As we previously pointed out in Of Machines and Tools, taps are cutting tools used to obtain internal threads (nuts) in mainly metallic parts, although they can also be made of soft metal, plastic or even wood. The taps are complemented by the so-called bearings or threading dies, which carry out the external thread of parts to get bolts and screws.
The operation of threaded can be done in the form Handbook or machine (using a lathe or a milling machine) and finds its main applications in industrial and mechanical maintenance for thread dressing, as well as in electrical installations and assemblies.
As in all machining work, the applications and, therefore, the correct choice of taps corresponding, depend on various factors that must be previously taken into account, since there is a wide selection designed to cover each of these aspects or a combination of them. Let’s briefly look at those factors.
1) Threading method to be used
Generally speaking, the hand threaded It is used in unit jobs and to make threads that do not require too much precision, while the machine threaded It is carried out in serial work or when high precision threads are to be obtained. Although we will not deal with it in this article, it should be added that there is also the roll threading, whose difference with the previous ones is that it is done with taps specials without chip removal.
Therefore, the taps They must be acquired according to the threading method to be used, although in some cases and certain conditions, their use may be interchangeable.
2) Use of accessories
The employment of drill bits, lubricants me refrigerants is critical for good results. The first step of any threading operation, whether manual or machined, requires drilling a smooth hole with a suitable drill bit and then machining the hole with the tools. taps appropriate.
There are tables and simple formulas that relate the diameter and thread pitch with the dimensions of the drill recommended for each material, but in general we can point out the following:
- A smooth hole that is too small causes the tap to fracture.
- For cast irons, brasses, bronzes, and aluminum alloys, drill diameters should be somewhat smaller than for steels or zinc alloys, for example.
- Blind holes require drilling a little deeper than you want to tap.
More details on this are offered in the following video:
In addition, the entire threading process must be adequately lubricated or cooled to facilitate the passage of the tool, removing the chip, to dissipate the heat generated and to obtain a good thread finish. The lubricant/coolant (oil, emulsion or compressed air with oil mist) must be appropriate for the material to be threaded and the method of lubrication or cooling can be either external or internal.
On the market, for example, taps with axial coolant supply with axial outlet and axial coolant supply with radial outlet in the grooves of the inlet area. The taps what are you offering internal lubrication are superior to those who have external lubrication, since they facilitate a better evacuation of the chips, which are transported out of the threading area. Cutting edge wear is reduced, because the cooling effect in the cutting area is greater than the heat generated.
3) Tap material
The vast majority of taps are built with Fast steel (HSS) and only in certain cases, with heavy metal (tungsten carbide). Some manufacturers also offer them in cobalt high speed steel (HSSE), which allows them to withstand high temperatures to work at very high cutting speeds, although they are fragile and can break. Others market males in steel HSSE to vanadium, which provide greater resistance to temperatures, wear and tear.
4) Work piece material
The type and characteristics of the material to be machined, as well as the type of chip it generates, define many of the existing varieties of taps. Manufacturers offer their selection of taps based on the material qualification given by the ISO DIN 513 standard (which groups them by a letter and a colour) and recommend cutting speeds based on the material and its tensile strength.
Furthermore, the type of chip generated is grouped into three main classes:
- long, typical of materials with low hardness and/or high plasticity in general, such as low-alloy steels, stainless steels and low-silicon aluminum.
- Intermediate, typical of materials of intermediate hardness and/or medium carbon content, or easy-machining steels.
- Very short or dust, typical of foundries.
The table below provides guidance on recommended speeds based on some of the parameters we just looked at:
5) Characteristics of the thread to be obtained
The type threaded, that is metric fine or coarse thread, whitworth thread (BSP, BSPT), american thread (NC, NF, NPT, etc.) or special thread (trapezoidal, sawtooth, etc.), as well as the nominal diameter, the depth, the He passed (that is, the number of threads or fillets per unit length or the distance between two adjoining profiles) and the tolerance to obtain are essential to select the tap. For this reason, manufacturers often print the thread designation on the tool, for example, M30, R 3/4″ or M20x1.5 for identification purposes.
Another parameter that is also usually recorded in the male is, as we saw in point 2, the recommended drill size to make the first hole that we will later machine with the taps to obtain the desired thread.
6) Type of hole to obtain
As we already pointed out in the previous article, the threading of parts can include two types of holes:
while the through holes need no further definition knockout holes They are those that, when machining the thread, extract the chip in the opposite direction to that of advance, that is, towards the grooves of the tap, and then cut it when the tap is extracted from the hole.
Obtaining blind or through holes is a very important condition to determine two key factors of the tap:
a) Type of entry point or chamfer
The complete machining work is done by the teeth of the entrance chamfer, and the input length, given by the number of threads, is determined by different opposing factors. To avoid overloading, premature nicking, and oversized threads, the number of input threads should not be too small. However, an initial cut that is too long increases the torque and thus the risk of the tap breaking.
The following table shows the different shapes and input lengths of the taps according to DIN 2197 and recommended applications.
b) Geometry of the groove, channel or groove
The geometry of tap (ie shape, number and direction of the grooves, cutting angle, etc.) depends on the material to be machined and the application. Taps up to M16 (ISO metric threads) or for the civil engineering industry are typically 3-flute, and above this size 4-flute or more. On the other hand, for through holes a straight flute shape is normally chosen, while for blind holes right hand helical flute shapes are preferred.
The table below shows the geometries of taps used (there are many more) as well as their main features and applications.
As a practical application of everything we have just detailed, the following video shows us a simple way to carry out the manual tapping of a blind hole with a set of three taps, as described in the previous article.