"Loving" how customers state dimensions and tolerances like this . . . - Page 2
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  1. #21
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    Quote Originally Posted by eKretz View Post
    That's adding COST to the job for no good reason.
    Fixed it for you.

    Additional time figuring it out to machine and inspect, additional scrap parts, perhaps wrong reamer or pin gauges ordered, all adds $$.

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  3. #22
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    Quote Originally Posted by mhajicek View Post
    Fixed it for you.

    Additional time figuring it out to machine and inspect, additional scrap parts, perhaps wrong reamer or pin gauges ordered, all adds $$.
    Yeah, time is money... All that was meant to be implied; but absolutely correct.

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    I can't really see the big deal here with +/+ ...

    you add both + together and divide, add this to the nominal and you have your median, and the divided tolerance number is your +/- if you really need it that much

    but if you start off with calculating the abs low, then abs high and then you look for the median between these 2, you just made the possibility of an error quite a bit higher

    the +/+ actually tells me something, it tells me that something with that nominal size has to fit in there, sliding fit judging from the numbers but not loose, but a 0.28255 +/-0.00125 says just that and nothing else

    and I find it also funny when you work with 5 digits behind comma and complain about metric dimensions, we have 2 behind comma for general machining, and 3 if you can afford it, much easier on the eyes this way IMO (probably a habit)

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  7. #24
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    Quote Originally Posted by mhajicek View Post
    The purpose of a print is to communicate the part requirements to the machinists and inspection. Anything that makes it more difficult to machine and inspect is suboptimal.
    No... The purpose of the print is to describe a functional part..

    Lets go simple. A plate with 4 bolt holes in it. In a square. Simple.. 1 thing
    bolts to those 4 holes...

    You are saying that those bolts holes should be dimensioned from the upper left hand
    corner.. But what if the outside of the plate doesn't matter, it getting welded to the
    end of a square tube, but the bolt holes have to be ±.010" from each other?

    So now we have a dimension from a corner of ±.060" to the first bolt hole, and then the
    bolt holes are ±.010 from each other. A little harder for the machinist, probably easier
    for QC, but we are describing a functional part.. Now lets throw a little GD&T in there.
    If you drill the hole this size, you can be .015 out this way, but only .006 out that way.
    The functionality is described even better. Harder for QC, and theoretically harder for the
    machinist.

    So lets stick with the same square plate, and kick that 4 hole bolt pattern off at a 30 degree
    angle.. If we dimension from the corner, we now can't use a shitty plasma cut square.. We
    need a true edge.. So lets dimension it from the first bolt hole.. Using only X and Y dimensions.
    Now we are easier for the machinist, but a lot harder for QC.. And we would now probably have to
    tighten up the tolerance.

    If we dimension to one hole from the corner, and rotate, we make it harder for the machinist,
    easier for QC.. Add some GD&T into that, and its pretty much harder for everybody, but more
    accurately describes a functional part. This holes center has to be within this circle.. If
    the hole is bigger, it can be within this bigger circle. (which usually makes it easier for the
    machinist if they know how to read it).

    Even on a turned part.. Ever seen a dimension that doesn't come from the end? Lets say
    ± .030 to a shoulder, and then coming back +.003-.000 to a c-clip groove that holds a bearing
    up against that shoulder. Where that bearing is is barely important, but where that C-clip is
    compared to that shoulder, is important.

    To make that work with "simple" dimensioning from the end, the location of the shoulder would have
    to be ±.0015 and also the location of the groove.

    The purpose of the print is to describe a functional part at the limits of where it can work.

    If it wasn't GD&T wouldn't exist.

    I'm not saying there aren't places where they couldn't make it easier, and still say the same thing,
    I'm not saying that every print is perfect, or is dimensioned properly or has the correct datums..
    I've met a LOT of prints that if made "TO PRINT" a certain way, would not work. Say a slot that
    was dimensioned on both sides from the edge of the part, which potentially could make a slot
    that the mating part would not fit into.

    I'm also not saying that there aren't a TON of times where a simple, dimensioned from the corner
    print is more than adequate. Or a turned part, where everything is dimensioned from the end. 90%
    of the time, that is fine and dandy. Functional, easy to inspect, easy to make.. But.... Not always.

