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Optimum Design of Special Tap for Processing Titanium and Titanium Alloy Parts

Titanium alloy has become one of the most important engineering materials in mechanical processing, especially in the aerospace industry, and has been used more and more widely. However, titanium alloy is a typical hard-to-machine material with poor cutting performance. In titanium alloy materials, tapping internal threads with taps is particularly difficult, and need to be optimized.

First, the cutting performance characteristics of titanium alloy
1. Characteristics of titanium alloy materials
Titanium and its alloys have three significant advantages over other metal materials: High specific strength, good medium temperature performance and corrosion resistance. At room temperature, the specific tensile strength of titanium alloy is 1.26 times that of high strength structural steel and 1.38 times that of high strength aluminum alloy. In the temperature range of 400-550 °C, the titanium alloy has better specific strength, specific creep strength and specific fatigue strength than heat-resistant stainless steel.
Titanium alloys can be classified into metal phase structures: α phase titanium alloys, β phase titanium alloys, and (α + β) phase titanium alloys. The hardness and strength increase in the order of the α phase, the (α + β) phase, and the β phase, and the machinability decreases in this order.

2. Titanium alloy cutting characteristics
Titanium alloys with hardness greater than 350HBS or 300HBS are difficult to machine. However, the reason for the difficulty lies not in the hardness but in the synthesis of the mechanical, chemical and physical properties of the titanium alloy itself. The characteristics of the cutting process are as follows:
Small deformation coefficient;
High cutting temperature;
The main cutting force is small and the back force is large;
The chips are squeezed, and the processed surface is easy to form a hard and brittle metamorphic layer;
The sticking knife phenomenon is serious.

3. Titanium internal thread tapping characteristics
Due to the many characteristics of titanium alloys, internal thread tapping on titanium alloy materials is the most difficult process in the processing of titanium alloy parts. Especially for small hole thread processing is more difficult, and inseparable from tapping. The main performance is that the total torque of the tapping thread is large, about twice the tapping torque of 45 steel; Tooth of tap cutter wears out too fast, breaks the edge, and even is "bitten to death" and breaks. The main reason is that the elastic modulus of titanium alloy is too small, and the machined thread surface will produce a great rebound, which will cause a great normal force behind the side and top of the taper cutter teeth, thus causing a great friction torque. In addition, the cutting temperature is high, the chip has a sticky knife phenomenon, it is difficult to eliminate, and the cutting fluid does not easily reach the cutting area; The machined surface is prone to stress concentration and forms a hard and brittle layer, and the surface quality of the machined thread is poor.

Second, the design features of NORIS titanium and titanium alloy special taps
1. Calibration of partial taper
All the calibration parts of NORIS titanium alloy special taps have larger taper to reduce friction and prevent the rebound of titanium alloy from "biting" the taps and breaking them. The comparison with the taper taper of the general tap is as follows:

NORIS titanium and titanium alloy special taps

2. Front angle
In order to ensure the sharpness and strength of the tool, the rake angle is generally 5°-7°.

3. Reasonable back angle
4. Through holes and blind holes
For optimum processing performance, threaded taps for through holes (NORIS patent) and spiral groove taps for blind holes. Considering the strength, the helix angle should not be too large, and a 15° helix angle is generally used.

 5. Tap material
In order to ensure the quality of the tap and good economic performance, NORIS titanium alloy taps generally use high-performance high-speed steel (HSS-E) and powder high-speed steel (HSS-PS).
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