In the household, it is quite often required to make a part with a metric internal or external thread. For this, special tools are used - a tap and a die.
Selecting a workpiece for threading
The diameter of the bar or tapping hole is not equal to the nominal value of the finished product. When cutting an external thread, a bar is selected that has a diameter that is several hundredths of a millimeter smaller than the thickness of the finished product. This is due to the fact that when removing the chips, the metal is partially squeezed out, completely filling the threaded profile of the die. For internal threading, the hole is also made with a small allowance to the smaller side. The exact values of the diameters of the blanks for cutting metric threads are specified in GOST 19257-73.
If drills of the appropriate size are used to drill holes with an inaccurate diameter value, then it is rather problematic to find a bar suitable for cutting external threads. This problem can be solved by pre-grinding the standard thickness bar. To do this, the product is clamped into a drill chuck and processed with emery paper evenly along the entire length, periodically making measurements with a caliper.
External threading
Before threading the bar, mark the cutting depth and clamp it vertically in a vice. The surface should be liberally lubricated with machine oil or lithol. The die of the appropriate size fits into the holder. Start threading with the starting side of the die. It has a gradually increasing depth of cutting edges, while on the back of the die, these edges are completely reduced in one and a half turns. The die must be put on the bar and cut off until an increased metal resistance appears. In this case, the handle of the holder must be strictly perpendicular to the workpiece. After threading, the die must be turned back a third of a turn, after which the full threading can be started by slowly turning the holder handles. The cut threads are lightly processed with fine emery paper and cleaned with a fine metal brush.
Tapping
Internal threads can be cut in two ways. A through is done in one pass, while blind taps will require a roughing and finishing taps used in two successive passes. The part must be firmly fixed and the nose of the tap, lubricated with machine oil, installed in the hole or recess of the part. After turning it one and a half to two turns, it is necessary to perform a sidecut, after which the tap is unscrewed by half a turn and threading continues to full depth. From time to time, it may be necessary to completely unscrew the tap to clear any accumulated chips from the hole.
