One of the most common in industry and in everyday life is metric thread. That it is used in most fasteners. It has a profile in the form of an equilateral triangle, all dimensions are measured in millimeters.
Metric thread - main dimensions
The specified thread is indicated by the letter "M" and the numbers that indicate the diameter of the rod. Thus, the designation M8 indicates that a metric thread is applied to the product, and the diameter of the rod is 8 mm.
Another indicator of such a thread is its step, which is indicated after the first digit and the sign "x." Large steps are usually not written, since they are basic, the step itself is the distance between the vertices of the turns that are located nearby. Experienced craftsmen determine the step by eye, and there are also special templates called thread gauges.
If there is a metric thread on the bolt and you need to determine its pitch, and there is no thread gauge at hand, then this can be done using any measuring device, for example, a caliper, but you can use a bench scale. To do this, measure the distance between several turns, and then divide the resulting number by their number.
The determination of the thread pitch is necessary during various operations, for example, for the correct selection of a threading tool or drill. Since during the threading process on the rod the metal is drawn, a rod of a slightly smaller diameter is taken than the required thread size.
There are times when you need to make a hole for a thread to be cut, but there are no necessary references. This problem can be dealt with using simple mathematical calculations. For example, you need to drill a hole for a thread with a diameter of 14 mm. We found that the thread pitch is 1 mm, then 14 - 1 = 13. That is, the diameter of the hole in which the metric thread will be cut should be 13 mm.
A metric tapered thread that has a taper of 1:16 is typically used for tapered joints, and an external tapered thread can also be connected to an internal cylindrical thread.
Metric threading can be performed in several ways, for example, on a turning or drilling machine, on special machines for threading, and can also be performed manually. When manually threading a rod, a die is used, and a tap is used to cut the internal thread.
In the latter case, taps of several sizes are used. The first and second numbers carry out preliminary cutting, and with the help of the third give the final shape and size. Only two sizes can be used - fine and rough tap. They are made of durable, high carbon steels.
Dies can be either round or sliding, they are also called prismatic. When using round dies, they are pre-clamped into the collar, and then an external thread is cut.
Cut the internal thread
First, prepare the hole. For better cutting, it is necessary that the drill has a slightly larger diameter than the internal diameter of the future thread. If you make a hole that will be equal to the internal diameter of the thread, then when it is cut, the removed metal particles will put a lot of pressure on the teeth of the tap and it will overheat. The result will be poor-quality thread with torn scallops, as well as possible tool breakage.
When cutting an external thread with a die, problems usually do not arise. From the very beginning, it is necessary to accurately set the workpiece and tool, then you will get high-quality thread.