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    Quote Originally Posted by Bobw View Post
    No... The purpose of the print is to describe a functional part..
    ..to the guys who have to make it (the machinists and programmers) and the guys who have to inspect it (inspection). As I said.

    Quote Originally Posted by Bobw View Post
    You are saying that those bolts holes should be dimensioned from the upper left hand
    corner..
    I am saying nothing of the kind. I can read GD&T just fine. Put your datums where you think is best, that's fine. Just don't make me waste my time and your money playing math games and increasing the chance of error (by however little, is still an increase) for no benefit. Using asymmetric tolerances doesn't tell me a dang thing that I need to know, I'm going to shoot for the midpoint of the tolerance band every time, because that maximizes my chances of making acceptable (salable) parts.

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    Quote Originally Posted by TeachMePlease View Post
    Just use the spec MIL-TFP-41. You'll never have another issue.
    Most important Mil spec EVAH!

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    I gotta go with Bob on this one. Maybe it's just because the shop I'm in uses ISO stuff (not all the time, but a lot of our drawings come from Germany) and so that's how I was taught, you double check your tolerances, and if it's +/+ or -/- it adds absolutely ZERO time or cost, because I was already looking for it. If the designer is good, it shows design intent. If I have two mating parts, say a hole and a shaft, I expect the hole to have a +/+ and the shaft to have a -/-... It clearly conveys to me that no matter where they are in the tolerance band, that shaft absolutely MUST fit in that hole.

    I dunno, different worlds, I guess, but to me, it's just another drawing, nothing to get worked up about.

    Now, if they give me 4 holes on a plate, dimensioned to each other, but not to any datum on the plate itself... Then I get annoyed.

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    Quote Originally Posted by mhajicek View Post
    Using asymmetric tolerances doesn't tell me a dang thing that I need to know, I'm going to shoot for the midpoint of the tolerance band every time, because that maximizes my chances of making acceptable (salable) parts.
    Some people say to shoot as close to the zero point as you can.. +0 -5, they say
    to get as close to that zero as possible.. 99% of the time, I do the same as you,
    shoot for the middle.

    Though on occasion, especially if I know where its going, I stay as far away from that
    zero as possible, since that is usually the limit of where the part will work.

    On the unilateral vs symetrical tolerances. I do a ton of parts that all eventually
    go together, and almost all of them have cross holes for roll pins.. The holes are
    all .126 to .129... And its written about every possible way you could imagine.
    .126 +3-0 129 -3+0 .1275 ±.0015 .127 +2 -1... And its the same hole for the same
    pin, just on different parts.. Must be what the engineer felt like that day.

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  16. #29
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    Quote Originally Posted by TeachMePlease View Post
    I gotta go with Bob on this one. Maybe it's just because the shop I'm in uses ISO stuff (not all the time, but a lot of our drawings come from Germany) and so that's how I was taught, you double check your tolerances, and if it's +/+ or -/- it adds absolutely ZERO time or cost, because I was already looking for it. If the designer is good, it shows design intent. If I have two mating parts, say a hole and a shaft, I expect the hole to have a +/+ and the shaft to have a -/-... It clearly conveys to me that no matter where they are in the tolerance band, that shaft absolutely MUST fit in that hole.

    I dunno, different worlds, I guess, but to me, it's just another drawing, nothing to get worked up about.

    Now, if they give me 4 holes on a plate, dimensioned to each other, but not to any datum on the plate itself... Then I get annoyed.
    Gotta disagree with you here, respectfully of course.

    bullshit.jpgbullshit1.jpgbullshit2.jpgbullshit3.jpg

    Just saying, not a hard and fast rule. I worked on this part and I've got a total of 9 one way tolerance dimensions. 27.5MM +1/0 (WHOA!), 50MM +.3/+.2, 2x 36.2MM +2./0. (ANOTHER WHOA!), a slot width called out for 8.1MM +1./+.5, a bunch of stuff called out for max and designed at max.

