Decimal places and accuracy.

[loren]

If you were given a print for a part and it stated a dimension as 2.3", what are you to hold the high and low sides to? 2.4" and 2.2"? Loren

[D_R]

If it were me and the title block of the drawing didn't show the general tolerancing, I think I'd ask. Not a good idea to assume anything. The single place decimal could be a mistake.

The most common tolerancing I see on prints is:

.X = +/-1/32"
.XX = +/-.010"
.XXX = +/-.005"
.XXXX = +/-.0001

This is almost a standard in the USA it's used so widely. I've wondered who originated it. And why +/-.005 and not.004 or .006, kind of arbitrary.

[Jacin]

I agree that one should ask the print supplier to clarify.

With that said, while taking drafting in school, we were also taught that precision is implied by the number of decimal places. In anotherwords, if you asked for a dimension of .125 with a tolerance of plus or minus .005 you have a range of .130-.120. Good so far??? ok ok - so what if your part comes in to measure .1304 - is it rejectable since it's over the range?? NOPE! - you've only specified 3 places of accuracy - that means you are technically giving them up to .1309 and so on and so forth. What I am also getting at is that if you ask for a dimension .1 or .10 or .100 or .1000 (same??)- these are all vastly different requirements - we don't drop zeros as one might if you were an accountant. Where this adds confusion is when we use standard fractional conversions - like .0625 ( or a "sixteenth") we can easily get carried away with out ever really considering the implication.

Food for thought anyways!!!

[JEB]

jacin,

i don't know about drafting school but in the working world, if you are given a dimension of .125 plus or minus .005, it would have to measure .1200 to .1300. or however many decimal places you were reasonably expected to measure. anything outside that could or would be rejected.

regards, jim ..... where's harold v when we need him?

[_pete913]

On a properly done print, all tolerances should be shown in the title block, and any deviations should be clearly marked on the print. One of my pet peeves is when I get a print from some 'boy engineer' sans half the necessary dimensions and tolerances, and have to call someone for some numbers that should already be there and then hearing on the other end of the phone, " well, umm, I dunno, what do you think? Hey, what's the difference what I think? TELL ME WHAT YOU WANT HERE. Back in my toolmaking days, we wouldn't even start on a job until all the dimensions and tolerances were verified and initialed, usually by at least 3 people, all the i's dotted and t's crossed, because at that point, when we started cutting steel, we had to know that all the numbers were correct. It just saves a whole lot of confusion and frustration ( not to mention a whole bunch of added costs to the customer, because someone designed something wrong) It is my job as a machinist to build something to print, but it isn't my job to guess at dimensions, tolerances, finishes, fits, etc. If the guy doing the print doesn't know, i'm sure not gonna start guessing at it for him. Hey, I lost my magic wand,, you know, the one that magically informs me of what the customer wants when he can't inform me on the print.

[Jacin]

Sorry Jim, You're gonna have a tough sale on this one. And while I learned this in school I'm talking real world here. EVERY shop we deal with will take EVERYTHING to the limit - PERIOD. There are simply NO implied tolerances (to them) UNLESS it helps them. Bottom line a print is a contract - plain and simple - if you need four decimal places of accuracy you HAVE to ask for it. Plus or minus .005 0 is three places of accuracy - NOT four - and as such you CANNOT reject that part by going to the fourth decimal place.

Is this BOGUS??? It is saplittin hairs ??? Is it simple "grey matter" ??

Well ALL may be true however IF you wanna have the contract upheld you HAVE to make it clear. The ONLY exception to this is if you deferred the grey matter to a standard - like ANSI Y14.5M (and the appropriate revision of such) where there is an attempt to identify and clarify any grey matter.

I KNEW this one was gonna get a "rise" or two - and that's why I threw it out [img]/ubb/images/graemlins/crazy.gif"%20alt="[/img]

Not to be a trouble maker but rather show something that we infact HAVE to deal with in the REAL WORLD. This type of thing HAS bit us before and will likely again, but we do try and avoid it.

Now on the other hand if you are askign what any "self respecting" machinist would interpret that plus or minus .005 really means then I will submit to your point, but the point I wanted to make was that we don't always have the luxury of people "understanding" what we want - we gotta spell it out or bring in the mediators!!!!

