Int J Adv Manuf Technol (2005) 25: 888–894 DOI 10.1007/s00170-003-1950-1

ORIGINAL ARTICLE

Young-bong Bang · Kyung-min Lee · Seungryul Oh

5-axis micro milling machine for machining micro parts

Received: 18 August 2003 / Accepted: 3 September 2003 / Published online: 20 February 2004  Springer-Verlag London Limited 2004 Abstract This paper presents a PC-based 5-axis micro milling machine, which can be used for machining micro-sized parts, and be easily constructed a low cost. Micro cutting is a method for manufacturing three-dimensional micro parts; however, machine tools for micro machining are expensive. The micro milling machine presented in this paper is mainly composed of commercially available micro stages, and an air spindle and PC-based control board. An effective method for initializing the spindle position is proposed. Test results of the micro milling machine are presented, which include machining of micro walls, micro columns and micro blades. Keywords 5-axis milling · Micro cutting · Micro machining · Micro milling · Micro parts

production. Though the feed systems of the precision machine tools move with nanometre or sub-nanometre resolution, cutting is not performed at this resolution. Also, the machine size is not so small when compared to ordinary machine tools, despite the workpiece being much smaller than ordinary workpieces. A little research has been performed to produce small machine tools; however, it has been focussed mostly on making a ‘small’ machine that does not perform close to the precision machine tools on the market. In this research we constructed a precision 5-axis milling machine of compact size (about 300 mm in height) and at a low cost (about 1/10 of the cost of precision milling machines on the market), which is available for machining micro parts.

2 Constructed 5-axis micro milling machine 1 Introduction With the on-going development of technology, needs for micro parts are increasing. Etching processes can create tiny parts, but etched parts are fundamentally restricted to two-dimensional shapes. The research in this paper aims to manufacture micro thermal systems composed of micro pumps and micro turbines that are related with fluid flow. In many cases, it is advantageous for fluid machinery to be composed of threedimensionally curved parts, which cannot be produced by the etching processes. Micro cutting by precision machine tools is an effective method for producing 3D micro parts. Therefore, research related to micro cutting is being performed [1–3], and some companies are producing precision machine tools, whose feed resolution is within nanometre or sub-nanometre order. Such precision machine tools are very expensive mainly due to their limited Y.-B. Bang · K.-M. Lee School of Mechanical and Aerospace Engineering, Seoul National University, San 56-1, Shinlim-Dong, Kwanak-Gu, Seoul 151-742, Korea S. Oh Samsung Techwin, 69-2, Sinchon-Dong, Changwon City, Kyungnam 641370, Korea

Many of precision milling machines on the market are 3-axis machines. 4-axis and 5-axis machines are available at higher price. The followings are some of the advantages of 5-axis milling machines [4, 5]. 1. Some 3D parts with curved surfaces simply cannot be machined without using a 5-axis milling machine. 2. The error caused by the re-clamping can be eliminated: once the workpiece is clamped, there is no need for re-clamping in a different direction. 3. In case of machining cylindrical shapes, A-axis rotation can be used without X and Y -axis interpolation; therefore, precise machining can be easily performed. 4. The 5-axis machine has a fast material removal rate and improved surface finish. This research was performed to produce curved 3D parts for micro thermal systems; therefore, we constructed a precision milling machine equipped with 5-axis. 2.1 Make-up of the machine The constructed system is a column-type 5-axis milling machine. The overall size of the machine is 294 mm × 220 mm × 328 mm

889

(W × D × H). The feed system of this machine is composed of three precision linear stages (X, Y and Z-axis) and two precision rotary stages (A and C-axis) (Fig. 1). Corresponding stepping motors drive each stage, and Table 1 shows the specification of the stages. These stages have a high feed resolution as they are produced mainly for optical instruments. Each stage has no encoders or other displacement sensors, which can increase the overall cost. This is possible because the micro milling machine only performs micro cutting, and this cutting force is very small compared to the driving force of the stages, thus there is no risk of missing steps. The spindle is fixed on a rotary stage (C-axis). It is preferred to use a small and light spindle unit such as an air motor spindle chosen here. Table 2 shows the specifications of this air motor spindle. When the machine is small and the machined parts size is smaller than several mm, clamping becomes a problem. Here, a vice using a precision collet and chuck for the machine tool was developed to clamp the workpiece, aligning the rotary centre of the A-axis with the centre of workpiece (Fig. 2).

