Dynamic behavior (eigenfrequency)
Vibrations, tool wear, surface quality and machining capacity: Eigenfrequency analyses help
with optimization
Overview
Analysis with and without standard load
Example: T1-507510 fix, excitation only in the X-direction
1
EA
+M
2
4
TF
T1
Every object has several natural frequencies (so-called eigenfrequencies)
that depend on the object's shape,
mass and material. If the machining
frequency matches an eigenfrequency,
e.g. that of a rotary table, the result
is chattering or whistling noises. A
vertical machining center has its first
eigenfrequency in the range of about
100 Hz. It is important that the machining frequency does not match the
eigenfrequency.
Amplification /dB
Background
NEW
Frequency / Hz
MOT / KAB
WMS / CNC
1
3
3
RFX/LFX
The eigenfrequencies were determined using FEM modal analyses.
The accompanying BODE diagram
shows the result of the HARMONIC
analysis. The first 20 vibration modes
and eigenfrequencies of all rotary
tables listed below were determined.
Experience has shown the mode 1
and mode 2 are the most important in
actual practice. These values can be
found in the table below.
Example: T1-507510 fix
Workpiece
clamping
Dynamic analysis
Analysis of the operating directions X, Y and Z
Frequency / Hz
The lowest eigenfrequency is usually the most critical one. The above illustration shows clearly that this
frequency is excited in the X-direction. For this reason, machining that could cause vibration should be
performed in the Y- or Z-direction. The first two eigenfrequencies are given in the table below.
Example of machining frequency calculation
Angular milling cutter ø40 mm, number of teeth 4, speed 1'900 min-1 = 4 x 1'900 = 127 Hz
60
General
accessories
If the frequency of the machining
process starts to increase, change the
following:
+ Tool speed
+ Number of teeth on tool
+ Machining strategy
+ Workpiece orientation
Amplification /dB
Optimization options
T2
/ T3
The above peak values 1–4 can be found in the table below. This example shows clearly how a
change in the standard load can shift the eigenfrequencies. This shift also occurs during machining, of
course, since the weight of the workpiece is changing.
Eigenfrequency of mode 1 and 2 (approximate values)
Condition: Rotary table is mounted in the specified manner and both axes are clamped with 6 bar compressed air.
without standard load
Mode 1
fix
T1-507510
T1-510520
T1-520520
TF-507510
TF-510510
TF-510520
2
0°
292
267
181
198
152
203
90°
310
252
177
207
164
214
varioX
0°
90°
224
224
203
184
139
132
126
122
106
101
137
137
with standard load
Mode 2
fix
0°
4 397
309
234
210
169
222
90°
372
341
265
218
168
234
varioX
0°
90°
284
269
229
244
180
187
138
147
107
114
159
158
Mode 1
fix
0°
1 197
158
109
139
121
117
90°
217
166
118
147
124
136
varioX
0°
90°
178
188
137
136
95
96
98
97
87
85
89
94
Mode 2
fix
3
0°
265
182
141
151
124
140
90°
261
197
151
169
139
152
Technology /
engineering
Hz
varioX
0°
90°
237
230
170
179
134
140
113
121
91
99
110
114
83