F4e. press and shrink fit fastening - can be a low-cost method for permanently fastening parts together. The method involves the use of heavy force to drive one part, usually a pin, shaft, stud, or other round part, into a hole where the fit is tight or where there actually may be an interference fit. In such a fit, the diameter of the male part slightly exceeds the diameter of the female part. The disadvantage of press and shrink fits is that the dimensions of the mating parts must be closely controlled. Pins to be inserted are often centerless ground to provide an accurate diameter, and the holes to receive them are normally reamed or bored to insure an accurate internal diameter. Often, the end of the part to be inserted is tapered slightly or the hole is beveled slightly to permit easier initial insertion. In a typical situation, the part to be inserted is manually positioned in the hole and then driven into position with a hand, foot, or powered press. In a shrink fit, the receiving part is heated sufficiently to expand it so that the two parts can go together. When it cools, the outer part shrinks around the inserted part, holding it securely. Press and shrink fits are more common in heavier machinery. Fig. 7F4e illustrates a simple press fit of a small pin and two alternative designs that lessen the amount of precision needed to insure a satisfactory fit.

Fig. 7F4e Three varieties of pins press to fit into a metal component. The roll pin at the center and the groove pin at the right allow a lesser degree of precision in the diameters of the hole and the pin; for these pins, reaming of the hole after drilling is not normally required. ( from Design for Manufacturability Handbook, James G. Bralla, ed., McGraw-Hill, New York, 1998. )

F4f. staking, seaming and crimping – are other methods of fastening parts together. Staking is similar to riveting except that, instead of using a separate rivet, one of the parts is configured to fit into a hole in the other, and is upset to hold it in place and thereby hold the parts together. Fig. 7F4f illustrates a typical staking operation.

Seaming is a means for fastening sheet metal parts together at their edges. Fig. 7F4f-1 illustrates cross sections of a group of typical seam joints. These joints can be made by a series of operations on a press brake or, in high production situations, by dedicated tooling that fits the parts involved. (See chapter 2, sections C and D for sheet metal shearing and bending processes.) Containers such as drums, cans, and pails are routinely fastened by seaming. The approach is also common on all kinds of sheet metal work including the manufacture of ducting for buildings for heating and air conditioning systems.

Crimping , when involved in assembly, involves the bending of sheet metal parts to lock them into place. The term usually applies to cylindrical parts like caps, which fit over smaller, more rigid, cylindrical parts. The diameter of the cap is reduced where there is a circumferential groove in the inner part. Crimping is usually a fairly simple operation, performed with a hand or foot-operated lever tool or a light punch press. Electromagnetic forming is also sometimes used. (See 2J3.) Crimping is often less costly than using fasteners to hold the parts together, because it avoids the need for holes and screw threads and eliminates the need to maintain a stock of some kind of fastener. The most common applications of crimping are for the attachment of connectors to electrical wires, fittings to the ends of mechanical wires and cables, the attachment of hose and tubing to end fittings, and shells to bullets. Fig. 7F4f-2 illustrates a typical crimped assembly.

Fig. 7F4f Sectional view of a staked assembly.

Fig. 7F4f-1 Sheet metal seam joints. a) outside seam, b) inside seam, c) compound seam, d) double seam for containers, e) double seam for containers with recessed end.

Fig. 7F4f-2 Crimping used to fasten a length of tubing to the end of a rod.