A tap is a device that makes an internal thread inside a pre-drilled hole, so that a bolt with an external thread can be screwed into it. Metric taps have a numbered size system which denotes the size of the thread you can tap. The hole and drill size must be slightly smaller than the tap size to allow for the tap to cut into the metal and create a thread. Metric taps all have the letter “M,” meaning metric, in front of the relevant number, such as M6x1.
Metric Tap Overview
On first glance, the sizing system for taps can appear a little daunting, irrespective of whether the sizes are metric or U.S. However, once the various numbers are explained, it is fairly simple to understand the system and immediately recognize what it means. As previously mentioned, metric tap sizes all start with the prefix “M.” The number that immediately follows the letter relates to the maximum diameter of the tap in millimeters. For example, if the number “1” is written after the letter “M,” then the diameter is 1 mm. If the number is 2, then the diameter is 2 mm. After the number is the symbol “X,” followed by another number. The second number indicates the number of threads per mm. If the tap size is M1x1, then this means that the diameter of the tap is 1 mm and there is one thread per mm.
Metric Tap Thread Hole
The size of the drilled hole is directly proportional to the size of the tap. It must be slightly smaller than the tap diameter; otherwise, the tap has nothing to tap into. As a rough guide, the size of the drill used to make the hole before tapping begins needs to be about 20 percent smaller than the metric tap. This means that the drill size used in order to use a M2X1 tap needs to be 1.6 mm while the drill for an M4X1 tap needs to be 3.2 mm. However, it is wise to refer to a table to ensure you use the correct drill, because if you make the hole too large, there is nothing you can do to rectify the situation. There are many tables available to view on the Internet. One example is: customthermoelectric.com/Thread_drill_taps.html#Metric. There are many more.
Metric Tap M6X1
Metric tap size M6X1 simply means the tap has a 6 mm maximum diameter and there is 1 thread per mm. The higher the first number, the greater the size of the tap, so M6 is smaller than M6.3. The second number, which relates to how coarse or fine the thread is, works in a similar way; M6X1 is coarser than M6X4, because X1 has one thread every 1 mm while X4 has four threads every 1 mm.
Metric Tap M6.3X1
The only difference, then, between a M6X1 metric tap and a M6.3X1 metric tap is the diameter of the tap. The number of threads is identical. Remember, you also need to use a slightly larger drill to make the initial hole.