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  1. #21
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    I'm not sure if this applies only to metric stuff, but I am surprised more people are not familiar with the ISO fits/interference charts? IE H6/h6 etc. It's not hard...

    As far as a solid model, it should be modeled at 'nominal' IMO, but it is the machinist/programmers job to check the prints and adjust offsets at the machine to make the part to print, doesn't really matter if the dim is +0/-.005, the 'chinist should check part and make offset adjustment to get the part right.

    The only time I see a problem is if two features are cut with same tool and one is +xx/-0.0 and the other is +0/-.xx --- then the programmer needs to check/adjust model and program both at the mean so the 'chinist can more easily comp tools to get both features in spec.

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    Quote Originally Posted by Mike1974 View Post
    The only time I see a problem is if two features are cut with same tool and one is +xx/-0.0 and the other is +0/-.xx --- then the programmer needs to check/adjust model and program both at the mean so the 'chinist can more easily comp tools to get both features in spec.
    This "only problem" is in basically every case I've come across. Then there's a surfaced boss tangent to the shaft that was modeled at (or beyond) the tolerance limit, which has to be completely remodeled before machining, and then you find that it changes other called radii to exceed their tolerance limits. How hard is it to just model the thing at the midpoint of tolerance? Maybe the designer can save a few seconds by modelling it at 5" exactly and slapping on an "H6" note, but then that few seconds is multiplied many times over by everyone else down the road that has to work with it. It's lazy, and you're telling everyone that looks at the print that you think your time is several times more valuable than theirs.

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    Yes !
    No.

    The part should be modelled perfectly/nominal/not-accurate, unless You are able to model and spec to bearing and shaft fits, and callout to them.
    Once modelled to nominal, callouts should define desired fits and sizes.

    E.g. 7210 bearing, P2, should have a callout 40.000 mm, +0.000, -0.003 mm.
    And the bore in the model should be 40.000 mm unless you can do more accurate,
    and the shaft/spindle should be 40.000 mm theoretical unless you can do more accurate actually in your 3d model.

    Modelling to real-world accuracy in precision assys is hard.
    The models become very big, like 1GB in size.
    Specialist CAD PCs are required to use them efficiently.
    And modelling edge breaks, etc. is a lot of work.


    Quote Originally Posted by mhajicek View Post
    This "only problem" is in basically every case I've come across.
    Then there's a surfaced boss tangent to the shaft that was modeled at (or beyond) the tolerance limit, which has to be completely remodeled before machining, and then you find that it changes other called radii to exceed their tolerance limits. How hard is it to just model the thing at the midpoint of tolerance? Maybe the designer can save a few seconds by modelling it at 5" exactly and slapping on an "H6" note, but then that few seconds is multiplied many times over by everyone else down the road that has to work with it. It's lazy, and you're telling everyone that looks at the print that you think your time is several times more valuable than theirs.

  6. #24
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    [QUOTE=mhajicek;3458148]This "only problem" is in basically every case I've come across. Then there's a surfaced boss tangent to the shaft that was modeled at (or beyond) the tolerance limit, which has to be completely remodeled before machining, and then you find that it changes other called radii to exceed their tolerance limits. How hard is it to just model the thing at the midpoint of tolerance? Maybe the designer can save a few seconds by modelling it at 5" exactly and slapping on an "H6" note, but then that few seconds is multiplied many times over by everyone else down the road that has to work with it. It's lazy, and you're telling everyone that looks at the print that you think your time is several times more valuable than theirs.[/QUOTE]

    That is the definition of engineering, ya?

    Seriously though, if a company is paying an engineering firm/contractor $200/hr or so, they are probably directing them to do it as fast as possible. Right or wrong, seems to be the way it works...

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    Quote Originally Posted by hanermo View Post
    Yes !
    No.

    The part should be modelled perfectly/nominal/not-accurate, unless You are able to model and spec to bearing and shaft fits, and callout to them.
    Once modelled to nominal, callouts should define desired fits and sizes.

