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Allowances and Tolerances

09/24/2009 6:53 PM

I had a discussion with another engineer, and we talked about a tolerance,

It was .452 ± .002 , I check part and got an accurate .4548 and said the part is out of tolerance by .0008, he said that the part is good, because that tolerance is calling out to the .XXX place, so tenths don't matter.

Who is correct?

Daniel

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#1

Re: Allowances & Tolerances

09/24/2009 6:55 PM

He is.

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#2

Re: Allowances & Tolerances

09/24/2009 7:02 PM

The actual as-produced dimension value must not be greater than .454. The actual value may not be rounded down to .454 and considered to be within specification.

All limits are absolute. Dimensional limits, regardless of the number of decimal places, are used as if they were continued with zeros.

-Right from Y14.5M-1994 (except I changed the numbers to be relevant)

So yes, he is correct.

You always seem to beet me to the punch LL.

Basically, You can only use the same number of decimal places as you are given with the limit, anything past the limit's # of decimal places is irrelevant, and considered to be within the err of measurement.

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#3
In reply to #2

Re: Allowances & Tolerances

09/24/2009 7:07 PM

Your answers always add enlightenment. That takes more time.

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#6
In reply to #2

Re: Allowances & Tolerances

09/24/2009 8:25 PM

The part is out of tolerance indeed. If you use an adequated instrument to measure it to the precision it is calling, it will indicate .455 for sure - and you will find out it is .001 out of spec.

But, of course, it sometime depends on the glasses of your boss while double checking your dimensions, if your room temperature has varied too much between manufacturing and inspection, and if your company is supposed to pay for another part instead of accepting it - those factors all interfere in measurement - .

After all, why the hell are you using a micrometer to tens of milesimums if you have a very comfortable 4 mils to play with? Too small difference, indeed, maybe the use of a less accurate instrument will give you an acceptance. Maybe not. Is it used in critical applications (like an aircraft engine part)? If so, just scrap it and stop scrumbling...

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#7
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Re: Allowances & Tolerances

09/24/2009 9:21 PM

Sorry, our universe stops at .xxx. Nothing beyond the third place counts. Why would measure to a degree of accuracy not required by the note?

If the designer needed four place precision, he would have specified it.

We went to the moon with slide rules, keep it simple.

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#8
In reply to #7

Re: Allowances & Tolerances

09/24/2009 10:24 PM

I think his point is that with calipers that just read to three decimal places, he would get a reading of 0.455.

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#9
In reply to #8

Re: Allowances & Tolerances

09/24/2009 10:34 PM

Oh, That? Now you've done it! You've confused befuddled me to the point that I can't muster a good response. You didn't say if it was ID or OD.

My glasses are getting foggy!

On the farm we'd just pound it in anyway!

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#11
In reply to #8

Re: Allowances & Tolerances

09/25/2009 6:56 AM

I don't think so. He measured the part to 4 places, by whatever means.

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#48
In reply to #7

Re: Allowances & Tolerances

09/27/2009 12:41 AM

Doesn't .0008 round off to .001?

It did 35 years ago!!!

But you youngsters may have different definitions.

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#53
In reply to #48

Re: Allowances & Tolerances

09/27/2009 3:28 AM

You are dead right, it still does.....

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#73
In reply to #53

Re: Allowances & Tolerances

09/28/2009 2:20 PM

Standard states: Values must be truncated, not rounded... no rounding allowed. Period.

But this only applies if you are conforming to the standards, If you make your own standards, then, well... you have your own standards, and round away.

I Repeat: According to ANSI standards... Rounding does not exist, you must truncate ALL values to the exact number of decimal places in the tolerance.

I'm terribly sorry for sounding like a broken record, but I suggest that everyone needs to take a look at our National standards ANSI(If in the USA) or ISO(If otherwise), they were created for a reason: so no one needs to spend time arguing about tolerance standards.

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#88
In reply to #73

Re: Allowances & Tolerances

09/28/2009 8:25 PM

Values must be truncated

This means any value above or under the set tolerance need not be noted but doesn't preclude the part meeting the required tolerance value.

Check it out

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#13
In reply to #2

Re: Allowances & Tolerances

09/25/2009 11:49 PM

"The actual as-produced dimension value must not be greater than .454. The actual value may not be rounded down to .454 and considered to be within specification." (my underlining of the 'not')

I agree, BUT!

Either the 0.4548 (I INSIST on the zero before the decimal point) or the rounded value of 0.455 are above 0.454, so the part is OUT OF TOLERANCE!

"So yes, he is correct."

I interpret that statement to say that Daniel's friend is correct ('he' referring to the 'he' of Daniel's post, and that Daniel is wrong.

Your own original statement indicates that Daniel is correct, and his friend is wrong!

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#34
In reply to #2

Re: Allowances & Tolerances

09/26/2009 6:32 PM

Correct, its 8 - 10,000ths over.

