Fastening methods: Turning Performance

Most machine shop time is spent turning raw materials into parts, but the next manufacturing step is often assembling and fastening those parts together. Cost, weight, strength, availability, reliability, corrosion resistance and simplicity are some of the many factors that influence the fastening method. There are more than 100 different fastener designs and tens of thousands of variations when size, finish and material are counted.

Three factors determine whether a fastener is a screw or bolt.

• Size: Screws are typically under ¼ ” in diameter; bolts are larger.

• Mating threads: Screws fit threads tapered into a workpiece; bolts fit into a mating nut. This is usually, but not always, true.

• Head design: Screws are often slotted; bolts often have square or hex heads.

In connection with the difference between drive and head design, drive design is the external or internal shape on the fastener head that couples the screwdriver, socket wrench or other driving tool to turn the fastener. The head design is the overall shape of the head that makes the fastener better suited to a particular application, such as a round, flat, oval or fillister head. However, there is some overlap: All external hex drives are hex heads, but internal hex drives, all called Allen heads, may take various shapes even though they are commonly called hex or Allen heads. Therefore, these two terms are imprecise.

There are four common screw drive designs.

• Slot heads have a single slot and are driven by a flat-bladed screwdriver. Their main drawback is that they do not work well with power screwdrivers because flat blades have a tendency to come out of the slot and damage surrounding work. Slot heads are used on screws and small bolts.

• Phillips heads, sometimes called cross-head screws, have a “+”-shaped slot. They were originally designed for use with power screwdrivers in production. The rounded corners in the tool recess make the driver “cam out” when the fastener is tight and also makes unscrewing difficult. Phillips heads are used on screws and bolts.

• Combo heads accept either a Phillips screwdriver or slot-blade driver. Combo heads are used only on screws.