herman miller
Document Name: Dimensional Verification Requirements
Document Number: PD-ENG-008, Work Instructions, Revision 4, November 19, 2009
1. Purpose
1.1 The intent for quality of product at Herman Miller is zero defects. Recognizing that it is not practical to check every feature on every part for all possible defects, the Tool Potential Characteristic Symbol, the Process Potential Characteristic Symbol and the SPC Characteristic Symbol are to be used to communicate the necessary proactive and ongoing dimensional verification requirements.
2. Scope
2.1 This specification covers the definition of the Tool Potential Characteristic, the Tool Potential Characteristic symbol, the definition of the Process Potential Characteristic, the Process Potential Characteristic symbol, the Ppk calculation, the definition of Statistical Process Control (SPC) characteristic, the Cpk calculation and the SPC characteristic symbol on Herman Miller engineering specification drawings.
3. Definition of Process Potential Characteristics
3.1 Tool Potential Characteristics and Process Potential Characteristics are those dimensions and/or specifications on the engineering specification drawing that are designated to control any of the following:
· the mating of parts in the assembly process
· the function of the part
· the aesthetic value of the finished product (gap, flush, etc.)
Note: the Tool Potential Characteristic symbol or the Process Potential Characteristic symbol are not intended for use in conjunction with finish (i.e., gloss, color . . .) requirements.
3.2 Dimensions/specifications are selected based on a need for tooling/machinery approval data.
4. Definition of the Tool Potential and Process Potential Characteristic Symbols
4.1 Tool Potential Characteristics and Process Potential Characteristics require a short-term machine/tool study to evaluate the potential of the particular machine or tools to produce parts within engineering specifications and to project their ability to assist in the achievement of long-term process capability (Ppk). Tool potential studies and Process potential studies should reflect only the variability coming from the specific machine/tool combination and should not include variability from other sources such as operators, variation in material lots, etc. The evaluation methods and requirements are described below:
4.1.1 The Tool Potential Characteristic symbol, , when used on the engineering specification drawing indicates that the dimension/specification which it precedes has been identified as a Tool Potential Characteristic, requiring a tool capability study conducted on 30 units taken consecutively from a production run using production tooling and processes with no adjustments made during the sampling period. A Ppk of 1.67 or greater is required. The measurement of the identified Tool Potential Characteristic may be made with any device appropriate to ANSI/ASME Y14.5. These measurements will result in variable data The Herman Miller Ppkformulas.xls spreadsheet should be used for the recording and processing of the data for tool potential studies.
4.1.2 The Process Potential Characteristic symbol, , when used on the engineering specification drawing indicates that the dimension/specification which it precedes has been identified as a Process Potential Characteristic, requiring a process potential study conducted on a sample of at least 50 units taken randomly from a production run of 300 units using production tooling and processes. A Ppk of 1.33 or greater is required. All the requirements of 4.1.1 must be met prior to initiating the 300unit run. The measurement of the identified Process Potential Characteristic may be made with any device appropriate to ANSI/ASME Y14.5. These measurements will result in variable data. The Process Potential Characteristic symbol is used for high volume parts where a characteristic that affects fit or function is considered to be at significant risk due to process variation. Due to run quantity and study sample quantity, the cost and time associated with process potential studies will be greater than tool potential studies. It is important to properly align process potential study investment toward business risk. Participation of appropriate roles, while creating engineering drawing specifications, is one method of assuring proper alignment. See PD-PPD-REF-09, Engineering Drawings Procedure for more information. The Herman Miller Ppkformulas50.xls spreadsheet should be used for the recording and processing of the data for process potential studies.
4.2 Some prints may include Control Characteristic symbols () with a number 1, number 2, or no number in the circular portion of the symbol.
If no number appears inside the Control Characteristic symbol (example: ), measurement of the identified Control Characteristic may be made with any device appropriate to ANSI/ASME Y14.5M. These measurements will result in variable data.
If the number “1” appears inside the symbol (example: ), measurement of the identified Control Characteristic must be made with a Herman Miller approved dedicated gage. These measurements will result in variable data.
If the number “2” appears inside the symbol (example: ), the measurement of the identified Control Characteristic must be made while the part is staged in a Herman Miller approved holding fixture. These measurements will result in variable data.
4.3 Requirements for the Control Characteristic symbol differ from those of the Tool Potential Characteristic symbol or the Process Potential Characteristic symbol. Control Characteristics require a tool capability study conducted on 30 units taken consecutively from a production run using production tooling and processes with no adjustments made during the sampling period. A Cpk of 1.33 or greater is required for features identified as Control Characteristics. The formula used is currently called the Ppk formula, see Section 5.5 .The Herman Miller Cpkformulas.xls spreadsheet should be used for the recording and processing of the data.