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#38
In reply to #2

Re: Allowances & Tolerances

09/26/2009 6:54 PM

He is wrong, Daniel is correct and I don't know who you're talking about

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#4

Re: Allowances & Tolerances

09/24/2009 7:11 PM

Thanks for your answers, I felt he was right, but I guess I am a little stubborn and needed to hear it from someone else.

Daniel

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#37
In reply to #4

Re: Allowances & Tolerances

09/26/2009 6:50 PM

To the contrary "you" are correct. Please refer to Geometric dimensioning & tolerancing (GD&T) standard ASME Y14.5M-1994

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#5

Re: Allowances & Tolerances

09/24/2009 7:12 PM

I might suggest that you try to adopt a GD&T (Geometric Dimensioning & Tolerancing)standard rather than the old +- standard... It will remove any confusion about anything related in any circumstance in the future.

ADV/DM (Advanced Dimensional Management LLC) is a good company that coaches business on how to adopt and transition into the "new" way of doing things which completely and un-ambiguously define all tolerances, and geometric features on a part. I took some classes from the guy who owns the company, and see real value in adopting the practice, especially if you are outsourcing, or working with companies overseas.

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#31
In reply to #5

Re: Allowances & Tolerances

09/26/2009 11:59 AM

RV he did not metion that it was a circumfrence it could be machined two flats. We should never assume but think of all possiblities if not all the information is given to us.

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#75
In reply to #31

Re: Allowances & Tolerances

09/28/2009 4:10 PM

I don't believe the tolerance on the drawing cares what the feature is, its only a tolerance. 0.002 for a hole is the same as 0.002 for a flat, which is the same as 0.002 for a peg, its simply a tolerance. The precision of the measurement is called out by the number of decimal places in the tolerance. In this case the number of decimal places is .XXX (three) therefore, the measurement used to define whether the part is within spec must also be to the same precision (.XXX, again only three places, not .XXXX four places).

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#81
In reply to #75

Re: Allowances & Tolerances

09/28/2009 5:27 PM

I strongly disagree. I once made some 6.0000" standard bars (@65°F) for the department of Fish and Game. That's what they specified - accurate to the ten thousandth of an inch. They were for checking abalone gauges. Nobody measures abalone (or ever did) to that kind of precision, but if they were fining people for having undersized abalone, they needed to be able to prove that their instruments were precise and accurate, to avoid lawsuits.

At least two different posters before me have indicated that the measuring instrument needs to be more precise than the measurement required, when tolerances are involved.

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#82
In reply to #81

Re: Allowances & Tolerances

09/28/2009 5:37 PM

Hey, I'm just gleaming information from the American National Standards Institute. If anyone disagrees with the National standards which are currently in effect, you should attend the meetings, get on a committee, and push for the appropriate changes to the standards. I'm only providing what the national standards say.

Again, much of the time, companies have their own set of standards, which may or may not comply with standards from other parts of the country/world which is perfectly acceptable. The company I work for does not comply with ANSI standards, we have written our own standards.

That said, IF I get a customer who gives me a drawing with a note that says it complies with ANSI standard Y-14.5 XXXX, I'm contractually obligated to follow the standard If I accept the job. If I don't, I'll be in breech of contract and be liable for any and all repercussions.

The Bottom line to the OP: It will entirely depend on what standard you are required to follow.

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#89
In reply to #75

Re: Allowances & Tolerances

09/28/2009 8:30 PM

But you are ignoring the fact that a 3 digit instrument would read 0.455 because that would be the closest mark.

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#95
In reply to #89

Re: Allowances & Tolerances

09/29/2009 11:40 AM

I Absolutely agree 100%, and have said, that I would guess, the standard is simply allowing the tool to do the rounding, not the human, removing any human error.

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#98
In reply to #95

Re: Allowances & Tolerances

09/29/2009 11:50 AM

No the tool is incapable of true accuracy beyond x.xx, this is the point.

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#101
In reply to #98

Re: Allowances & Tolerances

09/29/2009 11:55 AM

I don't know the logic (or lack thereof) behind the standard, and have argued it plenty while the Instructor attempted to teach the class. I simply know what was being taught(as silly as some of it was)... Whether it makes any logical sense or not is not the question, the standard (If anyone decides to comply with it is up to them) just attempts to keep people from loosing their business/licence to a nasty lawsuit.

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#10

Re: Allowances & Tolerances

09/25/2009 1:46 AM

There is normally10% of tolerance consumption allowed for measuring instruments,So I would except the part if it is only off by 10% of 0.002, thats about 0.0002.

eating 40% of tolerance just because of someone has not added one more zero at the end of tolerance value is not justifiable. and how about if tolerance is 0.001, In that case deviation from tolerance can be 80% of tolerance.

