Engineer Seeking Knowledge on Turned Parts
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    Default Engineer Seeking Knowledge on Turned Parts

    Hey all,

    I have had a question for a few years regarding parts that get turned. If this is this is the incorrect place to ask this on the forum, please move according and I apologize.

    As an example part, lets keep it easy. A 10" long cylinder with 3 different diameters along it. One being 2", next being 3", and the last 3.5" in diameter. The lengths of those sections are arbitrary. If the tolerance on those diameters is +/-.005 without using further tolerance like GD&T, what guarantees on a print that they are made about the same axis? I know I can use GD&T to make one of those a reference and do total run-out on the other two surfaces, but without doing that, whats to say one of those diameters is machined on one axis, and the next on an axis .5" offset (exaggerate).

    Thanks all!

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    You need to specify concentricity on the D&T drawing. Mark one of the diameters as "A" or whatever, then specify that the other diameters need to be concentric to A within x.xxx or whatever.

    See this -- Concentricity – GD&T Basics

    Regards.

    Mike

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    Thanks Mike. What I gather, without GD&T on a round part, nothing is guaranteeing it is being turned about the same axis? Is there nothing in ANSI that says if an axis is drawn on a round part, the part is to manufactured around that axis? I realize the GD&T gets you better function and mass distribution, but what if the part is stationary for example.

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    Without GD&T nothing is guaranteed, although I have seen some prints with a note specifying " all features on a common axis must be within .xxx TIR"

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    If it's a turned part, concentricity is going to be as close as the machine spindle runs out. GD&T has not been the help to industry that the engineering community had hoped for. Notes on the drawing are usually much more meaningful than GD&T can EVER be. A note that says diameters "A,B &C" must be concentric within +/- xxx is much more helpful and meaningful.

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    So before GD&T, were all round parts assumed about a common axis? Or, were people putting notes similar to that saying "manufacture the part based on a common axis"? These are all questions that have been on my mind forever regarding round parts. A square plate, while yes I know it can be GD&T, can get by with ordinate dimensioning none critical features. The round part really throws a wrench at that, especially when I think about a round part more like a pipe! We don't define an axis other than the centerline mark, at least I have not went beyond that in my few years designing.

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    Hi All:
    Joe Miranda wrote:
    GD&T has not been the help to industry that the engineering community had hoped for. Notes on the drawing are usually much more meaningful than GD&T can EVER be.

    The main problem with GD&T that I've become aware of over the years is how badly and inconsistently it's applied.
    There are an awful lot of engineers and designers who are poorly trained, and the drawings they produce are more ambiguous and more confusing than if they'd just left all the goddam spaghetti off the picture in the first place and put in some notes as Joe suggests.

    GD&T is a wonderful concept, but it is arcane and complex; and therein lies its fatal flaw.
    In order to describe a part and the allowance for it's variability from nominal you need three things one or more of which are usually missing:
    1) A designer who has actually thought about what matters and most critically what he can get away with without making unusable stuff.
    2) A designer who is fluent in ALL of the details of how the GD&T code works so the intent is clearly and unambiguously applied
    3) An audience who is as fluent in the language of GD&T as the author is, and can understand what the designer requires.

    We have all seen ample evidence of that disconnect between what is designed, what is wanted and what is produced.

    Given that reality, I'm not a fan, never have been, and probably never will be.
    But it's the reality we work in, so I've sucked up my natural distaste and learned enough about it to be dangerous.
    However, every time I see a drawing with poorly applied GD&T pretensions I groan first before I buckle down and decide how much effort to put into the quote.

    Cheers

    Marcus
    Implant Mechanix • Design & Innovation > HOME
    Vancouver Wire EDM -- Wire EDM Machining

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    Marcus, I 100% agree with what you said. I was not taught GD&T in my bachelors program. In industry, what I find is people copy what past people did for GD&T and use that to defend why they are making their part that way. My question for you then is the same I have been asking Marcus, what guarantees a round part is made about a common axis without GD&T? Does the centerline/axis carry heavy weight on a round part defining a centerpoint even though it is not dimensioned?