4.4 Prints released prior to February 28, 1995, may include Control Characteristic symbols () with a number between 1 and 34 in the circular portion. This number corresponded with a measuring device as specified in previous revisions of this document. The listing of numbers and corresponding measurement devices follows, and is included for reference only.
1 - Dedicated Gage—Approved by Herman Miller
2 - Bevel Protractor
3 - Bore Gage
4 - Calipers—Dial, Vernier, Digital
5 - Coating Thickness Gage
6 - Coordinate Measuring Machine (CMM)
7 - Depth Gage
8 - Dial Indicator
9 - Gage Blocks
10 - Gloss Card
11 - 60° Gloss Meter
12 - Hardness Tester
13 - Height Gage—Dial, Vernier, Digital
14 - Micrometer
15 - Microscope (Toolmakers)
16 - Munsell Card
17 - Optical Comparitor
18 - Pin Gage
19 - Plug Gage
20 - Profilometer
21 - Radius Gage
22 - Ring Gage
23 - Screw Pitch Gage
24 - Snap Gage
25 - Spring Caliper
26 - Spring Divider
27 - Steel Rule
28 - Surface Roughness Scales and Specimens
29 - Tape Measure
30 - Taper Gage
31 - Telescoping Gage
32 - Thickness Gage
33 - Thread Plug Gage
34 - Thread Ring Gage
5. Requirements for Process Potential Characteristics (Tooling/Machinery Capability)
5.1 At minimum, the following information must be supplied for each Process Potential Characteristic studied:
· the nominal specification off the drawing
· the actual measurement for each piece sampled
· the mean ( )
· the standard deviation (s)
· the projected range ( ± 3s)
· the tool capability index (including Ppk and Pp)
See 5.5 for information on calculating standard deviation and tooling capability indices of Ppk and Pp.
5.2 The Herman Miller provided Excel spreadsheet Ppkformulas.xls, or Ppkformulas50.xls, should be used for the calculation of Ppk.
Ppkformulas.xls users should proceed by first accessing the Ppkformulas.xls tab titled “Instructions.”
Note: Ppkformulas.xls is for characteristics; Ppkformulas50.xls is for characteristics.
5.2.1 Exceptions to using the Ppkformulas.xls are intended in these instances:
· Where the study source does not have the necessary Excel software. They can fill in all the data/information required on a hard copy of the Ppkformulas.xls.
· Where the study source has the ability to mathematically calculate elliptical representation of data for the purpose of examining compliance to cylindrical tolerances. These particular analyses will supersede the Herman Miller cylindrical tolerance analysis method. Data from the CMM print out using the elliptical analysis technique should accompany the Ppkformulas.xls submission.
· Where the HMI quality engineer authorizes, cylindrical tolerance Ppk may be calculated in the X and Y axis independently.
· Where the distribution of the data is non-normal, alternative methods of calculating Ppk may be used, with authorization from the HMI quality engineer.
5.3 These requirements may be waived upon approval of Herman Miller engineering when prior tool/machinery capability has been established.
5.4 When approved by Herman Miller engineering, the capability study requirement for parts with minimal differences, such as length dimensions only, may be limited to only the parts reflecting the extremes, such as largest/smallest.
5.5 Calculations for Tool Potential and Process Potential capability indices (Ppk):
Standard Deviation Calculation:
where:
s = Standard deviation for the study
å = Sum
xi = Individual measurements of the study
= Mean value of the study
n = Total number of measurements taken
Calculating Ppk and Pp:
· For equally distributed plus/minus tolerances or for geometric tolerances that are equally bilateral:
Pp Index = Tolerance
6s
Ppk Index = Lower Figure of Ppu or Ppl
where:
· For geometric tolerances that are total wide (i.e., perpendicular within .030) or for unilateral tolerances:
Record the nominal specification off the drawing, the actual measurement for each piece sampled, the mean, and the standard deviation. Calculate the Pp and Ppk as indicated:
Note: In these instances, the LSL = 0. The design target is also 0.
· For geometric tolerances that are cylindrical zones:
Record the nominal specification off the drawing, the actual measurement data, mean, standard deviation, 3s, Sx, mean + 3s, mean - 3s, and Sxi2. Using graphical representation a Pp and Ppk can be obtained and must be recorded.
