Choosing the proper fastener head style often boils down to either the engineer’s preference, what’s currently popular, or what’s needed for a very specific (non-standard) use. All of these situations can be dangerous and costly.
Internal Recess Drives
For internal recess drives, the most commonly available styles for smaller machine screws (1/4″ or M6 and smaller) are slotted, cross recess (Philips), cross recess (Pozidriv), Torx and Torx Plus. Internal hex drives are common in what we refer to as socket head screws, but they are intended more for tooling than for use in production. The matrix below should give you some points to consider when choosing an internal drive screw:
|No tool required*||BEST||NR||NR||NR||NR|
|Production hand assemble||NR||NR||OK||OK||OK|
|Production power assemble||NR||NR||OK||OK||BEST|
*No tool required: In some designs, such as access doors on equipment, tools may not be readily available, so anything from a coin to a car key may be used to rotate the slotted head and gain access.
**NR: Pozidriv and Torx Plus drives are easily confused with Philips and Torx, even though they require different drivers. For consumer assembly, it is recommended to steer away from these.
Internal Recessed Drive Screws
For internal recess drive screws, the head style is imperative to consider. Many options exist, but not all are readily available. The most common head style is the pan head and it is available in nearly every size up to 1/4″ or M6. Less common styles are round head, cheese head, fillister head, hex head and hex washer head.
For larger screws, hex head, hex flange head and hex socket head are the most common styles available. Hex socket heads are generally only used when there is a clearance issue where a socket will not fit on the head to drive it. Hex flange screws are generally more expensive than hex heads, but with their larger bearing surface, they can often eliminate the need for washers which can ultimately lead to a cost savings. Hex heads may be used without washers when bearing up against a very hard surface with a properly sized hole, but in many cases the surface pressure limit of the mating part is not high enough to support the load without adding hardened washers or a flange.