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#12

Re: Allowances and Tolerances

09/25/2009 11:43 PM

Guys, I'm going to have to respectfully disagree. I'm with Daniel, when I put a three decimal dimension plus or minus .002. I expect the part to not deviate from the theoretically perfect max and min because those were the numbers I used in my tolerance stack-up study. Frankly I don't give a damn what ANSI says about it. They're wrong. Full stop. End of story. The designer gave the machinist a whole .004 to play in, and he STILL can't hit the target. That is unacceptable. I MIGHT could be convinced to accept .0002 out of tolerance, Hell that could just be a layer of skin oil or the part warmed up in my hand, but not .0008. I would be writing up an NCR so fast it'd make the inspector's head spin.

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#28
In reply to #12

Re: Allowances and Tolerances

09/26/2009 10:58 AM

One other point that was touched on in later responses the needs to be expounded upon. If you are comfortable accepting a part that exceeds your stated tolerance band by 20%, you really need to ask yourself if you did YOUR job by giving the machinist the biggest tolerance band possible.

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#49
In reply to #12

Re: Allowances and Tolerances

09/27/2009 12:44 AM

Yep!!

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#77
In reply to #12

Re: Allowances and Tolerances

09/28/2009 4:33 PM

"Frankly I don't give a damn what ANSI says about it. They're wrong."

Bold.

Will that stand up in court?

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#78
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Re: Allowances and Tolerances

09/28/2009 4:46 PM

It won't have to, if it is my project and my parts I am the law.

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#79
In reply to #78

Re: Allowances and Tolerances

09/28/2009 5:03 PM

I would direct you to this:

http://www.qualitycouncil.com/samples/CQI_s.pdf

Yeah I'm pretty sure it'd stand up in court too.

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#91
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Re: Allowances and Tolerances

09/28/2009 9:48 PM

I like that link, it's a good brush-up on things long forgotten

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#80
In reply to #78

Re: Allowances and Tolerances

09/28/2009 5:05 PM

If the drawing has any note saying that it complies to any ANSI standard, you are under a legally binding contract to follow those standards completely. If you fail to comply to the stated standard on the drawing, your law will not hold up in court and you will be financially responsible for any and all damages caused by your failed part.

If the drawing does not state any standard, feel free to do what you wish.

Careful note to the bold IF... it really doesn't matter if no standard is stated, but IF it is, its a legal contract.

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#90
In reply to #80

Re: Allowances and Tolerances

09/28/2009 8:32 PM

You're off the beam a bit, this truncation you've sited as a basis of your argument doesn't mean what you think it does...

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#93
In reply to #90

Re: Allowances and Tolerances

09/29/2009 3:51 AM

Exactly: why off topic?

I really don't know any thing about this: but this is my guess at what this whole thread boils down to.

If a designer says that a part must be 0.452 ± .002 he means that it must be greater than 0.45 and less than 0.454

If he/she has used any software or methodology to total up the tolerances then that software/methodology will not have put any tolerance on the tolerance.

If the ANSI spec. (and of course I don't have access to one to check the wording) says that the tolerance is truncated: then it means that 0.002 means 0.0020000000.....

How the guys in production ensure that they meet the spec. is up to them. First note that there is a huge difference between resolution and accuracy in a measuring tool, but lets forget about that for now:

If their tool can measure to three decimal places then parts which measure 0.451, 0.452 and 0.453 are acceptable. Parts which measure 0.450 or 0.454 may or may not be acceptable, but must be rejected (at least initially, they may be subjected to a more accurate measurement).

If production have a tool which which can measure to 5 places: then parts which measure greater than or equal to 0.45001 AND less than or equal to 0.45399 are acceptable. Parts which measure 0.45000 or less OR 0.45400 or more are rejects (and I wouldn't bother trying to re-evaluate the two borderline cases).

The ten to one rule which people keep referring to is suggesting that production's most economic way of proceeding is to get a tool which will measure to 4 places: then parts which measure greater than or equal to 0.4501 AND less than or equal to 0.4539 are acceptable. Parts which measure 0.4500 or less OR 0.4540 or more are rejects.

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#99
In reply to #93

Re: Allowances and Tolerances

09/29/2009 11:50 AM

"then it means that 0.002 means 0.0020000000....."

Yes. Well said Sir.

I'm sorry for pounding ANSI into this thread, It's just been pounded into my head so much in the past 6 months it makes me sick. The funny thing is, I completely disagree with many of the things being taught in a standard GD&T class (and spent plenty of time arguing with the instructor about these exact topics), such as the truncation rule, and only measuring to the degree of accuracy as the tolerance... the problem is, they have attempted to create a system where no one needs to think anymore, and common sense is a thing of the past. It's just such a Lawyer happy world out there, that they have made the book so black and white with no room left for actual logical thinking.

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#100
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Re: Allowances and Tolerances

09/29/2009 11:54 AM

We all need reminding that "common sense is genius in it's work clothes." I Don't recall who said it.

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#112
In reply to #100

Re: Allowances and Tolerances

09/29/2009 1:33 PM

A+++ Ain't that the truth!

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#113
In reply to #100

Re: Allowances and Tolerances

09/29/2009 2:07 PM

...then you also are not a genius!!!

