13 Replies Latest reply on Oct 1, 2010 9:19 AM by John Ehrhardt

    Dual Dimensions

    Barna Madau
      I thought I'd start a new thread regarding the proper way to use dual dimensions on drawing. First off, I strongly dislike dual dimensions, but the company I'm contracting for insists my drawings conform to their company standard. My hands are tied, though I will question their decision for using them...

      So, I understand now, though I had not previously given it any thought, how dual dimensions create contradictory specifications. Simple example- You have ".039/.035 [1/0.9]" on a drawing- the part comes in at .0393", do you reject it? it's outside of the .039", but still inside of 1mm (which is approximately .03937"). Is there any precedence for calling out the alternate units as reference? That would indicate that the inspector should inspect the parts to the primary units, and ignore the tolerance on the alternate unit.

      I can see why some companies use them. Most US CNC machines all operate in inches. Most of the tooling available in the US is made in inches (end mills, drills, reams, etc..). So, if you give a machinist a drawing in mm, you are forcing them to convert units which costs time (which = $$).

      BM
        • Dual Dimensions
          Charles Culp
          I can't speak for "proper" use, but we will occasionally use [mm] in our drawings. If we do there is a note in the title block defining units as being in:

          DIMENSIONS ARE IN INCHES [mm]
          MILLIMETER UNITS FOR REF. ONLY

          But soon you will get plenty of comments telling you what we do is wrong
            • Dual Dimensions
              Eddie Cyganik

              I agree Charles,

              Secondary units should be considered reference. Inspection should be done to the primary units only. If there is a deviation to either one of these statements, then a deviation would need to be specified in the company procedure for Dual Dimensioning.

              Barna,

              As I suggested to you before, you need to get the company procedure from the company you are contracting for as there isn't any commercial, industry or military specification to follow.

              We can make all knids the suggestions but in the end, a company procedure is what you must follow and what we have to say may be contradicted.
                • Dual Dimensions
                  Tony Cantrell
                  We use dual dimensions at my company, but when converting we use method B rounding which is rounding inward. If you take your dimension and add the upper limit tolerance and round that number down then take your dimension and subtract the lower limit tolerance and round that number up you will be within the original dimensions limits.
                  We also put a note on the drawing stating the primary and secondary dimensions. Design is done in the primary dimensions.
                    • Dual Dimensions
                      Robert Berry
                      Barna,

                      "Most US CNC machines all operate in inches. Most of the tooling available in the US is made in inches (end mills, drills, reams, etc..). "

                      Actually most, (if not all), CNC machines are made overseas and all the machinist/programer has to do is flip to metric. They are just more comfortable working in inches.

                      You can get virtually any tool in metric as they are all made overseas also.

                      Specific factory tooling is another matter all together, all of our tooling here is designed in inches. As we produce more and more metric product we are slowly switching in house tooling to metric.

                      we will be forced to go all metric in the not too distant future, YAY!


                      Regards,
                      B.B.
                        • Dual Dimensions
                          Eddie Cyganik

                          Bob,

                          You're right, on most machnes it is merely a flip of the switch. The bigger issue is tools, fixtures, gages, etc., that are all imperial. To duplicate your existing imperial tooling is a costly requirement.
                            • Dual Dimensions
                              Robert Berry
                              Eddie,

                              We are going through that painful experience as we speak.

                              Best Regards

                              B. B.
                                • Dual Dimensions
                                  Barna Madau
                                  Thanks all the replys... This forum has been a lot of help. Previously I've always had co-workers to discuss issues with but now I'm the only employee in my company (by boss is not an engineer/drafter, can't ask him)...

                                  I'm getting accustomed to thier way of doing things. Right now I'm still working out how to make a drawing cluttered w/ dual dimension come out clean and understandable. It definately takes up a lot of room on the drawings.

                                  I've seen some of thier older drawings and they were all in inch dimensinos. Maybe dual dimension is their baby step towards going all metric. All (well, most) of their products are accessoried that will end up on other products that were all designed in metric units, so it just makes sense.

                                  BM
                                  • Dual Dimensions
                                    Roland Schwarz
                                    In a similar discussion on another site, I asserted that the dual dimensions are always reference. Unfortunately, none of the "standards bearers" on that site came through with chapter and verse to back this up.

                                    If anyone has ANSI or ISO standard to quote on the matter, it would be helpful.

                                    Meanwhile, I suggest a note like Mr. Culp uses.
                                      • Dual Dimensions
                                        Eddie Cyganik

                                        Roland,

                                        That is the problem, there isn't a current standard.

                                        My last suggestion on the topic would be to go to the ASME or IHS website and buy the last standard that covered Dual Dimensions, ASME Y14.5 1973 @ 50 bucks, it may be worth it.
                                • Dual Dimensions
                                  Eddie Cyganik

                                  Tony,

                                  When we convert we use this method also, the only problem is that your tolerance zone is smaller. It will always work but you lose tolerance.
                            • Dual Dimensions
                              Josh Brady
                              As far as I know, the dual dimension is always reference. A note such as the one Charles suggested couldn't hurt, though.
                              • Dual Dimensions
                                Matthew Lorono

                                Barna Madau wrote:

                                 

                                I can see why some companies use them. Most US CNC machines all operate in inches. Most of the tooling available in the US is made in inches (end mills, drills, reams, etc..). So, if you give a machinist a drawing in mm, you are forcing them to convert units which costs time (which = $$).

                                BM

                                I don't agree with this. Metric tooling is as available in the US as Inch tooling. However, this is a good argument when working with Europe. Inch tooling is not readily available to most shops there, so using Inch may actually cost them more.
                                • Re: Dual Dimensions
                                  John Ehrhardt

                                  The ASME 14.5M discourages the use of dual dimensioning by omitting it from the standard.

                                  The ASME 14.100 4.30 states that brackets are for internal information such as processes.

                                   

                                  A derived and rounded secondary dimension will, I believe, have to be indicated as reference.  Solidworks, and other CAD, use brackets without a clear standard statement for guidance. There is no option for reference parenthesis on dual dimension display.  People do create a parentheses enclosed dimension with a text line in the dimension edit area.

                                   

                                  There may be an Enhancement Request for this.  I will check.