    Come on man, one or two things are reasonable, but sometimes it just gets way out of hand.

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    Quote Originally Posted by ttrager View Post
    Yes, I'm being a crybaby this morning. Haven't had my coffee yet.

    DIM: ".2813 +.001 / +.0015". Therefore, .2813 means nothing to the part feature. .2823 is the LOW, .2828 is the HIGH. .28205 is median.

    We re-stated the DIM and tolerance differently on our reprint, and it got past our re-printing review because in the moment brain-translations did the math wrong, between two people.

    Our shop foreman has his own policy of reprinting, so that's not going away (stated because others here have mentioned in the past stop reprinting).

    I'm annoyed. Annoyed two of us MISSED this on our reprint (caught in time, job OK). But I'm also annoyed that a physical DIM statement on a blueprint baselines to nothing unless you math the whacky tolerance to find What the DIM is actually supposed to be. .2813 actually is not the DIM.

    Is there some esoteric Voodoo I'm noobishly ignorant of that suggests it is good practice to state a DIM and Tolerance like this?

    /pre coffee crying over
    Hey every engineer is going to tolerance a drawing differently depending on their mood that day. Some of our engineers tolerance a precision bore for a bearing as 2.4320 +0.000 -0.0004 so basically you better not hit the nominally called out number for the dimension or it is SCRAPPED very likely. I've even seen somethihg stupid like - - callouts eg. 2.4320 -0.0001 -0.0004 so if you hit the nominal callout you DEFINITELY fucked up

    They're all stupid and they all suck so it's better to just accept it and move on. Unless the callout is egregious.

    What actually pisses me off though is an engineer who doesn't bother tolerancing anything. 2.50"...okay cool. Make it to 2.5000000000000" and they come back asking for a rework because that feature is actually a slide fit for a mating component. OOOOHHHHHHH that would be nice information to have....

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    at least you guys get specs, had a customer have on his print, " machine to general tolerances". ok, sorry wtf is that? what tolerance would you like? an inch or a mile? or am I just going with banana specs?

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    Quote Originally Posted by metalmadness View Post
    Hey every engineer is going to tolerance a drawing differently depending on their mood that day. Some of our engineers tolerance a precision bore for a bearing as 2.4320 +0.000 -0.0004 so basically you better not hit the nominally called out number for the dimension or it is SCRAPPED very likely. I've even seen somethihg stupid like - - callouts eg. 2.4320 -0.0001 -0.0004 so if you hit the nominal callout you DEFINITELY fucked up

    They're all stupid and they all suck so it's better to just accept it and move on. Unless the callout is egregious.

    What actually pisses me off though is an engineer who doesn't bother tolerancing anything. 2.50"...okay cool. Make it to 2.5000000000000" and they come back asking for a rework because that feature is actually a slide fit for a mating component. OOOOHHHHHHH that would be nice information to have....
    These are some of the reasons our shop foreman reprints. As an example, when a customer states an OAL on a 8" to 14" molding pin as 9.7988 -0 / +.0005, it's reprinted as 9.7993 -.0005 / +0. Similarly for IDs: stated at the low (-0) to a high (+.0002) so stock is available to the machinist to sneak into with a Go / No-Go check using a Class X gage pins, or a Sunnen Bore Gage on larger IDs.

    I understand everything you've said, and don't necessarily disagree with it.

    Thanks for the reply.

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    When an engineer uses tolerances like +.0004/+.0010, it's been modeled at +0. That means a part machined perfectly to the model is bad. Now if all you're doing is round shafts, or plates with holes, that can work. But for complex 3D shapes, that usually (in my experience) results in impossible geometry. Sometimes even for 2D shapes.

    Last place I worked we had a medical hexalobe driver (Torx-like), where the engineer did this. Machining was outsourced to a shop with Swiss machines, their demand was high so lead times were long (four to six months). We got batch after batch that didn't pass receiving inspection, and couldn't figure out why until I interrogated the model that we sent them, and then tried to make a model that matched the print. The tolerances described a geometric impossibility. When the shop was asked about it, they said they fudged it best they could because our model and print were F'd up. They'd been doing it for years, and we only began to catch it when we improved our inspection methods. This put the company in a bind, with no hexalobe drivers to put in our device kits.

