With some helpful tips, he explained: "Standard hexagon head is one of the most common form of screw or bolt head used throughout industry in typical sizes from M4 upwards but there are also countersunk screws with recesses of many types, e.g. multi-splined, hexagon socket, pozi and slotted. These have particular purposes since countersunk screws for example finish flush so need a recessed drive. There is also pan head which is effectively surface mounted and tends to be used for M1 up to M8 sizes with many drive forms available."
For both stock and custom needs in recent years it seems that the tendency has been to avoid slotted drives because they do not lend themselves to power tools, either manually or automatically operated. Consequently a wide variety of other designs have been created primarily to aid fast and accurate machine-driven assembly with less slippage and more control of speed, torque and pressure.
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Standard external hexagon drives suit many kinds of automated assembly, these and hex socket drives are both very positive options for larger sizes. However, auto-feed tooling is required for high speed assembly and this presents the screw to the driver which best utilises multi-spline recessed heads for fast positive performance.
Socket head cap screws are a good example of a screw design suitable for manual or semi-automated production and feature an internal hex drive in a circular head, often with external knurled finish which aids manual starting of the thread allowing a good finger grip for positioning. Similarly, flange head bolts which have a flange that stands in place of a washer to spread the load. A related variant is the range of socket set screws (grub screws) with internal hex drive, which are used for location, e.g. collars on shafts and similar.
Carriage bolts are a good example of a very useful and specialist head type, in this case very popular with the construction industry. They are formed as a dome with square underneath designed to locate into a square hole in metal or to pull down and lock into wood. This enables the bolt to be tamper proof on the head side and for the nut to be done up/undone without needing a tool at both ends of the bolt/assembly.
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