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  • Engineering

    I am not professionally trained as a machinist or a mechanical engineer.

    I've completed a fair amount of repair work, and do have good mechanical amplitude. My issue is in designing things. Well, not designing per-say, but dimensioning.

    You see, I can visualize an assembly in my head. I can see how the parts will work together, how they will interface to produce a subassembly.

    I can also take a dimensioned print, and usually figure out how to make it into a part. The problem lies between.

    When you understand how a subassembly will work together, how do you go about figuring out concrete numbers for the parts in question? Technical drawing via pen, paper, and T-square? Do CAD systems allow you to interface multiple parts? In repair work, since I'm working around a number of already created parts, I can usually extrapolate out the dimensions of the part I need. This is not so when starting from scratch, however.

    Any suggestions of places to start, texts to read, or helpful hints would be most appreciated.


  • #2
    A book is a good idea, but you need to look at drawings of stuff similar to what you want to draw to see what the conventions are. It's like a language, and engineering is even a different language than architecture. I was baffled when I started working in the costume industry as how abstract the designer drawings were, but then I realized they used a symbolic vocabulary that other designers and makers understood. Same thing.
    Location: Jersey City NJ USA


    • #3
      At some point, the dimensions are arbitrary. At a higher level they are governed by computations of the rigidity required of the assembly. I've seen a lot of people who call themselves mechanical engineers who just manage to put together assemblies that fit with no particular rhyme or reason. I also remember an intern who had the requirement to make a keying bar 1/2 thick but it was 3/8 thick when he designed it and thus it didn't keep out the plugs that shouldn't fit in the slot. Common sense will go far.

      I was trained as a general engineer and I'd go with instinct if there isn't a specific specification that the assembly has to meet. The really hard core mechanical guys here might have more insight.

      Usually, a sketch and some trigonometry will yield critical dimensions if you pick a starting point: Pick the dimension for one part and then work from there making sure that the rest of the parts will fit. A good CAD system will allow you to check for interferences and put assemblies together on the computer although this can be a lot of work for a simple thing.

      The carefully drawn and scaled technical drawing basically documents what you did. It's won't tell you what stuff should be but it will tell you when something won't fit if you read it correctly.

      A text on ANSI geometric dimensioning and tolerancing might help but it will really only help with communicating the dimensions in an appropriate manner and perhaps in keeping the cost of a widely manufactured part down. If you want to learn the black art of computing strengths for things and sizing parts that way, it will be uphill but covered in statics and mechanics of materials textbooks.

      You sound like you've got the skills you need. Just pick a dimension for a critical part and make the rest of the assembly fit and then write down the dimensions.



      • #4
        Nick---I have been a design engineer for 44 years. My lifes work has been designing and detailing prototype machinery and automation. A good rule of thumb for dimensioning is ---If its a "General Arrangement" drawing, which shows the final assembly, (or even a complex sub-assembly), put just enough dimensions on it to tell somebody how tall, wide, and long the assembly is, (these are referred to as "Envelope" dimensions) and dimension any mounting surfaces or holes that the end user will need to use to either mount the part to a floor, wall, ceiling, or a mating part. If it is a detail (shop) drawing, put enough dimensions on it that someone can immediately see the overall length, width, and height of the part, and enough dimensions that a machinist can put the part up on a mill or lathe and manufacture it from the given dimensions. A machinist should never have to add or subtract dimensions to come up with information required to make the part.--That is just adding a "possibility of error" that should not fall on the machinist. A good book, (and one I have used as a teaching textbook over the years) is "Engineering Drawing and Design" by Jensen, published by McGraw-Hill company, Canada Ltd.---Brian
        Last edited by brian Rupnow; 01-26-2009, 03:10 PM.
        Brian Rupnow


        • #5
          The visualising part is often the hardest, so well done on that score.

          Another old adage that often proves a truism is that very often if it looks right, it may well be right. Doesn't always work, but more often than not, and it's surprising how well your mental sense of proportion works.

          Also, when designing something there is usually a critical interface part that has to mate with something else, and this can often set the scale for things. An example would a plain bearing or guiding surface, where a general rule of thumb is that the bearing length should be at least 2 x diameter, and working with this as a base can then set the dimensions for other components built up around it.

