Raskatnik (tap-nakatnik)
Unlike taps, which cut off the allowance, knurlers create an internal thread by deforming the material and forming a V-shaped profile. A common misconception is the opinion that there is an axial shift of the material when rolling. In reality, the thread is formed within the entrance part of the knurler during its rotation in the hole. In the process of deepening the tool, plastic displacement of the material in the radial direction is carried out between the side surfaces of the thread profile of the knurler until it is cut into the entire length. After that, the thread with the appropriate height will be fully formed.
Knurlers have many advantages over traditional taps:
1) The most obvious of them is that the knurlers do not generate chips. Winding chips around the tap shank is typical for threading operations in blind holes in parts made of materials forming long chips. The use of knurlers on such operations will avoid difficulties with chip removal.
2) Since there are no problems with chip removal, we have the ability to process threads in deep holes (from 3 × D and above)
3) The output thread turns out better than the cut one – denser, smoother, strong to crumple and resistant to wear, more accurate with low surface roughness (high quality threads even on viscous metals and alloys).
4) The next advantage of the knurlers is their increased strength and resistance to breakage, since there are no grooves in the chipless tool, this increases its strength (we get a significant reduction in defective parts due to broken taps).
5) High thread rolling performance is one of the main advantages of this type of taps (thread rolling speed can reach 30m/min).
6) The rollers are more versatile, one type of tap can be used for processing various materials (aluminum, soft brass, copper, bronze with a predominance of copper, zinc, lead alloys, low-carbon steel, plastic stainless and heat-resistant steel).
7) The same tap-knurler can be used for both deaf and through holes, which, in conjunction with the previous paragraph, can significantly reduce the range of tools.
8) The service life of the tap is much longer than usual due to the long threaded part (makes it possible to re-sharpen the tap several times) and the application of a protective coating (titanium nitride, chromium nitride, titanium carbonitride, oxidized and others).
But like any other tool, the taper has a number of features and limitations associated with them.
To begin with, the knurlers can only be used for processing plastic materials. Due to the increased friction compared to traditional taps, the knurlers require higher torque and more powerful equipment.
Knurled taps are available with and without coolant supply channels. In most cases, it is necessary to use process fluids, while the percentage of oil in them should be about 6-12%. The use of oil-based lubricants can be inconvenient for CNC machining centers using water-soluble coolant. In this situation, the lubricant concentration should be increased.
The diameter of the rolling taps can vary from 0.5 mm to 19 mm. Large models require the use of machines with a high level of power.
A more thorough preparation of the hole with strict tolerances is required. Since the knurlers displace the material, the preformed holes should have a larger diameter. This is especially important to take into account when switching from traditional taps to knurlers. When the thread is rolled in the hole intended for the cutting tap, the displaced material will "clog" the thread of the knurler, which will lead to breakage. The dimensions of the pre-holes are given in the corresponding tables for the knurlers.
There is a limit on the hardness of the processed metal or alloy (up to 40 HRC), which can also affect the choice of this type of tool.
As a rule, we have a higher cost of the tool compared to traditional taps. However, due to the high processing speed, versatility and durability of the tool, in the end, we get that the rollers are a more cost-effective solution. And therefore, if the conditions and tasks allow the use of chipless taps, then it is more profitable to use them. From all of the above, it is not surprising that these taps are widely used, at least in mass production.
When creating the article , the following were used:1) Technical manuals and catalogs Kennametal, Dormer, Pramet, Sandvik Coromant.