Invited paper

Development of an Autonomous Machine Tool
without the Need to Prepare an NC Program
Keiichi Nakamoto1, and Keiichi Shirase2
1

Tokyo University of Agriculture and Technology, Koganei, Japan
2
Kobe University, Kobe, Japan

Abstract – An autonomous machine tool has been developed to solve fundamental issues with the current command method using
NC programs, and some key technologies for its realization have been devised. In the process planning system, various process plans
are prepared, and the appropriate process plan can be selected to achieve flexible machining operations. Digital Copy Milling,
digitizing the principle of copy milling, has opened up new possibilities for machine tool control. The NC machine tool can bedirectly controlled with the 3D CAD data of the product shape. Direct machining without the need to prepare an NC program,
adaptive control which changes the cutting conditions in accordance with the cutting load, and fault detection can be performed.
Machining shape simulator and cutting force simulator are integrated to achieve simultaneous prediction with milling operation forproviding the possibility of milling process control and fault detection.
Key Words: autonomous machine tool, computer aided process planning, real-time tool path generation, milling process simulator

1. Introduction

The Strategy module contains a Digital Copy Milling (DCM) function

As opposed to traditional NC machine tools, which machine

which generates the tool path during machining operation in real time,

according to pre-set programs, intelligent machine tools do based on

and it controls the machining process by providing feedback on thetheir own decision making. The evolution in machine tools ismachining conditions predicted by the Prediction module or measured

summarized as shown in Fig. 1 [1]. Despite the evolution of machineby the Observation module. The Prediction module has a function fortools themselves such as the handle of multiple axes and various tasks,

predicting any changes to the machining shape during machining

the NC programs which use letters and numbers in machine tools’

operation, and for estimating the cutting force. Hereinafter, the details

operating commands are being used.

of the process planning, the copy milling and the machining process

To generate NC programs, the machining sequence, tool paths, andsimulation which are the distinctive functions are explained. They do

cutting conditions all need to be determined in advance. That means

not work independently but rather by complementing each other tothat once its automatic operation has begun, no changes can be maderealize an autonomous machine tool.

to the tool path or cutting conditions, or depth of cut in particular,

Its main feature is DCM, which controls the NC machine tool by

during milling operation. Moreover, even if cutting overload orgenerating the tool path during milling operation. There have beenchatter vibration is detected during milling operation, the milling

numerous studies done on intelligent machine tools and adaptive

process control according to the situation is not possible. On the othercontrol [2], but the concept of an autonomous machine tool which

hand, it requires a great deal of time and effort to prepare safe andactively performs machining operation without the need for a NC

secure NC programs.

program is not found elsewhere.

An autonomous machine tool has been developed to solve thesefundamental issues with the current command systems using NC
programs. Also, some key technologies devised for its realization areintroduced in this paper.
2. Proposed Autonomous Machine Tool
Existing NC machine tools perform machining operation byfaithfully following the NC program in a passive manner. However,autonomous machine tools do their operations in an active manner,
based on their own decision making according to the 3D CAD data of
the product shape without the need to prepare NC programs.
A proposed autonomous machine tool is comprised four function
modules: 1. Management, 2. Strategy, 3. Prediction, and 4.
Observation. The Management module contains a process planning
function (CAPP: Computer Aided Process Planning) and determines,
in accordance with the 3D CAD data for the product shape, theFig. 1 Evolution of machine tools toward intelligent machine tools [1]milling sequence as well as the cutting tools and conditions to be used.

ISUPEN 2013
International Symposium on Ultraprecision Engineering and Nanotechnology, Tokyo, Japan, March 13, 2013.

