Machine Rigidity - Industrial Engineering and Productivity Aspects

Lesson 59 of Industrial Engineering ONLINE Course.

Lesson 16 of Process Industrial Engineering FREE ONLINE Course (Module)

Machine Rigidity is the first mentioned issue in "5 Essential Machine Features for Enhanced Productivity" according to an article in Modern Machine Sop Online article. The five features highlighted are:

1. Machine Rigidity, 2. High-Performance Spindles, 3. Rapid Chip to Chip cycle, 4. Unattended Reliability and 5. Integration Features with other Automated Systems.

Productivity Science of Machining indicates the factors responsible for increase in productivity or decrease in productivity.

In study of the current state of productivity science in various elements of machining, industrial engineers have to remember the initial consolidation of the topic done by F.W. Taylor

IE Research by Taylor - Productivity of Machining  - Part 1 - Part 2  - Part 3  - Part 4 - Part 5 

Rigidity in the machining is required to reduce chatter of the tool. Chatter of the tool leads to poor surface finish and productivity. Taylor investigated this topic.

In industrial engineering literature, a synthesis comparable to Taylor's initial work has not appeared so far in the area of productivity science of machining. These collection of lessons are an initial attempt to present the productivity science of machining to demonstrate the possibility of culling out the relevant aspect from the production technology books and various papers published by faculty in the industrial engineering departments. I welcome all industrial engineers to give suggestions and comments and put forward their point of view.

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. Chatter was researched by F.W. Taylor and it is covered in Part 5 .

Machine tools are composed of several components and therefore can be considered multi-mass
vibrators.

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.

There are two prevailing techniques for chatter prediction: “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 best toolholder device for the particular tool or application, and reducing the tool overhang length
3. Selecting the proper bearing types, configurations, and installation geometry to provide maximum stiffness and damping
4. Isolating the system from vibration forces and using active or passive dynamic absorbers
5. Increasing the effective structural damping and using tuned mass vibration dampers
6. 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
7. If the wavelength of chatter marks is small, increasing process damping by reducing the surface cutting speed
8. 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
9. Increasing the part stiffness
10. For forced vibration, decreasing cutting force, increasing part stiffness, changing the tooth passing frequency to be far away from resonance frequency of structure.
11. 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


Active Vibration Control

1. Chase Control Method

2. Predictive Control

3. Multivariable Control Schemes

4. Harmonizer System

5. Forecasting Control

Application Examples

2019

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

21.3.2019

Smart damping solutions to boost stock removal capacity during machining 

Machine tool productivity is associated with material removal capacity. A number of parameters, some relating to the machine itself, such as its power output and dynamics, and other process-related criteria such as the tools, the material to be machined and the part itself determine the removal rate that may be obtained. However,  problems concerning vibration can limit the ideal machining conditions, and lead to noise problems, poor finishing or even tool breakage. Elimination of chatter is a challenge for any operator. 

SORALUCE has developed a number of solutions to eliminate self-excited vibrations or chatter, boosting machines´ cutting capacity and improving the efficiency of machining processes. This is a field in which the company has been working successfully for several years and in which it has been recognized with prestigious awards. 

 SORALUCE  has recently developed the Dynamic Workpiece Stabiliser (DWS). This system eliminates the chatter which usually occurs when flexible parts are being machined. DWS, patented, is an active damping device which consists of a controller and one or more inertial actuators placed over the part to be machined.  

“One of its main benefits is a better surface finishing in areas where the part is more flexible, and the chance to boost productivity thanks to in-depth passes with no vibration problems. Its user-friendliness, adaptability to various parts and its portability are some of the other advantages of the device”, says Xabier Mendizabal, Head of R+D at SORALUCE. 

DWS, a pioneer development on the market, provides an effective solution for the problems associated with machining slim-format parts. 

Another in-house SORALUCE development is the DAS+, a smart system which oversees the machining process and selects the best technological alternative to eliminate any chatter that may arise. DAS+ has a user interface to control realisation of the process and avails itself of several strategies to eliminate chatter such as active damping of the ram, spindle speed tuning by automatic selection of optimum speed, and harmonic oscillation of spindle speed. 

The solution, now available for new machines, can be fitted on request to machinery already operational. The system can boost productivity by up to 300%, with 100% cutting capacity through the complete workpiece volume. Besides, DAS+ also improves the surface quality of the parts produced, extends tool life, makes processes sturdier and reduces wear of the machine´s internal components. 

In addition to DWS or DAS+ products, SORALUCE offers customised advisory services aimed at improving the stability of the machining processes. It consists of the study of existing machining methods and strategies, an analysis to determinate the origin of the vibrations and the approach of solutions to deal with chatter problems that may appear during machining. 

https://www.soraluce.com/en/soraluce-portal-summit-2019

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


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 29 July 2021

Pub 16 September 2020