Lesson of
Industrial Engineering ONLINE Course
Machining Science - Machining Productivity Science
https://nraoiekc.blogspot.com/2020/07/machining-science-machining.html
Machining Dynamics
Adopted from Metal Cutting Theory by Stephenson and Agapiou
During cutting, high forces are used leading to deflections of the structural components of the system and to vibrations (relative motion between the tool and workpiece). These vibrations should be minimized because they degrade machining accuracy and the machined surface texture. They also lead to chatter, which can cause accelerated tool wear and breakage, accelerated machine tool wear, and damage to the machine tool and part.
Machine tools are composed of several components and therefore can be considered multi-mass
vibrators.
The dynamic performance (or stiffness) of a linear mechanical system can be described by a set of transfer functions (TFs) or by the resonance frequencies and their associated displacements at the different points, which are called associated modes of vibration. Several methods have been proposed to examine the dynamic behavior and system stability, including: (1) the s-plane approach, (2) the frequency (j) plane approach, and (3) the time-domain
approach.
Machine tools are subject to three basic types of vibration: free, forced, and self-excited vibrations.
Free or natural vibrations occur when the stable system is displaced from its equilibrium position by shock; in this case, the system will vibrate and return to its original position in a manner dictated by its structural characteristics. Machine tools are designed for high stiffness and this type of vibration seldom causes practical problems.
A forced vibration occurs when a dynamic exciting force is applied to the structure. Dynamic forces are induced by one of the following sources:
(1) alternating cutting forces such as those induced by (1a) inhomogeneities in the workpiece material (i.e., hard spots, cast surfaces, etc.), (1b) built-up edge (which forms and breaks off periodically), (1c) cutting forces periodically varying due to changes in the chip cross section, and (1d) force variations in interrupted cutting (i.e., in milling or turning a nonround or slotted part);
(2) internal source of vibrations, such as (2a) disturbances in the workpiece and cutting tool drives (caused by worn components, i.e., bearing faults, defects in gears, and instability of the spindle or slides), (2b) out of balance forces (rotating unbalanced members, i.e., masses in the spindle or transmission), (2c) dynamic loads generated by the acceleration/deceleration or reversal of motion of massive moving components; and
(3) external disturbances transmitted by the machine foundation.
“Self-excited vibration” or “chatter” is induced by variations in the cutting forces (caused by changes in the cutting velocity or chip cross section), which increase in amplitude over time due to closed loop regenerative effects.
Chatter prediction actually means the prediction of the stability limits of the machining process.
There are two prevailing techniques for this purpose: “limit chip analysis” and the “stability chart method.”
VIBRATION CONTROL
The dynamic behavior of a machining system can be improved by reducing the intensity of the sources of vibration for the machine tool, toolholder, and cutting tool. Several sources, primarily stiffness and damping, have a significant impact on forced and self-excited vibrations.
The stability of the cutting process against vibration and chatter can be improved by several approaches, which can be categorized as methods for selecting cutting parameters in the stable zone within the stability lobe diagram and methods of avoiding chatter by changing the system behavior and modifying the stability of the system:
1. Optimizing the design of the machine tool using both analytical and experimental methods
to provide maximum static and dynamic stiffness
2. Selecting the proper bearing types, configurations, and installation geometry to provide
maximum stiffness and damping
3. Selecting the best toolholder device for the particular tool or application, and reducing the
tool overhang length
Apart from the above the following approaches are discussed in more detail.
1. Increasing the part stiffness
2. Isolating the system from vibration forces and using active or passive dynamic absorbers
3. Increasing the effective structural damping and using tuned mass vibration dampers
4. Selecting special cutting tool geometries; minimize the length of the cutting edge(s) in contact with the part; Reduce the nosing radius of the insert; Increasing the rake angle at the cutting edge; For milling, reducing the number of teeth on the cutter
5. Selecting optimum cutting conditions, especially the spindle speed; using high-speed milling to machine between stability lobes. Some commercial software packages simplify testing and offer automatic predictions of the stability lobe diagram. Reducing the depth of cut to perform machining under the stability limit. If the wavelength of chatter marks is small, increasing process damping by reducing the surface cutting speed. For forced vibration, decreasing cutting force, increasing part stiffness, changing the tooth passing frequency to be far away from resonance frequency of structure. For self-excited vibration, decreasing doc and number of teeth in cut or changing the tooth
passing frequency to match resonance frequency.
Stiffness Improvement
Isolation
Damping and Dynamic Absorption
Tool Design
Variation of Process Parameters
Stiffness Improvement
The static stiffness, the ratio of the deflection to the applied static force at the point of application,
can be measured for all three coordinate axes of the machine tool. The main contributors to deformation between tool and workpiece are the contact deformations in movable and stationary joints between components of the machine structure and fixture, the toolholder–spindle interface, and
tool–toolholder interface.
