining, MRBs (material review board) and ECOs
(engineering change orders) to achieve the same goal. Failing to put
forth the effort at the beginning more than doubled the cost and ramp-up
time:
Cost Comparison Summary
|
Cost Related to Drawings
|
G&K Engineer A
Ambiguous
Drawings
|
G&K Engineer B
Non-Ambiguous
Drawings
|
|
1,600
|
3,440
|
|
2,300
|
2,300
|
First run
inspection cost
|
300
|
300
|
First
re-engineering cost
|
1,820
|
0
|
|
2,090
|
0
|
Second run
inspection cost
|
300
|
0
|
Second run
rework charges
|
980
|
0
|
Second run
extraordinary in-house cost
|
3,060
|
0
|
Second run
re-engineering cost
|
1,200
|
0
|
Total cost
investment to produce
|
13,650*
|
6,040*
|
|
|
|
|
1 Week |
2 Weeks |
|
6 Weeks |
1 Week |
Total required
time frame to produce
|
7 Weeks* |
3 Weeks* |
*Cost and time overrun is $7610 and 4 weeks
|
The above example reflects greater than a 50% decrease in overall
cost and lead time due to utilizing an engineering language that clearly reflected the design intent and could easily be understood.
The following are a few of the key areas that recognized cost savings
and reduced overall cost in this analysis:
|
Less engineering time |
|
Less material rework |
|
Reduced product deviations |
|
Fewer meetings |
|
Less supplier time (confidence/communication) |
|
Fewer CRs/ADRs/DMRs |
|
Less inspection time |
|
Reduced lead time |
|
Reduced tool maintenance |
Five-Year Projection
Based on a survey of 20 technical buyers and procurement engineers
within one organization, the above example is reflective of roughly 50%
of the designs and builds for tooling. The remaining 50% varies to a
lesser degree depending on the supplier and the length of time they have
been doing business together. Current dollars spent by this organization
for tooling alone was in the range of $10 million, with
projections for increase to be greater than 20% each year. Note:
tooling dollars reflect roughly 3.5% of gross sales for this
organization. Based on the above noted percentages, the following matrix
outlines approximated potential savings over a five-year period:
Five-Year Projection
|
Year
|
Dollars
(in Millions)
Spent on Tooling
|
Projected
Savings Based on 50%
|
1994 |
$10.0 |
$5.0 |
1995 |
$12.0 |
$6.0 |
1996 |
$14.4 |
$7.2 |
1997 |
$17.3 |
$8.7 |
1998 |
$20.7 |
$10.4 |
Total for 5 years |
$74.4 |
$37.3 |
Over a five-year period, if no changes were made to the technical
competencies and proficiencies of the organization, roughly $37.3 million
would be unnecessarily spent. If this organization recognized
only 25% of the projected savings (which was 50% of the total dollars
spent), they still would recognize a savings of $9.3 million.
Most would agree this savings would be significant enough to get one's
attention.
These are factual projections that did, in fact, get this company's
attention and based on this, a tremendous project was initiated to
integrate into the organization standardized methods for dimensioning
and tolerancing.
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