Chapter One

Background

1.1 Introduction

Retrofitting is the practice of adding new technology or new features to older systems. It is

customary to replace some components of an older machine to make it up-to-date with the new

technology, reducing the cost spent required for buying a new one. In Ethiopia, machine tools

retrofitting, especially lathe and milling machines, has been started in Hibret Manufacturing and

Machine Building Industry (HMMBI) with the technical support of GSK, China. The

retrofitting work involved replacing the existing drive elements and systems with servo systems

such that they become Computer Numerical Controlled (CNC).

Servomotor sizing is an element of CNC retrofitting of machine tools which involves

determining the servo motor size, in terms of torque, required for driving the axes of machine

tools. In the process of servomotor sizing, to accurately determine the different loads involved

in each axis and analyzing them enable the retrofitting personnel to select the optimum

servomotor size. Meaning that a further cost reduction is achieved in converting an existing

normal machine tool to a CNC system.

Therefore, the thesis aims at selecting the optimum servomotor size required for retrofitting a

medium duty lathe with the minimum possible cost and the desired functionality.

2.1 Objective of the Study

The general objective of this thesis is selecting the smallest but effective servomotor size for

the two axes of a lathe machine, namely the longitudinal feed motion and the cross slide

motion. Determining the optimum size of the motor enables to determine the smallest amplifier

size to drive the motors, hence cost reduction opportunity in retrofitting of a lathe machine is

achieved.

The Specific objectives of the thesis are:

To have a machine tool with the desired functionality and performance

To develop engineering guidelines for servomotor selection in CNC retrofitting

To reduce retrofitting cost of machine tools

To reduce energy dissipation of machine tools

1.3 Methodology

The research methods employed to meet the objectives are:

Literature Survey: papers, journals, books and other written materials pertaining to retrofitting and machine tool design will be assessed.

Investigation of a medium duty lathe:

o Examining the drive system of the slide and carriage

o Taking measurements of each component and making sketches

CAD modeling of the slide and carriage components with their respective

assemblies

o The CAD modeling is done using CATIA v5R19 software

Static and dynamic load analysis

o Determination of mass moment of inertia of each slide and carriage

component

o Cutting force analysis during turning

o Static loads on carriage and slide

o Determination of maximum torque requirement on each axis

o Determination the top speed and acceleration in each axis

Servomotor selection

o Assessment of servomotor sizes available in these global market (using the

internet)

o Matching the system requirement with the available servomotor sizes

Conclusion

o Results obtained from servomotor sizing in CNC retrofitting will be

presented.

Recommendation

o Problem areas which are not included in the research work will be

presented for further study.

1.4 Significance of the Study

Currently, in Ethiopia, Metals Engineering Corporation (MetEC) takes the biggest share of

the country’s industrial expansion. Considering it is being involved in transfer of technology

in retrofitting machine tools, this thesis will have a great importance in full transfer of the

technology and making it our own.

Chapter Two

Literature Review

.In this thesis the literature review part consists of the following main parts: the lathe , retrofitting

of machine tools and servomotor sizing.

2.1 The Lathe

2.1.1 Design and Basic Construction

Although lathes have changed greatly in both design and appearance, the fundamental principles

of design, construction, and operation have remained virtually the same. Therefore a thorough

understanding of any type of lathe enables a skilled worker to operate almost any lathe,

regardless of its age or make.

2.1.2 Size of Lathe

The maximum size of the work that can be handled by the lathe is used to designate the size of

the lathe ( that is, the diameter and length of the work). Manufacturers use the word swing to

designate the size as the maximum diameter of the that can be machined in the lathe.

Manufacturers also use the length of the lathe bed, rather than the distance between centers, to

indicate lathe size [1].

Figure 2.1 The size designation of an engine lathe.

2.1.3 Basic Construction

A lathe is made up of many parts. But the principal parts are the following:

Bed Headstock Tailstock Carriage Feed mechanism Thread-cutting mechanism

Bed

The lathe bed is the stationary part that serves as a strong, rigid foundation for a great many

moving parts. Therefore it must be scientifically designed and solidly constructed.

The ways of the lathe serve as a guide for the saddle of the carriage as it travels along the bed,

guiding the cutting tool in a straight line. The v-ways are machined in the surface of the bed, and

are precision-finished to ensure proper alignment of all working parts mounted on the bed.

The two outer ways guide the lathe carriage. The inner v-ways and the flat way together provide

a permanent seat for the headstock and a perfectly aligned seat for the tailstock in any position. A

slight twist in the bed of a lathe can cause the machine to produce imperfect work. The lathe

should be carefully leveled in both lengthwise and crosswise directions.

Figure 2.2 Lathe bed and guide way

Headstock

The headstock is mounted permanently on the bed of a lathe at the left- hand end of the machine.

