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PAULKORENKIEWIC1-Visitor
1-Visitor
July 7, 2009
Question

accuracy and merges...

  • July 7, 2009
  • 12 replies
  • 1897 views
Guru's,

Occasionally when merging two parts Pro/E will give me an error telling
me to change one of the parts to some new accuracy value OR to use
absolute accuracy. I've somewhat blindly always just followed orders
and changed whatever it told me to. I am wondering if I would be better
off going to the absolute method instead? Would I simply change the two
parts to the same absolute accuracy value? And would it matter what
that value is? Any thoughts? Please respond to the list to maintain
the thread...
thanks in advance...

Paul Korenkiewicz
FEV , Inc.
4554 Glenmeade
Auburn Hills, MI, 48326-1766

    12 replies

    P
    ptc-5936191-Visitor
    1-Visitor
    July 7, 2009
    I have had a lot of problems solved by changing to absolute accuracy, so
    much so that I am wondering why I don't make this the default. Why do we
    have accuracy values?


    R
    RodGiles1-Visitor
    1-Visitor
    July 7, 2009
    Dear Paul,

    I'd go along with what Richard said. I'd also point out that PTC
    recommends that if you are using geometry from one part in another one,
    then they should both have the same accuracy. The only way to ensure
    this is to use absolute accuracy. My preferred method is to set absolute
    accuracy in the start part. There is a good explanation of accuracy on
    the PTC web-site - see
    D
    dgschaefer21-Topaz II
    21-Topaz II
    July 7, 2009
    I have used nothing but absolute accuracy (except where required to by
    customer constraints) for over 10 years with great success. Accuracy is
    a way of limiting how much math Pro|E will do to solve the geometry. It
    does so by limiting the smallest edge size.

    Using relative accuracy, you are setting a ratio between the longest and
    shortest edges in a model. That's why you hear occasionally of folks
    designing a long extrusion and then having trouble adding small holes to
    it. The length of the edge of the extrusion limits the size of the hole
    allowed. The theory there was if you have long edges, you have a large
    part and you don't want tiny details on it because the scale of the part
    makes tiny details hard to manufacture. However, now very large
    injection molded parts are quite common and they can have lots of tiny
    details.

    Using absolute accuracy, you are setting the absolute shortest edge
    allowed. No more large parts that can't have small details. It also
    makes it easy to match the accuracy between models. Since relative
    accuracy is a ratio, it isn't actually setting the accuracy but
    providing a formula to do so. The actual accuracy value is calculated
    by Pro|E (and recalculated as the part regens) and is unavailable to the
    user. So two parts with the same relative accuracy can have very
    different actual or effective accuracy values.

    I've found that relative accuracy is limiting in other ways as well.
    Because it's a ratio, you really need to establish the size of your
    model early in the tree. If not, the drastic change in model size may
    cause a drastic swing in accuracy. Suddenly, the rounds you want to add
    don't work because the edge is too small. Also, if the design changes,
    say two smaller parts are now to be merged into one to save assembly
    labor. You pick the more complicated part and roll back the tree to add
    in the geometry of the other part. Now that the model is much larger,
    the calculated accuracy goes up. Suddenly, rounds that should be
    unaffected, simply won't regenerate. The message is 'Failed to
    intersect with part' or something equally vague. This is due to a
    change in accuracy.

    Lastly, the farther along a model is, the harder it will be to change
    from relative to absolute accuracy. Because relative accuracy changes
    as a model is regenerated, there may not be one absolute value that will
    work at every point in the tree. It may be effectively 0.0001 at the
    start, 0.001 in the middle and .0005 at the end. Finding an absolute
    value that allows the model to regen can be a long trial and error
    process and it may change the model as the new accuracy value forces
    features to be calculated differently.

    Bottom line, I have set my absolute accuracy to 0.0001 for inches and
    0.00254 for metric years ago and haven't looked back.

    Doug Schaefer
    H
    HugoHermans12-Amethyst
    12-Amethyst
    July 7, 2009
    Also related to accuracy settings and merges, when confronted with STEP
    or IGES, alligning the accuracy of your model with the accuracy of the
    source system greatly enhances the import process.

    Regards, Hugo.
    B
    BobFrindt1-Visitor
    1-Visitor
    July 7, 2009
    For what it is worth, I have taken a different approach due to the
    accuracy issues and it has worked very well for me. I prefer to use
    published and copied geometry of the part surfaces and then solidify them
    to create the merge. I have not found any drawbacks yet (I'd like to hear
    them if anyone knows of any or I'm setting myself up for problems down the
    road) and the accuracy issue never comes into play.

    It is an alternative to changing the accuracy of a part for no other
    reason other than to get it to merge.

