Design of spindle cutter connection of the hottest

Spindle/tool connection design of high-speed machine tool

high speed machining has a cutting speed of 5 ~ 10 times greater than ordinary machining. Its advantage is that it can reduce the processing time and achieve the machining accuracy and surface quality that ordinary machining requires several processes. The main problems related to high-speed cutting are: tool material and design, dynamic balance of high-speed machine tool spindle, thermal dynamic performance and reliability of machine tool, etc. The connection problem between the tool and the spindle will seriously affect the reliability of high-speed cutting and the dynamic balance of the machine tool spindle, which has become one of the weak links limiting high-speed cutting

I. requirements for tool setting/shaft connection in high-speed cutting

requirements for high-speed machining ensure that the connection state between the spindle and the tool cannot change at high speed. However, the conical hole at the front end of the high-speed spindle will expand due to centrifugal force, and the expansion amount will increase with the increase of the rotation radius and speed. The expansion amount of the standard 7/24 solid tool handle is small, so the stiffness of the standard taper connection will decrease, and the axial position of the tool will change under the pull of the pull rod. The expansion of the spindle will also cause the deviation of the mass center of the tool and clamping mechanism, thus affecting the dynamic balance of the spindle. To ensure that this connection still has reliable contact at high speed, a large amount of interference is required to offset the expansion of the spindle end during high-speed rotation. For example, the Standard No. 40 cone requires an initial interference of 15 ~ 20 μ m. Plus the interference of eliminating the taper fit tolerance zone (the taper tolerance zone of at4 reaches 13 μ m) Therefore, this interference is very large. Such a large amount of interference requires the pull rod to produce a large tension. It is generally difficult to achieve such a large tension, which is also very detrimental to the tool change. It will also expand the end of the spindle and have a negative impact on the front bearing of the spindle

high speed machining requires very high dynamic balance, which requires not only precision dynamic balance of spindle components (above g0.4), but also precision dynamic balance of cutting tools and clamping mechanisms. However, the keys and keyways that transmit torque are easy to damage the dynamic balance. Moreover, the standard 7/24 taper handle is long, so it is difficult to achieve full-length gapless fitting. Generally, only 70% of the front part of the fitting is required to contact. Therefore, there will be a certain gap in the rear part of the fitting surface, which will cause the radial runout of the tool and affect the dynamic balance of the structure. The key is used to transmit torque and carry out angular positioning. In order to solve the dynamic balance problem caused by the key and keyway, we can try to study a method that the tool/shaft coupling is strong, and now the mating part produces a lot of friction to transmit torque, and use the method of marking on the tool handle to realize the angular positioning of installation, so as to achieve the purpose of canceling the key. 2、 Advantages and disadvantages of standard 7/24 cone connection

the standard 7/24 cone connection has many advantages: because it is not self-locking, it can realize rapid tool loading and unloading; Under the axial tension of the pull rod, the cone of the tool handle is in close contact with the inner conical surface of the spindle, and the solid cone directly supports the tool in the inner conical hole of the spindle, which can reduce the overhang of the tool; This kind of connection has only one size, that is, the cone angle needs to be processed, which has opened up a variety of new extruded products to high precision, so the cost is low and reliable, and it has been widely used for many years

however, 7/24 connection also has some disadvantages; The taper is larger and the taper handle is longer. The conical surface plays two important roles at the same time, that is, the accurate positioning of the tool relative to the spindle and the realization of tool clamping and the provision of sufficient coupling stiffness. Because it cannot be positioned simultaneously with the spindle end face and inner cone, the standard 7/24 cutter/shaft cone connection has a large gap between the spindle end face and the knife handle flange end face. In the 7/24 taper fit specified in the ISO standard, the angular deviation of the taper hole in the main shaft is "-" and the angular deviation of the tool handle cone is "+" to ensure the contact of the front section of the fit, so its radial positioning accuracy is often not enough, and a gap will be generated in the rear section of the fit. For example, the tolerance value of the angle specified in the typical at4 (iso1947, GB) taper is 13 ", which means that the maximum radial gap in the rear section of the fit is as high as 13" μ m. This radial clearance will cause the tool tip to jump and destroy the dynamic balance of the structure. It will also form the condition that the contact front end is the fulcrum. When the bending moment of the tool exceeds the friction torque generated by the axial tension of the pull rod, the tool will swing with the contact area of the front section as the fulcrum. Under the action of cutting force, this swing of the tool in the taper hole of the spindle will accelerate the wear of the front section of the taper hole of the spindle, form a bell mouth, and cause the axial positioning error of the tool

7/24 taper connection is sensitive to the change of cone angle and axial tension. When the tension is increased by 4 ~ 8 times, the stiffness of connection 3. Pressure test can be increased by 20% - 50%. However, excessive tension will accelerate the wear of the inner hole of the spindle in the process of frequent tool change, make the inner hole of the spindle expand, and affect the service life of the front bearing of the spindle

in addition, as mentioned above, when the cone connection of this solid tool handle rotates at high speed, the expansion of the spindle end is greater than that of the cone handle, and the high-speed performance is poor, which is not suitable for the connection of ultra-high speed spindle and tool

III. The expansion rate of typical high-speed spindle/small crack is much higher than that of long crack tool connection design

in the design of high-speed spindle, there are two main types of designs that have been successfully studied on tool shaft connection at present. One is to abandon the original 7/24 standard taper and adopt the alternative design of new ideas in today's automotive interior design, such as HSK Series in Germany and KM series tool taper in the United States. The other is to adopt the existing 7/24 taper for improved design in order to reduce the cost. This design can realize the transition from the existing spindle structure to high-speed, such as the WSU series tool handle in the United States

1. Alternative design

"curve coupling" structure: this structure is composed of two parts, each part is processed with the same number of spiral teeth, and is fixed with the front end of the spindle and the tool handle respectively. The connection accuracy between the tool and the spindle is high, and the connection stiffness is also good. The axial movement required for loading and unloading the tool is very small (5 ~ 10mm). However, there are high requirements for the accuracy of the spiral tooth profile used for coupling, and there are also high requirements for the fixation of the two parts of the structure with the spindle and the tool handle. In addition, the end of the spindle and the tool handle need to be redesigned. When changing the tool, it takes a long time to make the two parts of the tooth profile mesh accurately, which affects the tool change speed

sandvik's triangular pyramid structure: this kind of tool handle is not conical, but triangular pyramid. Its edges are circular arc-shaped, and the taper is a hollow short cone structure of 1/20. It realizes the simultaneous contact positioning between the conical surface and the end face, and the three arris structure can realize torque transmission, which eliminates the dynamic balance problem caused by the transmission key and the keyway. But triangles, especially principal triangles