DOI 10.1007/s00170-003-1950-1

O R I G I N A L A R T I C L E

Int J Adv Manuf Technol (2005) 25: 888–894

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 millingmachine, which can be used for machining micro-sized parts,and be easily constructed a low cost. Micro cutting is a methodfor manufacturing three-dimensional micro parts; however, ma-chine tools for micro machining are expensive. The micromilling machine presented in this paper is mainly composedof commercially available micro stages, and an air spindle andPC-based control board. An effective method for initializing thespindle position is proposed. Test results of the micro millingmachine 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

1 Introduction

With the on-going development of technology, needs for microparts are increasing. Etching processes can create tiny parts,but etched parts are fundamentally restricted to two-dimensionalshapes. The research in this paper aims to manufacture microthermal systems composed of micro pumps and micro tur-bines that are related with fluid flow. In many cases, it isadvantageous for fluid machinery to be composed of three-dimensionally curved parts, which cannot be produced by theetching processes.

Micro cutting by precision machine tools is an effectivemethod for producing 3D micro parts. Therefore, research re-lated to micro cutting is being performed [1–3], and some com-panies are producing precision machine tools, whose feed reso-lution is within nanometre or sub-nanometre order. Such preci-sion machine tools are very expensive mainly due to their limited

Y.-B. Bang · K.-M. LeeSchool of Mechanical and Aerospace Engineering, Seoul National Univer-sity, San 56-1, Shinlim-Dong, Kwanak-Gu, Seoul 151-742, Korea

S. OhSamsung Techwin, 69-2, Sinchon-Dong, Changwon City, Kyungnam 641-370, Korea

production. Though the feed systems of the precision machinetools move with nanometre or sub-nanometre resolution, cuttingis not performed at this resolution. Also, the machine size is notso small when compared to ordinary machine tools, despite theworkpiece being much smaller than ordinary workpieces. A lit-tle research has been performed to produce small machine tools;however, it has been focussed mostly on making a ‘small’ ma-chine that does not perform close to the precision machine toolson the market.

In this research we constructed a precision 5-axis milling ma-chine of compact size (about 300 mm in height) and at a lowcost (about 1/10 of the cost of precision milling machines on themarket), which is available for machining micro parts.

2 Constructed 5-axis micro milling machine

Many of precision milling machines on the market are 3-axis ma-chines. 4-axis and 5-axis machines are available at higher price.The followings are some of the advantages of 5-axis milling ma-chines [4, 5].

1. Some 3D parts with curved surfaces simply cannot be ma-chined without using a 5-axis milling machine.

2. The error caused by the re-clamping can be eliminated: oncethe workpiece is clamped, there is no need for re-clamping ina different direction.

3. In case of machining cylindrical shapes, A-axis rotation canbe used without X and Y -axis interpolation; therefore, pre-cise machining can be easily performed.

4. The 5-axis machine has a fast material removal rate and im-proved surface finish.

This research was performed to produce curved 3D partsfor micro thermal systems; therefore, we constructed a precisionmilling 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

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(W × D × H). The feed system of this machine is composed ofthree precision linear stages (X, Y and Z-axis) and two precisionrotary stages (A and C-axis) (Fig. 1). Corresponding steppingmotors drive each stage, and Table 1 shows the specification ofthe stages. These stages have a high feed resolution as they areproduced mainly for optical instruments. Each stage has no en-coders or other displacement sensors, which can increase theoverall cost. This is possible because the micro milling machineonly performs micro cutting, and this cutting force is very smallcompared to the driving force of the stages, thus there is no riskof 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 airmotor spindle chosen here. Table 2 shows the specifications ofthis air motor spindle.

When the machine is small and the machined parts size issmaller than several mm, clamping becomes a problem. Here,a vice using a precision collet and chuck for the machine tool wasdeveloped to clamp the workpiece, aligning the rotary centre ofthe A-axis with the centre of workpiece (Fig. 2).