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    Quote Originally Posted by prangy89 View Post
    So before GD&T, were all round parts assumed about a common axis? Or, were people putting notes similar to that saying "manufacture the part based on a common axis"?
    For run-of-the-mill work it is an assumption. When it matters, people put a note on the surfaces or in the title block indicating T.I.R.

    "Total Indicator (or Indicated) Runout" will getcha there.

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    Quote Originally Posted by Joe Miranda View Post
    Notes on the drawing are usually much more meaningful than GD&T can EVER be. A note that says diameters "A,B &C" must be concentric within +/- xxx is much more helpful and meaningful.
    ^^^^^^^^GD & T is a well-meaning attempt to circumvent language barriers in engineering. Unfortunately it has never in practice become a common language, because it lacks nuance. A concisely written note in a drawing more accurately conveys the design intent which is helpfulness defined.

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    Hi prangy89:
    Often a perceptive designer who has been around machine shops will understand enough about how the part will typically be made to know where he (or she) must specify a callout to get what is intended.
    In your example with stepped diameters on a round shaft, the processing method is usually pretty obvious and since the biggest uncertainty is typically around re-fixturing for a second op, a designer who knows his shit will be sure to include an alert there.
    A good example is a shaft with three short steps each of a larger diameter than the last, and then a neck down back to a smaller diameter.
    If the proportions of the part allow turning all three steps of ascending diameter in one chucking, it's pretty common to assume they will be as concentric as the machine is capable of spinning, whereas the last neck down step is the problem one and will either be re-chucked, or back turned.

    That's an obvious case and a single callout in the form of a note specifying the concentricity of all features will ensure proper attention is paid to this issue and the manufacturer then chooses his method to satisfy the stated requirement but still make money on the part.
    Even though the ascending steps will all be as good as or better than the callout, the requirement does not invoke special consideration for those features because the maker gets most of it for free; in other words those three steps will be concentric simply by virtue of the fact that any competent machinist will turn them all in one chucking, and will do them on a decent lathe.
    The callout defines the care with which the LAST step must be processed relative to all the other steps.

    This is a pretty common assumption when things are not explicitly stated but it creates potential for an unusable part to fall within the stated constraints of the drawings, which is why GD&T began in the first place.
    Just like a common assumption that all dimensions called out to however many decimal places implies a tolerance band of whatever: lots do it but it's risky unless explicitly stated on the drawing, especially if things proceed to a lawsuit over a failed or unusable part.

    So yeah, the centerline and some common sense are quite commonly assumed to adequately define a part, but when the shit hits the fan, it's probably not defensible in a court of law.
    If the drawing is ambiguous in any way, the onus is typically on the producer of the document so far as I know, but any lawsuit is such a shit-miserable experience to go through that most smart makers will ask first and save themselves some pain.

    Cheers

    Marcus
    Implant Mechanix • Design & Innovation > HOME
    Vancouver Wire EDM -- Wire EDM Machining

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    Quote Originally Posted by prangy89 View Post
    Marcus, I 100% agree with what you said. I was not taught GD&T in my bachelors program. In industry, what I find is people copy what past people did for GD&T and use that to defend why they are making their part that way. My question for you then is the same I have been asking Marcus, what guarantees a round part is made about a common axis without GD&T? Does the centerline/axis carry heavy weight on a round part defining a centerpoint even though it is not dimensioned?
    Nothing really. BUT I would say unless the shop is a real hack and slash I would *expect* them all to be within a thou or two (depending on manul lathe or cnc and condition of machine). That said, I think most people would assume you want them all on the same axis / centerline unless otherwise noted. When turning a part from both ends I think most people would check it with an indicator and make sure runout was minimal. Now minimal can have different meanings depending on your skill level and type of work you are familiar with.

    For example, when you see a rectangle or square shape, most people would assume the angles are 90deg unless otherwise specified. But you rarely (well I've not too much) see them dimensioned like that.

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    Marcus, thanks for the thoughtful reply. That example you gave is perfect, as the part I am doing right now are very similar. Great bringing up the the courts as that really gets down to what I was looking for. If I can not point to a specific standard from ASME Y14.5 in court saying that axis is to be followed for all features without a note, then a note should defiantly be there!

    Mike, that is a good analogy on the 90° for a plate.

