Birla Institute of Technology and Science Pilani, Hyderabad Campus

First Semester 2019-2020

ME G532 Machine Tool Engineering

Part A-Close Book Comprehensive Examination 04.12.2019

60 minutes 15%weightage

Answers should be crisp and to the point, Vague or incomplete answers will not get ANY credit

Q1. A milling machine is required to be design with stepless drive system to provide continuous

variation of speed of the cutter. Provide a circuit diagram of a hydraulic system to achieve this

purpose. Also provide the specific application/significance/importance for each of the component

used in the circuit, pointwise. [7]

Q2. For the problem stated in Question 1, suggest at least three alternate stepless drives. Also

mention their common advantages (at least 4) and limitations (at least 3). [5]

Q3. What are the modes of deformation or deflection in a lath machine spindle? Derive the

expression for the deflection of spindle for the condition shown in Figure Q3, and find its

numerical value. Stiffness at end A and B are measured as KA=300N/mm, KB=200N/mm. All

bearings are anti-friction type. [5]

Figure Q3

Q4. (a) How a machine tool can be represented using a close loop system for dynamic analysis?

Mention the components of equivalent elastic system of the machine tool and cutting process

model. [3]

(b) Being a complex system, the dynamic analysis of a machine tool is difficult. Suggest a

proper methodology for dynamic analysis of multi- degree of freedom machine tool with

necessary steps. [4]

(c) Considering a machine tool as a single degree of freedom system, derive the expression for

limiting unreformed chip thickness. [6]

***END***

a=12cm

l=32cm c=16cm

P2=1600 N P1=1200 N

A B

Birla Institute of Technology and Science Pilani, Hyderabad Campus

First Semester 2019-2020

ME G532 Machine Tool Engineering

Part B-Open Book Comprehensive Examination 04.12.2019

120 minutes 25%weightage

Answers should be crisp and to the point, Vague or incomplete answers will not get ANY credit

Q1. The duty cycle for an intermittent application

machine tool drive is given here. Determine

the duty factor (ε) for this the process. What

will be the maximum and equivalent power required if the machine tool uses, V-belt drive, Helical gear and

Jaw clutch. The prime mover is general purpose DC motor. What will be the power required if a similar

motor of standard duty factor of 15% is used for the same machine and application? [8]

Q2. Obtain the expression and limiting values of characteristics (Xu), number of speed steps (zu) and range

ratio (Rn). The common ratio (ϕ) of the geometric progression of speed is 1.06, and 1.41, and speed steps in

the last transmission group (Pu) are three. [8]

Q3. A special purpose machine tool is designed, and at the end it is observed that the tail stock is happened to

be stiffer than the head stock. The machine tool is subject to turning operation of cylindrical block. Develop

a mathematical model for the deformation of the workpiece, staring from the free body diagram to variation

of deflection curve (yx/ymax vs x/l). [10]

Q4. On a lathe machine, turning operation is performed on a cylindrical workpiece of 40cm diameter, which

covers half of the gap available between chuck center and lathe bed surface. The chuck rotates at 20rpm,

the cutting power required is measured as 120 watt (considering only cutting forces), the axial feed is

6cm/min, and the radial feed is 2.4mm/rev. The pulling force is measured as 200N, and the maximum twist

measured under saddle is 150Nm, at this condition. The workpiece weight is 20N. The coefficient of

friction at slideways can be considered as 0.2. Compute the cutting forces. [12]

Q5. It is proposed to replace the slideways of an existing lathe machine with hydrodynamic guide ways.

Presently a flat-flat slideway with carriage length of 24cm is employed, which moves linearly with a

velocity of 12cm/min. The width of the slideway faces are 6 cm for all the three faces. The maximum

pressure experienced at tool post side is obtained as 10kPa. For the new carriage assume length and width

as 8 and 6 cm, thus three hydrodynamic wedges can be accommodated in the carriage length with slight

increase in total carriage-length. The oil viscosity is 0.430 Ns/m2. Calculate the gap created between

carriage and guideway in the provided condition. What will be the compliance value for this system? [12]

***END***

Power (Watt) 10 8 0 10 8 0 8 10 9 Duration

(min) 2 1 1 2 1 2 1 3 2

Solution

Close Book

Q1.

Q2. Stepless drives: PIV drive, CVT drive, Friction coupling cone etc.

Advantages

1. Carrying out machining operations at optimum cutting speed: Improves productivity, and tool life

2. Ease of changing the speed or feed rate without stopping the machine tool. This helps in

increasing productivity as it cuts down the non-productive time.

3. Noiseless and more smooth rotation and absence of jerks which are characteristic of geared drives.

These features explain why stepless devices are generally employed in high-speed machine tools.

4. Generally more expensive than the devices providing stepped regulation.

5. Employed in high-speed machine tools

6. Hybrid speed variation: stepless drives in stepped drives

Limitations:

1. Commonly used in small-and medium-sized machine tools than hydraulic and electrical devices.

2. Mostly friction type, and therefore, their application is restricted to power transmission of up to 15 kW:

For high values dimensions increase

3. Speed adjustment on the running machine. Extremely difficult to adjust the speed on a stationary

machine

Q3: Modes: Deflection, deformation, tool holder taper joint deflection [1M]

Derivation: [2M]

Numerical value:

Ydef=1600x120+1200(320+160)(1+160/320)/(320x200)+(1600x200-1200x160)x160/(320x320x200)

=18.667mm [2M]

Q4. Close loop figure [1M],

EES components: Machine structure, Motor, Fractional effects [1M]

CP components: Tool face, Tool flank [1M]

(b) Explanation: Nyquist Method [1M] Step 1: open loop system with figure [1M]

Step 2: estimation of A and ϕ [1M]

Step 3: Polor plot, stability evaluation using combine A and ϕ value [1M]

1 M

1 M 1 M

1 M

Components: (each 0.5M), total 3M

Motor:

Pump:

Throttle valve:

Tank

Bypass line

Pressure gauges

(c) Derivation: Considering EES, A and ϕ values [2M],

Considering CP, A and ϕ values [2M]

Combining A and ϕ values [1M]

blim, expression [1M]

Open Book

Q1.

Q2.

Q3.

Q4.

Q5.