Design Margin -
Statistical Tolerance Part 1
for use with DFSS
- Design for Six Sigma
You cannot expect to get to six sigma, (for
detail information why, see our six
sigma page), on if your specification limit and/or
manufacturing process was based around a 3-sigma design
margin. If you manufacturing equipment has a tolerance of 3
sigma, there isn’t a whole lot you can do about it except
change your calculations by doing estimates, or change the
specifications. The latter is the only real way to get to a
six sigma design margin.
You need to know what your actually process
is capable of. Then compare that to this table
as a first step. You do this by using control
charts that look at X
and a distribution
of how your process
If we were to describe the
meaning of +/-6 Sigma and Processes, we might describe it as
the most wanted attributes that can be used 99.9997%, (3.4 PPM),
of the time in an end product and any process that will not
fabricate any damage to the end product in excess of 3.4 PPM.
Note the saying “as the most wanted attributes”. You do
not and probably cannot get 3.4 PPM on every parameter
for every piece part, sub-component or assembly that you
manufacture it depends on the parameter
design. Trying maybe just a big waste of time and could
directly contribute to many company’s failures, (sometimes
caused by external
failures), to introduce a six sigma program.
The product design can and does contribute to
product failures. The design of the product, to include the
supplier’s designs, is the major determinant of a persistent
which must be discovered and corrected. Lack of operating
margin, (design margin), or lack of an adequate amount
of overstress tests
tends to appear as a continuous failure rate in the long term.
Reduction of defects
levels may be accomplished through these steps.
- Ascertain the product characteristics that are
crucial to satisfying both physical and functional
requirements of the consumer, in other words, the customer
- Ascertain the exact product components that
directly contribute to accomplishing these crucial
- For each ascertained product component
establish the process step(s) or process choices that
affect or control the required characteristics.
- Ascertain the upper and lower allowable
tolerance, (real), for each of the product(s)
characteristics and process(s) steps, which will insure
the absolute best performance.
- Ascertain the product capability part(s) and
processes components that will control the absolute best
- If Cpk,
(process capability), is not equal to or
greater than 2, then make the necessary adjustments to the
product and/or process to ascertain it.
Some points to consider when designing
a new product or processes and retrofitting a legacy
product or processes to aid with the above steps.
- Use the fewest number of piece parts,
sub-components or assemblies. Mathematically, the fewer
points than can go wrong will go wrong.
- Use parts and/or designs with a know
competence that have been proven.
- Create designs for maximum intolerance of
- Use the lowest possible stress levels. Do not
design for any piece parts, sub-components or assemblies
to operate at the edge of their envelope.
- Know and supply maximum likely operating
When you design a process and product to be
manufactured from the beginning using the above guidelines,
you can improve reliability due to…
- Lack of early life failures determines the
customer’s perception of the product and the company.
- Latent defects are reduced, thus reducing
improving first time yields. Latent defects are defects
that are not easily found by normal test and inspection
- No test or screen can find all defects. If
there are none to be found, it is much more desirable.
Only integrated designs for manufacturability and tight
process control can eradicate latent defects so there are
none to be found.
If you have piece parts, sub-components or
assemblies that need
repair after the product has started the manufacturing process,
the cost goes up and the latent defect rate also rises because
of the need to handle the product more. This can cause
additional manufacturing errors. Solder shorts, product may be
dropped in the repair process causing defects that have
already been test or inspected for. It also forces you
maintain a larger inventory, thus, inventory turns are larger.
An inventory turn is the amount of time it takes for a
facility to turn it’s entire stock of inventory and be
replaced. Example, a turn of 1.25 could mean 288 turn days, a
turn of 40 could mean 9 turn days. Maintaining a large
inventory can get rather costly in terms of dollars that are
not accessible. Smaller inventories give you those dollars
back for operating capital, etc.
A lot of manufactured products these days are
non-repairable. This means that if you do not get it right the
first, you have all scrap. But, it’s not just the cost of
the scrap, you also have labor involved in producing the
non-useable material. And of course the more points of test or
inspection the product has gone through before it is detected,
if it is detected, adds exponentially to all costs.
While there are formulas to calculate all these
items, it is beyond the scope of this document to provide
In summary, defects cost lots of dollars due
- The need for additional analytical time
- The need for additional repair time
- The need for additional retest time
- The need for additional re-inspection time
- The need for additional inventory
- The need for additional labor and associated
- The need for additional materials
- The need for additional capacity (production
equipment / floor space)
- The need for additional support
- The need for additional warranty repairs
- The need for additional replacement of
Defects and product design margins have a major
role in early
life failures, cycle
time, inventory and resources and these are key components
in competitive issues, which can improve your companies
Our Six Sigma software ZeroRejects
can help you in many of these areas. Because our histogram
chart not only shows you the mean
and +/- 3 sigma
of the process you are measuring, but also the nominal and +/-
3 sigma of your specification, arranged in units of sigma.
This allows you to easily reckon what you specification should
be to meet any desired design margin.
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