This document introduces a high speed machining algorithm called look-ahead interpolation that improves machining speeds for CNC machine tools. It does this without requiring additional hardware by analyzing upcoming small line blocks to generate smooth feedrate profiles. Simulation and experimental results showed the algorithm increased machining speeds by up to 6 times compared to conventional methods. The algorithm works by reading multiple upcoming blocks, calculating maximum feedrates, and interpolating points to generate minimum command distances every 8ms for axis control.

1. A High Speed Machining Algorithm for CNC Machine Tools
Guk-ChanHan, Dong-I1 Kim, Hyo-Gyu Kim
KibeomNam, Byeong-Kap Choi and Sung-kwunKim.
Mechatronics Engineering Group Production EngineeringCenter
SamsungElectronics Co., Ltd.
SuwonCity, KOREA
Abstract - This paper introduces a high speed
machining algorithm based on look-ahead
interpolation technique for the machining of 3D
surfaceobtained by CAD/CAM system. Generally,
specification of CNC system for high speed
machining requires an additional hardware and it
burdens end users with additional cost. The
purpose of this research is to develop the low cost
CNC controller having the high speed machining
ability.The experimental results demonstrate that
the proposed algorithm improves the machining
speed in the program consisting of small line
blocks without any hardware support. To
investigate the performance of the introduced
algorithm, we implemented it to the machining
center equipped with Samsung SNC that consists
of a 64bit main processor and a 32bit floating
point DSP as the motion control CPU.
I. TNTRODUCTION
In the CNC machine tool, the acceleratiod
deceleration profile of each axis of motion is
determined by the commanded traveling speed and
the capability in acceleration. In the conventional
method, acceleratioddeceleration is performed on
each unit line block and the unit line blocks featuring
acceleratioddeceleration are overridden. This
method causes difficulty in smoothing continuous
unit line blocks which are shorter than the minimum
distance required for the acceleration /deceleration.
Hereafter, those blocks are defined as the small line
blocks. The small line blocks in machining program
results in speed reduction in machining.
Generally, the machining program generated
from the CAD/CAM system appears as the
continuous path data composed of such small line
blocks. Incorporating the acceleratioddeceleration
characteristic into consecutive small line blocks in
high speed requires controlling those blocks above
the speed limit. This can be realized by the contour
analysis of the path to be machined. This means that
hundreds of small line blocks should be read and
analyzed in advance to obtain the smooth continuous
motion in the machine tool. According as machining
speed increases, the machine tool has low
acceleration capability, or the length of the unit line
block is short,the number of the block to be read and
analyzed in advance increases.
The following two figures show the concept of
the look-ahead interpolation algorithm. Fig. 1 shows
an example of the path to be machined, which is
composed of consecutive small line blocks. Fig. 2
shows the comparison between each feedrate profile
of consecutive small line blocks for Fig. 1. Three
different feedrate curves obtained from the block
oriented acceleratioddeceleration,the application of
feedrate overriding to acceleratioddeceleration, and
the proposed method based on look-ahead
interpolation algorithm respectively. From Fig.2, we
can see that the fastest machining speed for
consecutive small line blocks is obtained in case of
the look-ahead interpolation algorithm. Fig. 3 shows
the detailed feedrateprofiles versus time for the look-
ahead interpolationalgorithm.
In this paper, an efficient look-ahead algorithm
is introducedto realize the smooth high speed motion
control of the machine tool for the consecutive small
line blocks without any hardware support. Some
simulation and experimental results demonstrated
that the proposed algorithm improves machining
speed considerably.
Fig. 1 Consecutive small line blocks
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2. override
‘IAfter
I After look-ahead
i Block oriented
Fig.2 Comparison of feedrate profiles for
consecutive small line blocks
Speed
Fig. 3 Feedrate profile versus time by look-
ahead algorithm
11. LOOK-AHEAD INTERPOLATION
ALGORITHM
1.Calculationof thefeedrate
As CNC developed, a buffer was added to allow the
control to read a block of data before it was ready to be
excuted, thus speeding up operation. This buffer gave a
sort of look-ahead to anticipate the next move and
minimize dwell time before the execution of each move.
In the Iook-ahead interpolation, N line blocks must be
read in advance and stored in the buffers The data of the
blocks are stored in N buffers continuously during
operation of the machine tool. The purpose of this
approach is to analyze the data in the buffers and then
generate the maximum and smooth feedrate profile. The
feedrate is calculated at every N-th block when N buffers
become filled with data. When the current moving block is
between A and B, the feedrate becomes the limit speed Fk
for the total length L, between A and B as in Fig. 4. If L k is
less than the minimum distance required for
acceleratioddeceleration, the maximum speed is not
reached. As the number of the buffer increases, the
maximum speed becomes larger.
