Due to a jump in work requests recently, please pardon the delay in getting this out to you. (If you have been thinking about having some design work done, it would be a good idea to contact me soonest, as I work on a first-come first-served basis.) Now picking up where we left off last time, we were discussing DFM (Design For Manufacturability) and DFA (Design For Assembly), let’s start with a few simple examples in order to really get the concept. Obviously, the main criterion for any part is to do what it is supposed to do. Once you know that, the initial design usually presents itself.
For a fishing pole, you want a long slender rod that has a good amount of flexibility. You can use a simple branch from a tree, a piece of bamboo, a tapered, fiberglass rod or a carbon fiber rod with different orientations of the various layers (plies) of the material. All will work. Next look at what kind of load you might expect. Are you fishing for little fish in a lake or very large fish you are likely to find out in the ocean that will be more than a little annoyed at being hooked. The next point to consider is the anticipated selling price. What is the public willing to pay for it?
For a hammer, you want a heavy, compact mass with a handle. Because a hammer is used for hitting things, you need to know if the things you are hitting are relatively hard or soft. In some cases you might want the hammer to leave impact marks in the piece being hit and sometimes that is definitely unacceptable. The heads of hammers have been made out of materials ranging initially from rocks to tightly rolled leather and various woods, metals and plastics. If you had to get the most mass into the absolutely smallest envelope, your best choice would be iridium, followed by platinum and then gold. Observe how the cost factor immediately enters into the picture (unless you are an entity of the government of course).
Let’s say that due to various functional requirements, the part will be made out of a specific plastic. To make one part, just get a block of that plastic and machine it. In this case, the design parameters are wide open. This works for a relatively small number of pieces. If the anticipated annual production volume increases greatly, one may consider using the already mentioned injection molding (IM) process. If so, there are other considerations that enter the picture immediately, namely draft and wall thickness.
With a machined plastic part, having all the surfaces parallel and square is the norm, while IM parts must have some draft (taper) to allow the part to be removed from the cavity. Yes you can have an IM part with no draft, but you will pay a hefty premium in mold cost. Also, a machined part designed with greatly varying wall thicknesses is not a problem. However, with an IM part, one tries to maintain a uniform wall thickness to prevent the part from warping after it comes out of the mold and is cooling. So, taking a drawing for a machined plastic part and giving it to a mold maker and telling him that that is exactly what you want, without considering the IM process, will result in a mold that is extremely expensive that makes unacceptable parts.
One last point is dimensioning. Let’s say you have a 2” thick wedge that is a right triangle with the long leg measuring 5 inches. Let’s say that the smaller angle is 22 degrees. You can call out the angle as 22.000 degrees, plus or minus .005 degrees. This will define the part. Or, you can call out the angle as 22 degrees, plus or minus 3 degrees. This defines the part too. Obviously, the part with the tighter tolerance will be a lot more expensive. Do you need it so “tight?” If it is for the alignment of some kind of laser device, yes! However, if it is to only hold a door open, no! Tight tolerances cost. Do you need them so tight?
I’m out of space and out of time. Definitely to be continued. Stay tuned.