Fig. 2. Designed vice

Fig. 3. Schematic diagram of micro milling machine system

For the positioning control, a TurboPMAC2 controller that can control five axes simultaneously is used. To facilitate the link with CAD/CAM, it is operated with a PC. Figure 3 is a schematic diagram of this system. Fig. 1. Constructed 5-axis milling machine

2.2 Feed system performance Table 1. Specifications of micro stages

XY Z A C

Model

Travel range

Resolution

Suruga Seiki, K201-20MS K302-30 K401-60 K402-75

20 mm each

50 nm

30 mm 360 deg 360 deg

50 nm 0.002 deg 0.0012 deg

We measured the feed resolution of the three axes after the complete assembly and setup under the actual working condition. A capacitive gap sensor (Microsense II, ADE Tech) was used. Figure 4 shows the measured results of X, Y and Z axis. Here, the three axes display a resolution of 50 nm/step. The X, Y and Z stages are driven by lead screws and are susceptible to backlash. There is a function in the PMAC controller for compensating the backlash. Inputting the backlash quantity measured by the capacitive gap sensor can minimize the backlash error. Figure 5 shows the result of backlash compensation.

Table 2. Specification of the air motor spindle Model

NSK, MS-1930R

2.3 Tool path conversion

Speed Maximum output Maximum torque Runout

20 000 ∼ 30 000 rpm 63 W 0.8 kgf · cm Max. 2 µm

Before performing machining, we must draw the part and generate the tool path from the part drawing. The tool path that is generated from the CAD drawing consists of the position (X 0 , Y0 , Z 0 ) and their direction vector (U, V, W). The position and

890

Fig. 4. Movement of linear stages a X-axis, b Y -axis, c Z-axis

Fig. 5. Stage movements before and after backlash compensation Fig. 6. Figures to explain tool path conversion

the direction vector must be converted into the movement of each stage (X d , Yd , Z d , Ad , Cd ) of the machine. The origin of the X, Y axis and the centre of the A stage are in the same axis; therefore, from the direction vectors V and W, the angle of rotation of the A stage can be found. The angle of rotation of the C stage is obtained from U, V , and W. The movement of the X, Y , and Z stage is calculated using these angles and the position values, and the offset error between the centre of C axis and the tip of the tool occurs every time the tool is changed. The equation below also takes this offset error into consideration when producing the output (Fig. 6).

offset = the distance between the centre of C stage and the end of the tool

2.4 Matching of tool and A-axis centres

Ad = θ ,

When assembling a column-type 5-axis milling machine, it is important to coincide the tool axis (spindle axis) with the A-axis as the zero position. Unless the zero position coincides, when machining a circular cylinder, the machined diameter may differ from the required diameter. The initial position of large-sized machine tools can be measured by, for example, a ball bar system [6]. However, it is difficult to measure the home position of a micro milling machine due to its small size. In this paper a new and easy method for coinciding the tool axis with A-axis is proposed. Figure 7 shows this procedure. First, a workpiece is clamped and the cylindrical surface is machined while the Aaxis is turning and the Y -axis is moving back and forth. When the radii of the machined workpiece and tool are added, the Xdirectional distance between A-axis and tool axis can be found (Fig. 7a). That is,

Cd = φ ,

dX =

where,

where,

θ φ

d X = X-directional distance between A-axis and tool axis Dw = machined workpiece diameter

(X 0 , Y0 , Z 0 , u, v, w) → (X d , Yd , Z d , Ad , Cd ) , θ = arctan 2(v, u) , 
 u 2 + v2 , w , φ = arctan 2 X d = X 0 cos(θ) + Y0 sin(θ) + offset · sin(φ) , Yd = −X 0 sin(θ) + Y0 cos(θ) , Z d = Z 0 − offset · (1 − cos(φ)) ,