19

Invited paper

Se lectio n ofC utti ng To ol

N um b er o f
D ivisi on P lan

D eci si on o f
C uttin g Pa ra m ete rs

Ge ne ra tio n o f
M a ch in ing P rim itiveEstim a tion of
M ach ini ng Tim e

Process planning

E xtr action of
To tal R e m ova l Volu m e

N um b er o f
M ach ini ng Fe atur eD e cision of
M ach inin g S eq ue nce
R e co gn ition of
M ach ini ng Fe atur e

Fig. 2Operation planning

In iti al sha pe &Fin ishe d sha peof w orkp iece(a) Bilateral zigzag paths(b) Contouring paths(c) Change of stepover(d) Change of cutting depth

Se lectio n of
S uita ble Pla n

Flowchart of flexible process planning system [3]Fig. 4

Advantages of real-time tool path generation

The cutting tool types used for milling are determined beforehandfor each recognized machining feature. When determining the cutting
tool types to be used, the cutting tool diameter suitable for milling themachining feature is determined on the basis of the relation between
machining feature dimensions and the cutting tool diameter held by
the machine tool. The cutting tool information held by the machine
tool has been prepared in advance.
The cutting conditions are determined using Case Based Reasoning
Fig. 3Machining primitives generated from TRV(CBR), referring to past machining cases stored in a database. With
regard to the required milling details (workpiece, cutting tool type,finishing conditions), cases with similar details are extracted from the

3. Flexible Process PlanningStudies using the concept of machining features for the automation

database. Differences from the cutting conditions in the extractedof process planning are wide-ranging. However, there are many

machining cases are reflected, and the cutting conditions arerestrictions on machining features and product shapes, and thedetermined based on those average values. The database contains

selective extraction of machining features is not taken into

1182 machining cases focusing on end milling.

consideration. In this study, the total removal volume (TRV) from theThe advantage of computer aided process planning is the possibility

material shape and the 3D CAD model of the product shape isof evaluating the machining time and costs of several process plans.

extracted. By taking the shape during milling operation as the material

In the process planning system introduced here, various process plans

shape, the TRV can be similarly extracted, even when process

can be prepared depending on the division options. The total

re-planning is required due to a machining problem, for instance.

machining time and the count of cutting tool change are predicted,

Figure 2 shows the flowchart of flexible process planning systemdeveloped here [3]. Considering the milling height in the z-axis

and the appropriate process plan can be selected to achieve flexible
machining operations.

direction, the TRV is first divided by planes parallel to the XY plane,
and then divided by planes parallel to either the YZ plane or the ZX

4. Digital Copy Milling

plane. Depending on the division sequence, various division options

For the realization of flexibility and autonomy in machining, tool

can be prepared. Machining primitives are the smallest unit inpaths need to be generated in real time, and NC machine tools need tomachining, and are obtained by dividing the extracted TRV. Examples

be directly controlled. Therefore, the Digital Copy Milling (DCM)of machining primitives generated according to the 3 division options

function, digitizing the copy milling principle, has been developed.
In copy milling, a mock-up is prepared, and when the operator

are shown in Fig. 3.
The milling sequence is determined considering to the spatial

manually copies this model operating a probe, a tool of the same

relationship between the generated machining primitives. When theshape as the probe moves simultaneously and a product of the same

milling sequence of the machining primitives has been decided, theshape as the model is produced. In DCM, a tool path is generated inmachining features can be recognized from the number of open faces

real time while the virtual probe, modeled on the computer,and the connections of the edges. The “open face” mentioned in theautomatically copies the virtual model which is a 3D CAD model ofrules indicates a surface which is in contact with the air. While milling

the product shape. As a result, the NC machine tool can be directly

operation proceeds, new open faces appear, but these are also taken

controlled with the 3D CAD data.

into consideration and used to recognize machining features.

20Figure 4 shows an example of an automatically generated tool path.

ISUPEN 2013
International Symposium on Ultraprecision Engineering and Nanotechnology, Tokyo, Japan, March 13, 2013.