    Bob Frindt
    Sr. Designer
    Parker Hannifin Corporation
    Parker Aerospace
    Gas Turbine Fuel Systems Division
    9200 Tyler Boulevard
    Mentor, OH 44060 USA
    direct (440) 954-8159
    cell: (216) 990-8711
    fax: (440) 954-8111
    -
    www.parker.com



    "Korenkiewicz, Paul" <->
    07/07/2009 07:19 AM
    Please respond to
    "Korenkiewicz, Paul" <->


    To
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    cc

    Subject
    [proecad] - accuracy and merges...






    Guru's,

    Occasionally when merging two parts Pro/E will give me an error telling me
    to change one of the parts to some new accuracy value OR to use absolute
    accuracy. I've somewhat blindly always just followed orders and changed
    whatever it told me to. I am wondering if I would be better off going to
    the absolute method instead? Would I simply change the two parts to the
    same absolute accuracy value? And would it matter what that value is? Any
    thoughts? Please respond to the list to maintain the thread...
    thanks in advance...

    Paul Korenkiewicz
    FEV , Inc.
    4554 Glenmeade
    Auburn Hills, MI, 48326-1766
    P
    ptc-19327451-Visitor
    1-Visitor
    July 7, 2009
    Doing experiments with both copy/geometry, and merge/cutout some time
    ago (WF2), I decided that copy geom. was the only way to go. Besides
    the accuracy issues that are non-existent using copy geom., I ran into
    problems updating dies using merge and cutout because the molded parts
    used for the merge/cutout had to be in session for the cavity to update.




    Otherwise, when doing a regen, or even rolling the die model back and
    forward again produces no changes to the cavity.



    Some more obscure config.pro options need to be set for the copy-geom
    method to work as expected (automatic update, bring all referenced
    models in session as needed, recursively regen parents of current model
    as needed, etc...) but it is worth it.



    As an example, if I have a model approx 2x2x4" that I want to put into a
    die that is 8x8x8, using merge / cutout the proper accuracy may not be
    possible (either because the die relative accuracy had to be less that
    1e-6, ) or using absolute accuracy would cause the die model to fail.



    Secondly, when trying to create an EDM electrode from the die model, the
    accuracy is changed again (and for the worse)



    Using the copy-geom method, If I open the EDM electrode model and hit
    regen, all of the changes are updated. Using the copy geom. method, I
    would have to make sure that all of the relevant models are 'in-session'
    first. (Not a problem if you have PDM, but a royal pain otherwise)







    Christopher Gosnell

    TRIGON INC.
    FPD Company
    124 Hidden Valley Road
    McMurray, PA 15317
    PH: 724.941.5540
    FX: 724.941.8322
    www.fpdinc.com
    M
    merrill.rosenow1-Visitor
    1-Visitor
    July 8, 2009

    How much extra effort is required for Pro/E to go from relative to the finest resolution of absolute accuracy? I have used relative accuracy without problems except for two parts. I simply changed from relative to absolute and continued working. Now that I know what to look for in regards to accuracy problems, I won't get stumped anymore. (It was already mentioned before, but features failing to intersect or in my case, graphical issues and failures for no apparent reason). In most cases is it so terrible to leave the accuracy as relative and change it if required?

    One odd thing I've noticed, is the smallest absolute accuracy of one part I am working on is not the same as another part.

    P
    ptc-5936191-Visitor
    1-Visitor
    July 8, 2009
    That was my question - if running at absolute accuracy solves a lot of
    problems, why run at relative accuracy? What are the pros for relative
    accuracy that you would want to leave it as such? Why not make your
    start part at absolute accuracy, what do you lose?


    P
    ptc-19327451-Visitor
    1-Visitor
    July 8, 2009
    Unless you are referencing features from one part to another, I don't
    think that the 'accuracy' settings affect assemblies.



    One thing to watch for is problems in Manufacturing, Pro-Man uses
    absolute accuracy. We have had surface finish issues on highly sculpted
    4-5 axis parts that 'seemed' to be related to the accuracy.



    For the parts we do (~10x10x20" max) we use .0001 absolute accuracy in
    our start parts / assemblies with good results.



    Christopher Gosnell

    TRIGON INC.
    FPD Company
    124 Hidden Valley Road
    McMurray, PA 15317
    PH: 724.941.5540
    FX: 724.941.8322
    www.fpdinc.com
    H
    HugoHermans12-Amethyst
    12-Amethyst
    July 8, 2009
    AFAIK, from the point of view of PTC, it's difficult to set up a generic
    configuration if they don't know in advance the general magnitude of the
    parts that will be designed. OOTB, ProE has to be capable to tackle
    details below 1 mm to several meters. If the order of magnitude is
    mostly the same, what's the case for most of the users, starting from a
    template with a specific absolute accuracy is by far the best approach.
    But if you can't decide on a common absolute accuracy, a relative
    accuracy might be an option.

    The disadvantage of an accuracy that's too small for most of the parts
    (relative or absolute), is that these parts become too big on disk and
    to heavy to process.

    We design industrial machines with parts from a few grams upto 250 kg,
    our template has an absolute accuracy of 0,01 mm. So, details below
    0,01 can't be modelled.

    Regards, Hugo.
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