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#115
In reply to #113

Re: Allowances and Tolerances

09/30/2009 12:55 AM

If I dress up one of the first things I'll do is spill something

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#116
In reply to #115

Re: Allowances and Tolerances

09/30/2009 2:39 AM

LOL!!

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#108
In reply to #93

Re: Allowances and Tolerances

09/29/2009 12:16 PM

This be a lesson to me to not argue when you do know about something

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#96
In reply to #90

Re: Allowances and Tolerances

09/29/2009 11:43 AM

If you are indeed correct I definitely would like to be corrected. If I am misunderstanding the text, and the teaching the GD&T instructor was teaching, I would like to understand it as it was intended. I read through the posts, and I must apologize for repeating myself so many times, I don't recall posting that many times, and I apologize.

Again, If I am wrong, or have misunderstood the teachings/text, please correct me.

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#103
In reply to #96

Re: Allowances and Tolerances

09/29/2009 12:04 PM

It's good to comment and I'm sure we're all in agreement and respect you for admitting of a possible mistake.

It's fine to allow a device to assign values if it's the correct tool of proper calibration and resolution.

Randall post #93 has developed this concept well I think.

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#14

Re: Allowances and Tolerances

09/26/2009 12:12 AM

0.4548 rounds to 0.455 not 0.454 so it's always out of tolerance by 0.001 no matter what.

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#24
In reply to #14

Re: Allowances and Tolerances

09/26/2009 10:13 AM

I'm with you on that one. Its out of tolerance in my book.

If the rulebook (or whatever) stated that it would only be measured to 3 decimal places and the fourth ignored, no matter what it was, then the other Guy is right......

If I was making the part, I would have made another cut......

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#15

Re: Allowances and Tolerances

09/26/2009 12:12 AM

My vote is that the part is out of tolerance.

I would be tempted to accept the part if it were 0.4544 as it would be a case of rounding down and a machine that measured accurately to the required 3 decimal places would find the piece acceptable.

Another possibility is that the part was still a little hot in coming off the machine (maybe?). If you leave it over the weekend somewhere cool, will it get to the right dimension?

A final question is always: so what? does it matter?

Ask the specifier whether the part is acceptable. Perhaps the specification asked for a stringent tolerance but the detail is still workable with the slight out-of-tolerance.

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#16

Re: Allowances and Tolerances

09/26/2009 2:08 AM

just goes to show why america is having so much trouble in the "quality" side of things. the guy, which ever one it was, that wanted to pass that part should be fired. that type of attitude is killing this country. but of coarse there is always time to "do it over"........

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#36
In reply to #16

Re: Allowances and Tolerances

09/26/2009 6:40 PM

I guess the Quality issue is the reason why Air Bus is having to change their Pitot tubes for American made units. Sorry if I stepped on any EU toes.

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#52
In reply to #36

Re: Allowances and Tolerances

09/27/2009 3:25 AM

Safety/quality, especially in the air industry, is too important for anyone to worry about having their toes stepped on......but there is always one who feels that this aspect is more important!!!!! Total idiocy to my mind.

At least the aircraft should be far safer once the tubes are all replaced (if not already achieved).

Air France needed a short sharp lesson of some kind as Airbus had recommended strongly that all tubes be replaced many months (8?) before that tragic accident......

Not making manufacturers recommended changes also contributed to the tragic AF Concorde accident.

A BA Concorde would not have been damaged under the same conditions as special deflectors had been built in after a BA Concorde had a similar accident in the USA, but luckily the Kerosene did not catch fire. This was some years before I believe, so AF had plenty of time to get the parts installed......safety is not a strong point with AF!! I will never, ever fly with them again as it is obvious to me that they will never change......its only peoples lives???!!!!??????

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#54
In reply to #52

Re: Allowances and Tolerances

09/27/2009 7:51 AM

Andy, I guess my unspoken words are, when working within a set of given tolerances it inherently bring in QA into the picture. And the last time I checked Quality Assurance, or the lack of, is not limited to one particular country nor is it limited to one particular person in a particular country. As far as the commit (#20) made about, "taking it with a little grain of "engineering" and keep on trucking" says a lot about that particular person, whether he/her is American, Asian, or what ever. But to make a statement, "this just goes to show you why america is having so much trouble in the "quality" side of things". Every country has their own QA problems, and the last time I checked on the market, Asia's are taking the brunt of the QA problems right now. I have too many years in aviation to know the importance of QA and maintaining tolerances. But, that goes with any industry where your product could hold you liable in a law suite. And when lives are at stake, you have to weigh how much money you can make by " keep on trucking" to how much money your going to lose if a part or system fails because it was out of specs.

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#56
In reply to #54

Re: Allowances and Tolerances

09/27/2009 10:05 AM

This is old #20 here, I am glad that someone finally verbalized all the objections to the statement that I made. The English language is a beautiful thing until it is placed in the wrong hands. The question becomes "what did I say?" versus "what did you read?". I make no claims to being right or wrong in the English department. So let me restate what I intended to say. There were two paragraphs to that message;

The first paragraph was to offer the opinion that with the stated facts you could go from a slide fit (with the right surface finish) to an .0008" interference fit. The significance of that would be the diameter involved (less than 1/2") would be of more consequence than on say an 80" retaining ring with a .250" fit.

