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Int J Adv Manuf Technol (2001) 18:399–403
ó 2001 Springer-Verlag London Limited
A New NC Machine Tool Controller for Step-by-Step Milling
J. Balic
University of Maribor, Faculty of Mechanical Engineering, Intelligent Manufacturing Laboratory, Maribor, Slovenia
The paper describes the design solution, operation and analysis of a new NC controller for a new step-by-step milling pro- cedure. A step-by-step milling device ensures that the milling of workpieces by end or conical milling cutters, where the ratio between the depth of milling a (mm) and the milling
cutter diameter D (mm) is greater than 1.5 (a/D > 1.5) results
in the increased wear resistance of the cutting edge. Breaking of the milling cutter is minimised and is not frequent and the milling forces are reduced, which results in smaller deflections of the milling tool and higher accuracy of machining. The machine tool use is better.
Keywords: NC machine tool controller; Step-by-step milling
1. Introduction
Technological problems occur when machining some compo- nents using end or conical milling cutters of small diameters at large cutting depths. According to the recommendation of tool manufacturers the depth of milling should be within the range of 1–1.5 ′ D, where D is the milling cutter diameter. Frequently, the depth of milling required for the manufacture of the parts may be as much as 6D.
This problem is particularly severe when machining parts (dies and moulds) with a conical milling cutter (cutting depths is 4D, conical angle 3° to 6°), where rough milling with a special rough milling tool is not possible and in the case of machining dies (the material of the dies is steel according to DIN 1.2343–0.4% C, 1% Si, 5% Cr, 1.3% Mo, 1% V), for
the extrusion of aluminium bars, pipes, and beams.
2. State of the Art and Existing Solutions
The state-of-the-art technology in the area of machining offers satisfactory solutions for almost all types of machining. The
Correspondence and offprint requests to: Professor J. Balic, Laboratory for Intelligent Manufacturing, University of Maribor, Faculty of Mech- anical Engineering, Smetanova 17, SI-2000, Maribor, Slovenia. E-mail: joze.balic@uni-mb.si
main problem arises when, for the industrial use, it is necessary to optimise the choice of technological solutions with respect to the manufacturing time, resistance to tool wear, quality of surface, geometrical precision and manufacturing costs.
The literature contains some solutions concerning this area for ensuring higher quality of NC milling [1–11]. That literature contains discussions of step-by-step milling carried out by tools of special shapes. For each shape to be made by step-by-step milling, a specially shaped tool is used. In this process the programmed motion of the tool does not change, only the manufactured shape changes, influenced by the predeterminated step shape of the milling cutter. The US patent [12] proposes a very high-speed adiabatic face milling machine. The efficiency of the chip removal system is such that chip recutting is nearly eliminated and the tool life is improved. This solution required a new machine tool and a high investment.
The above mentioned devices and methods for milling do not include the new solution for step-by-step milling described in the paper.
3. Step-by-Step Milling Method
The step-by-step milling method features the step motion of the milling cutter (f1) in the direction of cutting and the related
Fig. 1. Milling by the step-by-step procedure. a, depth of milling; f1, motion in the direction of machining; f2, motion in the direction opposite to machining; D, diameter of milling cutter.
New NC Machine Tool Controller
403
Fig. 2. New tool path generated along the programmed trajectory. f1, motion in the direction of machining; f2, motion in the direction opposite to machining; Pst-t, start-point of the motion trajectory; Pend- t, end-point of the motion trajectory; Pst, start-point of each movement forward; Pend, end-point of each deviation; Pint, intermediate point (end-point of the previous deviation and start-point of the next movement).
movement of the milling cutter (f2) in the direction opposite to the cutting direction (Fig. 1) [13,14].
4. NC Machine Tool Controller
4.1 Description and Operation
The step-by-step NC control consists of an input communi- cation module, a generator of the step code, an appropriate control program, an output communication module and a microprocessor. The generated path of the milling cutter is such that a ratio of the depth and milling cutter diameter of up to 5D is achieved, which cannot be achieved using conven- tionally controlled milling machines.