It is held in alignment by the ways of the bed and contains the gears that rotate the spindle and

workpiece

.Figure 2.3 A lathe gearing mechanism enclosed in the headstock housing

Tailstock

The tailstock assembly is movable on the bed ways, and carries the tailstock spindle. The

tailstock spindle has a standard Morse taper at the front end to receive a dead center. The

tailstock hand-wheel is at the other end to give longitudinal movement when mounting the

workpiece between centers. Reamers and taper-shank twist drills can be mounted in the tailstock

spindle when required. A spindle binding lever clamps the spindle in any position in its travel.

clamp bolt nuts are used to clamp the tailstock assembly in any position of its travel on the ways

of the bed. The dead center or any other tool mounted in the tailstock spindle can be removed by

turning the tailstock hand-wheel counterclockwise.

Figure 2.4 Tailstock assembly

Carriage

The carriage assembly is the entire unit that moves lengthwise along the ways between the

headstock and the tailstock. The carriage supports the cross slide, the compound rest, and the tool

post. The two main parts of the carriage are the saddle and the apron.

Saddle The saddle is an H-shaped casting that is machined to fit the other ways of the lathe bed.

The saddle can be moved along the ways either manually or by power, through the

gearing mechanism in the apron. The apron and the cross slide are bolted to the saddle.

Cross slideThe cross slide is a casting that is mounted on and gibbed to the saddle. The cross slide

screw is located in the saddle and is connected to the cross slide. The cross slide can be

moved either manually or by power across the saddle in a plane perpendicular to the

longitudinal axis of the spindle.

Figure 2.5 The carriage assembly including saddle, cross slide, compound rest, and apron

Compound rest

The compound rest is mounted on the cross slide. It consists of two main parts –the base

and the slide. The base can be swiveled to any angle in a horizontal plane; the slide can

be moved across the base by making hand adjustments with micrometer dial. The

compound rest supports the cutting tool, and makes it possible to adjust the tool to

various positions.

The tool post assembly is mounted on the compound rest. The rocker or wedge has a flat

top, and is convex in shape on the bottom to fit into a concave ring or collar so that the

cutting tool can be centered.

Apron

The apron is bolted to the front of the saddle. The apron houses the gears and controls for

the carriage and the feed mechanism. The carriage can be moved either manually or by

engaging the power feeds.

Feed and thread cutting mechanism

The same gears that move the carriage are involved in the feed and thread cutting mechanisms.

These gears are used to transmit motion from the headstock spindle to the carriage.

Lead screw

Some lathes have two lead screws – one for turning operations and one for thread-cutting

operations exclusively. The lead screw is very strong and has coarse, accurate threads

Figure 2.6 Apron internal gearing

Quick- change gearbox

On most lathes, the quick change gear box is located directly below the headstock on the

front of the lathe bed . A wide range of feeds and threads per inch (TPI) may be selected

by positioning the gears.

The reversing lever is used to reverse the direction of rotation of the screw for chasing

right- or left-hand threads, and for reversing the direction of feed of the carriage

assembly. Levers on the quick-change gearbox should never be forced into position.

2.2 Machine tool retrofitting

Retrofitting converts the conventional machine into a CNC machine by replacing its old

gear boxes and lead screws with ball screws and servomotors. During the 1983-98,

commonly retrofitted machines were only small lathes and knee type milling machines.

These retrofits never worked as full-fledged CNC machines. Compromises in accuracy and

performance in these low cost retrofits were accepted in those days. Customers never took

retrofitting beyond this level.

The present day scenario in retrofitting is substantially different, thanks to improved skill

levels and customer awareness. Customers are ready to retrofit more complicated machines

like cylindrical grinders, borers, crankshaft milling and grinding machines, camshaft

grinders, vertical turret lathes, huge roll turning lathes, roll grinders, tool and cutter grinders,

gear cutting and grinding machines, etc. Not only has the confidence of customers increased

in retrofitting; they have also started demanding productivity and accuracy from the

retrofitted machines as good as the new machines ( Prof Karunakaren and S Meyyapan,

2005 ).

When identical parts are produced rapidly with the required precision, conventional machine

tools are not efficient. Such tasks require numerical control of the machine tool. Numerical

control is based on the use of numerical data for directly controlling the position of the

operative units of a machine tool in machine operation[Zin Ei Ei Win, Than Naing Win, Jr., and

Seine Lei WinnZin Ei Ei Win, 2008].

In retrofitting of a lathe machine, the cast structure remains intact except that the guide

ways on the bed will be scraped to reach the desired surface roughness value. The apron will

be totally removed from the system since it contains the lead screw and gearing for thread

cutting and longitudinal feed motion. The lead screw will be replaced by the ball screw, and

made to be driven by a servomotor. Thus, control of carriage motion will be achieved.

Adaptation of the mechanical components is also required in controlling the motion of the

cross slide. In this case also, the lead screw should be replaced by ball screw and a servo

motor should be selected to drive the ball screw.

