Process Selection in Making a Simple Part
You should first determine the part's function, the types of load and environment to which it would be subjected, the dimensional tolerances and, surface finish required, and so on, For the sake of discussion, let's assume that the part is round, that it is 125 mm (5 in.) long, and that the large and small diameters are 38 mm and 25 mm (1.5 in, and 1 in.), respectively. Let's further assume that, because of functional requirements (such as stiffness, hardness, and resistance to elevated temperatures), this part should be made of metal.
Which manufacturing process would you choose, and how would you organize the production facilities to manufacture a cost-competitive, high-quality product? Recall that, as much as possible, parts should be produced at or near their final shape (net- or near-net-shape manufacturing), an approach that largely eliminates much of the secondary processing (such as machining, grinding, and other finishing operations) and thus reduces the total manufacturing time and manufacturing cost.
This part is relatively simple and could be suitably manufactured by different methods such as :
- Casting or powder metallurgy
- Upsetting,
- Extrusion,
- Machining, and
- Joining two separate pieces together.
For net-shape processing, the two logical methods are casting and powder metallurgy; each process has its own characteristics, need for specific tooling and labour skill, and costs. This part can also be made by cold, warm, or hot forming. One method, for example, is the upsetting of a round bar 25 mm (1 in.) in diameter in a suitable die to form the larger end (heading). Another possibility is the partial extrusion of a bar 38 mm (1.5 in.) in diameter in order to reduce its diameter to 25 mm. Note that each of these processes can shape the material with little or no material waste.
This part can also be made by machining a 38-mm-diameter bar stock to obtain the 25-mm-diameter section. Machining, however, will require longer time than forming, and some material will inevitably be wasted as metal chips. On the other hand, machining does not require special tooling (unlike net-shape processes, which generally require special dies), and this operation can easily be carried out on a lathe. Note, finally, that this part could be made in two separate pieces and joined by welding, brazing, or adhesive bonding. Because of the possibility of using different production methods, the raw material to be used in each case would be different. This would affect the cost of raw material and hence the cost of finished unit. For example
- Square bars are more expensive than round bars,
- Cold-rolled plate is more expensive than hot-rolled plate or sheet, and
- Hot-rolled bars are much less expensive than powders of the same metal.