For metalworking materials, the hardness value HRC36 is generally considered to be the boundary between soft and hard materials. HRC36 is used as the starting point of the hard material because the elongation of the material is usually less than 10% after the hardness is greater than this value. In general, materials of this hardness are no longer suitable for processing threads with cold extrusion forming taps or roll forming taps. Therefore, threading of hard materials usually requires the use of cutting taps or thread milling cutters.
However, such general restrictions are not absolutely insurmountable. Some cold extrusion forming taps can also machine threads on materials up to HRC44. The cutting taps can be used to machine threads on both soft and hard materials. However, when tapping hard materials, some changes to the structural design of the tap are required.
Application field
Manufacturers need to perform threading on a variety of hard material workpieces, including aerospace parts and medical parts. For example, CIM Corporation of the United States requires threading of medical parts with hardness HRC32-39 (such as bone saw guides) and materials of hardness HRC48-52 (such as stainless steel laser devices used in the optical industry). CIM was originally a turbine manufacturer, and turbine parts were usually made of nickel-based alloys (such as Stellite 31) with a hardness of HRC 48-52 after heat treatment.
Moldmakers also need to machine threads on hard materials. Molds are usually made of tool steel (eg H-13, P-20, etc.). The hardness of H-13 is HRC48-52, while the hardness of P-13 can reach HRC50-52. Hard materials for molds also include heat treatable alloys and cobalt-based alloys.
Although HSS-E high-speed steel (E refers to "special grade", indicating cobalt-rich high-speed steel) taps can be tapped for materials with hardness up to HRC55, manufacturers should use carbide taps that can process higher hardness. Carbide taps can be used to machine high-hardness martensitic stainless steels (400 series hardness HRC50-60). The main alloying elements of these stainless steels are 11.5% to 18% chromium.
Hard material tapping can also be applied to the machining of some process control parts. For example, nickel-based alloys can be used to make parts that measure the flow rate of chemicals, including acids, which must be well tolerant to corrosive chemicals.
Tap design
Taps that process hard materials require a harder coating (such as TiN and TiCN coatings) than soft materials. Such taps also include other structural features.
One feature is the number of slots in the tap. In order to process hard materials, it is necessary to use a larger number of slots. For example, a two-fluted tap is suitable for processing aluminum, while a tap for processing a hard material may require 5-6 slots. More slots can create more cutting edges to disperse and reduce cutting forces and tool wear.
Another feature is the amount of tooling that is used to machine soft materials. The amount of tapping is small, because reducing the amount of shovel can improve the stability of these materials. Taps that process hard materials require a large amount of shovel. The purpose of increasing the amount of shovel is to form a negative (radial) cutting surface, while the tap that processes the soft material forms a positive cutting surface, somewhat similar to a sharp tang, which may be Harder workpiece material causes damage. In addition, by chamfering the tap guide or cutting edge, eccentric shovel (which affects the pitch and diameter of the thread tapping) and the shovel (which affects the large diameter, pitch and diameter), Get a large amount of shovel. Increasing the amount of shovel can reduce the cutting resistance of the tap, which is beneficial to the processing of hard materials. Because the elasticity of the hard material is small, if the cutting resistance is too large, the tap may be broken. In addition, reducing the cutting resistance can increase the free cutting ability of the tap and reduce the torque required for tapping. However, reducing torque should not be confused with small torque. Tapping on hard materials requires a lot of power, and the tap cutting edges also have to withstand large cutting forces.
In addition to increasing the amount of shovel, a tap that processes hard materials also requires a larger rake angle. The influence of the taper rake angle on the stability of the cutting edge is greater than that of the back angle. Reducing the rake angle improves the stability of the cutting edge motion and generally results in a more favorable chip shape. However, reducing the rake angle, which is also necessary for machining hard materials, increases cutting forces and torque.
In addition, when processing hard materials, manufacturers should expect that even if the tap structure is specifically modified, its service life is still relatively short. The service life of the special tap depends on the depth of the threaded hole and the hardness of the workpiece material. Manufacturers should also expect that the price of taps for processing hard materials is more expensive than for taps that process soft materials. For example, in order to process workpiece materials with particularly high hardness, it is often necessary to use more expensive carbide taps.
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