Thread taps are used to cut thread inside of a hole, which can be used for cutting new thread, and also can be used for restoring an internal thread.
There are many kinds of thread taps, for example , Metric taps, UN thread taps, BSPP thread taps, BSPT thread taps, ACME thread taps, and Trapezoidal thread taps etc, which are used to make different thread form’ thread.
Hand taps have three kinds of styles, taper , plug and bottoming, which are distinguished by the quantity of chamfered threads at the end of the tap. Usually taper taps have 8-10 chamfered threads, plug ones have 3-5 chamfered threads, and bottoming taps have 1-1.5 chamfered taps. Machines taps usually just have bottoming taps.
You can check the tap drill size from chart table, you also can account out it by yourselves. For example, M1.8×0.2, the tap drill size is 1.6mm. How to get it? the rule is subtracting pitch from diameter, then get it. 1.8-0.2=1.6 mm For example, M1.8×0.35, the drill size is 1.45mm. (1.8-0.35=1.45 mm). For United Inch thread, the rule is subtracting 1/thread per inch from the diameter. For example, 1-8 UN, 1- 1/8= 7/8″, 7/8″ is the tap drill size! 1/4-20 UN, 1/5″ is the tap drill size, (1/4-1/20=1/5″ )
When tapping, make sure use enough lubricant.For all carbon steels, it is a good idea to use the sulpherized cutting oil as lubricant, which is often used for pipe threading. For aluminum material, it is better use WD-40. Cast iron usually no need any lubricant.
Some tips need to be noticed. taps generally can’t be threaded all the way to the bottom of a blind hole.so a blind hole need to be a little deeper than the required thread depth. When tapping, try to keep the tap straight as you turn,which is very important, the tap should be kept perpendicular to the workpiece.Taking few turn, you need to back the tap to clear away the chip,which is specially important in a blind hole. Small taps are easy to be broken, you should pay more attention using small taps.
Most taps are produced using HSS(high speed steel), HSS thread taps are often used for softer materials. If tapping hard material, for example, stainless steel, it is better use HSSE material. some taps are plated with special coatings to increase the cutting ability and using life.