    E.g. 7210 bearing, P2, should have a callout 40.000 mm, +0.000, -0.003 mm.
    And the bore in the model should be 40.000 mm unless you can do more accurate,
    and the shaft/spindle should be 40.000 mm theoretical unless you can do more accurate actually in your 3d model.

    Modelling to real-world accuracy in precision assys is hard.
    The models become very big, like 1GB in size.
    Specialist CAD PCs are required to use them efficiently.
    And modelling edge breaks, etc. is a lot of work.
    Huh? Only if it is complex 3d work IMO. I can throw a fillet/chamfer on edges in a very old version of Mastercam pretty damn fast. That said, I don't often need to see/have a chamfer modeled on the part as it is a pre-defined(?) toolpath in Mastercam -

    contour>chamfer>parameter settings (chamfer size, tip offset)...

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    Quote Originally Posted by hanermo View Post
    Modelling to real-world accuracy in precision assys is hard.
    The models become very big, like 1GB in size.
    Specialist CAD PCs are required to use them efficiently.
    And modelling edge breaks, etc. is a lot of work.
    Modelling something at 4.9976" does not take one iota more computer resources than modelling it at 5.0000". The CAD software is internally calculating everything to umpteen decimal places already regardless.

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    Quote Originally Posted by hanermo View Post
    The part should be modelled perfectly/nominal/not-accurate, unless
    Once modelled to nominal, callouts should define desired fits and sizes.
    Huh? Then why the [email protected] do you bother creating a model in the first place?
    Doodle something on a napkin, and type up a spec sheet...

    Quote Originally Posted by hanermo View Post

    E.g. 7210 bearing, P2, should have a callout 40.000 mm, +0.000, -0.003 mm.
    And the bore in the model should be 40.000 mm unless you can do more accurate,
    and the shaft/spindle should be 40.000 mm theoretical unless you can do more accurate actually in your 3d model.
    And in my shop, that gets you a bonus charge for having to re-create the solid model for you.
    I always interrogate the critical dimensions on any and all incoming models.
    If I find a single feature that is not modeled to MEDIAN dimensions, I will deem the entire solid model as not trustworthy, therefore useless for manufacturing.


    Quote Originally Posted by hanermo View Post

    Modelling to real-world accuracy in precision assys is hard.
    WTF?
    Modeling to "real world" is hard?
    Have you tried to machine to "pipe dream" accuracy and precision ?

    And as mentioned before, the word "Nominal" must be either not used, or it's meaning clearly defined and agreed upon.
    In our case, in our shop, the word NOMINAL DIMENSION refers to the MEDIAN, MIDDLE, TARGET ( or whatever else you want to use ) dimension
    around which a symmetrical +/- tolerance is given.
    Intent of Engineer with a 40mm -x / -y tolerance be damned.

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  12. #28
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    Quote Originally Posted by mhajicek View Post
    Modelling something at 4.9976" does not take one iota more computer resources than modelling it at 5.0000". The CAD software is internally calculating everything to umpteen decimal places already regardless.
    Quote Originally Posted by SeymourDumore View Post
    Huh? Then why the [email protected] do you bother creating a model in the first place?
    Doodle something on a napkin, and type up a spec sheet...



    And in my shop, that gets you a bonus charge for having to re-create the solid model for you.
    I always interrogate the critical dimensions on any and all incoming models.
    If I find a single feature that is not modeled to MEDIAN dimensions, I will deem the entire solid model as not trustworthy, therefore useless for manufacturing.
    I get it, I really do, I am a cad/cam guy-programmer now, more than a tool&die person (these days)... buuuttt.... I learned a long time ago that I needed to use both -cad/cam/solid models AND prints to properly produce a part...
    I'm not trying to defend the guys that need a 3.996" part +/-.001 and model it to 4" 'cuz they are lazy...buuutt (see more below)---

    [/QUOTE]
    WTF?
    Modeling to "real world" is hard?
    Have you tried to machine to "pipe dream" accuracy and precision ?