The intent of the second paragraph was to imply that if the only engineering you were going to use was based on some "book" definition of tolerances and your only input to the quality involved was based on some rigid set of numbers, then that is apparently what you do for a living, It would appear to be that it is a limiting factor with you so keep on doing what you are doing. No one is going to change you here.

To further elaborate, one contributor offered the opinion that he (QC' guy?) would evaluate the situation and if it gave him any doubt he would write a Non Conformance Report, and you would have to live with it until you cleared it. That person is more right than the rest of us.

Well guess who the NCR goes to, probably, "Engineering", need I elaborate any more or do I still appear to be a jackass.

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#57
In reply to #56

Re: Allowances and Tolerances

09/27/2009 3:27 PM

Hey otha, I'm probably that QC guy and being I don't what your back round is, I'm not going to get into a pi**ing contest with you. I'm sorry, all I seen was a simple question about a tolerance of .452 ± .002, I didn't see were the OP stated the measured units in either inches or millimeters, let alone the part flat, bored or even a joggle. The question was " is .4548 out of tolerance. And that is an Engineering decision. NCR's are a written notice to the Engineering Dept and Production line that Non Conforming parts are being produced, which relieves QA/QC of the responsibility of the part that's not to specs. Every industry I've worked in that uses NCR's also have in place, paper work to allow "Deviation" for a said print. There is usually a reason for tight tolerances. If production can not maintain a ±.002 then it's up to the Engineering Dept to sign off on a Deviation Order and reevaluated the tolerances, then revise the print to reflect the new tolerance. Unfortunately we have people that are sue happy and Tort lawyers to back them, that leaves our world in Black and White and by " The Book". So when it's all said and done with, and it comes down to it, everything is in black and white, there can not any grey area where you left yourself open to a liability suite. With that I'm done with the subject. dj

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#58
In reply to #57

Re: Allowances and Tolerances

09/27/2009 3:56 PM

We still aren't communicating, I said you are you are the only right one in the group. You just elaborated on why you were right.

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#84
In reply to #16

Re: Allowances and Tolerances

09/28/2009 7:38 PM

I grew up, "If you can't do it right the first time, how are you going to find the time to do it again"

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#17

Re: Allowances and Tolerances

09/26/2009 2:54 AM

0.4548 rounds upto 0.455, if it were 0.4544 it would have been rounded to 0.454. So it is a reject.

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#45
In reply to #17

Re: Allowances and Tolerances

09/26/2009 9:39 PM

This thread sounds like the "Who's on first routine"

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#18

Re: Allowances and Tolerances

09/26/2009 5:59 AM

I agree with all those who say the permitted tolerance has been exceed and therefore the part is a reject.

But go back a step to consider why the tolerance has been set at that amount.

Is it because it is critical to the component itself, or is a measure to trigger off an adjustment to compensate for wear-and-tear to the machine that made it.

And what of the reject protocol. Is the part scrapped completely or returned for reworking - there are so many things to condsider - but to answer the original question - as stated above - it is a reject.

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#47
In reply to #18

Re: Allowances and Tolerances

09/26/2009 11:13 PM

Being the part came in over sized, rework is always an option, but the part still needs to be constantly manufactured to the print specs, other wise you'll have a lot of consistent rework on your hands.

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#19

Re: Allowances and Tolerances

09/26/2009 6:48 AM

Guys,

the one thing that seems to be missing in the discussion so far is the tolerance of the measuring equipment. This has to be taken into account in the YES/NO game that you are playing.

I assume that layers of oil on the part and instrument have been considered?

And that temperature of the part and the instrument are within specification.

A good, calibrated set of slip gauges might settle arguments.

Sleepy

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#50
In reply to #19

Re: Allowances and Tolerances

09/27/2009 12:46 AM

good points.

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#60
In reply to #19

Re: Allowances and Tolerances

09/28/2009 2:29 AM

The OP said he got an accurate .4548 so it's reasonable to assume that the part is between 0.45475 and 0.45485: both of these "round" to 0.455 so the part is out of tolerance.

I don't know anything about this sort of thing but I would have assumed that a part which was 0.454000001 was out of tolerance: if the tool I'm using to do the measurement with can't give me the confidence that the part is >0.450 and <0.454, then I'd better get a better measuring stick.

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#20

Re: Allowances and Tolerances

09/26/2009 7:38 AM

Let me answer a question with a question, if .454 represented a size and size fit with the mating part (assuming a circle), would you folks still be so comfortable with a .0008 shrink fit on this diameter?

So my take on it is tolerances need to be taken with a little grain of engineering. So if you are simply measuring pieces, accept the part and keep on trucking.

