Precision & accuracy

Discussion in 'Technologies and Hardware' started by FreeRangeBrain, Apr 22, 2013.

  1. I'm curious to know the precision and accuracy available with the various materials. WSF would be a good start though, if a complete list is unavailable.

    For those unfamiliar with the difference between precision and accuracy, consider darts thrown at a dart board. If a total of six darts are thrown and all hit in the bull's eye, that's high precision and high accuracy. If they all hit the doubles ring at the perimeter, that's high accuracy and low precision - the shot locations all average to the exact desired value. If they all hit the triple twenty, that's high precision and low accuracy - the shots all err by the same amount.

    The printer's resolution will have a strong influence, I'm sure, but I'm curious to know if there's any firm (if not hard) data. I'd like to make some small mechanisms that would be printed as unassembled, as the gap requirements during printing as assembled would be too great for the nature of the device.
  2. *bump*

    Anybody? Anybody?
    JACANT Well-Known Member
  4. Youknowwho4eva
    Youknowwho4eva Shapeways Employee Community Team
  5. Thanks, Jacant. I've seen each of those before but can't seem to glean any repeatability data from them. Mostly they're talking about clearances while printing as-assembled parts. I'm more interested in generating parts consistently over a series of print runs, and generating parts that fit closely together that were not printed in an assembled configuration.
  6. Thanks, Youknowwho4eva, but those pages aren't giving me the data I'm looking for. I need to know about run-to-run consistency, and dimensional accuracy across differing parts intended to fit together but printed separately.
  7. Youknowwho4eva
    Youknowwho4eva Shapeways Employee Community Team
    If you go to the design guidelines for each individual material, the accuracy is listed. For instance WSF is ± 0.15mm, then ± 0.15% of longest axis So if you intend parts to fit together, and they are 100mm long, then you would expect a .3mm possible variance. So if you doing a peg and a hole, you would want the peg to be designed .6mm smaller than the hole you intend it to fit in to make sure that it should fit every print. This could result in a snug fit with no gap or a gap up to 1.2mm (if the peg goes to -.3mm small and the hole goes to .3mm larger)
  8. stonysmith
    stonysmith Well-Known Member Moderator
    There is an issue with run-to-run consistency that you should be aware of.

    For WSF in particular, there are two different models of printers, one that can print LARGE items, and another one that is more limited in size.
    Unfortunately, the tolerances between the two printer models are different, and you can't be guranteed which particular printer your item will be printed on for any one order, or even that two items on the same order will be printed on the same printer at once. Just because it's a smaller item won't necessarily mean that it's always printed on the SMALL printer. Therefore a hyper-exacting repeatablity won't be possible.

    It is also highly likely that the printers have to be regularly recalibrated, and that the accuracy for even a single printer could drift slightly over time.

    Someday, we'll have sub-micron accuracy on parts 1000 meters long. The technology just isn't developed to that level yet.

    On many of the material pages is listed "Accuracy". I'd go with that for now as the allowable run-to-run differences.

    But, it sounds to me like a request for "high definition WSF" or the like would be a good idea. I'm sure that will be added to the list (at extra cost), but it'll be somewhere lower on the priority list than the request for user-selected print orientation (which, by the way, also affects accuracy).

    I also doubt that Shapeways has incurred the expense of measuring a statistical sample of printed items such as to be able to satisify your "dart" example. These are "professional" machines, not the home-grown devices you may have seen elsewhere. Shapeways depends in large part upon the specifications and accuracy provided by the manufacturers.
  9. Thanks, Stonysmith. Those are the kind of details I was looking for. (Not sure how I missed the precision numbers on the materials pages, but there you go.)

    As a machine designer and engineerig technologist, I thoroughly understand the recalibration and statistical drift of automated production. I expect to be printing mating pieces in unified orders, but most won't be printed "as assembled," thus the need to know. FUD may be the preferred material if it gets critical, though other design considerations may prevail.