MIMS: Web-based micro machining service

MIMS: Web-based micro machining service

SUNG-HOON AHN,{* DONG-SOO KIM,{ WON-SHIK CHU,{ and CHA-SOO JUN{

{School of Mechanical and Aerospace Engineering, Seoul National University, Shintim-Dong San 56-1,Kwanak-Gu, Seoul, Korea

{Division of Industrial and Systems Engineering and ReCAPT, GyeongSang National University, Jinju,Gyeong-Nam, Korea

Presented in this paper is a Micro Machining Service (MIMS) based on the World Wide

Web (WWW) technologies. Taking advantage of the bi-directional communications of

the WWW, the fabrication process of micro machining can be expedited and becomes

more viable for students or researchers. In order to ensure an easy access to the service,

web browsers are used as the user interface of MIMS. A three dimensional geometry

generated from a commercial CAD system is uploaded as an STL (Stereo Lithography)

file, and the process parameters for three-axis CNC micro milling are to be selected via

the user interface. The communication architecture is based on the three-tier client-server

model. Depending on the user’s knowledge on micro machining, novice or expert,

controllable input parameters are differentiated, but at any case an STL-based process

planner automatically provides NC codes. To achieve high precision, scanning toolpath

and pencil-cut toolpath are created by a curve-based polyhedral machining method. The

hardware system for micro machining was established, and a couple of sample parts were

fabricated by micro endmills. The parts fabricated by scanning toolpath followed by

pencil-cut toolpath resulted in less form error (within 1.5%) than the parts fabricated

only by scanning toolpath. This service is available at http://fab.snu.ac.kr/cam.

Keywords: Micro Machining; Web-based; CAD; ST2 (Stereo Lithography)

1. Introduction

Micro machining is becoming more important than ever in

various industries such as semiconductor, printed circuit

board, mold making, and bioengineering. Although the

terminology of ‘micro machining’ is commonly used for the

processes that fabricate parts in micro scale, it may mean

different manufacturing technologies. In the semiconductor

industry, mainly electromagnetic waves, such as UV or

electron beam, have been mainly applied for its micro

machining process while in ‘mechanical’ micro machining,

the material removal process resembles macroscopic

machining processes such as drilling and milling.

Although the micro machining processes used in the

semiconductor fabrication have been well developed for a

mass production of Very Large Scale Integrated circuits

(VLSI) or Micro Electro Mechanical Systems (MEMS), the

selection of major substrate material is limited to silicon

wafers. In contrast, the mechanical micro machining allows

almost the same variety of stock materials as the materials

used in mechanical machining at macro scale.

In terms of prototyping ‘service’, the VLSI industry has

pioneered with Metal Oxide System Implementation

Service (MOSIS) ( University of Southern California’s

Information Sciences Institute) whose groundwork had

been developed in the late 1970s by Mead and Conway

(1980). Today it provides students at research universities

the opportunity to obtain prototype chips during semester-

long CAD/CAM courses in VLSI design and fabrication.

Their chips can be fabricated by a number of semiconduc-

tor companies who offer their services to the MOSIS

bureau.

*Corresponding author. Email: [email protected]

Int. J. Computer Integrated Manufacturing, Vol. 18, No. 4, June 2005, 251 – 259

International Journal of Computer Integrated ManufacturingISSN 0951-192X print/ISSN 1362-3052 online ª 2005 Taylor & Francis Group Ltd

http://www.tandf.co.uk/journalsDOI: 10.1080/09511920400030187

Inspired by the success of the ‘‘VLSI MOSIS’’ project,

US National Science Foundation (NSF) workshops in the

early 1990s addressed the possibilities of a ‘‘Mechanical

Implementation Service’’ (MIS) (Mukherjee and Hillibrand

1994, National Science Foundation 1995). As the Internet

boom burst in the mid 1990s, researchers have applied the

Internet technologies to mechanical manufacturing pro-

cesses (Dong and Agogino 1988, Adamczyk and Malek

1998, Huang and Mak 1999, Gadh and Sonthi 1998,

Adamczyk and Kocioiek 2001). For example, in the

CyberCut project, a web browser was used as a user

interface to download a Java-based CAD, named Web-

CAD (Ahn et al. 2001). The main advantage of the web-

based CAD/CAM systems is the utilization of distributed

software and hardware resources. Thus, CAD/CAM tools

can be accessed anywhere and anytime via the Internet in

the web-based paradigm. Furthermore, manufacturing may

be performed at remotely located machining centers, and

the final part can be shipped through express mail.