Invited paper(a) 3D CAD model (STL format)

Fig. 6

Cutter-workpiece engagement(b) Finished surface
Fig. 5

Direct machining using Digital Copy Milling (DCM)

The feature that distinguishes it most clearly from the current
command method using NC program is the fact that stepover or theaxial depth of cut can be changed during milling operation. As
illustrated in Figs. 4(b), (c), (d), even when cutting conditions areFig. 7

Example of overhung milling shape simulation

changed, a new tool path is autonomously generated from that point
up to the end of the milling operation. Studies have so far verified thecomparison test with direct machining, the current method for which

effectiveness of DCM as follows [4]:an NC program is required was performed. Where the same shape was1㸧 As the NC machine tool is controlled with the 3D CAD data on

machined with exactly the same machining equipment, the operator

the product shape, no NC program needs to be prepared.

required approximately one hour for preparing an NC program with2㸧 Autonomous machining becomes possible based on 3D CAD

CAM, and the total machining time came to 98 min. and 12 sec. The

data of the product shape, through integration of the process

total machining time for direct machining was longer than for theplanning and the operation planning functions.

current method, but for single item machining, such as prototyping or

3㸧 As tool paths are generated during milling, adaptive control isrealized for cutting conditions under cutting load feedback, andmold machining, comprehensive lead time including milling
preparation time can be reduced in direct machining.

tool breakage is avoided.
When using the DCM, and after preparing 3D CAD data for the

5. Machining Process Simulation

product shape and the material shape, machining can start

An example of milling shape simulation using a voxel, a model

immediately without generating an NC program in CAM (hereinafterbuilt up from minute 3D cubes, can be seen in Fig. 6. Though shapecalled direct machining). Direct machining requires process planning

representation and shape processing are simple, the cubes needed towhere milling sequences, cutting tools used and cutting conditions arerepresent detailed shapes must be small, and a vast amount of memory

automatically determined. Therefore, a commercial process planningis needed for the representation of shapes. Therefore, a milling shape

software package called Mill-Plan, developed by Toyota Central R&D

simulator that requires only small amount of memory since represents

Labs. Inc., has been integrated with the DCM. By performing direct

shapes in hierarchically structured cubes, representing only themachining, its effectiveness is verified.

detailed parts of the shape in small cubes has been developed. Voxel

In this case, process information (e.g. milling operation sequence,models can also express shapes with an overhang, such as the impeller

milling area for each operation, cutting tools used, and cutting

shown in Fig. 7, and can be applied to milling shape simulations forconditions) is automatically decided by Mill-Plan from 3D CAD data

5-axis machining.

of the product and material shapes. In DCM, the tool path is generated

To predict cutting force in this study, an instantaneous cutting force

in real time based on this process information, and machining

model with which realistic simulation results can be obtained

operation can start immediately by sending NC commands generated

relatively easily was used. With the instantaneous cutting force model,

by the DCM to the machine tool. For that reason, process planning orchanges in cutting force during one tool revolution can be calculated

NC programming in CAM performed by the operator are unnecessary,from the actual cutting thickness obtained from the geometricaland time and effort spent at the machining preparatory stage isrelation between the tool and the workpiece as well as from thesignificantly reduced.

cutting coefficients obtained experimentally. If cutting conditions are

The machining results are shown in Fig. 5. CAM operation time

obtained, such as the radial and axial depths of cut required for thewas 0 min., total machining time came to 113 min. and 25 sec., andmilling shape simulator’s detection, then the cutting force can be

there were no particular issues during milling operation. As apredicted in real time.

ISUPEN 2013
International Symposium on Ultraprecision Engineering and Nanotechnology, Tokyo, Japan, March 13, 2013.

21

Invited paperof the milling process control and fault detection are effectivefunctions to realize an autonomous and intelligent machine tool.
6. Conclusion
In this paper, an autonomous machine tool have been developed tosolve fundamental issues with the current command method using NC
programs, and some key technologies for its realization have beenintroduced. DCM in particular, digitizing the principle of copy milling,Fig. 8

