Machine concepts for high speed cutting Prof. Dr.-Ing. H. Schulz Institute of Production Engineering and Machine Tools (PTW) Darmstadt University of Technology Darmstadt, Germany
KEYWORDS: High speed machining, hsc-machine design. ABSTRACT:Based on specific advantages high speed machining has many economic applications. HSC-machines need a lot of new components, e.g. high frequency motor spindles, dynamic feed drives as linear motors, faster CNC-controls. The moved parts must be designed in light-weight construction. The rules of safety regulations must be regarded.
1. Introduction Although that High Speed Cutting (HSC) is established in the delivery program of many machine tool manufacturers, this technique only recently started to be accepted in the application field. Several useful application areas for this technique are based on the advantages of this cutting technology. These are: • Relatively high chip volumes, which occur mainly in the aerospace industry and in die and mold making. • Manufacturing of high precision parts: in the optical industry, precision mechanics and in precision industry. • Production of difficult vibration and temperature draft sensitive parts: Precision parts, household appliances and automotive parts. Every time the technology of the cutting process influences the development of the required tools, the machine tool and its components as well as the design of the whole process chain.
2. Machine concepts Regarding the existing knowledge it can be seen, that basically there are several application fields for HSC-machines (Fig. 1). Steel and cast iron HSC milling, especially roughing, can be done by using new CNC-machines, which are equipped with modern up to date controllers. But for the machining of light alloys, fiber reinforced plastics and similar materials and also for the finishing of steel and cast iron a more sophisticated real HSC-machine tool has to be used. These can be distinguished from conventional machine tools by applying the following components: • high frequency motor spindles • fast controls • high dynamic feed drives • light weight design • safety technology
m/min
50
40
feed rate
30 I : cast iron/steel II : light metals III: fibre-reinforced plastics
20
10
II
I 0 1000
2000
3000
4000
5000
m/min 7000
8000
cutting speed
Figure 1. Application areas for high speed cutting
2.1 High frequency motor spindles Due to physical relationships the power of motor spindles decreases with increasing revolutions per minute. Figure 2 displays this significantly for the spindles available on today’s market.
spindle performance [kW]
100
80
60
40
20
0 0
20,000
40,000
60,000
80,000
100,000
frequency [1/min] performance and frequencies of over 400 presently available spindles
Figure 2: High frequency spindles available on the market
120,000
But for reasons of economic machining, higher amounts of power are necessary. This leads therefore today into two different application fields: HSC (High Speed Cutting) and HVM (High Velocity Machining)(Fig. 3). HSC is applied in the areas of very high cutting speeds, but only for medium material removal rates, therefore this is the application field of real HSC-machine tools. HVM means the use of cutting speeds in the transition area between conventional and actual high speed machining but realizing high material removal rates. 100 90 80 70
conventional milling High Velocity Machining (HVM) High Speed Machining (HSM)
maximum feed rate [ m/min ]
60 50 40 30 20 10 0 0
20000
40000
60000
80000
100000
spindle frequency [ min -1 ]
Figure. 3. Power profile of milling machines However a dream of the production engineer is the combination of both techniques. This can be realized in the next two to three years, by doubling the available power of the spindles (Fig. 4). Then these two application fields can be again reunited. As the interface to the tool, the HSK-tool holder will be the only one to be used for HSC-machines.
spindle performance (kW)
120
100
80
ap=0.6d 60
40
present boundary
20
ap=0.3d 0 0
20000
40000
60000
80000
100000
120000
frequency (1/min)
Figure 4. Demanded application area of high frequency motor spindles
2.2. Fast Controls Contrary to rotational drive systems, the static and structural dynamic behavior of the linear direct driven machine mainly depends on the position control cycle time and therefore upon the abilities of the CNC control. An important requirement to achieve high Kv values and thus a sufficient load stiffness, are very short position control cycle times with neglectable short lag times (