    And as mentioned before, the word "Nominal" must be either not used, or it's meaning clearly defined and agreed upon.
    In our case, in our shop, the word NOMINAL DIMENSION refers to the MEDIAN, MIDDLE, TARGET ( or whatever else you want to use ) dimension
    around which a symmetrical +/- tolerance is given.
    Intent of Engineer with a 40mm -x / -y tolerance be damned.[/QUOTE]

    Sometimes it really is a PITA to make a model to say--- 2.9933 (nominal (I know!) LOL) and have to dimension everything off centerline (for example), and have to divide 2.9933 by 2 to produce centerlines and such... much easier to make it 2.994 (easier to divide by 2) then call it out as 2.993 +/- .xxx


    I know, pc makes it easy to divide/multiply/add/subtract etc etc, but when designing something, easier to use whole numbers... at least for me..

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    Quote Originally Posted by hanermo View Post

    E.g. 7210 bearing, P2, should have a callout 40.000 mm, +0.000, -0.003 mm.
    If your part is 40 and minus only where would you model it?
    If your maker or source can not understand a minus only tolerance perhaps you need a better place for making your parts.
    If as a manufacturing shop this is a problem from the model then perhaps you should stay away from such work.
    This is like brain dead. There is a print, model and tolerance. If this is all confusing go sit on the porch and watch the big dogs run.
    If you come to me with this as a machinist I will make you unemployed. Getting this is the job that you are paid for. If too complicated for you, go serve burgers.
    There is no excuse for not understanding a one sided tolerance or the model at zero. This falls under not thinking and without thinking I have robots.
    Bob

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    Quote Originally Posted by Mike1974 View Post

    Sometimes it really is a PITA to make a model to say--- 2.9933 (nominal (I know!) LOL) and have to dimension everything off centerline (for example), and have to divide 2.9933 by 2 to produce centerlines and such... much easier to make it 2.994 (easier to divide by 2) then call it out as 2.993 +/- .xxx
    Mike, you have got to be fucking kiddin' me ( and everyone else ) with this!!!
    It is the damned B/P that has to say 2.9933 +/-.0002 ( or whatever ) for the tolerance, but the fucking modeling process shouldn't give one shit or another!
    If you can model the fucking hole to be 2.994 for your personal easement, then why can you not model it to be 2.9933, which is exactly what you fucking want the fucking thing to be!!!

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    Getting heated. I wonder who is going to win...

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    Quote Originally Posted by Mike1974 View Post
    I get it, I really do, I am a cad/cam guy-programmer now, more than a tool&die person (these days)... buuuttt.... I learned a long time ago that I needed to use both -cad/cam/solid models AND prints to properly produce a part...
    I'm not trying to defend the guys that need a 3.996" part +/-.001 and model it to 4" 'cuz they are lazy...buuutt (see more below)---

    WTF?
    Modeling to "real world" is hard?
    Have you tried to machine to "pipe dream" accuracy and precision ?

    And as mentioned before, the word "Nominal" must be either not used, or it's meaning clearly defined and agreed upon.
    In our case, in our shop, the word NOMINAL DIMENSION refers to the MEDIAN, MIDDLE, TARGET ( or whatever else you want to use ) dimension
    around which a symmetrical +/- tolerance is given.
    Intent of Engineer with a 40mm -x / -y tolerance be damned.

    Sometimes it really is a PITA to make a model to say--- 2.9933 (nominal (I know!) LOL) and have to dimension everything off centerline (for example), and have to divide 2.9933 by 2 to produce centerlines and such... much easier to make it 2.994 (easier to divide by 2) then call it out as 2.993 +/- .xxx


    I know, pc makes it easy to divide/multiply/add/subtract etc etc, but when designing something, easier to use whole numbers... at least for me..


    Not to be a jerk... But your shop uses nominal incorrectly.

    Nominal = Name = What's written.

    Mean = Average = Midpoint

    For a 1.0" +0/-.010" dimension, nominal is 1.0", mean (where most of us would program/cut to) is .995".
    Period.

    You can use them however you like, but semantically, you are incorrect.

    That said, nobody in my shop uses them correctly either, not even the engineers, which drives me nuts.

