The hottest high-speed machining and dynamic balan

High speed machining and dynamic balance

high speed machining, due to the use of high cutting speed and feed speed, greatly shortens the processing time and obtains high processing accuracy and surface quality, and leads to the saving of processing procedures, the simplification of production process flow and the reduction of production equipment. Therefore, high speed machining has significant technical and economic benefits

since the rise of high-speed machining in the mid-1980s, it has developed rapidly in about 20 years, and has been widely used in aviation, aerospace, automotive industry, mold manufacturing and other fields

in the automotive industry, high-speed machining technology began to be applied in the early 1990s. At that time, the cutting speed of using polycrystalline diamond milling cutter to process aluminum alloy cylinder heads had reached 3000m/min, while by the mid-1990s, the milling speed of milling cylinder heads had individually reached 7000m/min (H ü ller Hille company). When using polycrystalline diamond face milling cutter with high-strength aluminum alloy cutter head and better dynamic balance (balance quality grade g2.5), Considering the cen/safety standard draft (i.e. taking half of the breaking speed of the cutter body as the maximum allowable speed of the milling cutter), the cutting speed when milling aluminum parts can even be increased to 8000m/min (milling cutter diameter ≥ 200mm, MAPAL company). This cutting speed is almost 2.6 times higher than that in the early 1990s. In the mold industry, in the late 1990s, the milling speed of hardened steel (HRC50) has reached (370 ~ 400) m/min

Figure 1: definition in dynamic balance technology

in order to meet the needs of high-speed machining, machining centers and milling machines currently have high spindle speeds. The spindle speed of machining center is generally within the range of 15000r/min and 24000r/min. For machining centers or milling centers used for mold processing, because milling cutters with smaller diameters (2mm ~ 12mm) are often used during processing, the maximum spindle speed is generally 42000r/min, and some are as high as 60000r/min

obviously, the residual unbalance (asymmetric mass) of the tool system with high-speed rotation will produce centrifugal force that is squared with the rotation speed. This dynamic load will arouse the vibration of the tool and machine tool, resulting in the decline of the machining surface quality, tool life and spindle bearing life, and even affect the normal progress of the machining process. In order to reduce or limit the influence of dynamic load caused by residual unbalance, the tool system should be dynamically balanced

causes of tool system imbalance

in machine tool spindle chuck tool system, mass asymmetry (imbalance) is mainly caused by the following reasons

1. Asymmetric shape of the tool

2 Asymmetric shape of tool holder

3 Processing error of system components

4 Inaccuracy of connection clearance and clamping of system components

5 Circular runout and wear of spindle

6 Impurity particles are stuck on the taper hole of the spindle and the tool

7 Pull rod in main shaft - offset of laminated spring

8 The influence of cooling lubricant, etc.

these imbalances will adversely affect high-speed finishing by turning the hand wheel of the oil delivery valve to open the oil delivery valve. Especially when using long cantilever tools for high-speed finish machining, its impact is particularly serious. In the past, in order to reduce the adverse impact of tool system imbalance on machining accuracy, we had to use a lower speed for machining, so that the machine tool capacity can not be fully utilized. Therefore, in order to make full use of the advantages of high-speed machining, it is necessary to use 4-degree and 10 degree tool systems for spindle above M20 for dynamic balancing. In order to reduce the vibration load and transfer the higher accuracy to the cutting edge of the tool as much as possible

the concept of dynamic balance technology

dynamic balance is to make the mass of the rotating body of the tool system evenly distributed, so as to reduce the centrifugal force generated by the imbalance when the rotating body rotates

in dynamic balance technology, the unbalance (U) is the product of the mass of the rotating body (MR) and the displacement of the mass center of gravity (E):

u= Mr. e (gmm ,kg μ m)

mu - unbalanced mass (g)

mr - mass of rotating body (kg)

e - mass center of gravity offset（ μ m)

ru -- distance of Mu mass center of gravity relative to the axis of the rotating body (mm)

n -- rotating speed of the rotating body (r/min)

ω—— Angular velocity of rotating body (1/s)

ω= 2 π n/60

f - centrifugal force generated by imbalance (n)

f=u ω 2

the unbalance (U) is the product of the unbalanced mass (MU) and the distance (RU) of the center of gravity of the mass relative to the rotation axis:

u = mu ru.......... (GMM)

the balance quality (g) when the rotating body rotates is expressed by the following formula:

g=ew/1000=u/mr. π n/30 (mm/s)

balance quality or balance quality grade (g) indicates the linear speed of the center of gravity of the rotating body when the rotating body is in an unbalanced state, which is a mechanical comparison quantity used to judge the dynamic load of the rotating body. The smaller the value of G, the more stable the rotation of the rotating body

it is difficult to determine the g value of balance quality for tools processed at different speeds. The residual unbalance (U) has nothing to do with the speed, and this unbalance can be measured directly on the dynamic balancing machine

through conversion, the above formula of balance quality can be rewritten into the following relationship:

e=9549.296g/n ( μ m)

Figure 2 is a nomograph drawn according to this relationship. According to the determined balance quality and the actual working speed, the allowable residual unbalance per unit mass of the swivel (gmm/kg), that is, the allowable offset of the center of mass (E), can be found from the curve on the graph or calculated by using the above relationship

