To Exist or Not to Exist

Discussion in 'Pro/Engineer & Creo Elements/Pro' started by mnmca, Nov 14, 2006.

  1. mnmca

    mnmca Guest

    It has been proposed to me to open a debate on the theme of relatively
    accurate bevel gear tooth creation in Pro/ENGINEER.

    This same topic, under identical headings, is being posted to the
    following discussion boards:
    www.eng-tips.com
    www.mcadcentral.com
    www.cadproe.com
    www.cad-portal.com
    www.tech.groups.yahoo.com/group/pro-e/
    groups.google.com/group/comp.cad.pro-engineer
    www.proe.com

    So, if a reader also frequents any of the above that is not the same as
    this group, please only contribute to one.
    After two weeks I will compile the results and post a follow-up t all
    groups.

    It is my wish that any moderator post this as expediently as possible.

    Now, on for the main attraction:
    1) Should certain features of models be simplified so as to not over
    complicate the creation process (long regen time)? Some examples would
    be cosmetic threads for fastener or ball screw threads or circular arcs
    for spur type gear teeth or trapezoidal sections for bevels.

    2) To see it from the other side, should models be created to be as
    realistic as possible?
    Please respond with a brief opinion, 10 sentence max., as to why you
    think this way. A separate response, same length, to each is welcomed;
    some people can see merits in both.

    Thank you,
    Mark N. McAllister
     
    mnmca, Nov 14, 2006
    #1
  2. mnmca

    John Wade Guest

    I don't think this is something which should be governed by blind
    imposition of policy. The designer should use their judgement to decide
    whether the feature is significant, or whether it can be simplified
    without negatively impacting the value & utility of the model.

    Designers who consitently exhibit good judgement should be promoted.
     
    John Wade, Nov 15, 2006
    #2
  3. mnmca

    Jeff Howard Guest

    High fidelity or simplified geometry?
    Isn't asking for a one-size-fits-all answer being a little simplistic?
    What serves the practical purpose(s) of the model?

    Term paper?
     
    Jeff Howard, Nov 15, 2006
    #3
  4. mnmca

    David Janes Guest

    Oops, passive attribution ('it has been proposed to me'). Naturally, we'd
    all like to know who the "proposer" is; if it be one of us, feel free to say
    so. I've had my go rounds with Mark on this issue. Maybe I'm the "proposer"
    or maybe Mark was just reaidng my mind as I just thought, today, that
    getting the issues we were discussing into a larger forum would be
    beneficial. Doesn't seem likely, though, does it?
    Good idea to broaden and widen the forum but can't find the discussion/topic
    on any of these. Maybe you could be more precise in referencing these
    locations, IOW, the locations where you actually posted the discussion. Why
    would you make us hunt for it if you really wanted us to participate?
    Sorry, no 'moderators' here; stay posted
    Yes, they exist, they are very commonly used instead of the more realistic
    looking helical swept features. Shorter regen times are a strong factor in
    their use; a stonger one is that those of us who have, at one time or
    another, used these advanced modelling techniques, have discovered the WTFC
    zone, that place where everyone, from users to in touch management, says
    'what does this buy us' and finds a void.
    I think the audience here is reality based, experienced and quite practical.
    The sentiment is against absolutist rules. It favors the contextual, the
    local, the skilled user and the application of a broad brush to particular
    applications. In this case, (gearing) the application is so exclusive, that
    outside input (from Pro/e or any solid modelling software) is difficult.
    I think I'll repond a little later on the issue of substance. But, hey,
    thanks for getting the ball rolling. You read my mind.

    David Janes
     
    David Janes, Nov 16, 2006
    #4
  5. mnmca

    Polymer Man Guest

    If it truly serves no purpose, I see no reason to accurately model a
    component. But it often does serve a purpose. If the threads on a screw
    make it into a work instruction illustration, or the hex recess on a
    SHCS into a presentation rendering then they're useful because they
    prevent confusion when people view these things. A BHCS with no details
    may make the mass properties work out, or the BOM correct, but they
    look a lot like a rivet. But accurately modeling a gear, when a simpler
    tooth profile would look basically the same is probably a waste of
    time. Unless you're using it in an analysis or manufacturing it.

    Threads modeled as a simple revolve (no helix) look good enough most of
    the time.

    Sometimes I'll make hollow parts solid, like blow molded or rotationaly
    molded parts, to keep assembly complexity down.

