Step 1: Determine units

Start by asking yourself where the part was made, and more specifically where it was designed. A part originating outside of the United States is probably metric, but what if it was designed by an American company and simply manufactured overseas? Alternatively, what if it was designed overseas, but with the purpose of interfacing with an American product?

Let’s take a look at headphone plugs to illustrate the complexity of this problem. The original standard plug was an American design and the specifications call for a diameter of 1/4″ (6.35mm) on the barrel of the plug. The mini headphone jack, which became popular later and is probably what you have now, is exactly 3.5mm (0.137795″). I can’t find solid information on this, but I assume that’s because it was designed for international standards.

Once you have a hunch about the units being used, try taking some measurements in inches and millimeters on some major features, like the length, width, or diameter of the main body. See which (inches or millimeters) are closer to nice round numbers, while still keeping in mind that manufacturing is never perfect, and they probably won’t be dead on.

Step 2: Important primitives

Start by taking measurements of the main features of the part, and modeling those. It’s best to start with features that would be primitive solids, in order to get an accurate base. You also want your most accurate measurements (which are usually the first ones) to be the “important” features. These are features which affect the functionality of the part, such as where it will mate with another part.

Using the headphone plug as an example again, you can see that the plug barrel is very important to the functionality of the part. It’s what interfaces with the audio output device’s jack, and so it’s critical to get those measurements as accurate as possible. The body of the plug is used for two things: to house the wire connections, and to provide a gripping surface. Neither of those things is especially dependent on accurate dimensions. Therefore, you should start with designing the plug barrel — specifically by beginning with a cylinder based on the overall length of the barrel and its diameter.

Step 3: Unimportant Primitives

Next, it’s a good idea to go ahead and model the rest of the primitives that aren’t as important. The reason you want to do this before getting to the details of the important parts is simply a matter of logistics. It can sometimes be difficult to add major features without a nice “clean” primitive to reference. There are ways around that of course, but it’s usually best to have a complete “rough-in” of your part before you begin with the detail work.

Step 4: Important Details

Now that you’ve got your rough part, it’s time to start adding the important details. This is the most difficult part of the entire process, as those details are hard to measure but are still essential to the functionality of the part.

For this plug, you need to get measurements for each of the two revolved cuts into the primary cylinder that you started with. To do this, you need their diameters (5 and 6), as well as information on their positions (1 and 3) and their widths (the difference between 2 and 1, and the difference between 4 and 3). Why measure from the tip, instead of from the other end? Because the tip is what actually fits into the female jack, so the distance of these features from the tip is more relevant than the distance from the grippy end. You also need to be careful not to stack tolerances — all measurements should be taken from a hard point. That’s because each measurement you take will have a margin of error, and you don’t want those errors to add up.

The tip is next, which presents a problem: how do you measure the angle of the tip? You could use a protractor, but that’s not necessary and could present its own problems. One way would be to measure the distance from the tip to the widest point. But that will give you accuracy issues caused by the rounded tip and the beveled (fillet) edge. There just aren’t any “hard” edges to measure from. Instead, a better way is to make some inferences on the angle and the choices the original designer made. Drawing lines on a photo can be surprisingly helpful, especially if your software has 2D CAD-like measuring capabilities.

Right away, we can see that the angle between the center axis (blue line) and tip slope (green line) looks pretty darn close to 45 degrees. That’s about as round of a number as you can get, and it’s probably safe to assume that was the original design intent. But, did you notice that another problem has come up? The intersection of blue and green lines isn’t at the end of the tip (orange line). This is because instead of having a sharp tip, it was made with a blunt rounded tip. That means that when modeling the tip, you can’t easily use the green/blue intersection as a reference point for the revolved cut.

Instead, you can make the reference the intersection of the green and yellow lines. Now, this is also an imaginary point, as there is no hard edge there. The fillet makes it impossible to get a perfect measurement with calipers. But, it should be a little easier than the tip. Making a 45° revolved cut from there would leave you with a flat tip, which would then be rounded with an edge fillet (the fillet radius matching the radius of the circle of the flat tip).

For the rest of the fillets on the plug, you’re going to have to guess and make inferences. Try some different radii until they match those of the part. Getting this right takes some practice and experience, but it shouldn’t be too hard to get it close. Luckily, the radii of bevels (fillets) and dimensions of cut off edges (chamfers) aren’t integral to the functionality of the plug, because they’re just there for the spring clip to grip. So, it just needs to be close.

Step 5: Unimportant Details

This last step is pretty easy, because it’s not essential that you get it exactly right. On our headphone jack, the “grip” area could be completely different from the original part, and it would still work just fine as long as the wire connectors still fit inside. You can make it look like the original part, or you could take some artistic liberties (like knurling the entire area for better grip).

Now, I’ve obviously simplified the modeling of this particular part. In reality, it’s actually an assembly made up of a few parts to allow internal wire connections, and an electrical connection to the female jack. If you were actually trying to reproduce this jack (a TRS connector specifically), you would have to take it apart and model each part individually. But, hopefully this has given you an idea of the process you would need to use.

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Source: hackaday.com