Significant figures and tolerances 8

[microbus]

thread301-176582

I dug out the above referenced thread and hope to get the groups opinion on this real world example:

I have a customer that has requested a coating thickness of 2 to 4 microns. Their drawing doesn't reference any dimension or tolerancing standard.

Actual thickness of the part is measured at 4.2 microns. I have argued to my QA department that 4.2 should be rounded to 4 as the significant digit in the specification is a whole number. In other words, they did not require 2.0 to 4.0 microns.

Should the reading of 4.2 be rounded in this case?

 

[ewh]

As I tried to explain, this particular situation may or may not be realistic, but it doesn't matter as far as the real issue is concerned, the one attempting to legitimize the use of significant figures in limit dimensions. Your post explaining the problems of holding the tolerance may be true but in no way addresses this issue.

"Good to know you got shoes to wear when you find the floor." - [small]Robert Hunter[/small]

 

[mfgenggear]

the statement should read

cnc grinders can not hold that tolerance.

 

[mfgenggear]

then we do not agree, because I deal with these type of issues every day.

my opinion is when the tolerance has to be held
and is a practical tolerance then it will be held.
having experience & knowing the product line is required.

it's not uncommon to hold diameters within .0002" tolerance.
or closer when it's required.

this product that microbus is dealing with could be for toy,
for all we know.

 

[ewh]

I don't understand why you are so against the concept of limit dimensions. If they are unreasonable, it is the fault of the designer. The drawing should be revised to reflect reality. It does not give the fabricator the freedom to choose whatever tolerance he sees fit to use (regardless of his reasons) without notifying the responsible engineer, who then will see that the drawing is changed to reflect what is being made. That is what drawings are for, to unambiguously define the part, not to sorta define the part.

"Good to know you got shoes to wear when you find the floor." - [small]Robert Hunter[/small]

 

[KENAT]

It might be, but that wasn't his question.

His question was just about the sig figs V absolute limits.

How to specify/interpret something is different from whether it's possible.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[microbus]

Here are some additional facts to help clear the issue, or fuel the fire, as the case may be.

The coating is Zirconium Nitride being applied to a implantable dental device via PVD. The coating serves a functional service (prevents galling of threads and a press fit mating service)and also aids in identification of the part (color coding.

The customer came to us with a specification of "2 to 4" microns. The customer's really could allow "2 to 5" microns but when using a previous vendor that could only put on a maximum thickness of 4 microns, for whatever reason, the specification was changed to 2 to 4 microns. I don't fully understand the reason for the change to 2 to 4 but this is what I was told.

When I said that the issue has been resolved, I meant that the customer has accepted the parts with a thickness measured at 4.2 microns. They also understood that a specification of "2 to 4" microns could be confusing so they added clarity and opened the range by setting the new limits at "2.0 to 5.0". Now, if we get a reading of 5.1 or 2.1 microns we are out of spec. So from the customers point of view and our point of view the issue has been resolved. We now have a clear understanding what is needed to meet our customers expectations.

 

[ewh]

Which question? I was refering to the error in assuming that the issue was resolved because the customer opened up the tolerances. The issue of parts in hand may have been resolved, but not the larger issue of sufficient part definition if significant figures were allowed. That's another can of worms which seems to be an unwelcome guest in this thread.

"Good to know you got shoes to wear when you find the floor." - [small]Robert Hunter[/small]

 

[mfgenggear]

ok
apples & oranges

microbus did the correct procedure.
he contacted the customer & basicly submitted to MRB.
the MRB was use as is.

OK significant digits
in black & white tolerance must be in between.

 

[Compositepro]

To most people there is no difference between a tolerance of 2 to 4 or 3 +/-1 and how these are interpreted. Perhaps there is but if we are going to argue it would be useful to provide some references, otherwise it is just one opinion versus another.

On the other hand it doesn't matter what standards are out there if neither party in a conversation or transaction is aware of it.

 

[ewh]

The standard that I am most familiar with and based my comments on has already been mentioned as an example by KENAT. Granted, if you are not bound to follow any standard, then it is only a matter of opinion.

"Good to know you got shoes to wear when you find the floor." - [small]Robert Hunter[/small]

 

[gerhardl]

Hello everybody,

Landing late in on this discussion I cannot let be commenting. I believe several different issues have been somwhat mixed together in the heat of discussion.

From a pruduction and technological point of view, this whole discussion boils down to the normal ground principles:

a) A tolerance is the exact figures put down on a paper, (if this is stated as a tolerance or given as a limit) and without any additional figures or variations up or down.

If there is any doubt about that this is the (real) tolerance: contact customer as stated earlier by others to clarify.

b) Producer and customer has to agree on test methode (measuring procedure) : where to measure and how to measure, if several tests shall be done, on different parts of one piece, each piece tested or statistical sampling, series of measurements with main figures, single figures etc.

c) Producer and customer has to agree on measuring instrument, type, how and when to calibrate and calibrating tolerances.

d) Producer and customer has to agree on how to handle deviations.

Both a. b. c and d may be very simple, or more complicated if technical issues are difficult or consequences large.

 

[mfgenggear]

Yes, what is on the engineering drawing limits must be held.
that is what is contracted by both parties.

No, It was not microbus question. However, the error is so obvious, it stands out like a sore thumb.

Limits must always be held, & I not against that.
what I was suggesting that the drawing limits should have been re toleranced by owner of the drawing to a practical limits thats all.
My apology to all if I offended anyone.

 

[gerhardl]

mfgenggear:

...on the contrary, thank you for an interesting contribution!

 

[mfgenggear]

thank you gerhardl

you too!!