In this paper, a web-based Micro Machining Service,

named as MIMS, is proposed in order to provide time

effective process planning and fabrication. This integrated

system is the first attempt in mechanical micro machining

that may fabricate three-dimensional geometries in micro

scale.

2. Web-based interface

Figure 1 shows the communication architecture of the

micro machining system. In this system, the three-tier

architecture is applied for better scalability (Messerschmitt

1999). Both CAD and web browser play a role of the client

(1st tier). A CAD system is used as a modeling tool which

outputs an STL file. The STL file generated from the CAD

and the process parameters are uploaded via the web

browser located in the client PC (figure 2). The MIMS

server, 2nd tier, mediates the client and the 3rd tier

applications. The 3rd tier mainly consists of a database of

tools and an STL-based Computer Aided Manufacturing

(CAM) software. A toolpath is generated in an automated

fashion by the integrated STL CAM, which is represented

as G&M codes for a three-axis micro machining system.

2.1. Design for manufacturing (DFM)

DFM is an important concept to effectively integrate design

and manufacturing. By providing designers with the issues

or knowledge related to manufacturing, the cost and time

required for the entire design and manufacturing can be

reduced (Stoll 1991, Boothroyd 1994).

In the micro machining system, precision and accuracy

are the key parameters to evaluate the quality of fabricated

parts. The main dimensional discrepancy in micro ma-

chined parts is raised from the run-out of the cutting tool,

tool deflection by vibration and cutting force, and errors in

the linear stages (Lee et al. 2001). In the MIMS service,

generic information of the DFM on micro machining is

provided as a webpage, so that a novice user may

understand at least the issue of errors. In addition, a

database of micro endmills were provided to the designer in

order to have him/her aware of manufacturing domain of

micro machining.

Figure 1. Schematic diagram of the communication architecture (when machining is done in remote fabrication site).

252 S.-H. Ahn et al.

2.2. User levels

Several researchers have studied the characteristics of web-

based user interfaces (Hu et al. 1999, Lederer et al. 2000,

Miles et al. 2000, Moore and Moshkina 2000, Ahn et al.

2002), and the advantage of accessibility takes place in this

web-based machining service.

Not only computational speed and robustness, but also

minimization of user intervention and convenience are the

critical factors in the web-based service. To offer the users

Figure 2. A web browser as the user interface of MIMS.

Web-based micro machining 253

with both convenience and control, two different levels of

users are defined in MIMS: one ‘novice’ and the other

‘expert’. For a novice, the web-based user interface requests

only tool diameter and option for additional roughing.

Compared with user interactive commercial CAM soft-

ware, the time efficient process planning is one of the main

advantages of the MIMS approach.

For an expert user, as many as 16 parameters used in

machining can be decided by user’s selection, so that the

details of machining are to be controlled. Table 1 lists the

controllable parameters for novice and expert users.

2.3. Web-aided local fabrication

The ordinary users of MIMS upload a CAD file and get

machined parts by postal service (figure 1). In the web-

aided local fabrication, the micro machining system is

available at the client side, and the communication

architecture is designed as in figure 3. In this case, the

designer may use MIMS just to obtain a G&M code for

micro machining without actual fabrication having

happened in the server side. The engineer in the client

side can download the G&M codes and fabricate using

the local micro machining system.

Thus, MIMS provides (1) a complete micro fabrication

service and (2) a web-based G&M code generation service.