Figure 2: corresponding to different balance qualities g0.4~g250 (according to diniso1940), the maximum allowable residual unbalance at different working speeds (extracted from the data of water company)

here we can see that under the same balance quality, the higher the speed, the smaller the allowable mass center of gravity offset, that is, the smaller the circular runout of the rotating body (tool holder and tool, etc.) (Figure 2 and table 1). For example, for a tool weighing 1.4kg, according to its determined balance quality g6.3 and working speed n=25000 r/min, the offset of tool mass center of gravity e=2.41 can be obtained μ m。 The allowable residual unbalance u=mr is obtained. e=1400G × 0.00241mm=3.37gmm。 This unbalance can be measured by the balancing machine

moderate dynamic balance

users of high-speed processing should put forward a moderate balance quality grade (g) according to the specific processing task and considering the technical feasibility and economic rationality. The measurement of tool residual unbalance is limited by the measurement limit and measurement ability of dynamic balancing machine. At present, with the highest level of dynamic balancing machine, the tool residual imbalance that can be measured repeatedly has reached 0.3gmm. According to relevant reports in the German technical journal, in a draft dynamic balance limit standard of "using rotary cutting tools with a certain geometric angle" drafted by Germany, it is stipulated that μ The residual offset of M is the highest balance quality that can be achieved

therefore, users of high-speed processing cannot blindly pursue the stability of processing and put forward too high balance quality. Because the unrealistic high requirements are difficult to achieve technically and uneconomical economically. For example, for a hot expansion and cold contraction chuck with a tool handle hsk40 weighing 300g and working speed up to 60000r/min, when its balance quality is required to be g2.5, the allowable unbalance should be less than 0.119gmm, which means that the mass center of gravity of the chuck is only allowed to be from its 5 The electromagnetic clutch is a key component, and the rotation axis offset is 0.4 μ m。 At present, it is almost difficult to achieve <2 on the spindle of machine tools only with a tool or collet μ M, which is symmetrical in structure and especially suitable for high-speed machining. Its circular runout can only reach 3 μ M (measured at the 3XD overhang of its inspection rod). Thus, it is meaningless to require such balance accuracy. What is more practical is from 2 μ Starting from the residual offset of M, the unbalance u=300g x 0.002mm=0.6gmm is obtained, and the balance quality is g=12.56. This is not only technically feasible, but also economically reasonable

dynamic balance standard of manufacturer's products

at present, most tool and collet manufacturers have attached great importance to the dynamic balance problem in high-speed machining, and formulated the dynamic balance standard of their factory products according to the international standard ISO. The products are basically balanced at the speed of 10000r/min according to the balance quality grade g2.5~g6.3, For the hot expansion and cold contraction chuck with HSK E-type tool handle (symmetrical in structure), the dynamic balance is carried out at the speed of 25000r/min or higher (Table 2). If the user requires a higher balance quality than the standard balance grade of the product, or requires balancing at a higher speed, he can propose to the manufacturer to carry out fine balancing of the product

* according to the calculation of the cutting speed that can be achieved by high-speed milling of MAPAL company, it is estimated that it is in the range of 10000 r/min and 13000 R/min - the author

dynamic balance of tool system

for short and symmetrical integral tools, the weight to be corrected during balance is usually only a few grams, so only static balance is sufficient. For the cantilever cutter with asymmetric structure (the cantilever length is about 300mm), dynamic balance must be carried out on two correction planes to eliminate the imbalance error as much as possible. It is recommended that the tool, collet and spindle be balanced separately, and then the collet and the tool should be balanced again. For example, a controlled eccentric drive tool from Beck engineering works at a speed of 10000r/min. In order to ensure the processing quality of the workpiece, the tool is dynamically balanced in four steps. The first step is to carry out fine balance on the dynamic balancing machine at the speed of 1500r/min with the eccentric transmission cutter body and the installed balance correction tool. The second step is to balance the tool at the same speed on the dynamic balancing machine. Third, the eccentric drive cutter body and the installed balance correction tool are precisely balanced on the spindle at the speed of 10000r/min. Fourth, the tool installed in the tool body is finely balanced on the spindle at the same speed, so as to finally achieve a balance quality better than G2

in addition, for some high-speed machining tools and collets, if the structure allows, it should also be set on the tool body (cutter head) as fine adjustment mechanisms such as screws or balance rings for fine balancing or rebalancing in the future (such as the face milling cutter of Walter company and the WWS face milling cutter of MAPAL company, which are equipped with balance fine adjustment screws on the cutter head), or set multiple balance holes, so as to achieve the best dynamic balance effect of the tool system

automatic balance system

even if the tool and collet have been dynamically balanced, when the tool collet is installed on the spindle, the imbalance will be generated due to clamping inaccuracy (for hollow taper handle HSK interface, this value is generally 2 μ M and 5 μ M), in addition, there are the adjustment of adjustable tools, the imbalance caused by the offset of the pull rod - laminated spring in the spindle, and other imbalances that occur randomly. The inaccuracy of these connections and the changing imbalance cannot be corrected by mold opening. Therefore, for high-speed precision machining, it is best to use an automatic balance system to balance the whole tool spindle system in the working process to compensate for the above interference

the automatic balance system available in production at present is an electromagnetic dynamic balance system, which includes