The stiffness of the structure is determined primarily by the stiffness of the most flexible component in the loading path. This component should be reinforced to enhance stiffness.
The overall stiffness can be improved by placing the tool and workpiece near the main
column, by using rigid tools, toolholders, and clamps, by using rigid supports and clamps in the fixture, and by securely clamping all machine parts that do not move with respect to each other. Stiff
foundations or well-damped mountings are required.
The bearing design has a strong influence on the static and dynamic behavior of machine tools.
Reducing the tool–workpiece compliance is not always possible in practice, and all other
approaches, should be examined to increase stiffness before productivity is sacrificed by reducing the
depth of cut to ensure stable operation.
The static and dynamic analysis of the cutting tool and toolholder is important. An FEM of an end mill or boring tool can be used to perform the dynamic (modal) analysis to determine the natural frequencies of more complex tool bodies. Several approaches could be used to evaluate and improve the vibration of end mill or boring tools by increasing rigidity.
Isolation
Vibration isolation is the reduction of vibration transmission from one structure to another via some
elastic device; it is an important and common component of vibration control. Vibration isolation
materials, such as rubber compression pads, metal springs, and inertia blocks, may be used [6,163].
Rubber is useful in both shear and compression; it is generally used to prevent transmission of vibrations in the 5–50 Hz frequency range. Metal springs are used for low frequencies (>1.5 Hz). Inertia blocks add substantial mass to a system, reducing the mounted natural frequency of the system
and unwanted rocking motions, and minimizing alignment errors through an increase in inherent
stiffness.
Damping and Dynamic Absorption
The overall damping capacity of a structure depends on the damping capacity of its individual
components and more significantly on the damping associated with joints between components (i.e.,
slides and bolted joints. A significant damping increase can be achieved by filling internal cavities of the frame components with special materials (i.e., replicated internal viscous dampers).
Resonant structural vibrations can be reduced by applying a dynamic absorber or layers of damping
material on the surfaces of the structure. Increasing the damping ratio from 0.02 to 0.2 of a lathe having a resonant frequency of 60 Hz resulted in increase of the asymptotic borderline of stability from 0.04 to 0.48 mm, which corresponds to an increase of a factor of 12 in the effective cutting stiffness.
A dynamic absorber or tuned mass damper is an alternative form of vibration control. It consists
of a secondary mass attached to the primary vibrating component via a spring, which can be either
damped or undamped. This secondary mass oscillates out of phase with the main mass and applies
an inertial force (via the spring), which opposes the main mass. For maximum effectiveness, the
natural frequency of the vibration absorber is tuned to match the frequency of the exciting force.
Auxiliary mass dampers can be used on machine columns, spindles, and rams.
Tuned dynamic vibration absorbers have been used with considerable success in milling and boring applications. A tunable tool provides a controlled means of adjusting the dynamic
characteristics of the tool in a particular frequency range. A tuning system minimizes the broad-band
dynamic response of the tool without requiring cutting tests or trial-and-error tuning. A very common
vibration damper used in boring bars consists of an inertial weight or a spring-mounted lead slug fitted
into a hole bored into the end of boring bar; bars so equipped are often called antivibration boring bars.
The weight helps damp bar motion and prevents chatter. The chatter resistance of boring bars can also
be increased by using different materials in the bar structure. Impact
dampers can be also installed in the toolholder, spindle, or ram to absorb vibration energy.
Active control of structures can be also used to suppress vibration and chatter in machining.
Actively controlled dynamic absorbers use sensors and force actuators together in a closed-loop
control system to alter the dynamic characteristics of a structure so that it possesses greater damping and stiffness characteristics. Accurate system identification (both in terms of sufficient
model order and parameter accuracy), hybrid high-speed control, and proper power force actuators
are essential elements of any successful structural or tool vibration control system.
Tool Design
Reductions of both forced and self-excited vibrations for multiple cutting edges tool structures (such as
milling cutters, end-mills, and reamers) have been achieved by employing non-standard cutting tools
such as unequally spaced cutting edges (nonuniform tooth pitch), variable axial rake, variable helix
cutting edges (alternating helix on adjacent flutes), lip height error, or serrated and undulated edges. Nonuniform tooth pitch cutters are often called white nose cutters. All these tool design variations increase stability by reducing the effective tooth passing frequency and disturbing the regeneration of surface waviness (the phase between the inner and outer modulations). Their effects are most pronounced over particular ranges of cutting speeds which depend on the cutter geometry (L/D ratio, lead angle, helix angle, etc.), the configuration of the particular part feature to be machined, and the cutter path (up milling vs. down milling, end-mill diameter vs. diameter of circular interpolation, etc.). Alternating the helix increases pitch variations along the axial doc, improving both the dynamic performance and operative speed range of the cutter. An optimal distribution of spacings or helix angle variations between the teeth can be found, but the non-uniform pitch or helix cutters can be used within a limited speed range constrained by the dominant structural mode.