Figure 4 shows the G&M codes generated by MIMS and

displayed on the ‘NC Code Viewer’ at the client PC. The

NC code viewer can be downloaded from the MIMS site,

and was developed to offer the feedback of the MIMS data

in near real time.

3. STL-based CAM

The STL is a de facto standard format in CAD for Rapid

Prototyping (RP) systems. The advantage of using STL

over IGES or STEP format is its stability during data

transfer as well as universality. Thus more commercial

CAM systems have provided STL as the input format.

In this research the curve-based approach (Jun et al.

2002) is adopted to generate scanning and pencil-cut tool

paths for STL-type polyhedral models using flat endmills,

which are generally used for micro machining. The curve-

based approach can satisfy the above requirements of the

internet-based CAM system and generates more accurate

tool paths than the point-based approaches. Figure 5 shows

the difference of two approaches in case of a flat endmill.

The proposed method of generating tool paths for scanning

is as follows:

(1) Constructing topological relationship of the ele-

ments of the input model, and identifying the

attributes of the vertices and edges like ‘convex’,

‘concave’, or ‘flat’.

(2) Offsetting the polyhedral model by the local off-

setting scheme (Jun et al. 2002). While all the facets

of the polyhedron are offset to the offset facets, only

the convex edges and convex vertices are offset to

the sheared cylinders and the disks, respectively.

(3) Slicing each offset element with a series of drive

planes. The sliced curve segments are stored

separately in the corresponding drive planes.

(4) Trimming and linking the CL-curves on a drive

plane.

The CL-curves generated by the above procedure is

represented in NURBS curves, and is matched exactly to

the polyhedral model. The proposed method is efficient,

robust and easy to understand. There may remain large

cusps along the sharp-concave edges after scanning type

machining. To clean up the uncut volumes a pencil-cut

process can be added.

For a given polyhedral model and a flat endmill, the

pencil-cut paths are generated in the following three steps

in this research:

(1) Constructing a pencil-cut CL-surface by both x-

and y-directional scanning tool paths.

(2) Detecting pencil-points by the angle of the bi-

normal vectors at the sharp-concave junction points

of the CL-curves.

(3) Tracing the pencil-cut path by marching through

the pencil points.

Although the dimensional error is affected by machining

conditions and work piece materials, a web-page is

Table 1. Comparison of control parameters for two differentuser levels.

Novice Expert

Cut Mode *Plane Normal *Pattern Type *Tool Diameter * *Path Interval *Cutting Tolerance *Surface Offset *Start Point *Clearance Height *Approach and Exit Type *Path Connection *Linking Tolerance *Feed Rate *Spindle Speed *Boundary Machining *Roughing * *


provided to show the typical error in ordinary machining

condition using the machining system (figure 6). Thus the

user can expect a potential dimension of error and prevent

it by controlling machining parameters. In addition, the

recommended cutting condition for micro machining is

provided in the same webpage (table 2).

Figure 3. Web-aided local fabrication.

Figure 4. Display of G&M coded on the NC Code Viewer.


A scanning toolpath and a pencil-cut toolpath generated

for a channel are compared in figure 5. Note that in order

to make an NC data automatically, a feature recognition

function might have been required. The proposed method

can generate the NC data suitable to the prismatic features

such as steps and pocket boundaries in the micro

machining applications using flat endmills without user

intervention or any additional feature recognition pro-

cesses.

In order to integrate into the MIMS architecture, a fully

automated CAM, i.e. without any user’s interaction at

every planning step, was designed and implemented.

4. Micro machining stage

To guarantee accuracy and precision a specially designed

micro machining stage was constructed. As shown in figure

7, a three-axis stage is built on a granite frame to reduce the

effect of thermal strain. The maximum spindle speed is

43,000rpm and maximum run-out is 5 micrometers (Fischer

HEN40). Each x, y, and z axis has an optical encoder for

feedback control of the table positions (Parker Automa-

tion) and the resolution of each axis is as small as 1 micro

meter. PMAC controller is used for the control of the table

and the spindle. Besides the spindle, a digital microscope is

located and measures the dimensions of machined parts.