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#30
In reply to #20

Re: Allowances and Tolerances

09/26/2009 11:55 AM

Make a Go/NoGo Gauge! A 0.4548 part will NOT fit in a 0.454 hole!

The part is a reject!

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#41
In reply to #30

Re: Allowances and Tolerances

09/26/2009 7:21 PM
  • A Go/No GO gauge, is no good if you can't pass First Article Inspection within the tolerances specified in the print.
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#39
In reply to #20

Re: Allowances and Tolerances

09/26/2009 7:01 PM

Have you tried navigating that way? How's it working for you? What planet are you from? Are you sure you can find you way back??

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#44
In reply to #39

Re: Allowances and Tolerances

09/26/2009 8:55 PM

I must be missing something here. Can't correlate your remarks to what I stated. Must have something to do with being from the planet Uranus.

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#51
In reply to #39

Re: Allowances and Tolerances

09/27/2009 12:49 AM
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#59
In reply to #51

Re: Allowances and Tolerances

09/27/2009 4:35 PM

Eh? post 20 is ridiculous, that's my story an I'm sticking to it!

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#42
In reply to #20

Re: Allowances and Tolerances

09/26/2009 7:41 PM

With this kind of attitude, why even bother measuring the damn thing, let alone taking it to 0.XXX tolerances. Just look at it and save your self some time. Maybe just maybe, it won't come back and bite you in the A**.

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#21

Re: Allowances and Tolerances

09/26/2009 8:17 AM

You are correct. Even though the tolerance is called out to 3 places, the dimensions must be within the +/- .002. Your part is out of tolerance by .0008"

Lively Machine Co.

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#22

Re: Allowances and Tolerances

09/26/2009 8:36 AM

The dimension has a stated tolerance of 0.452 +/- 0.002. So, the minimum dimension is 0.450 and the maximum dimension is 0.454. This is clearly stated and there cannot be further discrepancy past these limits. Okay - let's think about that mandate, Robert.

A part dimension expressed as X.XXX would have a tolerance of +/- 0.003 in most machine shops since that seems to be general practice. But that is not an empirical rule - it is just a general practice. And since a tolerance for this particular dimension was stated, that tolerance overrules any general practice.

To me, the part drawing (or blueprint for us old-timers) is a contract between the manufacturer and the customer - whoever that customer may be. If the tolerance is stated we must manufacture the part within those tolerances.

Tenths do matter, although in this particular case it may not matter if the part is as much as 0.0009 below or 0.0009 above the limits of the applied tolerance. But that is where the engineer must determine the true permissible limits and keep them as broad as possible. And the shop must use tools that are capable of making these measurements accurately and repeatedly. If I had to use hand measuring tools for the task I certainly would not trust a vernier or digital caliper and would insist on using a calibrated micrometer that has a tenths Vernier scale.

This is where it can get really touchy. Let's say the machine shop uses calibrated micrometers (with a tenths Vernier scale) to measure these parts. Then I as the god of inspection (all hail!) slip off to my temperature-controlled sanctum and use a digital comparator to inspect the parts and find that some measure as low as 0.4499 or as high as 0.4541. Obviously these parts are out-of-limits so they are rejects; correct? Yes, the numbers say they are, but the calibrated micrometer that was used during the manufacture of these parts determined they were within specifications. So, who is right? Yes, this pissing contest occurs in some shops or between the shop and the customer's inspector and usually occurs because the customer did not specify exactly which measuring instruments must be used.

Since the dimension and its tolerance are expressed no finer than thousandths, I would just apply the round up, round down rules. So, any deviation of equal to or less than 0.0004 can not count, and any deviation that is equal to or more than 0.0005 does count. So with that idea the part could be correct if it measures from 0.4496 to 0.4544, and out-of-limits if it measures from 0.4495 to 0.4545. But since I am talking tenths here - I had better be able to prove it.

Best Regards,

Ing. Robert Forbus

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#23
In reply to #22

Re: Allowances and Tolerances

09/26/2009 8:57 AM

I am replying to my self here as well as the original poster. In my earlier post I presented no opinion for the original poster and that is what he seemed to really want. I became a bit of a novelist and merely presented some arguments about who or what is correct.

So, now I will express my true opinion here. In the end, the part is out-of-limits by 0.0008 and is a reject - no question about it. The drawing is the contract, and this is a violation of the contract.

Best Regards,

Ing. Robert Forbus

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#25

Re: Allowances and Tolerances

09/26/2009 10:36 AM

if your measurement is accurate enough then the tolerance is .002+.001=.003

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#26

Re: Allowances and Tolerances

09/26/2009 10:42 AM

I have given this some more thought and I wonder at the gauges being used, back when we knew that an outside micrometer had 40 TPI and how a vernier scale was developed, we approximated the value between graduations and that particular reading would have been so obvious it would have been recorded rounded up to the next value.

Of course we didn't look at a fellow arriving on the job with a tap extractor in his toolkit as having a lot of tools, we kind of wondered if he was planning on breaking a tap.

