Finally, 20 is the length of the bolt in mm, measured from the tip of the thread to the bottom of the head (with the exception of oval headed bolts).

The “M” indicates the “metric diameter” of its thread (see tap drill chart below), which in our case is 4 mm.However, the bottoming tap cannot start the threading on an uncut hole, so it can only take part in the cutting process as the second or third tap. RS PRO hand tap sets are manufactured from high speed steel (HSS), offering excellent corrosion resistance and the ability to withstand higher temperatures.

This formula works for both metric and imperial units, and for all 60-degree threads (all ISO, DIN, UTS, UNC, UNS, and UNF bolts). When manufacturing custom CNC machining parts, holes with screw threads are often required for fastening the finished part or facilitating the building. The data chart below shows the standard metric ‘coarse’ threads and their drill holes sizes were are most commonly used when tapping sheet metal fabrications.

Having determined the bolt that we want to use in the specific case, we already have a technical specification for the thread which will host it. Before you design your next product or make updates to an existing product, consider partnering with Optimas to take advantage of our many capabilities. RS PRO hand tap sets are manufactured from HSS (high speed steel), offering a durable and reliable selection of tools and are available in a range of thread sizes from M3 to M12.