Fig. 1. Constructed 5-axis milling machine

Table 1. Specifications of micro stages

Model Travel range Resolution

Suruga Seiki,XY 20 mm each 50 nmK201-20MSZ K302-30 30 mm 50 nmA K401-60 360 deg 0.002 degC K402-75 360 deg 0.0012 deg

Table 2. Specification of the air motor spindle

Model NSK, MS-1930R

Speed 20 000 ∼ 30 000 rpmMaximum output 63 WMaximum torque 0.8 kgf ·cmRunout Max. 2 µm

Fig. 2. Designed vice

Fig. 3. Schematic diagram of micro milling machine system

For the positioning control, a TurboPMAC2 controller thatcan control five axes simultaneously is used. To facilitate the linkwith CAD/CAM, it is operated with a PC. Figure 3 is a schematicdiagram of this system.

2.2 Feed system performance

We measured the feed resolution of the three axes after the com-plete 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 sus-ceptible to backlash. There is a function in the PMAC controllerfor compensating the backlash. Inputting the backlash quantitymeasured by the capacitive gap sensor can minimize the backlasherror. Figure 5 shows the result of backlash compensation.

2.3 Tool path conversion

Before performing machining, we must draw the part and gen-erate the tool path from the part drawing. The tool path that isgenerated from the CAD drawing consists of the position (X0,Y0, Z0) and their direction vector (U, V, W). The position and

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

Fig. 5. Stage movements before and after backlash compensation

the direction vector must be converted into the movement of eachstage (Xd, Yd, Zd, Ad, Cd) of the machine.

The origin of the X, Y axis and the centre of the A stage arein the same axis; therefore, from the direction vectors V and W ,the angle of rotation of the A stage can be found. The angle ofrotation of the C stage is obtained from U, V , and W . The move-ment of the X, Y , and Z stage is calculated using these angles andthe position values, and the offset error between the centre of Caxis and the tip of the tool occurs every time the tool is changed.The equation below also takes this offset error into considerationwhen producing the output (Fig. 6).

(X0, Y0, Z0, u, v,w) → (Xd, Yd, Zd, Ad, Cd) ,

θ = arctan 2(v, u) ,

φ = arctan 2(√

u2 +v2, w)

,

Xd = X0 cos(θ)+Y0 sin(θ)+offset · sin(φ) ,

Yd = −X0 sin(θ)+Y0 cos(θ) ,

Zd = Z0 −offset · (1− cos(φ)) ,

Ad = θ ,

Cd = φ ,

where,

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

Fig. 6. Figures to explain tool path conversion

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

2.4 Matching of tool and A-axis centres

When assembling a column-type 5-axis milling machine, it isimportant to coincide the tool axis (spindle axis) with the A-axisas the zero position. Unless the zero position coincides, whenmachining a circular cylinder, the machined diameter may dif-fer from the required diameter. The initial position of large-sizedmachine tools can be measured by, for example, a ball bar sys-tem [6]. However, it is difficult to measure the home positionof a micro milling machine due to its small size. In this papera new and easy method for coinciding the tool axis with A-axisis proposed. Figure 7 shows this procedure. First, a workpieceis clamped and the cylindrical surface is machined while the A-axis is turning and the Y -axis is moving back and forth. Whenthe radii of the machined workpiece and tool are added, the X-directional distance between A-axis and tool axis can be found(Fig. 7a). That is,

dX = 1

2(Dw + Dt) , (1)

where,

dX = X-directional distance between A-axis and tool axisDw = machined workpiece diameter

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

Dt = tool diameter.

Y -directional distance between A-axis and tool axis can befound by the same method (Fig. 7b).

This method can be performed precisely when the actual tooldiameter is known, and when the machine tool and the tool arecompletely rigid, and when the machined surface is perfectly incontact with the tool surface. However, the actual tool diametercan differ from the nominal diameter when the machine tool andthe tool are elastically deformed, and the cutting surface doesnot meet the tool surface at a point. This error may not add upto be large, but it should be excluded as this is a case of preci-sion machining. To eliminate this error, we propose the followingmethod, as shown in Fig. 8.

Again, the cylindrical surface is machined while the A-axis isturning and the Y -axis is moving back and forth (Fig. 8a). Aftermeasuring the machined workpiece diameter, the tool movesin the X-direction to the opposite side of the workpiece, andcylindrical surface is machined again while both the A-axis andY -axis are operating (Fig. 8b). The following are the relevantequations.

dX,1 = Dw,1

2+ Dt,n

2+ ε , (2)

dX,2 = Dw,2

2+ Dt,n

2+ ε , (3)

x = dX,1 +dX,2 , (4)

where,

Dw,1: workpiece diameter after first cuttingDw,2: workpiece diameter after second cuttingDt,n: nominal tool diameterε: error due to tool diameter error, elastic deflection, etc.dX,1: X-directional distance between A-axis and tool axis at the

first cuttingdX,2: X-directional distance between A-axis and tool axis at the

second cuttingx: X-directional displacement of tool.