          Of course you can always do it properly and work out loads/stresses/deflections and so on, but the above can provide a good starting point.



          • #6
            Thank you for the information - this is exactly what I was hoping to find.

            Brian - the local library has an available copy of Engineering Drawing and Design, 7th Edition. I will definitely check it out!



            • #7
              Nick---My 3D software does let me interface individual parts, and it does let me "assemble" these individual parts in cyberspace into working machines. It will let me check for static and dynamic interferances, and it will automatically create 2D dimensioned drawings of these parts. Unfortunately, it is very expensive (as in $5000 to $6500 per license), and it has a very intense and difficult learning curve. I have it because I use it professionally, and I can justify the expence. However for the average "home shop machinist" its just too much money and trouble. Not only does it let me design parts and assemblies and make drawings, it allows me to make machine animations of those assemblies after the machines are designed. For a good sample of what I am talking about, go to and click on any of the animations to see it run (there are 10 or 12 different ones). I did all of those animations about 2 months ago for that website.---Brian
              Brian Rupnow


              • #8
                One thing I might add . It helps a lot if you know what size stock material comes in. If you can keep from having to cut of too much excess material it helps a lot.
                Every Mans Work Is A Portrait of Him Self


                • #9
                  Originally posted by brian Rupnow
                  N A machinist should never have to add or subtract dimensions to come up with information required to make the part.Ltd.---Brian
                  You rock

                  Originally posted by lane
                  One thing I might add . It helps a lot if you know what size stock material comes in. If you can keep from having to cut of too much excess material it helps a lot.
                  You too lane I try and get this thru the heads of the metric guys, I have zero problem working with metric prints, but getting metric sized stock is tougher and cost more or I have to cut down some to make the part to print. Will the .7 really make a difference on a 12mm thick part, if not I can get 12.7mm(1/2") all day long right down the street.
                  Last edited by mochinist; 01-26-2009, 07:42 PM.


                  • #10
                    I have been a design engineer for a shorter time, "only" 30 years.... I am basically an EE, but started doing the whole product and have to some extent kept that up at least at the general level (I try to slough off the details on others... more efficient).


                    You almost ALWAYS have some "external restraint".... the unit or part has to fit somewhere, connect to other parts, mount in a particular way, pass through a certain minimum sized hatchway, etc.

                    And, you generally know some loading or demand on the unit or part that forces certain dimensions. A shaft must be a certain size, a transformer (in my case) is a certain size and has wires in certain places, etc.

                    In other cases, it must fit in a shirt pocket, or a dial must be readable, or a big guy's fingers have to fit between the knobs, etc, etc, etc.

                    So you can almost always find some limiting factors.

                    On top of that, there are usually some natural "flows" to the device. In my case, it is usually cooling ... In yours it may be something completely different.

                    Then you start taking the pieces, or estimates of the pieces size, and fit them together. Pretty soon you will get sizes for different elements, and it all starts to come together.

                    The absolute KEY is to set the "external" stuff, and work inwards to the innards that make the outer parts work. You will change things, and SHOULD. It's good for the design to have to "worry it" a bit.

                    External things you set will become obviously impractical, and you will change them. NEVER let a "set" item mess up the design unless something absolutely forces it. It's easy to change now, hard to fix later.

                    The result should be a good design

                    CAD is your friend. I don't use Solidworks or Pro-E, I consider them a trap for unwary engineers, allowing too much detail and "finality" to appear too soon in the design.

                    I use a 2D package, in my case Autocad LT, which is deliberately NOT a final drawing means for complex things.

                    But, if you can't reduce your device to 2D, you likely don't understand it, and you also probably will spend too much time messing with the 3D and too little getting the device right.

                    So I do 2D views of critical interfaces, defining everything, and overalls, and toss it to others to finish. But when I get rid of it, I know where stuff goes. The 2D allows me to very quickly move things and gain an understanding of how it should fit.

                    Maybe I am too good at visualizing, and that is why this works. I know guys who jump right into 3D and spend too much time chasing bad designs. And others who do fine straight into 3D. I just plain don't WANT to, so I get it laid out and farm out the rest.

