Significant figures and tolerances
Significant figures and tolerances
(OP)
thread301-176582: Significant digits on tolerance
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?
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?





RE: Significant figures and tolerances
I disagree with your logic, in your first line you stated that your customer required a coating thickness between 2 and 4 microns and thats what it should be, your tolernce band for coating thickness is 2 to 4 microns not 1.8 to 4.2.
desertfox
RE: Significant figures and tolerances
My logic would actually allow for a thickness range of 1.50 to 4.4 on a specification of 2 to 4. 1.5 would be rounded to 2 and 4.4 would be rounded down to 4.
What I am trying to understand is whether there is a difference as far as rounding and significant digits goes for a specification of 2 to 4 versus 2.0 to 4.0? Does "2 to 4" imply "2.0000000000+ to 4.0000000000+"?
RE: Significant figures and tolerances
No theres no rounding 2 to 4 is 2.0000000000000000000 to 4.000000000000000000000 thats it
desertfox
RE: Significant figures and tolerances
RE: Significant figures and tolerances
The statement that 2 to 4 means 2.00000000... to 4.000000... is not correct. That would require measuring instruments that are even more precise.
RE: Significant figures and tolerances
I agree about the number of zero's after the 2 and 4 however the point was that a customer had requested a coating thickness of between 2-4 microns and what I was saying was thats what it should be between those figures or the max and min figure
desertfox
RE: Significant figures and tolerances
RE: Significant figures and tolerances
Every dimension as a tolerance I agree, if you write on a drawing or specification that the thickness is between 2 and 4 whether it be microns, millimetres,inches then thats what it must be, anything above 2 and anything below 4 is acceptable if its 1.8 or 4.2 its not.
If your measuring something then you need to make sure the tool your using is adequate for that purpose.
I have no doubt that its a complicated issue but unless their is any mention in the specification or on the drawing about significant figures which the OP as not mentioned in any of is posts then he is wrong to assume that there is.
desertfox
RE: Significant figures and tolerances
RE: Significant figures and tolerances
Your get out is that your customer doesn't referance a standard (double check it's not in your or there standard terms or contract though, even if not on the drawing). However, do you really want to play that game? Is it worth winning this one at the expense of maybe upsetting the customer longer term? Yes, you'd probably win in a court of law but do you want it to get anywhere like that far, where is the cost/bennefit.
Maybe you go back to the customer, ask what they want (Absolute or rounding sig figs) or maybe just ask if the 4.2 is OK on some kind of waiver etc. and change your process to meet it next time. Find someway to meet in the he middle.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Significant figures and tolerances
FYI - I did check their terms before posting and there is no reference to any standard. The issue has been resolved and the customer is happy to accept the parts at 4.2 microns. After discussions with the customer, they actually want >2 microns not to exceed 5 microns. The "2 to 4 micron" specification was a carry-over from a previous supplier's high end limitations. They are updating the specification to read "2.0 to 5.0" microns. This helps us both to have clarity.
There was no game being played here. You apparently misunderstood the motivation for my post. I was looking for technical advise about the definition and interpretation of a somewhat vague specification, not business or legal advice.
RE: Significant figures and tolerances
The concept of absolute limits is a fairly fundamental one in dimensional specifications on drawings, given that you weren't familiar with this I thought you may not have been familiar with other factors I went on to comment about.
Sorry if trying to take a broader view of the question upset you.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Significant figures and tolerances
All parties are correct.
The tolerance that is specified should be held.
but yet it's not a relistic tolerance that can be held.
if I remember 1 micron = .000039"
so 4 micron =.000156"
which = .000117 tolerance which is not gonna happen.
I would have the drawing change MM or Inches thick
that is reasonable. without knowing the fit, form or function is tough to say.
.0001-.0005 plate is resonable to me.
RE: Significant figures and tolerances
"Good to know you got shoes to wear when you find the floor." - Robert Hunter
RE: Significant figures and tolerances
the lower portion of the my previous post states
the drawing requires changed.
this would be the long term fix.
it is possible this is an old drawing & their customer is reluctent to change it. even if it's wrong.
RE: Significant figures and tolerances
"Good to know you got shoes to wear when you find the floor." - Robert Hunter
RE: Significant figures and tolerances
beating up microbus about this is just wrong.
The drawing is not practical.
and unless we know the fit form & function.
it's not worth arguing.
RE: Significant figures and tolerances
"Good to know you got shoes to wear when you find the floor." - Robert Hunter
RE: Significant figures and tolerances
the tolerance is 2 to 4 microns
in the inch = .000078"-.000156"
which is .000078" tolerance.
what type of plating can hold that tolerance.
my cnc grinders can hold that tolerance.
maybe if I lap them to 3 light bands flatness.
normal plating that I work with day in and day out
is normally .0005' tolerance (12 microns)
ok
RE: Significant figures and tolerances
"Good to know you got shoes to wear when you find the floor." - Robert Hunter
RE: Significant figures and tolerances
cnc grinders can not hold that tolerance.
RE: Significant figures and tolerances
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.
RE: Significant figures and tolerances
"Good to know you got shoes to wear when you find the floor." - Robert Hunter
RE: Significant figures and tolerances
His question was just about the sig figs V absolute limits.
How to specify/interpret something is different from whether it's possible.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Significant figures and tolerances
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.
RE: Significant figures and tolerances
"Good to know you got shoes to wear when you find the floor." - Robert Hunter
RE: Significant figures and tolerances
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.
RE: Significant figures and tolerances
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.
RE: Significant figures and tolerances
"Good to know you got shoes to wear when you find the floor." - Robert Hunter
RE: Significant figures and tolerances
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.
RE: Significant figures and tolerances
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.
RE: Significant figures and tolerances
mfgenggear:
...on the contrary, thank you for an interesting contribution!
RE: Significant figures and tolerances
you too!!