Sharp tools are more likely to chatter than slightly blunted tools. Therefore, a lightly honed cutting edge can be used to avoid chatter. Negative rakes and small clearance angles minimize chatter
occurrence due to process damping. It has been demonstrated that reduction in the relief angle and
an increase in flank wear of the cutting edge increases process damping. In general, it
is important to use the smallest possible tool nose radius that gives acceptable tool life, because a
small nose radius can alleviate the regenerative effect. Finally, combination tools which generate
complex surfaces should incorporate design features which inhibit chatter. For example, combined
reaming, countering, and chamfering tools, require a cylindrical land not only along the reaming and
counterboring margin sections, but also a narrow land along the chamfer section to prevent chatter on
the chamfered surface, which could occur with a conventional sharp chamfering design.
Variation of Process Parameters
Cutting conditions, especially the cutting speed, directly affect chatter generation. At a given spindle speed, the widths and depths of cut are often limited by the chatter threshold. As indicated in
the stability lobe diagram a small increase or decrease in speed may stabilize the cutting process. Small changes in the cutting speed are particularly effective in increasing stability in milling operations. In addition, speed and feed affect process damping.
Automatic regulation of the spindle speed for stable cutting can be used in CNC machine tools.
If it is known that there is a gap in the stability lobe diagram, then that frequency is used while
increasing the depth of cut until the maximum machine/spindle power is reached. This approach is currently only applicable to uniform pitch cutters. Chatter suppression systems employing this principle are discussed in the next section. Case studies indicate that the use of spindle speed variations provides more flexibility than the use of variable pitch cutters.
Active Vibration Control
1. Chase Control Method
2. Predictive Control
3. Multivariable Control Schemes
4. Harmonizer System
5. Forecasting Control
Application Examples
Optimize Machining with Active Vibration Control
A retrofittable, accelerometer-based system cuts tool-head vibration to maximize MRR.
AUG 28, 2019
An accelerometer-based active vibration-control system for CNC machine tools is designed to improve machining productivity by maximizing material removal rate (MRR), by eliminating tool-head vibration by NUM.
There is Tool Centre Point (TCP) vibration due to the various vibration modes of a machine tool's mechanical structure. Using NUM’s new active vibration-control system it’s possible to measure and dynamically alter the TCP acceleration in each of the main X, Y and Z axis directions, and so to damp the vibration very accurately.
https://www.americanmachinist.com/machining-cutting/media-gallery/21903157/optimize-machining-with-active-vibration-control
Behind the Scenes: Harmonizer Optimizes Machine Performance
Harmonizer
8/26/2013
with example
https://www.protomatic.com/about-us/blog/2013/08/behind-scenes-harmonizer-optimizes-machine-performance
The science of milling sounds
Author
Dr. Scott Smith
Published February 1, 2013
https://www.ctemag.com/news/articles/science-milling-sounds
3/15/2000
Find The Right Speed For Chatter-Free Milling
https://www.mmsonline.com/articles/find-the-right-speed-for-chatter-free-milling
Machines highlighting vibration control
June 25, 2019
Makino F5 Pro 6 vertical machining centre
Makino introduces the F5 Pro6 vertical machining center designed to provide stiffness and rigidity for chatter-free cutting, agility for high-speed/hard-milling and accuracies for tight tolerance blends and matches typical of complex, 3D contoured geometry.
References
Active Vibration Control
Method of controlling chatter in a machine tool (Cited in Stephen-Agapiou)
1990-12-06
Application filed by Manufacturing Labs Inc
https://patents.google.com/patent/US5170358A/en
Teager-based method and system for predicting limit cycle oscillations and control method and system utilizing same
1996-11-07
Priority to US08/745,014
https://patents.google.com/patent/US6004017
Device for stable speed determination in machining
1999-03-15
Priority to US12444199P
https://patents.google.com/patent/US6349600
Machine Tool Vibrations and Cutting Dynamics
Brandon C. Gegg, C. Steve Suh, Albert C. J. Luo
Springer Science & Business Media, 30-May-2011 - Technology & Engineering - 179 pages
“Machine Tool Vibrations and Cutting Dynamics” covers the fundamentals of cutting dynamics from the perspective of discontinuous systems theory. It shows the reader how to use coupling, interaction, and different cutting states to mitigate machining instability and enable better machine tool design. Among the topics discussed are; underlying dynamics of cutting and interruptions in cutting motions; the operation of the machine-tool systems over a broad range of operating conditions with minimal vibration and the need for high precision, high yield micro- and nano-machining.
https://books.google.co.in/books?id=C_eCeCsQps8C
Implementation of an Algorithm to Prevent Chatter Vibration in a CNC System
Marcin Jasiewicz and Karol Miądlicki
2019
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6803880/
Updated on 16 Sep 2020
