Screw HEADs types, Gauges, & lengths
What is Meant by The Gauge & LEngth of a Screw?
The gauge of a screw is defined by the thickness (diameter) of the un-threaded part of the screw known as the shank. Ordinarily, in the United Kingdom the head size is roughly twice the diameter of the shank, although there are some exceptions associated with hybrid, hardened, DIN Standard (metric) and imported screws.
Screws are also categorised and measured by length normally ranging from 1/8" up to 6" long.
For example a 2 * 1/4" screw (2 gauge by ¼" inch long) is a small wood screw, whereas a 28 * 6" (28 gauge by 6" inch long) is rather large both in gauge and length.
Head Types Explained
Wood screws are manufactured in many sizes and usually have one of 3 different head types; Countersunk, Raised, Round, all with slightly different uses.
Countersunk Head screws are used for general carpentry and joinery applications where the head of the screw is required to be either flush or recessed below the surface of the timber, hardware or ironmongery alike.
Round Head screws are dome shaped in appearance and are used for applications where it is desirable that the head of the screw is to be seen and therefore the screw of choice, when an ornamental finish is required, as with brass surface locks & decorative cabinet work.
Raised Head screws are very much used for the sake of appearance, when the head of the screw is to be tastefully displayed, all be it less prominent than that of a round head wood screw.
HOW ARE SCREWs MEASURED?
Screw lengths are measured differently depending on the head type (countersunk, raised or round head). When measuring the length of a wood screws the technical term for the overall measurement of the screw is called the effective Length.
Countersunk Head wood screws lengths, are measured in imperial (inches). The two effective measuring points for this screw are taken between the very top of the countersunk screw head (the widest point of the flat surface) down to the very tip (historically called the gimlet point) at the bottom of the screw.
Raised Head wood screws lengths, are once again measured in imperial (inches). The two effective measuring points for this screws are between the centre line or lip of the raised head, down to the very tip (historically called the gimlet point) at the bottom of the screw.
The way we would describe the lip of a raised head wood screw is, if you imagine looking at the top of the screw, then the lip is the widest part of the head, where the countersunk and raised parts meet.
Round Head wood screws lengths, are also measured in imperial (inches). The two effective measuring points for this screw are taken between the underside of the round head (the widest point of the flat surface) down to the very tip (historically called the gimlet point) at the bottom of the screw.
A less known and often undisclosed fact about traditional slotted wood screws is that, typically screws are manufactured to what is known as a +/- tolerance and thus screw lengths can differ very slightly between batches and also manufacturers, more so if a manufacturing standard is not adopted.
What is a TraditiONAL WOOD SCREW?
Traditionally, wood screws are manufactured in many sizes and usually have one of 3 different head types; countersunk, raised or round, all with slightly different uses.
Wood screws as we know them were manufactured as far back as the late 1700’s. Throughout the early part of the 20th century wood screws were produced principally with a slot cut into the head, so that the screw could be driven in with a flat headed, blade type screwdriver, however as technology advanced in the late 1950’s and early 1960’s different drive types were introduced to include Philips cross recessed drives, which require a different type of screwdriver.
Wood screws are usually manufactured from, brass, bright mild steel or stainless steel and can be found with a variety of metallic, decorative and protective finishes, including chrome, nickel, black japanning, sherardized, zinc and light B.M.A, all of which have various aesthetic, protective or functional advantages.
Historically wood screws were manufactured using ‘ingenious’ automatic machines in the late part of the 19th century, in a 3 stage process known as ‘cut threading’, on machines known as ‘slone machines’ (automatic lathes), whereby a wood screw was cut from a blank un-threaded screw, thus removing material (metal swarf), producing a wood screws with a shank that has tapered core and also a gimlet point (cork screw type point) at the very tip of the screw. This type of screws is often referred to by joiners and craftsmen of old as a 'traditional' cut thread wood screw.
As manufacturing processes evolved throughout the 1950’s & 60’s, wood screws were manufactured using a different technique called ‘roll threading’, whereby the blank screw is rolled between two metal plates known as rolling dies, to extrude the steel or brass, forming a either a parallel shank or sometimes tapered wood screw, not dissimilar to those of the earlier traditional cut thread screws.
Which manufacturing process is best, cut threading or roll threading?
Cut threading or roll threading is the subject of great debate between academics, engineers, joiners, cabinet makers and boat builders alike, with strong convincing arguments for both.
Joiners of old and traditionalists, would argue that traditional cut thread wood screws are more malleable and have a degree of flexibility, that make them more resilient to breakages, especially in hardwoods, however those involved with rolling threaded products would argue that that rolled screws are much stronger.
The historians and manufacturing economists amongst us would argue that the gradual shift away from the cut threading process was influenced by the inefficiencies of cut threading, whereby the old automatic lathes were much less efficient than modern rolling machines both in respect of output and material costs.