    If you have a rule that every model must be at the midpoint of tolerance, you'll never have this kind of (very expensive) problem.

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  23. #34
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    Quote Originally Posted by Bobw View Post
    Some people say to shoot as close to the zero point as you can.. +0 -5, they say
    to get as close to that zero as possible.. 99% of the time, I do the same as you,
    shoot for the middle.

    Though on occasion, especially if I know where its going, I stay as far away from that
    zero as possible, since that is usually the limit of where the part will work.

    On the unilateral vs symetrical tolerances. I do a ton of parts that all eventually
    go together, and almost all of them have cross holes for roll pins.. The holes are
    all .126 to .129... And its written about every possible way you could imagine.
    .126 +3-0 129 -3+0 .1275 ±.0015 .127 +2 -1... And its the same hole for the same
    pin, just on different parts.. Must be what the engineer felt like that day.
    You need to know the application to do things like this, but I agree. If you have no idea of the application, it's safe to shoot for the middle. If you know for instance that a shaft is going to be a sliding fit in an assembly, it may be a good idea to shoot for the side of the tolerance that gives the most clearance. There are times when one would rather prefer to favor the least clearance as well. But if you don't know, the middle is a good place to be.

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    So the next time I put a print into our shop, it'll have a dimension 10, -5, -4.999 inches. And let the fun begin!

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    Quote Originally Posted by mhajicek View Post
    The purpose of a print is to communicate the part requirements to the machinists and inspection. Anything that makes it more difficult to machine and inspect is suboptimal.
    This x 1BillTrillion


    Quote Originally Posted by Bobw View Post
    No... The purpose of the print is to describe a functional part..
    Respectfully no Bob.
    A print can just state
    Part #
    Revision
    Part Name
    Material/Spec
    Finish
    and general tolerance of +/-0.005" for all features inspected to CAD data.
    With a statement/note stating something like "Manufacture and Dimensionally Verify to CAD Model".

    So the drawing has no GD+T.
    No dimensions at all.

    Part can then be made, and verified via CMM checking to CAD Model (providing model is nominal which as a bitmaker we wouldn't know but the CAD jockey/customer should....)

    What you described, is the correct way to tolerance of course. Related features to eachother correctly toleranced to eachother, and not from one datum face and dims all struck from that face.


    Quote Originally Posted by Bobw View Post
    Some people say to shoot as close to the zero point as you can.. +0 -5, they say
    to get as close to that zero as possible.. 99% of the time, I do the same as you,
    shoot for the middle.
    Again +1xBillTrillion to this.
    We have had posts on this subject here over the years where it has caused big disagreements.
    Namely if a drawing states 1.000+0/-.01", the minority say the drafty detailing this obviously wants the part to be as close to 1.000 as possible because that's the size he has on the print, so make it to this.
    Where my take is with the majority, who say bollox, we aim for (0.995) mid limit.


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    Ours is not to reason why but to do or die.

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    Quote Originally Posted by CarbideBob View Post
    Ours is not to reason why but to do or die.
    and usually on a daily basis too....

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    Quote Originally Posted by Bobw View Post
    No... The purpose of the print is to describe a functional part...
    Bob is most correct here. The purpose of the print is to describe a functional part AND to document the intention of the nominal design. ie there is design information here, in perpetuity, for the engineering department. The print is not to make the machinist's job or the inspector's job easier. It calls for the nominal size and also provides acceptable tolerances.

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    Quote Originally Posted by BT Fabrication View Post
    at least you guys get specs, had a customer have on his print, " machine to general tolerances". ok, sorry wtf is that? what tolerance would you like? an inch or a mile? or am I just going with banana specs?
    I get this all the time. It basically refers to how the dimension is called on the print if it has no specified tolerance.
    One place decimal = +/- .050", two place decimal = +/- .015", three place decimal = +/- .005".
    At least that is how I handle it. Even on napkin sketches.
    When I get a solid and no print (quite often) I just ask what its for (if I can't tell) and use my best judgment.


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