13 Replies Latest reply on May 6, 2015 1:49 PM by Dwight Livingston

    Tolerances

    Andi H

      I was wondering if people could point me in right direction. I have this model I created from scratch. Its fairly accurate all the parts are within correct dimensions overall but they are exact dimensions hole to hole is same dimension etc. I have steel Aluminum and brass how do I go about determening the correct tolerances so parts will move and work correctly? Is there a general chart or rule of thumb ? Example Piston head is steel piston the cylinder that makes contact with the piston is brass the main body that holds everything is aluminum. How are tolerances calculated?

       

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        • Re: Tolerances
          Mike Pogue

          Andi,

           

          Unfortunately, Walter is right. Your question is spot-on, but it is way to big for an internet forum. You will need the help of an experienced engineer or mechanic/machinist.

          • Re: Tolerances
            Andi H

            I have many books dealing with mechanical engineering so it will take along time to understand what I am trying to but I was hoping if there were set of rules or tables that give the necessary information for the different materials.

              • Re: Tolerances
                Mike Pogue

                You can absolutely educate yourself on this. But there is going to be some pain and mistakes along the way. It's not just a clearance problem. Ensuring clearance is just addition and subtraction. You have high pressure moving pistons, thermal expansion, and lubrication, to name just a few serious issues. Even a titan of mechanical engineering like me would look for experienced help tolerancing an engine.

              • Re: Tolerances
                Lawrence Kiefer

                TolAnalyst is for stack tolerance up  after you have already determined what fits you want.

                To determine the actual tolerances, I have always referred to the Machinery handbooks Allowances and Tolerances section. You still need to know if you want a running, sliding, Clarence of transition fit, but once you determine this, the chart will help you choose the actual tolerance for the hole/shaft. See attachment as an example. There is a metric version of these charts as well.

                 

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                Once you know the type of fit you want, Solidworks has a feature ti input these dimensions based on if its a hole or shaft...See image below.

                fits.JPG

                • Re: Tolerances
                  Mike Pogue

                  I should be less negative and more constructive. Here you don’t have a Ferrari V-12, you have what looks like a lawn mower engine with a finite number of parts.

                  So:

                   

                  • Features that have no interface at the next assembly should be loose e.g. +/-.030 in, or as tight as they need to be to clear. A lot of this stuff will basically be as cast. Then sealing surfaces and things will need secondary machining operations
                  • Fit of the bearing on the output shaft will be driven by the stock shaft size and bearing  size. This information is readily available from the manufacturer.
                  • For the housing at the output shaft, Koyo’s bearing catalog (for instance) has detailed information on bearing tolerancing which should help you detail the housing. If you can’t understand it, call them and get an application engineer to explain it. Represent yourself as a designer and indicate you are working on a prototype.
                  • For o-ring seals, consult Parker’s o-ring catalog for tolerance information. You can read the answers right off of charts.
                  • For the piston, rings, sleeves etc., I’d be out of my depth. However, I’d wager if you asked very specific questions on an engine rebuild forum, you’d get good help from men who have actually fabricated these parts.

                  Good luck.

                  • Re: Tolerances
                    Andi H

                    now thats what I was looking. thanx guys much appreciated I know about the thermal expansion and lubricaiton and the other issues at the very least I wanted to get the right hole clerances. That is a Nitro engine for rc models

                      • Re: Tolerances
                        Mike Pogue

                        Screw clearances are addition and subtraction.

                         

                        (hole diameter - screw diameter) + (mating hole diameter - screw diameter) - hole position tolerance - mating hole position tolerance >= 0

                         

                        hole diameter = screw diameter for threads, so

                         

                        (hole diameter - screw diameter) - hole positional tolerance - thread positional tolerance >= 0

                         

                        All positional tolerances are on the diameter, so, if you are using +/- tolerancing for the position, which are on a square, rather than true position tolerancing, which is on a circle, multiply all position tolerances by 2*sqrt(2) before plugging into the above equations. You can take that on faith or you can research true position tolerancing to convince yourself of this.

                         

                        You want to maximize positional tolerance to make the part manufacturable, so distribute it evenly between the hole and mating hole.

                         

                        All hole diameters are minimums, so subtract out the lower deviation before plugging in. A perfectly reasonable hole diameter tolerance is +.004/-.001. Limiting the lower deviation on diameter tolerance allows you to add the savings directly to the position tolerance. And, assuming the tools are new, this is more or less free.

                         

                        There is a lot more to this, obviously. And, the reality is, with modern machining, a lot of very poorly toleranced hole-patterns wind up fitting together.

                      • Re: Tolerances
                        Christopher Sudlik

                        This kind of question has no easy answer. Your tolerances will not be as close as you would like if the materials were the same material. Because they are different, they grow and shrink at different rates, which will create interference and it will not work. What you should do is calculate the maximum interference from thermal expansion alone at different temperatures from your nominal dimensions, and then add the proper tolerances you find in the Machinist's Handbook on top of it. The design will lose efficiency, but that is a result of the different materials, not much you can safely do about it

                        • Re: Tolerances
                          Dwight Livingston

                          Andi

                           

                          One very useful reference is Design for Manufacturability (ISBN 978-0070071391). It covers many different manufacturing processes, with good descriptions of the processes and standard tolerances for each. For figuring out tolerances, it an excellent place to start.

                           

                          Dwight