    On the flip side, sometimes I'll temporarily add small fillets to
    components when, in reality, they're sharp. This creates an edge
    highlight in presentation renderings that helps the picture read.

    Is it just me, or has the power of computers over the last few years
    not kept up with the power needs of Pro? Feels slower than it used to...
     
    Polymer Man, Nov 17, 2006
    #5
  6. mnmca

    Fork Road Guest

    Oops, passive attribution ('it has been proposed to me'). Naturally, we'd
    When you visit the zoo, "Don't feed the animals."
    When you vist the newgroup, "Don't feed the egotist."

    One doanhaftabe campy ta go campin'
    GA
     
    Fork Road, Nov 19, 2006
    #6
  7. mnmca

    David Janes Guest

    Better ~ hobby, gear design theorist striving for the perfect tooth form.
    And, apparently, without ever considering AGMA standard tooth crowning, the
    sine qua non of gear making. So, perfection in the abstract. Perfection
    without ever touching, designing, making, or testing a real gear.

    David Janes
     
    David Janes, Nov 19, 2006
    #7
  8. mnmca

    David Janes Guest

    Put in this reasonable way, it doesn't seem controversial at all. Yet, we've
    been having one over, not so much the general issue of "blind imposition" vs
    "judgment", but over which products or geometries qualify for
    "simplification" or being made less than perfect.

    Two recent examples: screw threads and gear teeth. The latter sparked Mark
    to start this thread. Realism was the test in both cases. The thread issue,
    briefly, was about a more realistic way to terminate a blind thread on a
    screw as it reached the head, "like single pointing" was the test of
    realism. I pointed out a bunch of different manufacturing methods, some of
    which I'd done myself, e.g., that I had chased threads on a screw machine,
    single pointed them on a variety of lathes, milled them and ground them. Of
    these, only the screw machine could be considered a mass production method.
    Yet 99% of fasteners have rolled threads. And none of these different
    methods of thread production terminate with the same geometry. Of the 1%
    produced by some form of thread cutting, not one-thousandth are produced by
    single pointing. So much for the realism of the "single point threading"
    test. But it does point out the difficulty of having discussions of
    modelling and feature creation in a design vacuum.

    Advocating in favor of simplification (cosmetics on screws, hole feature
    with cosmetic threads), I pointed out that most people didn't have time to
    mess with a helical sweep or the huge regeneration time or debugging the
    failures, especially not when you get hole data, useable in a callout,
    virtually for free, plus 95% accurate geometry. And you can waste (my
    prejudice) 90% of your time trying to get that last 5% "perfect". I put
    perfect in quotes because of the flaws in the reality tests. The "reality"
    being tested to was NOT that realistic. But I think this didn't settle the
    question as Bruin presented a 3 point list of reasons to use helical swept
    threads. I didn't reply. Still, Bruin's remarks addressed some exceptional
    circumstances, not the common engineering one: why does one need fasteners
    in the assembly at all? Is it something fancy, such as, to do stress
    analysis, Failure Mode and Effects Analysis, some kind of analysis on the
    assembly? No, it's to principally get part numbers and quantities into a BOM
    for ordering; secondarily, to get these items into exploded views for
    assembly instruction (where do which fasteners go); thirdly, to do
    interference checking, especially on lengths of fasteners and potentially,
    on body size, the latter being made impossible by the inclusion of cut
    threads in hole/fastener (no, please don't anyone tell me they actually
    screw the fastener into the hole to assemble, the ONLY condition for their
    NOT interfering.) In the end, Jeff commented that, as the power of computers
    grows, we seem to be expecting more out of this process and invent new
    demands on the models. Anyone remember the drafting board/ACAD days when a
    screw thread was represented by a cartoon, a thread symbol (alternating,
    parallel long and short lines)? BTW, if you are seriously worried by an
    accurate weight of your fastener, DON'T depend on the accuracy of your
    geometry or the density of whatever convenient material you could find. Get
    a thousand screws, weigh them, divide by 1000 and assign this MEASURED
    weight in your MASS PROPS dialogue. This is the only really SERIOUS way to
    do it. Then compare it to the calculated one. And multiply the difference by
    a million such fasteners in a Boeing 787. Aren't we talking about how
    engineering gets done? isn't this how engineers work? aren't the models just
    rough, more or less accurate representations? and doesn't the level of
    disappointment over the difference between the model and the real thing
    depend on the EXPECTATION of absolute fidelity, unwarranted though it may
    be? Well, that's my position, anyway.

