The points are generally made sharp, so that they may penetrate the wood; the body of the screw is tapered, so that the deeper it is driven the more tightly it will fill the hole; the thread does not extend throughout the length of the screw, but a considerable portion below the head is left smooth • the thread is formed to an acute angle, and there is a considerable pitch or distance between the threads.
Each length is made in from 15 to 30 different thicknesses, identified by numbers.
The following are the thicknesses or diameters corresponding to some of the numbers. The thicknesses of the other numbers are interpolated between those given, varying in succession about 1/64 inch : -
Thickness or diameter in parts of an inch . .
Length from top of head to point in parts of an inch.
1/4 3/8 1/2 5/8 3/4 7/8 1 11/4 11/2 13/4
Numbers made. 0tol6 ltol6 ltol6 ltol8 2to20 3to24 4to26 5to28 6to30 7to32
Length from top of head to point in parts of an inch. 2 21/4 21/2 23/4 3 31/2 4 41/2 5 6
Numbers made. 8to36 9to38 10to40 llto40 12to40 14to40 16to40 16to40 18to40 20to40
Flatheaded Screws (Fig. 180) are used in wood for fixing all metal work or furniture whose thickness is sufficient to admit of the head of the screw being countersunk into them, so that the top of the head is flush with the face of the metal to be screwed on.
Round-headed Screws (Fig. 181) are used where the metal is too thin to be countersunk, as in some forms of locks, latches, etc.
1 Sc. Screw-nails.
Patent Pointed Screws are made with sharp points like that of a gimlet, as shown in Fig. 180. They resemble the general description given above, and are commonly used.
Fig. 182 shows an old-fashioned form of screw, with an angular thread and blunt point, formerly known as Nettle/old's Patent Screw. The advantage claimed for it was that the top side of the thread being horizontal or inclined upward, offers great resistance to the screw being dragged forcibly out.
Coach Sceews are large heavy screws used where great strength is required in heavy woodwork, and for fixing iron work to timber. They have square heads, so that they can be screwed home with a spanner or wrench, and a thread like that. shown in Fig. 183.
Handrail Screws are of a peculiar construction, and are intended for joining together two lengths of a staircase handrail, as shown in Figs. 184, 185.
The screws are from about 3 to 6 inches long, and are threaded at each end.
A square nut s is made for one end, and for the other end a circular nut c, the latter having at intervals deep nicks in its circumference to receive the end of a screwdriver.
The sketches at s and c show the form of these nuts. Deep slots are cut from the under side to the centre of the handrail, through which they are dropped into the positions 5 c in Fig. 184. A longitudinal hole, ab, is bored in the handrail, in which the screw is placed so as to pass through the nut at each end. The circular nut is turned on the screw by means of a screwdriver, so that the portion of the handrail in which it is fixed is drawn toward the other until the joint between them is quite tight, dd are dowels inserted to strengthen the joint.
Fig. 1861 shows another form of handrail screw, known as a dowel-screw.
Brass Screws may be obtained in nearly every form, at about three times the cost of iron screws. They are very useful for securing work which requires to be easily removable - such, for example, as the beads of sash frames (see Part I.)
Screws are made in several other forms besides those mentioned, for special purposes, which need not be further referred to.
Screws for Metal are made in different forms from wood screws; the diameter of the screw is the same throughout; the threads are close together, V-shaped, but with the points of the Vs rounded off.
The great difference between screws for metal and those for wood is that the latter, by the pressure of their threads against the fibres make a hole into which they will fit exactly, whereas in metal such a hole has to be tapped of the exact size to receive the screw.
Unless the internal thread of the nut, or of other metal into which the screw is to be driven, exactly fits the thread of the screw, one or the other will become distorted in screwing; they will bear unequally upon one another, and great loss of strength would ensue, together with difficulties in working.
1 Knight's Dictionary of Mechanics.