5. Test parts

In order to evaluate the effect of different toolpaths, two

micro channels with 200 micrometer width both in x and y

directions were designed and fabricated. A number of flat

Figure 5. Point-based (left) and curve-based (right) approaches.

Figure 6. Two types of toolpaths for channel machining.

Table 2. The recommended machining parameters used inMIMS.

Diameter

(mm)

Flute Length

(mm) Spindle (rpm)

Feed Rate

(mm/min)

0.127 0.254 15,000–18,000 10–15

0.254 0.381 15,000–18,000 15–20

0.381 0.572 12,000–15,000 20–40

0.508 0.762 10,000–15,000 40–80

1 3 10,000–15,000 50–100


micro endmills with diameters ranged from 127 micrometer

(0.005 inch) to 762 (0.030 inch) micrometer were used for

machining (figure 8). The work piece was 6061 aluminum

alloy, and normal spindle speed used in this test was 15,000

rpm.

As shown in figures 9 and 10, the channel machined using

scanning toolpath followed by pencil-cut resulted in more

dimensional accuracy and finer pocket boundaries.

Note that the commercial endmills with 127 micrometer

diameter inherently contain 3–5 micrometer error in

diameter. Furthermore, the measuring of dimension using

the digital microscope resulted in a 5–7 micrometer error at

200 times magnitude.

Table 3 summarizes the errors in two channels. While

scanning and pencil-cut offered less than a 1.5% error,

scanning only resulted in a 38% smaller channel width than

designed. From this result we can see that the scanning

Figure 7. Picture of the micro machining stage.

Figure 8. Micro endmills used in this research.

Figure 9. Micro machined channel only by scanning

toolpath.

Figure 10. Micro machined channel by scanning followed

by pencil-cut toolpath.


toolpath itself, which is commonly used for freeform

surface, is not accurate to be used in micro machining of

2.5 dimensional features unless additional pencil-cut is

applied.

Figure 11 shows a micro machined head on a polymer

work piece. Again, an STL file was generated from a three

dimensional scanner and submitted to the MIMS server to

get the tool path for micro machining. The ABS rotor in

figure 12 was injection molded from an aluminum mold

fabricated by micro machining.

Conclusions

The first micro machining service applying the World

Wide Web technologies has been developed. The main

concept in this research is to develop a web-based user

interface which contains DFM paradigm and fully

automated tool path generation that enables realtime

feedback. The overall usability and efficiency of the system

are enhanced by the web-based user interface. The STL

files that can be easily obtained from many commercial

CAD systems were selected as a standard input format to

the system, and this feature improved the integrity of the

data transfer.

The STL-based CAM module, which offers strong

stability and efficiency to the web-based integration, was

developed, and its capability was verified by actual micro

machining. We are conducting a trial web site of this

service, and users can try the service at http://fab.snu.ac.kr/

cam. Provided more accumulation of DFM knowledge on

micro machining, the functionality of MIMS will be further

improved.

Acknowledgements

This work was supported by the Brain Korea 21 Project

and New Faculty Support Fund (400-20030177) of Seoul

National University, and the Research Center for Aircraft

Parts Technology (ReCAPT) of the Gyeongsang National

University.

Table 3. Comparison of the errors in two channels.

Scanning + Pencil-cut Scanning only

Axis size (mm) error (%) size (mm) error (%) size (mm) error (%) size (mm) error (%)

X 200.0 0.0 198.6 0.69 126.0 36.99 123.3 38.36

Y 200.0 0.0 197.3 1.37 202.7 1.37 186.3 6.85

Figure 11. Test part of a scanned head.

Figure 12. Micro molded rotor (ABS plastic). The diameter

of pivot is 250 mm.


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MIMS: Web-based micro machining service