Figure 2.7 Lathe machine retrofit

2.7 Servomotor sizing

Servomotor sizing is the process of selecting the best motor for a servo application[Matt

Pelletier, 2009]. In machine tool retrofitting servomotor sizing is one of the main tasks as the

axes require servo drives.

Many secondary factors are important for servomotor sizing including: cost, encoder resolution,

environmental ratings, power requirements or space limitations. But the most critical factors in

the core process of servomotor sizing can be narrowed down to just four: inertia ratio, speed,

max torque @ speed and RMS torque @ speed. Understanding these critical four critical factors

is a vital step for the design engineer to select the best servomotor for the application.

Inertia ratio

The first key sizing factor is the moment of inertia ratio. Any rotating object has a

moment of inertia. The moment of inertia is a measurement of how difficult it is to

change the rotating velocity of that object. The entire moment of inertia of a servo system

can be divided into two parts: motor inertia and load inertia.

Motor inertia, JM, , is part of the design of the servomotor and is typically listed in the

manufacturer’s catalog. However, the load inertia, JL, often consists of many components.

Each component that is moved by the motor contributes to the total load inertia which is

determined by using proper equations for each component.

Inertia ratios around 5:1 are typical for many applications. Performance tends to go up as

the inertia ratio is lowered, often down to 2:1, 1:1, or lower [Yaskawa Electric America,

2009]. But when high performance is not as critical, ratios of 10:1 can be used for

servomotors [Wilfred Voss, 2007].

Bosch Rexroth, for instance, recommends the ‘good standards’ for inertia mismatch as

follows:

< 2:1 for quick positioning

< 5:1 for moderate positioning

< 10:1 for quick velocity changes

Torque - Speed curve

Several motors that provide a suitable inertia ratio may be available. So the task is to find

the smallest, most cost-effective motor that has the ability to produce the speed and

torque required for the application. A motor’s speed and torque capability is described in

a company’s catalog using an individual speed- torque curve for each motor.

The speed-torque curve displays several points of interest. Rated torque is the maximum

torque the motor can produce continuously at rated speed and lower, and is limited by

motor heating. This rated torque is given the value of 100 percent torque. Likewise rated

speed the highest speed at which rated torque is available. The motor can continuously

run faster than the rated speed, but the torque available drops significantly the faster the

Figure 2.8 Typical Torque vs. Speed for servomotors

TPS Stall peak torque

TPR Rated peak torque

TCS Stall continuous torque

TCR Rated continuous torque

ωR Rated speed

ωmax Maximum speed

motor runs. The motor’s attainable torque is denoted at the top of the speed torque curve,

and the motor’s maximum speed is at the far right ( see figure 2.8 ).The speed curve has

two regions, continuous and intermittent. If the combination of torque and speed required

by the motor is found in the continuous region, the motor can produce that torque and

speed forever without any chance of overheating the motor. If the

combination of torque and speed produced falls in the intermittent region, the motor can

only produce that speed and torque for a limited amount of time. If that speed is

exceeded, the motor will begin to overheat.

The application’s RMS torque must lie within the continuous region. If any combination

of speed and torque required lies outside both the continuous and intermittent region, the

motor is not capable of producing that combination of speed and torque. When selecting

a motor, it is imperative to ensure that the torque-speed curve is used effectively.

Motion profile

While a motor’s capability is described by the torque-speed curve, the application

requirements are best illustrated using a speed profile and torque profile (see figure 2.9)

Figure 2.9 Typical Speed and Torque Profile

The speed profile is a graphical representation of the motor speed versus time, and the torque

illustrates the motor torque required for the application to follow the speed profile during that

same time.

Max Torque

The torque at the beginning of the trapezoidal move is highest because mechanical friction

must be overcome and the load must be accelerated from rest. This point of highest torque is

called Max Torque. Once the traverse speed is reached, a nominal level of torque must be

applied to overcome friction and maintain speed. To decelerate the load, often a reverse

torque is required. The reverse torque during deceleration is not as high as the forward torque

during acceleration, since friction also helps to decelerate the load.

When friction torque is high, a forward torque may be required during deceleration so the

motor does not slow down too quickly. It is important to ensure that the motor can produce

the required Max Torque at the application speed. The Max Torque at application speed

ideally falls within in the intermittent region of the motor’s torque-speed curve. It may also

fall within the continuous region, but this may be an indication that the motor is oversized.

RMS Torque

Another torque calculation critical for sizing is RMS torque, the time weighted average of

the torque during a complete machine cycle or the equivalent of a steady-state torque

level. For example, a servomotor with 1.2N-m RMS torque will experience the same heat

rise if it produces 1.2N-m constant torque. So it is also important to ensure that the RMS

torque at the application speed falls within the continuous region of the torque-speed

curve.

Figure 2.10 RMS Torque at the Application Speed