Minimizing Tolerances - Using tolerance spread smaller than specified

Jim Wynne

Staff member
Admin
#11
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.
The OP is not referring to control limits, which are calculated based on the distribution of measurement data. He's referring to arbitrarily reducing the tolerance spread in an attempt (perhaps unwittingly) to avoid invoking statistical control and understanding the variation in the process.

It's something akin to guardbanding, which is the practice of adjusting spec limits to account for measurement uncertainty. There's more about that in these threads:
Guardbanding - How would one go about guardbanding a specification?
Calibration Uncertainty Philosophy - Reality vs. Basic Theory
 
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Jim Wynne

Staff member
Admin
#12
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
While I'm not defending the practice the OP is questioning, I think there are some false assumptions being made in the responses to him. First, the practice of process control traditionally assumes that we will try to get the (statistical) control limits as far inside the spec limits as is economically reasonable, and that the ideal center of the distribution is equidistant from bilateral spec limits. What the OP's bosses are doing is roughly the same thing, but without any statistical evidence to support it.

Also, there's nothing in evidence that says that parts that fall outside the tightened spec limits are thrown out or reworked. If they are I'd be very surprised. The OP's bosses are saying in essence that they know (or fear) that their processes will produce out-of-tolerance output, so they adjust the production tolerances so that output will remain within customer tolerances rather than invoking reasonable methods of process control.
 
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#13
What the OP's bosses are doing is roughly the same thing, but without any statistical evidence to support it.
Yes. It sounds like a case of making decisions based on something other than fact, and that is why I am so keen on more information.

Also, there's nothing in evidence that says that parts that fall outside the tightened spec limits are thrown out or reworked. If they are I'd be very surprised.
Exactly, and that is why I asked how this practice had been explained to atreyu.


Just a wee bit :topic:,

I might add that I have been subjected to the opposite extreme as well: In my first job after school and military service, I was told by a foreman that "the engineers always set the tolerances too tight in order to protect their backsides, so there will be no problems if we exceed a set tolerance twofold". :mg:

Needless to say, that was not entirely true, and the company suffered from the inevitable consequences. At the time, there was one ambulating inspector in the company, and I have to say that he tried his level best to oppose this "practice". He was soon deemed to be dead meat, and let go.

That company is long gone, btw. I left before it entered the final nosedive.

/Claes
 

Wes Bucey

Quite Involved in Discussions
#14
Great opportunity to be a hero!

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.
Call me a "glass half-full, lemonade-from-lemons" sort of guy, but this is a WONDERFUL opportunity for you to introduce the concept of SPC and help the organization do Control Limits/SPC to the organization's economic advantage.

These folks WANT to prevent nonconformance, they just need to be shown an efficient way to do it and derive sensible control limits fitted to the REAL capability of their machines and processes rather than plucking some arbitrary number from thin air (IF that's truly what is happenng.)
 

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#15
Actually, the old hands had it right - but they didn't know why. In precision machining (defined as an operation where tool wear is the main source of common variation) you cannot run to the tolerances. Well, you can, IF you measure every part! Anyone up for that? That is why you can't - you need the cushion to allow for variation between the sampling points.

Why 50%? Well, I agree, if the process was controlled correctly - and most are not - 75% would be just fine. It would yield a capability of 1.33. 50% yields a capability of 2.00. Might make a six sigma practitioner happy - but would gag a lean manufacturing practitioner. As for lean six sigma ? Well, it gets real ugly.

But, the key is that they should be running the process they way the old guys used to, also. For an OD, you start at the lower control limit, allow the tool to wear to the upper control limit, then readjust back to the lower control limit. That process yields a uniform distribution, whose capability is calculated by (USL-LSL)/.75(USL-LSL) if the control limits are set at 75%. Compression the limits to 50% only ensures more operator intervention to adjust the process with no economic gain.

Unfortunately, practitioners of SPC have polluted the machining processes with X-Bar-R charts, which make an operator believe the part has to be run to the mean. So they adjust like crazy to run the part near the mean. Classic overcontrol! :mg: The operator becomes the main source of variation - not the process. Sure, operators can easily generate a normal distribution. But, it is wrong and the statistics are wrong, and the whole thing frustrates everyone involved. :frust:

***DEAD LINK REMOVED***
 
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A

atreyu915

#16
Just to update you on my progressed understanding of what has been explained to me and what i've learned about this topic in the past couple days:
The spread that was being reduced was a "hot target" for a rubber extrusion. The engineering function have assisgned a "hot target" of x +/-x and the quality function would reduce the +/- by ~ 50% or so.

You probably know this but the "hot target" is the desired value off the extrusion line to accomodate the shrinkage of the extrusion. Studies are performed to analyze the shrinkage and thereby set the target.

I still don't feel this is a good practice, and it was brought up today. I was relieved to hear that my boss agreed, and most likely didn't know this was being done.....but that's a subject for a different time. ;)

Given this info, do you still feel this practice is detrimental to the overall process?

Thanks!
 
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