19 Replies Latest reply on Mar 13, 2009 12:25 PM by Jay Andrews

    .xx or .xxx

    Lamonte Bell
      I'm haveing a on-going debate with a co-worker about the standard on haveing some dimensions on a drawing at .xx decimal places and others at .xxx on the same drawing. I say it shoul be avoided whenever possiable, they should all be the same, but I'm not sure if that is a standard or not or even if there is a standand at all. I could not find it in the ASME Y14.100-2000
      Thanks for any help,
      LaMonte B.
        • .xx or .xxx
          Eddie Cyganik

          LaMonte,

          This is totally legit per ASME Y14.5M.

          • .xx or .xxx
            Chris Serran
            It was always my understanding that it depends on what tolerance you want to hold. I've always had a tolerance block where .xx, .xxx and .xxxx relate to a specific tolerance.
              • .xx or .xxx
                Jeff Holliday
                I agree there is no problem with having the different tolerances specified by the number of decimal places. In fact the ANSI Y14.5 from a while ago, states that the number of decimal places should be the same for both the dimen and the toler.

                Here is one thing I don't like regarding metric units. Have you noticed that when dimensioning with the metric units selected in SWorks that the "standard" and "smart" settings for trailing zeroes will automatically drop them? This is apparently the way ISO standards handle them. I have been changing that setting to "show" and adjusting each individually.
                  • .xx or .xxx
                    Eddie Cyganik

                    Jeff,

                    ISO & ASME Metric are quite clear on leading/trailing zeros. If you are adding zeros back on, just know that you are not meeting the standard.

                    For more information perform a search on dual dimensioning as this topic was discussed quite thoroughly.
                • .xx or .xxx
                  Alan Stoldt
                  3/8 = .375, if a machinist sees .38 on a drawing the can read that as .380. standard tolerance of +/- .01 now gives you .370 to .390 as being acceptable.
                    • .xx or .xxx
                      Eddie Cyganik

                      Alan,

                      3/8 is 3/8, a fraction, therefore, place a fractional tolerance on it, like; 3/8±1/64 or place a general tolerance in your title block or use a flag note and define the tolerance there.

                      There are options to treat fractions as fractions.
                        • .xx or .xxx
                          Jay Andrews
                          Every place I have worked in the last ten years uses the .xx vs .xxx to denote different standard tolerances, defined in the title block. As for .38 losing .005 accuracy in the tolerance range, I agree emotionally, but realistically, the .xx is a "sloppy dimension anyway, so most likely fine, and you can always on a specific case if need be dimension it as .375 plus/minus .020 if you wanted an accurate sloppy dimension...

                          Or if a little more advanced, use basic dimensioning, then all of this becomes moot. It's a little tricky to figure out all the nuances just by reading the spec, but when economically viable, it's the "right" way to do drawings. Of course, lots and lots of drawings are just easier to create and interpret with simple dimensioning.
                            • .xx or .xxx
                              Roland Schwarz
                              "More decimal places = more accurate" <> TRUE

                              It depends on many things, including drafting standard and tolerancing. Basic dimensions and ISO drafting standards do not generally use decimal places to dictate tolerances.
                              • .xx or .xxx
                                Jay Andrews
                                ...and by the way, "Basic Dimensioning" is not a generic term for simple dimensions, a basic dimension is indicated by having a rectangle around the dimension text. It is only a theoretical number without any tolerance on it. The tolerancing comes in when you use GD&T symbols on the feature you are dimensioning to.

                                For example you would use a basic dimension to locate a hole at it's perfect position, then in your tolerance block next to the hole call out, you show the allowance it can deviate from all of the "basic" locating dimensions. There are many great reasons to use this method on more complex parts.
                                  • .xx or .xxx
                                    Lamonte Bell
                                    Thanks everyone for the good info. I guess I should have been more specific about the debate. My stand is "in genreal" on the overall drawing unless noted all the dimensions should be either, or, not 5 or 6 .xx then 5 or 6 .xxx. My company standard is .xxx, I said we should not make one dimemsion .xx just for convinance, we should fix the model.
                            • .xx or .xxx
                              Jeff Norfolk
                              LaMonte, I think you lost that one. Generally, the more decimal places, the more accurate you want that feature. I have seen many times the overall size of a part defined by .xx dimensions with looser tolerances and holes, bosses, etc. dimensioned with .xxx. Dimension tolerances should be completely driven by the design. You also have to consider cost if you just assume that because one feature needs a .xxxx dimension that all other dimensions on the part need the same precision. Likely they won't, but the part will cost much more to manufacture.
                              • .xx or .xxx
                                Alan Stoldt
                                Sometimes we just hand the machinist a blank D-Sized sheet with "DESIGN @ ASSEMBLY" written on it.
                                • .xx or .xxx
                                  Alan Stoldt
                                  LaMonte,