    On to the gearing controversy or does the geometry of gear teeth need to be
    really accurate? The question that come to my mind anyway is why, what
    drives this issue, what need exists for very accurate tooth geometry? The
    real difficulty in discussing this issue is that, though people have found
    uses for solid models of gears, the gearing industry (design, analysis,
    testing, production of gears) has no use for those models. Take bevels, for
    instance, which I know fairly well since I was in charge of the bevels
    department at Allied Gear in Chicago for 5 years. Good gears depend on the
    setup of Gleason bevel gear machines. The parameters for setting up these
    machines are generated by specialized gearmaking software. The design of
    those gears depends also on highly specialized design software that can take
    some general torque, load and time numbers as well as some other variables,
    such as how smooth the transmission of power ought to be, and give
    recommendations on diametral pitch (smaller is better for smoothness), gear
    ratios, and come up with diametral pitch, pitch diameter, pitch cone angle
    (bevels), FA and RA. And, as an extra added bonus, you get the mounting
    distance of each member of the bevel gear set. The output of this software,
    given a Gleason, is gear ratios for cradle roll, workpiece roll, mounting
    distance, pitch angle and tool stroke length. The Gleason tables also give
    you a rough number for crowning length/location ~ it can be
    lengthened/shortened and moved up/down the tooth face. No model, however
    theoretically perfect, has anything to do with this setup which depends
    heavily on lookup tables based on experience gained over decades of use. The
    models in Pro/e are interesting visual references; they provide the WOW
    factor. But nothing in the manufacture of gears depends on them. No real
    gear will be more or less accurately made because of the more or less
    accurately modeled Pro/e gear. In fact, no practical use of the perfectly
    modeled Pro/e gear has yet been demonstrated. And, if one took into account
    manufacturing inaccuracies, the current methods of gear modelling are
    already about as accurate (less tooth crowning) as the manufactured ones.

    Finally, the "who cares" factor: I have the perfectly modeled gear to put in
    an assembly. I try to do animation or even, mechanism design in Pro/e ~ how
    does good or bad gear geometry come into play? Sorry, no, it doesn't. No
    dynamic analysis in Pro/e. In fact, the way you set up gears to run together
    in MD is to pretend they are rollers, with pitch cone geometry, rolling
    together, cone face to cone face. No tooth accuracy required here. How about
    stress analysis, also highly unlikely in a generic program like Pro/e. Well,
    it is static, depends on what you can define as contact surfaces. Certainly,
    the usual deflection will not enter into the calculation. And there's some
    other reasons not to trust its calculations: no tooth crowning as in the
    usual bevel gear so possibility of appearance of interference at toe/heel
    and so analysis of likely failure. Not so in real crowned tooth bevel gear
    sets which cancel the effects of deflection. Conclusion: one does not wish
    to do stress analysis, static or dynamic, with the "best" Pro/e models. One
    should do this with the specialized software to this purpose, invented by
    the gear manufacturers. (Ask AGMA). IOW, I don't give a crap about a
    "perfectly" modeled Pro/e gear, it is a worthless, pretentious waste of
    time.

    Here's hoping that I've stated my position clearly and unambiguously.
    Especially since no one seems to want to acknowledge there exists a debate
    or a debatable issue.

    David Janes
     
    David Janes, Nov 19, 2006
    #8
  9. mnmca

    Jeff Howard Guest

    Interesting. You should repost under a meaningful subject line
    (i.e. Essay on the Practical Considerations of Modeling Detail)
    instead of this Existential poohpooh one. Someone may stumble
    across it and find it enlightening.
     
    Jeff Howard, Nov 19, 2006
    #9
  10. mnmca

    David Janes Guest

    It's been kind of a trial recreating this discussion. Maybe you could follow
    it, if your were in at the start. Hard to say how anyone else would take it.
    Especially in the "one liners" world of NGs.
    Where might I post a summary that might not be blown, uncerimoniously, out
    of the water. In spite of opposing the formation of new groups
    (balkanization), I do think the general issues deserve more discussion.

    David Janes
     
    David Janes, Nov 19, 2006
    #10
  11. mnmca

    Polymer Man Guest

    David Janes wrote:

    snip


    Wow. You know a lot about gears too.
     
    Polymer Man, Nov 20, 2006
    #11
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