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#27

Re: Allowances and Tolerances

09/26/2009 10:45 AM

You need to measure .XXX at 4 places and report at 3 places.

---------------------------------------------------------------

Here's a couple of simple yet well respected explanations for the non-dimensional types.

Quality Digest

A number of factors come into play when choosing the right resolution for a measurement. A common one is the 10:1 rule, which states that the measurement resolution should be 10 times finer than (or 1/10th) of the specification being measured.

--------------------------------------------------------------------------------

George Schuetz, Mahr Federal Inc.

10:1 Rule. Whenever possible, the measuring instrument should resolve to approximately one-tenth of the tolerance being measured. So if the total tolerance spread is 0.01 mm (that is, ¦0.005 mm), the smallest increment displayed on the gage should be 0.001 mm. This amount of resolution allows you to make accurate judgments for borderline cases and makes it possible to observe trends within the tolerance band. Many people know of and can explain this rule, yet it can still sneak up on the best of us.

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#43
In reply to #27

Re: Allowances and Tolerances

09/26/2009 7:46 PM

Agreed and GA.

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#29

Re: Allowances and Tolerances

09/26/2009 10:59 AM

You are correct. I am not an engineer, but when I operated a lathe machine for a company that made parts for FoMoCo, that would have been a reject every time. Tenths do count with that kind of close tolerance. Ken Day

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#32

Re: Allowances and Tolerances

09/26/2009 12:38 PM

As an Inspector, some of you guys would not want me on your line. And as an Inspector doing First Article Acceptance, I DO NOT inspect Hot, Dirty parts! Hot, Oily, Dirty Parts are not ready for any type of Inspection! With that said 0.454(+) 0.0008 is out of "specs". I would request another cut. On the second try, the part is still out specs, I would go back to the machine that's trying to produce the said part and look at (1) the operator (2) the calibration of the machine producing the bad parts. 0.004 give you a wide margin to work within. You have to pass First Article within your print, other wise the said tolerance is Worthless and should not been specified on the print.

As an Inspector, I would expect the Engineer and myself to realize when production is having a hard time staying within said tolerances, there's something going on with the calibration, either the machine/operator is worn out or the specifications are to tight to work with and need to be revised to show (+/-) 0.0028

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#33

Re: Allowances and Tolerances

09/26/2009 3:33 PM

OP, OP, Oh, OP where did you go?

Did we scare him off?

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#35

Re: Allowances and Tolerances

09/26/2009 6:32 PM

you are correct see my comment

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#40

Re: Allowances and Tolerances

09/26/2009 7:05 PM

Both of you could be correct depending on the fit type. If it is force fit, your friend is in order and there could be further allowable tolerance. However, if otherwise he should remember that the maximum allowable tolrance is .454. THIS IS THE MAXIMUM ALLOWABLE therefore he is unlikely to be correct.

Munyeowaji E. Mbikan

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#46

Re: Allowances and Tolerances

09/26/2009 10:18 PM

Some of these answers scars me to think that an "Engineer", ( that's the form we're in, is not), would make statements that exceeding the min/max "Tolerances" is a permissible "Allowance" especially with any part that might have an interference fit or a tapper pin fit. And then try to assemble these parts that are of specs. and won't fit properly.

So Who's going to hang, if the said part has a premature failure due to the fact that the said part was out of specs, but was still excepted through Engineering and QA, is it going to be the Engineer, the Inspector, or will be the poor Machinist that made the "Bad" part to begin with? Who will be? I do know for a fact that will not be me, because I will not "Tolerate" parts that exceed the min/max limits that are specified on a print. In another words I'm going to cover my A** and the NCR's will fly. If it's not to print. Production should stop at that point, investigate into the problem before it manifests into a law suite and everybody loses, except for the Attorneys

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#55

Re: Allowances and Tolerances

09/27/2009 9:41 AM

.4541 is out of tolerance (let alone .4548)....................no question about it.

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#61

Re: Allowances and Tolerances

09/28/2009 8:41 AM

Wow - I have worked in manufacturing 30+ years, and I have to tell you that I am amazed by the answers I'm seeing.

Everyone I've ever worked with would tell you that the limits on this part would be 0.4500 to 0.4540 with as many zeroes as you'd like to stack on the end of those numbers. As someone has said, these numbers are limits, not fuzzy areas. To me a limit is as far as I can go, a line in the sand if you will.

It is certainly possible that I am wrong about this, but even the person who cited the ANSI standard has not shown me otherwise yet. If the number can not be larger than 0.454, it can not be 0.4548. To me it is just that simple.

If I am wrong, I have unnecessarily scrapped many parts, and rejected many more from vendors.

You are correct as far as I know.

Best regards,

Wade

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#62

Re: Allowances and Tolerances

09/28/2009 8:57 AM

Fundamentals of Geometric Dimensioning & Tolerancing (2nd edition - metric) by Alex Krulikowski

This book is based on ASME Y14.5M-1994

Page 8 Interpreting Dimensional Limits

"All dimensional limits are absolute. A dimension is considered to be followed by zeros after the last specified digit."