= The angle of rotation of A stage = The angle of rotation of C stage

1 (Dw + Dt ) , 2

(1)

891

Fig. 7. Tool position initialization procedure (basic). a, b

Dt = tool diameter. Y -directional distance between A-axis and tool axis can be found by the same method (Fig. 7b). This method can be performed precisely when the actual tool diameter is known, and when the machine tool and the tool are completely rigid, and when the machined surface is perfectly in contact with the tool surface. However, the actual tool diameter can differ from the nominal diameter when the machine tool and the tool are elastically deformed, and the cutting surface does not meet the tool surface at a point. This error may not add up to be large, but it should be excluded as this is a case of precision machining. To eliminate this error, we propose the following method, as shown in Fig. 8. Again, the cylindrical surface is machined while the A-axis is turning and the Y -axis is moving back and forth (Fig. 8a). After measuring the machined workpiece diameter, the tool moves in the X-direction to the opposite side of the workpiece, and cylindrical surface is machined again while both the A-axis and Y -axis are operating (Fig. 8b). The following are the relevant equations. Dw,1 Dt,n + +ε , 2 2 Dw,2 Dt,n d X,2 = + +ε , 2 2 d X,1 =

x = d X,1 + d X,2 ,

(2) (3) (4)

where, Dw,1 : Dw,2 : Dt,n : ε: d X,1 :

workpiece diameter after first cutting workpiece diameter after second cutting nominal tool diameter error due to tool diameter error, elastic deflection, etc. X-directional distance between A-axis and tool axis at the first cutting d X,2 : X-directional distance between A-axis and tool axis at the second cutting x: X-directional displacement of tool. The error (ε) can be assumed to be the same for the first and the second operations (Eqs. 2 and 3), because the cutting conditions are almost identical. From Eqs. 2 to 4, the distance between

Fig. 8. Tool position initializing procedure (precise). a, b, c, d

A-axis and the tool axis can be calculated as following,  1 2x + Dw,1 − Dw,2 , 4  1 2x − Dw,1 + Dw,2 . d X,2 = 4 d X,1 =

(5) (6)

d1 or d2 can be used as an offset value for initializing the tool position in the x direction. By the same procedure, the offset value in the Y direction can be obtained (Fig. 8c and d). This method is not only applicable for micro milling machines but also for ordinary-sized milling machines.

3 Machining test For a machine performance test, a carbide flat endmill of 100 µm diameter (NS, Japan) and a 200 µm diameter (Micro100, USA) tool were used. Although a brass workpiece was machined in this paper, harder materials can be machined without much difficulty. The constructed milling machine uses very small diameter tools and the cutting depth and the feed rate are very small compared to ordinary cutting; therefore, there is no problem associated with insufficient spindle torque or feed force. In case of machining hard material such as tungsten, diamond-coated tools will be preferred because they are durable and producing a high quality surface. 1) Micro wall As shown in Fig. 9, after dividing the height into several layers of a few micrometres, the layers are machined in sequence. In this process, the repeatability of machine position is crucial to the quality of the results. Figure 10 is an SEM (scanning electron microscope) photograph of the machined micro walls. The workpiece for machining the walls measures 2 mm × 2 mm at its rectangle cross-section. The cutting depth is 5 µm and the gap

892

Fig. 9. Machining process of micro walls

Fig. 12. Micro column with rectangle section (workpiece material: brass, cutting depth: 5 µm, step over: 5 µm) Fig. 10. Machined micro walls (workpiece material: brass, carbide flat endmill of 200 µm diameter, 25 000 rpm, cutting depth: 5 µm, federate: 1 mm/s)