Fig. 3. Flowchart for the generation of a new tool path. 1, reading of input NC sentence; 2, start of the block; 3, identification of functions G17, G18, G90, G91; 4, transformation of the system of coordinates; 5, definition of parameters f1 and f2; 6, rapid movement (G00); 7,
writing record of NC sentence; 8, end-point of trajectory Pend-t < f1;
9, movement to pend-t; 10, generation of movement f1 and f2; 11, writing record of the sentence for approaching and deviation; 12, transformation of coordinates; 13, writing record of the sentence; 14, the last sentence.
The device reads the program lines of the CNC program for a certain product, identifies the desired start-point Pst-t (Fig. 2) and end-point (Pend-t) of the step-by-step machining process and its parameters f1 and f2. The parameter f1 is the motion in the direction of machining, the parameter f2 is the motion in the direction opposite to machining and a is the depth of cut. The parameters f1 and f2 were determined by extensive tests in the laboratory and in real-time production and assure all the advantages of step-by-step machining. The optimal values found were f1 = 0.2–0.8 mm, f2 = 0.05–0.4 mm,
and the two parameters must fulfil the condition f1 < f2; the
stoppage time Ts = 0.01–1 s. Then the device transforms the values of the current position of the tool so that it corresponds to the desired limit values, and to the requirements of the new step-by-step milling method.
The generated path characteristic of the device, based on the invention [13,14], is a cyclic repetition of the step motion in the program trajectory. The tool moves f1 in the predetermined (desired) direction, stops in that position for the value Ts = 0.01–1 s and then moves f2 in the opposite direction. This step motion is repeated along the specified trajectory of the tool motion.
4.2 Computer Algorithm
The NC controller automatically performs the transformation of the tool motion and coordinates.
Fig. 4. Methods of incorporation of the step–by–step controller into the CNC unit of a machine tool (the layout of the CNC control unit is taken from [15]). 1, manual input; 2, input with punching tape; 3,
4, decoding; 5, computer; 7, positions memory (x,y,z,. . .); 8, functions memory (S, T, M,. . .); 9.1, step-by-step unit (in computer); 9.2, step- by-step unit (before memory); 9.3, step-by-step unit (before NC input); 10, interpolator; 11, function execution unit; 12, interpolated data flow; 13, comparison unit; 14, 15, conversion unit; 16, CNC unit, 17, position data; 18, measuring data; 19, functions data; 20, machine tool; 21, interface; 22, tacho-generator; 23, stepping (or servo) motor.
Fig. 5. Workpieces and direction of machining.
The computer model of the solution is based on the following assumptions and requirements:
Machining without correction (G40).
The programmer programs the NC machining “conventionally”; the start and the end of step-by-step machining are determined by two records in text form.
The program comprises the machining functions G17 and G18. The program recognises all the functions having an influence on the machining process (G90, G91, G54 – G59, G00, G01,
G02, G03).
Fig. 6. Graphical representation of forces.
The program includes simultaneous machining of all three coordinates (x, y, z).
The program changes only the statements inside the marked block.
Renumbering of statement numbers. Creation of output data file.
The program contains checking of machining correction.
The algorithm of operation for the computer program is shown in Fig. 3. Checking of the deviation from the desired positions is performed automatically.
4.3 Methods of Incorporation into Existing NC Control Units
The control can be incorporated onto the milling machine in three ways (Fig. 4):
Into the milling machine tool control unit, between the memory of positions and the interpolator as shown by position 9.1.
Into the NC control unit closely behind the reading-in of data module, as shown by position 9.2.
Before the NC control unit where it intercepts the input data of the NC program and suitably processes them, as shown by position 9.3.
5. Experiments and Tests
5.1 General
Tests with the step-by-step device were carried out for milling by the conventional and the new methods. The tests were executed in the laboratory [16] and in the real production of tools for the extrusion of aluminium [17].
A horizontal 4-axis machining centre (Heller BEA-05) with a pallet system and a tool magazine was used. Tool clamping was performed with ISO 50 clamping cones. Machining of workpieces was horizontal (G17).