Current Moving direction
block of Buffer
I r+I
V
... i i+l ... li+N I I ... i+2N -1 I
I
/ I
Fig. 4 Feedrate calculation and buffering
2. Interpolation
The interpolator generates the minimum command
distance from the consecutive block data and send it to
each axis in every interruption time. Usually, 8ms
interruption time is widely used in CNC controller. Now,
we describe how the interpolation data updated every 8ms
period is calculated after the feedrate calculation in the
previous step. Fig.5 shows the concept, where the small
line blocks are denoted as a dotted line.
The next interpolation point P,, is determined from
the current one P, as follows.
where PQ is the unit interpolation distance in 8ms,
the path length on consecutive blocks equivalent
to the unit interpolation distance. The unit interpolation
distance is obtained from commanded feedrate and
interruption time interval. The next interpolation point P,,
is obtained by projecting the point Q, to the path
composed of consecutive small iine blocks. As shown in
Fig. 5 the straight distance -becomes the actual
interpolation data and the relationship between unit
interpolation distance and actual interpolation distance is
expressed as follows.
n n
n
‘n‘n -I.1
‘n‘n +1
- -
‘nQn ’‘n‘n +1
Q.
Fig. 5 Interpolation method for consecutive
small line blocks
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3. From the above relationship (2), the actual feedrate
for the commanded F, is formulated as follows.
1000
BOO
-C
E
‘E 600
E
e
m
I
d 400
2 200
0 -
(3)
-
-
-
-
-
The minimum number of blocks, which is necessary
to prevent feedrate dropping compare to the commanded
value, can be set in case of the feedrate, the unit
interpolation distance and the average size of small line
blocks are known. When the small line blocks of 0.1 mm
are inputted continuously, the minimum number of blocks
is eight, which is to be processed by interpreter within the 8
ms interruption time to reach the command feedrate of
6000 mdmin. Since the more the number of blocks to be
processed by interpreter, the smaller the size of the small
line blocks to be processed by interpolator equipped with
the look-ahead algorithm, the enhancement of the
interpreter’s processing speed is the key point.
3) Acceleratioddeceleration
If signal x[n] is inputted into the filter featuring the
impulse response, output y[n] is obtained by convolution
operation between h[n] and x[n] from the digital filter
theory as follows.
where, x[n] is the interpolation data, h[n] the impulse
response. When Ch[k] = I, a linear-type
acceleratioddeceleration pattern is obtained. Feedrate
startsto decelerate from the moment ofx[n] =O inthe filter.
And a s-type acceleratioddeceleration pattern can be
obtained by convolution operation between x[n] and h[n]
again. According to the path deviation analysis in the
circular interpolation, the magnitude of the path error
reaches two times when a linear-type
acceleratioddeceleration pattern is adapted in comparison
with a s-type acceleratioddeceleration pattern.
Acceleratioddeceleration after interpolation
smoothes the discontinuous tool path and guarantees the
minimum machining error for the path to be machined.
Also, it enables the tool to machine the comer with the
commanded speed and the minimum permissible error.
Otherwise, to machine the comer precisely, the tool must
reduce the speed to pass the intersection of the comer.
Moreover, the acceleratioddeceleration reduces vibration
and jerk in machining the discontinuous path in the
machine tool.
Fig.6 shows the respective trajectories and path
deviation around a comer of the commanded path after
interpolation and after acceleratioddeceleration after
interpolation. In Fig.6, the point A represents the
intersection of the comer. Fig. 7 shows a part of the
Commanded path and that after interpolation and
acceleratioddeceleration. The commanded path after
acceleratioddeceleration is resolved to generate the
commandsto the servo drive of each axis of motion of the
machine tool and the spindle drive. The feedrate for Fig.7
appears as in Fig.8.
Before After
acceleratioddeceleration acceleratioddeceleratio
after interpolation n after interpolation
Fig. 6 Trajectories and path deviation for the
interpolatedpath after acceleratioddeceleration
around comer
0.0 B
l S . l . t . C . 9 . l
X axis
0 1 2 3 4 5
Fig. 7 The commanded path after
acceleratioddeceleration
1 Feedrate after interpolation
acceleratioddeceleration D 1
l ~ . , . ~ . ~ . , . , . ,
0 10 20 30 40 50 80
Time, [ x 0.008 msec 1
Fig. 8 The feedrate with respect to time
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4. 111. SIMULATIONAND
EXPERIMENTALRESULTS
From these two simulation results, we can see that the
number of the buffers must increase as the feedrate
increases to machine the desired path with good tracking
performance.
