14 Replies Latest reply on Jan 23, 2013 3:26 PM by John Burrill

    How to design and draw with tolerances?

    Michael Mora

      Hello guys

       

      I’m making a sheetmetal part and two of my fabricators work with different tolerances.

      One says he reaches 0.015 inches and the other says 0.020 inches.

       

      I’m trying to order parts from each of them that will ultimately make the final assembly and I want to make sure these part actual fit.

      What is the best way to design/draw/order the parts to minimize or eliminate the uncertainty? I’m scare the parts will generate some cascading tolerance effect or something like that.

      I dont know what are the best practices for this, I’m sure many people design parts that are made by different people or companies.

       

      (BTW, I’m having the parts made by multiple people to somehow protect the intellectual rights or my design)

       

      Thanks

       

      Mich

        • Re: How to design and draw with tolerances?
          Alan Stoldt

          Mich,

           

          Can you clear up eaxtly what the 0.015 and 0.020 tolerances apply to?

           

          If those apply to the fabricator's Laser Accuracy, then you may want to limit the amount of features being made by the Laser.

           

          If the tolerances are critical then they features should be machined in, if they are clearance, then perhaps Laser is acceptable.

           

          Here we handle this along these lines. If a feature has a dimension going to it, then it is to be machined, if it is left undimensioned, it is to be Lasered.

           

          You can arrange the Feature in your Feature Tree and put them in Folders and create different configs, such as "LASER" and "MACHINED".

           

          Just some general thoughts without having more specifics.

           

          Regards,

           

          Alan

          • Re: How to design and draw with tolerances?
            John Burrill

            Michael, the whole point of tolerancing in general and GD&T in particular to allow you to source components from different manufacturers and have them always fit and always be interchangeable.

            Part of this work in this approach is finding suppliers capable of hitting your design tolerances.  In the strictest sense, you should be applying tolerances to preserve the functional design of your product and as a guideline, you should make those tolerances as loose as the design will allow. If a supplier can't meet your tolerances in this paradigm, it means they can't make your parts work and you should get a different supplier.  That being said, it's sometimes more practical to accomodate your manufacturer's capabilities.  If you can design the part with an .020 allowance (which I hope is a profile tolerance on a bent part and not a hole to hole dimension) then that's what you should do.

              • Re: How to design and draw with tolerances?
                Michael Mora

                Sorry I didnt specify where the tolerance applies.

                The tolerance we are talking about is for bending sheetmetal (aluminum, to be more specific).

                They told me those values are for the bends.

                 

                Let’s say I have an assembly for an enclosure, only two parts (the bottom and the top), the bottom has flanges on the left and on the right, and each flange is apart by, let’s say 8 inches and that base is manufactured by Fabricator1.

                Now, the top, made by Fabricator2, needs to be also 8 inches wide, with flanges on the side,, those flanges has some small jog so they fit into the inside of the side flanges at the base, so the sides panels are flush (where the top side panel meets the bottom side panel)

                This is the thing, if I specify +- 0.020" for tolerance and of the fabricators make the flanges 8.020" apart, and the other makes them 7.98" I will have a .040" inches difference between them.

                Which is still not that much, but as assemblies gets bigger, that could cause functional and cosmetic problems.

                 

                How do you deal with those things?

                 

                Sorry, Im new to all this, and my background is definitely not mechanical engineering etc.

                 

                Thanks a lot!

                 

                Mich

                  • Re: How to design and draw with tolerances?
                    John Burrill

                    Make the top pannel

                    8.000+.020/-.000

                    make the bottom pannel

                    8.000+.000/-.020

                    If you're going to send the bottom part to supplier2 exclusively then you can reign in the tolerances on that part.  Otherwise, yes, you could end up with a .040 gap in a worst-case scenario (which is a rare occurence). 

                    If you're vendors are reliable you can probably use RMS (root mean square) instead of worst-case scenario to calculate your stack-ups.  As far as the cumulative effect of tolerance accumulation on your final product, you control that with the judicious use of dimensions and datums.

                    • Re: How to design and draw with tolerances?
                      Peter Farnham

                      Are the going to be painted or have some sort of coating?

                       

                      Textured powder coat paint can eat up that 0.02" tolerance easily.

                       

                      If so you will need to take this in to account.

                       

                      Get some finished samples and measure the finshed thicknesses with a vernier.

                        • Re: How to design and draw with tolerances?
                          Michael Mora

                          Oh yes,,

                          forgot about that,,

                          yes,, they will be painted,, powder coating, probably textured.

                          I dont know much about it, so you are right, Ill go to the painting place and get samples.

                          Dont have a vernier at hand,, I have a Mitutoyo caliper, but I dont think is good for that level of detail.

                           

                          Thanks for ponting that out

                           

                          Mich

                            • Re: How to design and draw with tolerances?
                              Peter Farnham

                              Glad to help.

                              Also get the tolerances that the paint shop uses for their powder coating (in writing) as this can vary lots depending on the painter's skill level and the temperature of the room, powder temperature, even the type of powder coat used gloss , matt, satin ect..

                               

                              Watch for inside corner paint build up if you are using sharp corners on top and bottom boxes, otherwise you will end up filing the paint out!

                               

                              If you can, give them a "jig" part to test the finished painted parts on.  

                               

                              Been there and suffered lol

                      • Re: How to design and draw with tolerances?
                        Jerry Steiger

                        Mich,

                         

                        First off, I would rather source parts that are going to fit together from the same company. It's much more likely that they really will fit and there is a lot less finger-pointing going on when they don't. Patents are how you protect intellectual property. 

                         

                        There are several approachs you could take. One is to design your parts with lots of allowance for tolerances, so that they will go together no matter how the parts are built. This usually requires a lot of experience so that you can estimate what kind of clearances are going to be required. A better approach for you would be to do tolerance studies. Use hand calculations or an Excel spreadsheet to add up the nominal dimensions as well as the tolerances to see what the range of clearances will be. Tweak your nominal values and/or your tolerances to make the range acceptable.

                         

                        Jerry S.

                          • Re: How to design and draw with tolerances?
                            Michael Mora

                            I totally agree with you Jerry,, but at this point I want to focus my energy on designing and not on doing patents (Im not familiar with it, and the process might need my full attention), furthermore, I have to send partial drawings for prototyping, before the whole design is completed, so I dont have much to patent. I’m using these steps to learn the process.

                             

                            Also, making all the parts on the same company might not be even possible; some of them have different capabilities, pricing structure or material they handle, so there have to be a way to handle multiple vendors in a logical way with minimum pain.

                             

                            I can live with tolerances of the order of 0.020",, I was just wondering how to handle this "multiple vendor situation" (so I protect the design) without having to involve a patents lawyer or a lot of mathematical calculations.

                            Still, very useful advice from all of you.

                            Im still in the search, but so far Im inclined to John Burrill’s comment.

                             

                            Mich

                          • Re: How to design and draw with tolerances?
                            Bjorn Hulman

                            Surely if you design with a tolerance of 0.020 in mind any parts manufactured to a tighter tolerance of 0.015 would fit within the greater tolerance?

                             

                            Secondly, if you have the right paperwork agreed and signed with a single manufacturer your design should be protected. For ease of prototyping working with a single (or least number) of manufacturers is best as the good ones will work with you to get everything fitting right.

                            • Re: How to design and draw with tolerances?
                              Christopher Estelow

                              Michael,

                                 You could have your sheet metal fabricator sign a non-disclosure agreement.  There are free templates on the web you can use and add/modify as you see fit.  As long as they sign it you are covered.

                               

                              Chris