Basics
dimstack.dim
Basic
A measurement is a single measurement of a part.
Parameters:
| Name | Type | Description | Default |
|---|---|---|---|
nom |
float
|
The nominal value of the measurement. Defaults to 0. |
required |
tol |
Union[SymmetricBilateral, UnequalBilateral]
|
The tolerance of the measurement. Defaults to SymmetricBilateral(0). |
required |
a |
float
|
The sensitivity of the measurement. Defaults to 1. If the nominal value is negative, the sensitivity will be multiplied by a -1 and the nominal value will be made positive. |
1
|
name |
str
|
The name of the measurement. Defaults to "Dimension". |
'Dimension'
|
desc |
str
|
The description of the measurement. Defaults to "Dimension". |
'Dimension'
|
abs_lower
property
The minimum value of the measurement. AKA, absolute upper
abs_upper
property
The maximum value of the measurement. AKA, absolute lower
abs_lower_tol
property
The absolute minimum value of the tolerance.
abs_upper_tol
property
The absolute maximum value of the tolerance.
Statistical
Bases: Basic
Statistical
Parameters:
| Name | Type | Description | Default |
|---|---|---|---|
target_process_sigma |
float
|
The standard deviation of the process represented as ±σ. Defaults to ±3σ. |
3
|
k |
float
|
The ratio of the amount the center of the distribution is shifted from the mean represented as a multiple of the process standard deviation. Defaults to 0σ. |
required |
distribution |
str
|
The distribution of the measurement. Defaults to "Normal". |
None
|
mean_eff
property
effective mean
stdev_eff
property
effective standard deviation "6 stdev" is the standard deviation of the distribution
process_sigma_eff
property
calculated sigma (# of eff_stdevs away fromm USL and LSL)
yield_loss_probability
property
Returns the probability of a part being out of spec.
Stack
WC: Basic
property
This is a simple WC calculation. This results in a Bilateral dimension with a tolerance that is the sum of the component tolerances. It states that in any combination of tolerances, you can be sure the result will be within the this resulting tolerance.
RSS: Statistical
property
This is a simple RSS calculation. This is uses the RSS calculation method in the Dimensioning and Tolerancing Handbook, McGraw Hill. It is really only useful for a Bilateral stack of same process-stdev dims. The RSS result has the same uncertainty as the measurements. Historically, Eq. (9.11) assumed that all of the component tolerances (t_i) represent a 3si value for their manufacturing processes. Thus, if all the component distributions are assumed to be normal, then the probability that a dimension is between ±t_i is 99.73%. If this is true, then the assembly gap distribution is normal and the probability that it is ±t_rss between is 99.73%. Although most people have assumed a value of ±3s for piecepart tolerances, the RSS equation works for “equal s” values. If the designer assumed that the input tolerances were ±4s values for the piecepart manufacturing processes, then the probability that the assembly is between ±t_rss is 99.9937 (4s). The 3s process limits using the RSS Model are similar to the Worst Case Model. The minimum gap is equal to the mean value minus the RSS variation at the gap. The maximum gap is equal to the mean value plus the RSS variation at the gap.
See
- Dimensioning and Tolerancing Handbook, McGraw Hill
- http://files.engineering.com/getfile.aspx?folder=69759f43-e81a-4801-9090-a0c95402bfc0&file=RSS_explanation.GIF
MRSS: Statistical
property
Basically RSS with a coefficient modifier to make the tolerance tighter.
Returns:
| Name | Type | Description |
|---|---|---|
Statistical |
Statistical
|
description |
Spec
median
property
median
yield_loss_probability
property
Returns the probability of a part being out of spec.
R
property
Return the yield loss probability in PPM
dimstack.tolerance
SymmetricBilateral
Bilateral tolerancing is a method of specifying a tolerance that is symmetrical about the nominal value. This is the most common type of tolerancing.
UnequalBilateral
Bilateral tolerancing is a method of specifying a tolerance that is asymmetrical about the nominal value. This can also be used for Unilateral tolerancing.
Bilateral(upper, lower=None)
Automatically determine the type of bilateral tolerance to use from the upper and lower inputs.
Parameters:
| Name | Type | Description | Default |
|---|---|---|---|
upper |
float
|
description |
required |
lower |
float
|
description. Defaults to None. |
None
|
Returns:
| Type | Description |
|---|---|
SymmetricBilateral | UnequalBilateral
|
description |
dimstack.dist
Uniform
Uniform distribution.
Parameters:
| Name | Type | Description | Default |
|---|---|---|---|
lower |
float
|
Lower limit. |
required |
upper |
float
|
Upper limit. |
required |
Normal
Normal distribution.
Parameters:
| Name | Type | Description | Default |
|---|---|---|---|
mean |
float
|
Mean. |
required |
stdev |
float
|
Standard deviation. |
required |
NormalScreened
Normal distribution which has been screened. e.g. Go-NoGo or Pass-Fail fixture.
Parameters:
| Name | Type | Description | Default |
|---|---|---|---|
mean |
float
|
Mean. |
required |
stdev |
float
|
Standard deviation. |
required |
lower |
float
|
Lower limit. |
required |
upper |
float
|
Upper limit. |
required |