between the walls is 200 µm. The thickness of machined wall is 25 µm and the height is 650 µm: this machining was done with the aspect ratio of 26. Compared to the typical machined surfaces, these micro walls have a much smoother surface; however, the machined surface may seem excessively rough due to the very high photographic enlargement scaling (Fig. 9). 2) Micro column with rectangular cross-section In order to machine a micro column with a rectangular crosssection of 30 µm × 30 µm, the base of 1 mm × 1 mm × 1.5 mm was machined first. This base was cut with a cutting depth of 5 µm and a 5 µm step over until the desired size is achieved and it then proceeds to the next stage. This method reduces the applied forces on the machined column, and reduces the deformation of the already machined section (Fig. 11). Figure 12 is an SEM photograph of the machined micro column. The size of the machined column is 30 µm × 30 µm × 320 µm, having the aspect ratio of 10.6.

Fig. 11. Machining process of micro column with rectangle section

3) Micro column with circular cross-section When we machine a cylinder with the method shown in Fig. 11 with a 3-axis milling machine, a high precision and circular interpolation function controller are required. But using the 5-axis machine, we can machine it with a relative ease and great precision. Figure 13 presents the machining method for a cylinder with 5-axis. By moving the workpiece in the X and Y direction, and rotating the A-axis, the milling machine can work as a lathe. Figure 14 is a photograph of the surface of the machined micro cylinder. The diameter of the machined cylinder is 30 µm and the height is 650 µm, its aspect ratio is 21.6. 4) Micro impeller and base To demonstrate the potential of utilizing a 5-axis micro milling machine to produce practical micro machine parts, a simple micro impeller and base are machined (Fig. 15). At first, the shaft of the impeller was machined as a cylinder. The impeller blades were completed using a similar method to machining curved micro walls. The diameter of the tool is 200 µm. The image of Fig. 16 is the micro impeller and base block in the assembly.

Fig. 13. Machining process of micro column with circular cross-section

893

Fig. 14. Micro column with circular cross-section (workpiece material: brass, cutting depth: 5 µm, step over: 5 µm)

Fig. 16. Assembled micro impeller and base block

turbine can produce a good machine overall performance. In Fig. 17a, the machined turbine has an overall diameter of 3.4 mm and a blade length of 0.35 mm. Figure 17b is the image of one blade.

4 Conclusion

Fig. 15. The shape of micro impeller and base

5) Micro blade A micro turbine can compose a micro power system with a micro generator. With its complex shape, a well machined micro

Fig. 17. Machined micro blade. a, b

In this paper we constructed a 5-axis micro milling machine for micro 3D parts machining. This precision machine can be constructed at a low cost with commercially available parts such as a micro stage, air spindle, and PC-based control board. A simple method to coincide the tool axis with A-axis was proposed; this includes cylindrical surface cutting and diameter measuring. Test machining of micro walls, micro columns, and micro blades showed that the constructed micro milling machine is capable of producing practical micro parts.

894 Acknowledgement This research was supported by the Micro Thermal System Research Center through the Korea Science and Engineering Foundation.

References 1. Sukawa H, Takeuchi Y, Sawada K, Kawai T, Sakaida Y (2002) Ultraprecision micro maching of structures with high aspect ratio. J Japan Soc Precision Eng 68(11) (in Japanese) 2. Takeuchi Y, Sawada K, Sata T (1995) Computer aided ultra-precision micro-machining of metallic materials. Proc of IEEE Int Conf on Robotics and Automation, Nagoya, 67

3. Yamagata Y, Higuchi T, Takashima Y, Ueda K (1996) Fabrication of micro mechanical and optical components by ultra-precision cutting. Proceedings-SPIE the International Society for Optical Engineering Microelectrics Struct and MEMS for Opt Process II, Issue 2881, pp 148–159 4. Jun C-S, Cha K, Lee Y-S (2003) Optimizing tool orientations for 5-axis machining by configuration-space search mechod. Comput Aided Des 35:549–566 5. Baptista R, Antune Simões JF (2000) Three and five axis milling of sculptured surface. J Mater Process Technol 103:398–403 6. Tsustsumi M, Saito A (2003) Identification and compensation of sustematic deviations particular to 5-axis machining centers. Int J Mach Tools Manuf 43:771–780