Minimizing Tolerances - Using tolerance spread smaller than specified

A

atreyu915

#1
I recently joined an organization whose quality function communicate tolerances to their operators tighter than the engineering function has specified for the part.
e.g. Nominal= .045 +/-.020 the run card will be made to read .045 +/-.010

I remember reading about this practice in the past and the disadvantages of such a practice, but I can't remember why this is disadvantageous other than causing the operator to react to a value that need not be reacted to. I need a refresher on this practice.

Thank you
 
Elsmar Forum Sponsor

Stijloor

Staff member
Super Moderator
#2
Re: Minimizing Tolerances

I recently joined an organization whose quality function communicate tolerances to their operators tighter than the engineering function has specified for the part.
e.g. Nominal= .045 +/-.020 the run card will be made to read .045 +/-.010

I remember reading about this practice in the past and the disadvantages of such a practice, but I can't remember why this is disadvantageous other than causing the operator to react to a value that need not be reacted to. I need a refresher on this practice.

Thank you
A silly practice, making products unnecessarily expensive. It may accommodate Material Review Board (MRB) negotiations concerning nonconforming product.... It becomes a game of "give and take" causing more and more confusion on part of the operators.

Very bad practice!

Stijloor.
 

Wes Bucey

Quite Involved in Discussions
#3
I recently joined an organization whose quality function communicate tolerances to their operators tighter than the engineering function has specified for the part.
e.g. Nominal= .045 +/-.020 the run card will be made to read .045 +/-.010

I remember reading about this practice in the past and the disadvantages of such a practice, but I can't remember why this is disadvantageous other than causing the operator to react to a value that need not be reacted to. I need a refresher on this practice.

Thank you
I think you are a victim of misunderstanding semantics here. In effect, what are being communicated to operators are "Control Limits," NOT "tolerances" and certainly not "specification limits." Most well-run organizations try to keep the control limits (outside of which they consider the process is out of control) tighter than the specification limits (design tolerances.) When performing SPC (statistical process control), they can identify "trends" which, if allowed to continue, could result in nonconforming product. Such trends may indicate tool wear or some problem with the machine itself which can be corrected before any nonconforming product is produced. When done consistently, this can be a very efficient process, generating little or no scrap.
 
A

atreyu915

#4
Thank you for your responses.

Allow me to clarify, the limits placed on the run cards by the quality function are not calculated control limits that will be used on an SPC chart. They are simply approximately half of the specification limits. They are not a percentage i.e. 75% as used in pre-control. They are just an off the top of the head number (as far as i've seen) to prevent the operator from allowing the process to approach the LSL/USL.
 
D

David DeLong

#5
I recently joined an organization whose quality function communicate tolerances to their operators tighter than the engineering function has specified for the part.
e.g. Nominal= .045 +/-.020 the run card will be made to read .045 +/-.010

I remember reading about this practice in the past and the disadvantages of such a practice, but I can't remember why this is disadvantageous other than causing the operator to react to a value that need not be reacted to. I need a refresher on this practice.

Thank you
I have also heard of this happening usually using 75% of the customer tolerance. Why would one want to make it more costly to produce the product? The tighter the tolerance, the more difficult and more costly the process becomes.

I would suggest using the customer tolerance as stated and then making sure that the process is capable of meeting this requirement using capability studies.
 
#6
They are just an off the top of the head number (as far as i've seen) to prevent the operator from allowing the process to approach the LSL/USL.
That sounds absurd.

An organization using this practice will be throwing its money away: Among many other things, they may force production to use equipment that is more expensive than necessary and thus could have been used for something more advanced, or throw away perfectly usable parts. The simple fact is that it is pure waste, as the customer cannot be expected to pay for the tighter tolerances: They do not add value.

Using SPC on the other hand, taking the capability of the process in account is (as Wes pointed out) a very good practice.

You made me curious: How has this practice been explained to you?

