Sorry for the delay in sending out these weekly info letters, but customer delivery requirements take precedence, (as I’m sure you understand). Okay, continuing on with design requirements as a function of process capabilities, let me give you an example of an investment-cast alloy steel part replacing the same part that was machined from bar stock. As mentioned previously, machining inherently provides straight (parallel) walls with sharp corners. In fact, the machined corners and edges, except for cutting blades for example, almost always have to be deburred in order to have a useable part.
Using the machining process, round holes are a “given” feature while square holes present a problem. The Electric Discharge Machining (EDM) process can make square holes (or any other shaped hole, for that matter) but it is a rather slow and costly process. On the other hand, investment casting is a lot like the injection molding process, where features like square holes are not an issue and rounded edges are the norm. The only difference is that the molten material, in this case alloy steel instead of plastic, is “injected” into a mold, cooled and removed from the mold. (Actually, the mold is a ceramic type material that is broken away, leaving the metal part.)
In this particular application, we had a drive shaft that connected an electric motor to a rotor. The end of the shaft of the motor was cross-drilled with a roll-pin inserted that stuck out on both sides. The motor end of the drive shaft had an axial hole with a radially-machined slot, which accommodated the shaft and the roll pin respectively. The other end of the drive shaft protruded from the machine. On this end there was a much larger cross-drilled hole that broke through the end of the drive shaft. Then, a milling operation cut a slot radially through the hole at an angle, so that part of the hole was machined away, leaving a slot with a sharp point (“hook”) that was the remnant of the hole on both sides. Finally, there were two grooves machined in the shaft to accept snap-rings, which held it in the machine. In use, a rotor was placed on the end of the shaft prior to starting the machine. The rotor had a hole to receive the shaft and a roll-pin, in a cross-drilled hole, that the “hooks” on the shaft captured, preventing the rotor from rising up in use.
I realize that this is rather complicated to visualize, so I’ll look through my records and dig up a photo showing the before and after design. I will have it for the next info letter. My whole point of this exercise though, is to give you an example of lazy engineering versus sharp engineering. Someone had the bright idea to change from a machined part to an investment-cast part. So far, so good. However, they just used the same drawing for the machined part from which the investment-cast part tooling was made. There was never an evaluation of how the design of the part might be changed to make use of properties that the investment-casting process gives you for free. Lazy, very lazy engineering.
I will continue this discussion and my solution that improved the drive shaft in the next info letter I’m planning to get out next week, subject to client demands for my services. It is starting to get busy, but I still have available time for new clients. Just call me for a no-cost, no-obligation appointment, so I can assist you with any of your product design or re-design issues. Tooling assistance is provided too.
(Originally published on 5-12-10)