Simulated milling shape and cutting forces

has opened up new possibilities for machine tool control. The
machining process simulator with integrated
machining shape simulator and cutting force
simulator, moreover, provides new functions,
includingmachiningreflecting

prediction

process
results

controland

fault

detection in the machining operation.
Specifically, through DCM which allows(a) Simulated cutting forces

for sequential control of NC machine tools
while generating tool paths during milling
operation based on 3D CAD data of theproduct shape, the following functions
which are not possible with the current
command method using NC programs havebeen performed: 1. direct machining without
the need to prepare an NC program before
milling operation, 2. adaptive control whichchanges the cutting conditions in accordance
withFig. 9(b) Measured cutting forces
Comparison between simulated and measured cutting forces in cutting test

thecutting

load

duringmilling

operation, 3. fault detection in the cutting
load and avoiding tool breakages, and 4.
flexible

processplanningfor

theFigure 8 shows an example of a milling process simulator withdetermination of milling sequence and cutting tools used in

integrated milling shape and cutting force simulators [5]. The image

accordance with changes in production planning or the production

on the left shows the change in the milling shape, while the instantlysituation.

calculated cutting force waveforms of that instant are displayed on theright hand side. The milling shape simulator detects the interferenceof the tool and the workpiece, calculates the radial and axial depths ofcut, and makes it clear that the cutting force during one tool
revolution has been predicted by the cutting force simulator.
Figure 9 shows the comparisons and verifications of the cutting
forces predicted by the milling process simulator and the actually
measured cutting forces. Omitting the details of the cutting conditions,the end mill machining was performed in the following sequence (Rd:radial depth of cut, Ad: axial depth of cut):
1. Rd: 5 mm fixed, Ad: 6 mm fixed, up cutting
2. Rd: 0 Ѝ 10 mm gradual change, Ad: 6 mm fixed, down cutting
3. Rd: 0 Ѝ 10 mm gradual change, Ad: 6 mm fixed, up cutting
4. Rd: 5 mm fixed, Ad: 6 Ѝ 0 mm gradual change, down cutting
5. Rd: 5 mm fixed, Ad: 6 Ѝ0 mm gradual change, up cutting
The cutting tool diameter was 10 mm, and the workpiece was
FC250 cast iron. The predicted cutting force and the measured one areconsistent for both the difference of cutting direction and the changes
in depth of cut. [6]Simultaneous cutting force prediction with milling operation
provides the possibility of milling process control and fault detection
by comparing the measured cutting force with the predicted one. Both22References[1] T. Moriwaki, Intelligent Machine tools, Journal of the Japan
Society of Mechanical Engineers, Vol. 96, No. 901, (1993), pp.
1010-1014. (in Japanese)[2] G. Proschow, Y. Altintas, F. Javane, Y. Koren, et. al., Open
Controller Architecture – Past, Present and Future, Annals of theCIRP, Vol. 50, Iss. 2, (2001), pp. 463-470.
[3] K. Nakamoto, K. Shirase, H. Wakamatsu, A. Tsumaya, E. Arai,
Automatic Production Planning System to Achieve Flexible
Direct Machining, JSME International Journal Series C, Vol. 47,No. 1, (2004), pp. 136-143.
[4] K. Shirase, K. Nakamoto, Direct Machining Operation Performed
by Autonomous NC Machine Tool Controlled by Digital Copy
Milling Concept, Transactions of the Japan Society of
Mechanical Engineers Series C, Vol. 74, No. 743, (2008), pp.
1901-1906. (in Japanese)[5] T. Shimada, K. Nakamoto, K. Shirase, Machining Strategy toAdapt Cutting Conditions under Digital Copy Milling Concept,Journal of Advanced Mechanical Design, Systems, andManufacturing, Vol. 4, No. 5, (2010), pp. 924-935.
[6] K. Nakamoto, T. Kouno, T. Koyama, T. Sakaguchi, K. Shirase,
Development of Virtual Machining Simulator by Using Voxel
Model, Journal of the Japan Society for Precision Engineering,Vol. 74, No. 12, (2008), pp. 1308-1312. (in Japanese)

ISUPEN 2013
International Symposium on Ultraprecision Engineering and Nanotechnology, Tokyo, Japan, March 13, 2013.