/Claes
 

Stijloor

Staff member
Super Moderator
#7
<snip>You made me curious: How has this practice been explained to you?

/Claes
When I was a young apprentice, an engineer explained to me how tolerances were calculated before putting them on the print. He told me that the actual allowable deviation (tolerance) for each of the dimensions was cut in half. When I asked him why, he said that if we (the operators) produced dimensions "slightly" oversized or undersized, our MRB had some room to negotiate. :mg:

True story. :(

Later, when folks got wiser, the practice was abandoned, but the MRB negotiations continued.

Two of the standard questions always asked were:
"Who is the Customer?"
"When does it have to ship?"

Sounds familiar?

Stijloor.
 
A

atreyu915

#8
You made me curious: How has this practice been explained to you?

/Claes
It hasn't really been explained to me yet. As I mentioned I just joined this company and this was my first week. It (topic) was one thing I found odd as I was bombarded with typical "first week overview/training"

But in a nutshell...the reasoning for doing this was like i explained earlier, Quality(who creates work instructions based on engineering data) always reduces the spec limits to "keep the operators from approaching the limits, if we narrow the limits then we won't have to worry about material being produced out of spec"

BUT

this was coming from sole technician, i'm not sure how this practice was implemented, if the technician was directed to do so, or just took it upon themselves thinking it might be beneficial. The reason for my original post was to confirm that this wasn't a good practice and gain some insight as to why.
I was confident it wasn't a good practice but wasn't confident that I could present that claim and have any factual information to back it up.
 
A

atreyu915

#10
Absolutely! I am really excited to be a part of this company. They are a very enlightened organization. Although the first week did uncover a few quirks that I think can be improved upon. I'm glad to be that new perspective.
I spent 8.5 years at a company that just recently came to understand that you can inspect quality into a product and that 100% inspection isn't 100% effective.:rolleyes:
I was looking for a major change of culture and vision, and I have found it!

Thank you all for the interest and your informative posts (especially on a weekend)
 