                                  We have also coined a phrase "Builder's Choice" for non-critical areas where it doesn't matter how it gets done, just get it done. It makes them feel better, and we are all about warm and fuzzy feelings, you know, cuz we are a family. Dysfunctional, but a family none the less.
                                    • .xx or .xxx
                                      Eddie Cyganik

                                      I guess I cannot see why one would ever want to limit any part to a single set of numbers of decimal places. The reason for one, two, three and/or four decimal places is to provide a higher level of accuracy when required and a lesser amount where features are less critical.

                                      Accuracy drives the tooling necessary to meet design requirements, example: a drill versus a reamer or a reamer versus honing or grinding. This information is required and any machinist that cannot interpret a blueprint due to a variety of decimal places is worthless.

                                      The number of decimal places is also a driving factor for:
                                      => Tooling and Fixturing - For instance, if there is a hole to hole dimension of .25, then the accuracy on the fixture might be to .250. If the dimension were .250, then the accuracy on the fixture might need to be .2500.

                                      => Inspection/Quality - The number of decimal places for any given dimension usually determines the tool or measuring device that must be used to check for compliance.

                                      So, using 3 decimal places across the board may be placing unnecessary requirements on the manufacture & inspection of a part where some feature & dimensions could be 1 or 2 place. Any part manufactured with nothing but 3 place dimensions will be more expensive than the same part manufactured with a mix of 1, 2 and 3 place dimensions.
                                    • .xx or .xxx
                                      Chris Challinor
                                      Simple answer I have is our standard.

                                      .xx = .01"
                                      .xxx = .005"

                                      We deal with plastic and metal parts for most of our products. Mainly we send out 2D drawings with .xx as the default tolerance and on specific measurements use the .xxx (mainly for mating surfaces or where a .xx tolerance would cause interference issues).

                                      For manufacturing it is cheaper to make a part with the .xx tolerance to be held so use that when it tolerances are not critical to the end product.
                                        • .xx or .xxx
                                          Lamonte Bell
                                          Eddie,
                                          Your last comment makes sense. My initial point of view my have been a remnant of a standard at a previous job and I just kept it in memory. I'm going to have to get out of that old train of thought. Thanks everyone for all the input it was very helpful.
                                            • .xx or .xxx
                                              Eddie Cyganik
                                              Lamonte,

                                              I believe my comments are more than just a "general rule", however, there are exception to all rules. Having said this, there may be product out there that could use a general single tolerance but I still contend that they are few are far between.

                                              I guess as examples,

                                              A black-smithing shop could use:
                                              ALL FRACTIONAL DIMENSIONS: ±1/8

                                              ...and an optical lens manufacture may be able to state:
                                              ALL .XXXX PLACE DIMENSIONS ±.0005

                                              See what I mean?

                                              So, ..."One size shoe does not necessarily fit all!"
                                                • .xx or .xxx
                                                  Jay Andrews
                                                  Lamonte,

                                                  I'm not sure how much experience you have, and forgive me if i am underestimating it, but in many cases, the wheel is already invented and evolved, so trying to reinvent it is swimming up stream. What I'm saying is I've seen (and been) young guys with a little experience figuring they will come into a place and evolve the place into a better and more efficient organization. But it turns out that it is usually better to take the advice of the veterans there, as they used to be bright like the new guys, then added a bunch of experience on top of it. In other words, consider alternatives, but follow and learn from the people that already work there for a long time before you tell them how they should be doing things. I don't have enough fingers and toes to count how many times when I was young and I thought I figured out why my way was better, but it turned out I was lacking a lot of the perspective that the older guys have.

                                                  Regards,

                                                  Jay