A Kistler measuring device for measuring cutting forces was fixed to the machining centre table. The measuring equipment and the software used for processing the measurement results had been developed at the Faculty of Mechanical Engineering in Maribor [18, 19].
The test material was aluminium AlMgSil (AC30T6 – pro- ducer’s factory designation) and steel with the designation according to DIN standard 1.2343.
Tool Depth
Spindle
Feed
Step
10
6
20g
20pg
(mm)
(min-1)
1A
15
2000
80
Off
1A
15
2000
240
On
1A
40
1500
30
On
1A
40
1500
20
Off
Table 1. Two planned tests for tool wear resistance for aluminium AlMgSil.
of cut speed (mm min-1)
Fig. 7. Deviation of the real shape (dotted line) from the ideal shape (solid line). Conventional milling process: (a) slot 3.1; (b) slot 3.2. Step-by-step milling: (c) slot 6.1; (d) slot 6.2.
Table 2. Measuring of surface roughness.
Fig. 8. (a) Conventional milling; (b) step-by-step milling.
The plan of tests for measuring the cutting forces and the selected parameters are given in Table 1. The tests comprised the measurements of cutting forces, wear, and resistance to
Material Procedure
AlMgSi1 Step off
Step on
Workpiece
3.1(break) 3.2
6.1
6.2
Measurements
wear, of the tool cutting edge, geometrical accuracy of machin-
ing, roughness of machined surfaces, and visual analysis of
Ra4
Results
– 4.99
3.73
4.31
Ra
– 6.6850
4.7150
5.6600
σn
– 2.8711
0.7579
1.2345
σn-1
– 3.3152
0.8751
1.4255
Ra1 Ra2 Ra3
– 4.14
4.28
6.98
workpieces.
– 6.10
5.68
4.55
– 11.51 5.17
6.80
5.2 Analyses of Cutting Forces
Average values of the measured Fx, Fy, and Fz were calculated. Maximum forces were determined and the standard deviation of cutting forces was calculated; then the results were diagram-
matically evaluated for the average cutting forces, maximum
cutting forces, standard deviation of cutting forces, feeding and cutting speeds (Fig. 6).
For machining we selected four types of KESTAG end milling cutters of cutter material: HSS–Co8 (of diameters 10 mm, 4–7 mm – conical, 20 mm and 20 mm for rough machining). The depths of milling were 1.5 D–4 D. Cooling by emulsion was used. The lengths of cutter paths for each test piece were two cuts of 50 mm long.
The shapes of the machining and the workpieces are shown in Fig. 5.
5.3 Measurement on the 3D Coordinate Measuring Machine
Measurements of the shapes of the slots were carried out in the laboratory on a UMC 850 (universal measuring centre) for all workpieces, and then compared with the ideal and/or desired shape of the slot. A programming package KUMVDA, which
automatically senses the points of the desired curve was used. Figure 7 shows the deviation of the real shape (dotted line) from the ideal shape (solid line) of the slot in the conventional process and when milling with the step-by-step method.
5.4 Roughness
The measurements of the surface roughness were carried out by means of a MITUTOYO SURF TEST 211. The results are processed by the mini processor DIGMATIC DP1-HS. The measurement results are given in Table 2 and show the differ- ence between the surface roughness in the case of the conven- tional milling process and that of step-by-step milling.
5.5 Visual Analysis
The results of conventional and step-by-step milling can be seen in Fig. 8.
6. Conclusion
By using the step-by-step milling device, ratios of the depth and the diameter of the milling cutter of up to 5D were achieved. These cannot be achieved on conventionally con- trolled milling machines. This method can machine components which previously could not be machined. The research shows that when step-by-step milling with the same machining para- meters, the cutting forces on the tool cutting edge are smaller, the deflection of the milling cutter is also smaller and the geometrical precision of machining is greater. These are posi- tive “side” effects of the use of the step-by-step milling device. The device mentioned is particularly useful in the tool and metal-processing industry where a large removal of material is required and the products have complicated shapes. It could be incorporated as an integral part in a system for cutting
condition optimisation [20].
References
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