The error (ε) can be assumed to be the same for the first andthe second operations (Eqs. 2 and 3), because the cutting condi-tions 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,

dX,1 = 1

4

(2x + Dw,1 − Dw,2

), (5)

dX,2 = 1

4

(2x − Dw,1 + Dw,2

). (6)

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

3 Machining test

For a machine performance test, a carbide flat endmill of 100 µmdiameter (NS, Japan) and a 200 µm diameter (Micro100, USA)tool were used. Although a brass workpiece was machined in thispaper, harder materials can be machined without much difficulty.The constructed milling machine uses very small diameter toolsand the cutting depth and the feed rate are very small compared toordinary cutting; therefore, there is no problem associated with in-sufficient spindle torque or feed force. In case of machining hardmaterial such as tungsten, diamond-coated tools will be preferredbecause they are durable and producing a high quality surface.

1) Micro wall

As shown in Fig. 9, after dividing the height into several layersof a few micrometres, the layers are machined in sequence. Inthis process, the repeatability of machine position is crucial tothe quality of the results. Figure 10 is an SEM (scanning elec-tron microscope) photograph of the machined micro walls. Theworkpiece for machining the walls measures 2 mm ×2 mm at itsrectangle cross-section. The cutting depth is 5 µm and the gap

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Fig. 9. Machining process of micro walls

Fig. 10. Machined micro walls (workpiece material: brass, carbide flatendmill 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 is25 µm and the height is 650 µm: this machining was done withthe aspect ratio of 26. Compared to the typical machined sur-faces, these micro walls have a much smoother surface; however,the machined surface may seem excessively rough due to thevery high photographic enlargement scaling (Fig. 9).

2) Micro column with rectangular cross-section

In order to machine a micro column with a rectangular cross-section of 30 µm× 30 µm, the base of 1 mm× 1 mm× 1.5 mmwas machined first. This base was cut with a cutting depth of5 µm and a 5 µm step over until the desired size is achievedand it then proceeds to the next stage. This method reduces theapplied forces on the machined column, and reduces the defor-mation of the already machined section (Fig. 11). Figure 12 is anSEM photograph of the machined micro column. The size of themachined column is 30 µm×30 µm×320 µm, having the aspectratio of 10.6.

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

Fig. 12. Micro column with rectangle section (workpiece material: brass,cutting depth: 5 µm, step over: 5 µm)

3) Micro column with circular cross-section

When we machine a cylinder with the method shown in Fig. 11with a 3-axis milling machine, a high precision and circular in-terpolation function controller are required. But using the 5-axismachine, we can machine it with a relative ease and great pre-cision. Figure 13 presents the machining method for a cylinderwith 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 machinedmicro cylinder. The diameter of the machined cylinder is 30 µmand 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 millingmachine to produce practical micro machine parts, a simplemicro impeller and base are machined (Fig. 15). At first, the shaftof the impeller was machined as a cylinder. The impeller bladeswere completed using a similar method to machining curvedmicro walls. The diameter of the tool is 200 µm. The image ofFig. 16 is the micro impeller and base block in the assembly.

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

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

Fig. 15. The shape of micro impeller and base

5) Micro blade

A micro turbine can compose a micro power system with a microgenerator. With its complex shape, a well machined micro

Fig. 17. Machined micro blade. a, b

Fig. 16. Assembled micro impeller and base block

turbine can produce a good machine overall performance. InFig. 17a, the machined turbine has an overall diameter of 3.4 mmand a blade length of 0.35 mm. Figure 17b is the image of oneblade.

4 Conclusion

In this paper we constructed a 5-axis micro milling machine formicro 3D parts machining. This precision machine can be con-structed at a low cost with commercially available parts such asa micro stage, air spindle, and PC-based control board. A sim-ple 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 bladesshowed that the constructed micro milling machine is capable ofproducing practical micro parts.

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Acknowledgement This research was supported by the Micro Thermal Sys-tem Research Center through the Korea Science and Engineering Foundation.

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