Screws come in a variety of lengths, diameters and head designs to meet the needs of many construction projects. Choosing the right screw size depends on material types and the hold or anchor required. Selecting the correct screw gauge is also important to prevent tearing or damage to materials.
Screw sizes are indicated by two numbers, the first of which indicates the diameter of the screw. The second number indicates the threads per inch. The threads per inch are determined by counting the number of peaks on a one-inch strip of the screw’s threads. The higher the thread count, the finer the screw; a smaller thread count results in coarser screws.
There are two standard systems of screw measurement: Imperial units and metric. Most screw manufacturers use a metric system of measurement and will list the screw’s length, thread diameter and head-bore (the dimension of the smooth part of the screw above the threads) in metric numbers. For example, a screw labeled 5 x 70mm is a screw with a 6 mm diameter and a thread count of 70 threads per inch.
Screws with coarse threads have larger spaces between the threads than screws with fine threads. Drywall screws are available in both coarse- and fine-thread versions to allow for a choice of maximum strength or minimum tearing when joining drywall with wood shafts. Wood screws typically have a slight taper on the shaft near their heads. For some applications, a screw with an unthreaded shank may be necessary, such as when fastening to concrete or sheet metal. 15mm in inches