The part is rejected because it is out of limits. What happens to the part depends on the situation and type of production.

If it is a high production run it might be cheaper to scrap than rework or it could be sold for after market parts.

If it is a job shop you could check with engineering and the assembly department to see if the part could be used and if the tolerance could be increased on the drawing. Very often there are tolerances on drawings that are are not required to be so tight and it would reduce cost to losen the tolerance.

The statement is often made that the machines can make parts to away more accurate than that so it does not cost to put a tolerance on a drawing. The cost is not always in the manufacturing but in the inspecting and the hours wasted deciding what to do with rejected parts that need not have been rejected.

It is a real shock the number of people who do not know enough about tolerancing to reject this part.

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#63

Re: Allowances and Tolerances

09/28/2009 9:09 AM

I think in this particular case if the tolerance is set as tightly as this it is for a reason. What is your measurement error (operator / micrometer)? Measure a few more times, do you get the same out of tolerance measurement? Grab 5 parts before and after the measured part if they also measure out you should be BLOCKING product and do and SPC study on on the parts being made. Perhaps it is a machine issue making the product. So I would not be as worried about only this one part but all others made before it!

Josh.

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#85
In reply to #63

Re: Allowances and Tolerances

09/28/2009 8:03 PM

I think in this particular case if the tolerance is set as tightly as this it is for a reason.

I would believe there is a reason but a tolerance set at ±0.002 is not considered tightly it is commonly thought of as ± a mile

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#92
In reply to #85

Re: Allowances and Tolerances

09/28/2009 9:48 PM

Right! I can do ±0.002" on my cheap home lathe or mill without much difficulty.

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#64

Re: Allowances and Tolerances

09/28/2009 11:33 AM

According to ANSI standards, when dealing with tolerances, there is no such thing as "rounding", either up or down. You can only measure to the precision which the tolerance is stated.

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#65

Re: Allowances and Tolerances

09/28/2009 12:48 PM

I am still here, reading all the responses and trying to figure out, In the grand scheme of things it seems i was correct.?

I write Quality Manuals and get companies certified for AS9100,.

I have been in the Aerospace business for over 25 years, I have been a process mechanic, machinist, programmer etc.. and have always held my belief that when called out to three places, the fourth place should be Zero. sure it will also depend on what that parts is for; Aircraft or clearance for a bolt head, on how much I will allow.

But as an Inspector, and ex-machinist, ± 0.002 was just that ± 0.002 period.

Dan

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#66
In reply to #65

Re: Allowances and Tolerances

09/28/2009 1:11 PM

Absolutely. Unequivocally. Without a doubt. You, sir, are correct.

Wade

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#67
In reply to #65

Re: Allowances and Tolerances

09/28/2009 1:12 PM

YEP... 0.002 IS 0.002 IS 0.002.

However, you should use a tool which is in the same range as the tolerance, and measure only to the degree of accuracy which the tolerance states.

(Standards aside, the logical approach would be to use your over precise measuring tool, and round up or down using a sensible logical approach... Silly standards.)

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#68
In reply to #67

Re: Allowances and Tolerances

09/28/2009 1:35 PM

I'm sorry but you are simply incorrect, you must use a measuring tool with an accuracy of at least ten times the specified measurement. that means a calibrated micrometer measuring to the fourth decimal place in order to measure three decimal places.

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#69
In reply to #68

Re: Allowances and Tolerances

09/28/2009 1:45 PM

Well, I do absolutely reserve the right to be incorrect.

However, that's not what the ANSI standards states. Only the number of decimal places which are given in the tolerance shall be considered(the rest simply get truncated), at least according to the standard. (let the tool do the rounding i suppose?)

Just went through a GD&T course last term, when we were deciding on whether or not to adopt GD&T, or stick with the old +- standards. (we stuck with the old way)

Don't shoot the messenger. (please?)

Anything past the given decimal place on the tolerance is considered to be followed by an infinite number of zeros. (again, just according to the standard)

I absolutely agree, that normally common sense should prevail, and If it were in my shop I would also use a caliper with more accuracy that needed, however, if you were to take it to court, you would most likely want to be able to say yes, I did comply with the standards.. (however, I don't know what standards the shop in question uses, maybe ISO... who knows... )

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#86
In reply to #69

Re: Allowances and Tolerances

09/28/2009 8:10 PM

The point is if the tool only measures to the third decimal place how can you substantively qualify that measurement without a means to do so.

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#70

Re: Allowances and Tolerances

09/28/2009 1:51 PM

Reject that out of tolerance part!

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#71

Re: Allowances and Tolerances

09/28/2009 2:00 PM

I would be more concern with the trend of this part. It is on the extreme outer limit of the spec and if it has been getting worse then it will eventually become out of spec. I also would pass it but it needs to be identified as a concern to address the potential of becoming Non conforming.

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