There are special requirements for HSK shank materials

Since the emergence of HSK hollow short taper shank technology, it has been widely used in the world. It is understood that some users in Europe and North America have experienced some problems with the HSK shank. The main reason for these problems lies in the poor choice of materials for making HSK shank. 

The DIN standard only specifies the geometry of the HSK shank, not the material to be used. As a result, almost all tool suppliers manufacture HSK shank with the traditional 7:24 taper shank. The "hollowness" of HSK hollow short taper shank itself indicates a potential problem: that is, the axial section is very small, which is obviously the weak point of HSK hollow short taper shank. 

Traditional tool-handles are usually made of alloy steel and then either hardened or surface-treated to obtain a hard, wear-resistant surface and a tough core (HRC32 ~ 36). These heat treatment processes (such as carburizing and ion nitriding, hardness up to HRC58 ~ 62, about 1mm deep) are ideal for 7:24 taper shank and inexpensive. Because it has enough cross section, it is not actually easy to deform. According to acceptable guidelines for industry, this relatively stable matrix of this size combines the two properties perfectly -- a soft, ductile core that prevents the hard surface from bursting and falling off due to deformation, and a very wear-resistant, hard surface.

But for HSK hollow short taper shank is different, it is made into a small structural section, for HSK63 below the small specification HSK hollow short taper shank has no strong heart to support the hard surface, in each clamping - release cycle will be greatly impacted, and the cutting process is subject to dynamic bending and torsional alternating load, Small cracks may occur at hardened brittle sites, and some parts of the shank of the HSK shank are very sensitive to large tensile stresses. These parts include the 30° clamping surface, the surface of contact between the keyway and the spindle during torque transfer, and the part closest to the bottom of the keyway and the bottom of the empty tool slot. These tiny cracks grow over time and are hard to spot, in fact, without microscopes and special tests. A shank appears to be working properly, but once overloaded, stress concentrates at the site of the crack and is likely to break when critical values are reached. This brittle fracture may occur at any time during use.

Some manufacturers use hardenability materials commercially. At this time, the properties of different points on the section are roughly the same, and the elongation, yield strength and impact strength of the shank may reach the best combination. However, relative to the surface hardened shank, the hardness will be the lowest (about 5 to 6HRC lower). The shank lacks impact strength and is not wear-resistant, so it has to be replaced frequently. 

So it is impossible to achieve the highest toughness, the highest wear resistance and corrosion resistance at the same time, but also low cost. We can't change the geometry of the product, it's standard. We are also constrained by the cost of mass production, which makes it impossible to adopt expensive "space materials and technologies". In order to make this new interface with many advantages widely used in production, we have to keep exploring. 

The manufacturer and the user must make a choice about the handle material. One is that the material requires high strength, toughness and moderate wear resistance; The other is to have high wear resistance and moderate strength. 

The analysis results show that the cross section with the maximum stress of HSK spindle mounting hole should have a larger section area, and try not to use the material sensitive to stress concentration. But must be able to adapt to high-speed processing, that is, the weight should be light, the radial deformation under the action of centrifugal force should be small. 

On CNC machining centers with automatic tool changers, the typical tool interface friction ratio is "many to one". That is, the shank is rubbed only once in the tool change cycle, while the main shaft hole is rubbed many times against the many different handles that perform the process. 

For example, suppose a CNC machining center has 32 tool locations in its tool library, all of which are in use. The cutting time of each process is 5 minutes, and the machine works for 10 hours a day. Each tool is used only once in the same workpiece processing. Thus, for the spindle, 120 times a day, but only 4 times for each shank. From this, it can be deduced that the ratio of the number of frictions during the working life of the machine tool. This number indicates that the spindle friction times are much greater than the tool shank friction times, and is the same for any type of tool interface. 

Traditional 7:24 taper spindles wear out as a result of "flaring", which is an imperceptible very slow process. However, the result of wear is tool vibration, runout increase, tool axial positioning size changes. At this point the main shaft hole will be reground. To reduce the rate of wear, relatively low pressure is required. 

When the HSK shank is clamped, there is a lot of pressure between the main shaft hole and the shank. Since its taper is 1:10 (half Angle is 2°51 '27 ") -- a self-locking Angle, unloading the shank requires an additional unloading force, which is also a friction process for the main shaft hole. Because the taper of 1:10 is very small, each radial wear of 0.0025mm will affect the axial clearance of 0.025mm, which will also affect the clamping state of the clamping mechanism, and may cause failure. Therefore, the HSK spindle wear should be minimized. Another important reason is the price of HSK spindle. Because the HSK spindle is actually unfixable, the regrinding taper will make the taper diameter out of alignment and affect the clamping position of the mechanism. And because of its high precision requirements, and to use a large length-diameter ratio of milling cutter machining drive key, so its price is very expensive, in order to extend the life of the spindle, the tool handle is best made of the same material, the minimum yield strength should be 14000kg/cm2, tensile strength should be 15400kg/cm2. And the hardness of the shank should be reduced (average HRC50), 6 ~ 8 degrees lower than the spindle hardness, to protect the expensive spindle. In this way, the surface hardness of the shank is reduced, and the manufacturing cycle is shortened and the cost is reduced