    Don't get me started on the funny looks I get when I say "radii" instead of "radiuses".

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    I'm still looking for the knob on the machine that sets the tolerance.

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    Quote Originally Posted by Conrad Hoffman View Post
    I'm still looking for the knob on the machine that sets the tolerance.

    It's the guy pushing the button.

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    Quote Originally Posted by TeachMePlease View Post
    Not to be a jerk... But your shop uses nominal incorrectly.

    Nominal = Name = What's written.

    .
    Teach, you are not a jerk, but you are actually wrong in this particular case.

    You see, when the job hits the floor, not only is it already processed, but an opsheet is created for the setup guys and the operators alike.
    If the incoming B/P called for a 4.000 -.001/-.003 dia shaft, when the opsheet is created it will have only a 3.998 +/-.001 on it.
    It will not have ANY mention of ANYTHING being 4.00 whatever, because it is completely irrelevant to anything we do on the floor.

    So, - in our shop - we aren't changing the definition of the word Nominal, rather redefining the actual Nominal dimension.
    IOW, your nominal 4.00 dia becomes 3.998, which is nominal for us.

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    And this is why use of the word "Nominal" is a mistake. It's not as if this is the first time it's come up. Literally the definition is exactly what it is defining, regardless of what it is in fact defining. It could be value associated with Bananas in a bunch, as long as the context is correct, the word used is correct.

    R

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    Quote Originally Posted by litlerob1 View Post
    Getting heated. I wonder who is going to win...
    No Rob, it isn't gettin' heated, nor is it about who is going to win.

    Obviously, the Engineer who can't be bothered to spend a little extra time designing the thing properly is going to win.

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    Quote Originally Posted by SeymourDumore View Post
    Teach, you are not a jerk, but you are actually wrong in this particular case.

    You see, when the job hits the floor, not only is it already processed, but an opsheet is created for the setup guys and the operators alike.
    If the incoming B/P called for a 4.000 -.001/-.003 dia shaft, when the opsheet is created it will have only a 3.998 +/-.001 on it.
    It will not have ANY mention of ANYTHING being 4.00 whatever, because it is completely irrelevant to anything we do on the floor.

    So, - in our shop - we aren't changing the definition of the word Nominal, rather redefining the actual Nominal dimension.
    IOW, your nominal 4.00 dia becomes 3.998, which is nominal for us.
    No he is right and you and your shop are wrong. Nominal implies no precision or even necessarily a true size. Raw bar stock would be nominal. Pipe sizing is nominal. 2” pipe has no dimension that is 2” and it’s literally called nominal pipe size. A 2x4 is nominal.

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    Quote Originally Posted by JCByrd24 View Post
    No he is right and you and your shop are wrong. Nominal implies no precision or even necessarily a true size. Raw bar stock would be nominal. Pipe sizing is nominal. 2” pipe has no dimension that is 2” and it’s literally called nominal pipe size. A 2x4 is nominal.
    You post a link to anything that backs you up. Sorry, but your information is not correct or accurate.

    Definition of the word in context of Engineering.

    The aeronautical sense of nominal derives from engineering where the nominal value is the specified dimension and the reference point for tolerances. ... (engineering) A permissible deviation from a specified value, expressed in actual values or more often as a percentage of the nominal value.

    But even the Web consists of conflict, because the very nature of the word is relative to the environment, the conversation, the background and the teaching chain.

    R

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    Quote Originally Posted by JCByrd24 View Post
    Nominal implies no precision or even necessarily a true size.
    Eggzatctly!
    It implies nothing more and nothing less than what is intended to imply.

    In OUR shop and OUR drawings, NOMINAL means what you're aiming for.
    If the word Nominal refers to a dimension, then it is the middle of a symmetrical tolerance band.
    If it is a material for say EDM fixturing, then it's nominally 17-4PH stainless first, then use whatever other SS you have laying about with AL as last resort.
    If it refers to tooling, then the nominal means the strongest and most appropriate on hand, else use whatever is available.

    See, there is no right or wrong, just interpretation of the intention.


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