Thread starter Similar threads Forum Replies Date
T Minimizing cost of product certification - int/ext CE certified Powersupply Unit? CE Marking (Conformité Européene) / CB Scheme 3
M SPC (minimizing defects) Vs Poka Yoke (zeroing out defects) Statistical Analysis Tools, Techniques and SPC 3
A Is there a standard on tolerances of substances ? Medical Device and FDA Regulations and Standards News 0
R Tolerances for Analytical Balances General Measurement Device and Calibration Topics 3
Skooma Deburring and Tolerances - Is there a Standard for deburring or removing material? Manufacturing and Related Processes 3
B FAI and ANSI Y14.5 Trailing Zeros and Print Tolerances Manufacturing and Related Processes 3
H Thread Plug Gauge Tolerances General Measurement Device and Calibration Topics 3
W Key functional characteristic definition - Tolerances and Deviations Definitions, Acronyms, Abbreviations and Interpretations Listed Alphabetically 0
M Best Practice for setting tolerances on a Drawing Inspection, Prints (Drawings), Testing, Sampling and Related Topics 8
DietCokeofEvil What is the general consensus on Caliper tolerances? General Measurement Device and Calibration Topics 1
Q M-282 - Engineering Standard for Tolerances - Is it ASTM, DIN? ISO 9000, ISO 9001, and ISO 9004 Quality Management Systems Standards 4
S MSA (Measurement System Analysis) - Process Tolerances Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 9
J Tighter Tolerances than Control Plan FMEA and Control Plans 1
P Applying Combined Tolerances of Two Scales - Balance Tolerance Question General Measurement Device and Calibration Topics 9
G MSA (Measurement System Analysis) and Dimensional Tolerances Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 1
B Changing dimensional tolerances to approve rejected product ISO 9000, ISO 9001, and ISO 9004 Quality Management Systems Standards 2
K Tolerances on Drawing Title Block Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 3
V GD&T Interpreting Datums in Two Single Segmented Position Tolerances Inspection, Prints (Drawings), Testing, Sampling and Related Topics 5
C ZZ Pin Gage Tolerances and Calibration General Measurement Device and Calibration Topics 2
W Thread Micrometer Standard Tolerances General Measurement Device and Calibration Topics 4
D ISO 8062-3 Angle Tolerances - The drawing states ISO 8062-3 DCTG 11 Other ISO and International Standards and European Regulations 1
M Thread Gage Tolerances ?Who?s The Authority?? General Measurement Device and Calibration Topics 9
N Injection Molded Nylon Part Tolerances Manufacturing and Related Processes 6
A Ppk Issue - Some Products have Unilateral Tolerances Capability, Accuracy and Stability - Processes, Machines, etc. 24
H GD&T - How to measure these Positional Tolerances on a CMM Inspection, Prints (Drawings), Testing, Sampling and Related Topics 10
S Improper Use of Tolerances & SPC Practices, or not? Misc. Quality Assurance and Business Systems Related Topics 5
M Vinyl and Cloth Cut Parts Tolerances? (Car Seats) Manufacturing and Related Processes 8
G Geometric Tolerance - Swedish Drawing which states that all tolerances are to NS 1430 Manufacturing and Related Processes 2
optomist1 ASME GD&T Certification Test - Converting Coordinate Tolerances to Profile Tolerances Inspection, Prints (Drawings), Testing, Sampling and Related Topics 8
S S-Type Tolerances: How to calculate Cs? Control Limit(s)? Statistical Analysis Tools, Techniques and SPC 8
S Typical Tools for Particular Tolerances Table? Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 15
M Recording True Positioning (Geometric Tolerances) on an FAI Report Records and Data - Quality, Legal and Other Evidence 15
D Statistical Tolerances in GD&T (Geometric Dimensioning & Tolerancing)? Various Other Specifications, Standards, and related Requirements 31
QMMike Quoted Tolerances Trumping Drawing Tolerances? Contract Review Process 13
D DIN 1683 Casting Tolerances - What does "GTB" mean? Other ISO and International Standards and European Regulations 7
P Gauge Design Tolerances - How to decide tolerances for gauge design? General Measurement Device and Calibration Topics 3
smryan Can Cpk be calculated without tolerances or limits? Capability, Accuracy and Stability - Processes, Machines, etc. 11
D Metric Print from Italy - Tolerances - 38.8mm -0.015/-0.045 Inspection, Prints (Drawings), Testing, Sampling and Related Topics 5
O Sum of Cpk - Updating Product Tolerances Capability, Accuracy and Stability - Processes, Machines, etc. 6
K Basis for Establishing Gage Tolerancing - "Gage Maker's Tolerances" General Measurement Device and Calibration Topics 4
E How to Determine Tolerances for Equipment Verification General Measurement Device and Calibration Topics 18
F Capability Calculation on 1 Population Multiple Tolerances Capability, Accuracy and Stability - Processes, Machines, etc. 5
L UN Class 5 Tolerances - Thread Setting Plug Gage 7/8-9 UNC 5 General Measurement Device and Calibration Topics 23
M No Tolerances given by Customer - Shirring (sewing operation) IATF 16949 - Automotive Quality Systems Standard 5
M DIN6930 What table to use - Tolerances of straight line and radii measurements Inspection, Prints (Drawings), Testing, Sampling and Related Topics 4
D Adjusting Tolerances because of available tooling? Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 9
U Calculating Pp for Unilateral Tolerances Statistical Analysis Tools, Techniques and SPC 17
S Opening Up Tolerances - We have over 6000 live drawings - help please Design and Development of Products and Processes 6
T How to Calculate Process Capability for Unilateral Tolerances? Statistical Analysis Tools, Techniques and SPC 21
R Definition Basic Dimensions - What are basic dimensions? Why don't they have tolerances? Definitions, Acronyms, Abbreviations and Interpretations Listed Alphabetically 14

Similar threads

Top Bottom