Refer to Application Example: Graphical and Mathematical Explanation for information on calculating Pp and Ppk relative to a cylindrical tolerance zone.
Note: In an effort to assure assembly, this method is to be used when MMC is invoked: The Ø positional tolerance allowed is determined by taking the mean - 3s value (for internal features) or the mean + 3s value (for external features), calculating the departure of that value from MMC (bonus tolerance) and adding the bonus tolerance to the Ø positional tolerance allowed at MMC (listed in feature control frame). See Application Example: Graphical and Mathematical Explanation.
For Ø positional tolerances invoked at RFS, the positional tolerance listed in the feature control frame is graphed. (No bonus tolerance is calculated.) All other calculations listed in the Applications Example apply.
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herman miller
Document Name: Dimensional Verification Requirements
Document Number: PD-ENG-008, Work Instructions, Revision 4, November 19, 2009
Application Example: Graphical and Mathematical Explanation
8 8
herman miller
Document Name: Dimensional Verification Requirements
Document Number: PD-ENG-008, Work Instructions, Revision 4, November 19, 2009
Application Example: Graphical and Mathematical Explanation (cont.)
Actual Measurement Data for Hole #1 of Application example
Print Nominal / 7.450 (x) / .415 (y) / Ø.188Actual measurement / 1. / 7.4584 / .4110 / .1937
data (30 piece study) / 2. / 7.4550 / .4128 / .1950
3. / 7.4587 / .4129 / .1931
4. / 7.4578 / .4116 / .1945
5. / 7.4572 / .4123 / .1945
6. / 7.4596 / .4112 / .1936
7. / 7.4561 / .4115 / .1963
8. / 7.4563 / .4122 / .1932
9. / 7.4570 / .4121 / .1946
10. / 7.4564 / .4137 / .1936
11. / 7.4559 / .4098 / .1929
12. / 7.4555 / .4135 / .1927
13. / 7.4571 / .4117 / .1931
14. / 7.4548 / .4112 / .1926
15. / 7.4555 / .4122 / .1942
16. / 7.4596 / .4139 / .1936
17. / 7.4557 / .4116 / .1948
18. / 7.4597 / .4111 / .1938
19. / 7.4562 / .4110 / .1955
20. / 7.4567 / .4120 / .1930
21. / 7.4600 / .4124 / .1949
22. / 7.4575 / .4124 / .1942
23. / 7.4556 / .4133 / .1935
24. / 7.4594 / .4113 / .1957
25. / 7.4560 / .4127 / .1926
26. / 7.4565 / .4145 / .1926
27. / 7.4568 / .4129 / .1941
28. / 7.4575 / .4117 / .1942
29. / 7.4573 / .4116 / .1940
30. / 7.4560 / .4113 / .1933
Study sample mean / 7.4571 / .4121 / .1939
Standard deviation / .0015 / .001 / .001
3s / .0045 / .003 / .003
Sx / 223.712 / 12.363 / 5.817
mean + 3s / 7.4616 / .4151 / .1969
mean - 3s / 7.4526 / .4091 / .1909
Sxi2 / 1668.2324 / 5.095 / 1.128
Application Example: Graphical and Mathematical Explanation (cont.):
Calculation for Allowable Positional Tolerance
Mean - 3s =MMC size = / Ø.191
Ø.178 / Mean - 3s / Positional Tolerance Allowed
difference = / Ø.013 / Ø.178 MMC / Ø.015
Ø.189 / Ø.016
Ø.180 / Ø.017
Tolerance at MMC / Ø.015 / Ø.181 / Ø.018
+ difference / Ø.013 / Ø.182 / Ø.019
= allowable positional tolerance / Ø.028 / Ø.183 / Ø.020
Ø.184 / Ø.021
Ø.185 / Ø.022
Ø.186 / Ø.023
Ø.187 / Ø.024
Ø.188 / Ø.025
Ø.189 / Ø.026
Ø.190 / Ø.027
Ø.191 / Ø.028
Ø.192 / Ø.029
Ø.193 / Ø.030
Ø.194 / Ø.031
Ø.195 / Ø.032
Ø.196 / Ø.033
Ø.197 / Ø.034
Ø.198 LMC / Ø.035
Application Example: Graphical and Mathematical Explanation (cont.):
Application Example: Graphical and Mathematical Explanation (cont.)
Plotting the Location of the Mean Position of the Sample Data
/ X / Y / R1 = √(.007)2 + (.003)2Print Nominal / 7.450 / .415 / R1 = .0076
Sample Mean / 7.457 / .412
difference / .007 / -.003