[JEB]

ok jacin,

it may be tough sale from me to you. and it might be a tough sale under contract law. but i don't think a supervisor would have any trouble selling it to an employee. [img]/ubb/images/graemlins/grin.gif"%20alt="[/img]

as to the original question... i remember the first time i had a print that gave a 2 decimal dimension with no tolerance stated. i asked the engineer what the tolerance would be. i think he told me plus or minus .020 or .030 ( i can't remember exactly.... i'm retired) but then he told me that if a tolerance was given, it was absolute. i can't imagine a machinist trying to argue that there is a tolerance to a stated tolerance.

jim

[Jacin]

Jim, The point ISN'T that a tolerance has a tolerance - although one could interpret it that way.

The point is that if you ask for 3 decimal places of accuracy - THAT'S what you get - not 4 not 5 not 6 - no matter how the fraction may work out.

And while your point of selling it from a supervisor to an employee may be sound - it has little to do with facts. I do what the boss wants - I don't necessarily always agree with it - that's not my charter.

My point while small, can and does raise it's head in the real world. Maybe not your world, but having been on both sides of the drafting board I have had to defend tolerances on drawings. When you order thousands of dollars of parts - the "supervisor" says it's right doesn't cut the mustard - prints OUGHT to be black and white aka crystal clear - problem is that they rarely are. Modern drafting tools Pro E for instance do not necessarily make things any better as they can automatically dimension a part and will not allow you to double tolerance something sounds pretty good !??!! They also assume center lines are on center without automatically allowing you to give a tolerance to them - which may or may not sound perfectly logical but what happens when the part isn't flat, round, symetrical etc. It gets uglier, but I am straying WAY off topic now [img]/ubb/images/graemlins/crazy.gif"%20alt="[/img]

Tolerances - absolute - sounds nice - even makes sense - won't fly if push comes to shove.

Here's a similar (in a twisted way) comparison. Today I wanted to drive a car I just purchased to a local show, but decided not to since I hadn't talked with my insurance agent yet. My buddy INSISTS that it's a STATE LAW that any newly purchased car is automatically covered for 30 days. And while that was an answer I truely would LOVED to have believed - I wouldn't risk it UNLESS I could see it in writing. To which he replied, "erhh I never saw it in writing - I just know....it's ALWAYS been that way"

nope - NO sale!!!!! Not worth the risk fo rmy new toy - I'll wait and be certain.

Machining in industry is 100 percent about reading the prints. Machinists can't make what I want unless I tell him - if he's got to call me on the phone to clarify - then I likely haven't done my job very well.


Your mileage (an opinion) may vary [img]/ubb/images/graemlins/grin.gif"%20alt="[/img]

[Lew Hartswick]

Pete,You obviously have never worked in a "research" type organization. The place I
worked at (as an electronic engineer) , the "prints" were quite often drawn on a napkin
or most any paper handy by the engineer and the shop person was eminently more
qualified to determine the tollerances required.
...lew...

[JEB]

alright jacin,

i have to admit that you bring up an intersting point. it never occured to me that an argument could be made that + .0015 would be acceptable when + .001 was stated. and yes, it would never have been made in my world. but since you have run into this problem, it's good to be aware that it could happen.

but i still say that the guy at the machine (which kinda relates to the orginal post) better hold the tolerance as stated. [img]/ubb/images/graemlins/smile.gif"%20alt="[/img]

jim

[gwrdriver]

Sounds like you missed a great opportunity to educate the guy so that next time he'll have a better understanding of what you need to do your job.

[pete913]

Lol, I know what you mean about the 'napkin prints'. I get prints sometimes that look more like they were drawn on a bar napkin about 10 minutes before closing time than by someone professing to be a mechanical engineer. Sorry GWR, but me educating these guys doesn't get it. It really isn't my fault that a lot of 'em majored in beer drinking and skirt chasing in engineering school, and somehow managed to sober up enough to pass the final exams. When I bid a job, and build the part, I'm entering into a legally binding agreement to supply said part the way it exists on the print, minus numbers and all. If I start taking things for granted, tolerances, finishes, fits etc, then it becomes MY liability if the part isn't correct. Trust me, that ain't gonna happen. I'd rather call and get numbers confirmed than guess at it, be wrong, and wind up eating the job. In most cases, I simply have no way of knowing what tolerances should be used at which locations on the part etc, because i don't have prints for the rest of the machine the part is being used in sitting in front of me. Believe me, I'm doing them a big favor calling for numbers, and have been told more than once that I really saved some high mucky muck engineers baby soft ass. Trouble is, it takes a lot of time for me to do this, that I don't get paid for, so don't expect me to go all touchy feely trying to educate the supposedly already educated about what they're supposed to already know.