                    Keep eye on ball.
                    Hashim Khan


                    • #11
                      dang...never really had to think about how things come together before. they just do.

                      I always start out by drawing the part that is to be made, or manipulated. Then I draw the maximum constraints. Then I just start working around things. Then I revise it 450 times and I have a part to make.

                      You get yourself really familiar with the products from carr lane, mcmaster carr, reid tool, danly, etc. Reid tool always had a CD that had all their products in cad format, so you could just plop them in. Generally everything else is just connect the dots.

                      I never was a "mathematical" engineer. I just built things that worked, and likely used about 2x the amount of steel actually necessary. For what I was doing, heavier never hurt.


                      • #12
                        A lot of designers quote "form follows function" I suggest you try "form follows restrictions". Function is but one of the restrictions as are money, available tooling, standard fasteners, bearings, material available, environment it will be used in, and today even the disposal of the item. Included on this list would be the abilities of the work force as well as the cosmetic/visual appearance of the product.


                        • #13
                          Junior draughtsman, draughtsman, senior draughtsman, senior design draughtsman, how many do you want to go Nick?? it's a long trail in WHICH field?? It's surprising how many have installed CAD packages and are now "Designers". Quite a few years ago, I decided that I needed the ability to be able to "draw on a screen" after thirty years of graphited and some times inky cuffs. On completion of a course and tests, the successful (all twelve of us) were presented with an industry recognised certificate to say we had completed a "Design draughting using Autocad release 12" examination. One guy was a bricklayer, another worked in a "Chain" store and various other jobs and made me wonder why I'd spent years suffering on "Further education" when thanks to the computer age I could become a"Designer" in a few weeks.

                          OK, what are you trying to achieve? Are the drawings you want to do to act as a record and reminder to yourself, or are they to amaze the rest of the world with your design prowess?? With the standard drawing packages, a dimensioning system is usually included so thats the easy part, as you quite rightly state, it's the where to where and why that takes the thinking about and time. You can ask the same question of a dozen "Designers" and get a dozen different answers, two things may get a common response. "All dimensions should start from two planes" and "A drawing is worth a thousand words".

                          Basically,-- bulls4*t and bulls4*t. If you have a baseplate with different "block" items fastened to it, it's important that you accurately dimension ONE point and then all subsequent dimensions pertaining to THAT part from it (Sub Datumning) and if you have a block that has one detail say a blind hole on a plan face, it's far quicker to WRITE 1hole drill Xdia x Ydeep than generate another view to "Fill" the page.

                          Metrication, that's a WHOLE new thread

                          Put the LEAST number of dimensions on a drawing, stand back and ask yourself "Have I got enough information to make the part"? and if you REALLY want to know, get someone ELSE to make the bit from your drawing.

                          Regards Ian.
                          You might not like what I say,but that doesn't mean I'm wrong.


                          • #14
                            At school I learned strength math, I would'nt know how to translate this.

                            It's about torsional load, bending etc. The teacher said: this is important, learn to understand the formula's.

                            I tried to, it teaches you that you can't transmit a thousand horsepower through a wrist-watch axle, that you can't mount a gear in free space, but that it should be mounted as close to a bearing as you can get.

                            What it also made me, is a disaster tourist. Worn, torn, you'll always see something.

                            My finest moment was with an agricultural machine.

                            They had drilled holes in the structural beams, out of the middle. According to the book this is a definite weakening no-no. I pointed this out, they said this is no problem.

                            The next day the machine broke at the exact spot, which I had pointed as the weakest point.

                            Being practical is one, but there should always be some underlying theoretical foundation. You can't make something work by shouting at it.


                            • #15
                              Journey's end?

                              Why draw it in CAD? What is wrong with a good free-hand sketch? The more so if a part or parts can be made from the "drawing"?

                              I have three CAD systems on my computer and other than a run-through to see how two of them worked I haven't used CAD in over 10 years.

                              I am not against CAD at all. But it must be a means to an end - not the end in itself as this is another version of "the end justifies the means" to be "the means justifies the end". Both MAY be true - but not always.