To evaluate the performance and efficiency of the
proposed look-ahead interpolation algorithm, various
machining tests were performed. The testing was
performed on Samsung Aerospace LCV40 machining
center equipted with Samsung SNC controller as shown in
Fig. 11. Samsung SNC controller consists of a Pentium PC
166MHzas a main processor, a 32bit floating point DSP as
the motion control CPU, 16hbytemain memory and flesh
memory of high storage memory capacity.
In this experiment, we performed machining test by
two ways: normal interpolation method and look-ahead
interpolation method. The former relies on the
acceleratioddeceleration before interpolation with block
oriented feedrate override and the latter relies on the
acceleratioddeceleration after interpolation.
Fig. 12 shows the photograph of machined workpiece
by using the proposed look-ahead interpolation algorithm.
In this experiment, the spindle speed of 4000rpm was used,
feedrate of 4000mm/min was used and tool diameter of
8mm was used. And the used material was chemical wood.
The elapsed machining time was reduced in the case
of look-ahead algorithm, 41 minutes, compared to the case
of normal interpolation algorithm, 244 minutes. There was
a considerable enhancement of machining speed about by
six time after adapting the proposed high speed machining
algorithm. The experimental results are shown to agree
well with the simulation results.
Now, we present the simulation result for the look-
ahead interpolation algorithm for the path composed of the
continuous small line blocks. A sinusoidal waveform is
used, which consists of 360 small line blocks and each
length of the small line block is between O.Olmm and
0.lmm.
Fig.9 shows the feedrate profiles with respect to the
number of the buffers at the feedrate2,000 mmlmin. While
the average feedrate is 500 mdmin at 20 buffers, it
approaches the commanded value at 200 buffers.
Meanwhile, Fig.10 shows the feedrate profiles with respect
to the number of the buffers for the same path at the
commanded feedrate 5,000 mdmin. At 200 buffers, the
commanded feedrate is obtained. The larger feedrate
requires the more buffers.
-50 0 50 100 150 200 250 300 350 400 450 500 550 600 650 700 750
Time. [ x O.OO8sec 1
Fig. 9 Feedrate profile with respect to buffer size
with command feedrate: 2000mdmin, small line
blocks: 0.001 -0.01.
-c 3000 -
E . .
2 2500 -
E
- 2000 -as-
E! 1500 -
1000 -
500 -
0 -
.-
-
w
50
0 50 100 150 200 250 300
Time, [ x O.OO8sec 1
Fig. 10. Feedrate profile with respect to buffer
size with command feedrate: 5000mm/min,
small line blocks: 0.001 -0.01.
Fig. 11 Samsung Aerospace LCV40machining
center equipted with Samsung SNC controller
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5. REFERENCES
Fig. 12Part milledby look-ahead algorithm
IV. CONCLUSION
Generally, the machining program generated from the
CAD/CAM system appears as the continuous path data
composed of small line blocks, which are shorter than the
minimum distance required for acceleratioddeceleration.
To realize high speed machining for this case in the CNC
machine tool, a look-ahead interpolation algorithm is
required. In the look-ahead algorithm, hundreds of small
line blocks should be read and analyzed in advance to
obtain the smooth continuous motion of each axis of
motion in the CNC machine tool.
In general, some company’s CNC controllers adapt
various kind of look-ahead algorithms as an optional
function and require an additional hardware support.
Through this investigation, we can be realized the high
speed machining performance with the aid of an efficient
look-ahead algorithm, which improves the machining
speed in the NC program consisting of small line blocks
without additional change of hardware specification.
The proposed algorithm is applied to the machining
center equipped with Samsung SNC. According to the
experimental results, it proved that the proposed algorithm
enhanced machining speed by sixtimes in comparison with
the conventional block oriented acceleratioddeceleration
feedrate override method. And the proposed algorithm can
be also improved the surface roughness with the aid of
smooth feedrate profiling ability which is a typical
characteristic of the acceleratioddeceleration after
interpolation method.
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3. Toddy Schuett, A Closer Look At Look-Ahead,
httD://www.mmsanl&39603.htmlin .
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