    All great thoughts, first time posting and I am very impressed. Much different then the old car forums where you would get harassed with a question!

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    Hi again prangy89:
    I think a really great learning exercise for engineering school would be to ask each mechanical engineering student to draw up a part for a real and existing mechanism, dimension and tolerance it to the best of their understanding and then have an in-house machine lab make one that is right at the edge of the stated envelope and see if it will work in the assembly.
    That would drive the lesson home like no other.
    If the student could be billed in beers for the machinist, for every extra hour that it took to make it to the spec, it would drive the lesson home even better and discourage gaming the system by overspecifying everything.

    What do you think?

    Cheers

    Marcus
    Implant Mechanix • Design & Innovation > HOME
    Vancouver Wire EDM -- Wire EDM Machining

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    prangy - as I and others have mentioned, a note is less ambiguous than the "application" of GD&T - ESPECIALLY in a court of law. In this world, words matter.

    Another thing I would like to mention, get to know a few suppliers and try to stick with them. Biggest problem most companies have is when the engineered product goes from concept to production it moves up to "supply base". Once it is in supply base it goes to "cheapest source". Often cheapest source is in a country that does not speak the same language - that was the whole intent of GD&T - to open up the world as potential production sources for industry.

    In reality, so much has been overlooked in terms of the problems this approach has opened up, that it would take a book to even come close to touching on them. I would heartily encourage you to find some local suppliers and develop relationships with these guys and stick with them. The intangible aspects that will be value added are difficult to even enumerate.

    For example, you find you are having a small problem with a part that you spec'd out as needing a .03 radius and that the radius works better if you hold it to the higher end of the tolerance or the lower end of the tolerance and a quick call to "Bill" or "Joe" and the problem is addressed simply, quickly, efficiently and production never even blips.

    Not so when you are working with "cheapest source" 2,000 or 20,000 miles away with a language barrier thrown in. The problem may or may not be solved and will likely take several meetings and several days to correct.

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    LOL, I do not disagree. My gripe with the bachelors program is I did not actually machine, weld, ect. However, I learned how to calculate feeds and speeds, weld strength. This is all great, but not having hands on or working with someone that runs the machine is ridiculous. I learned so much about making practical drawings at an automation company I worked for that had a fab and machine shop. LOVED working with those people.

    I see you do wire edm, do you do sinker edm? We design anvils of 5-8" diameter with patterns found on goods you are familiar with, diapers being one of them. Always looking for another edm source.

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    Oh yeah; while I'm at it:
    A drawing is part of a contract; together with an accepted purchase order it defines legally what the two parties to the contract are agreeing to do.
    Any ambiguity in the terms of the contract are potential grounds for a dispute which will be resolved by people with no knowledge and no interest in the details of what's involved.
    It's useful to reflect on that from time to time, when you get a big boner for the juicy gig that's just been dangled in front of you.

    Cheers
    Marcus
    Implant Mechanix • Design & Innovation > HOME
    Vancouver Wire EDM -- Wire EDM Machining

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    Quote Originally Posted by Joe Miranda View Post
    If it's a turned part, concentricity is going to be as close as the machine spindle runs out. ......
    Only if all features are created in the same operation. Many turned parts are processed in two operations where the accuracy of the second op workholding will introduce a concentricity error.

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    Joe, great advice. I do have local suppliers that do a great job. I am currently at a small company and keep everything local. However, we were bought by a larger company. When on the tour the CEO mentioned how some large weldments may get manufactured in mexico to save money. At that point, I knew what we got bought by, a company that is trying to be cheap. Thus far seems to be correct assumption.

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    IMO a lot has to do with developing a solid relationship with your preferred shop.

    Marcus may have a lathe with 3" spindle bore, so anything under that matches exactly with what he said about ascending dias.

    I only have 2" thru, so for the identical part in my shop it would need to be identified on the print or communicated directly to me because I'm doing a flip op.


    Another thing I've experienced, especially using CSS is the bar whipping in the spindle, even with a liner. I've gotten different runout from OD to ID on the same op. Gotta finish the OD and ID at the same RPM to hold concentricity.

    This is where notes on the print or communication is necessary.


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