Mechanical Technology

Four Stroke Engine

2009-02-21T11:04:00.003+06:00

Today, internal combustion engines in cars, trucks, motorcycles, aircraft, construction machinery and many others, most commonly use a four-stroke cycle. The four strokes refer to intake, compression, combustion (power) and exhaust strokes that occur during two crankshaft rotations per working cycle of the Gasoline engine and Diesel engine.

A four-stroke engine is characterized by four strokes, or reciprocating movements of a piston in a cylinder:

intake (induction) stroke
compression stroke
power stroke
exhaust stroke

In this example animation, the right blue side is the intake and the left yellow side is the exhaust. The cylinder wall is a thin sleeve surrounded by cooling water.

The cycle begins at top dead center (TDC), when the piston is farthest away from the axis of the crankshaft. On the intake or induction stroke of the piston, the piston descends from the top of the cylinder, reducing the pressure inside the cylinder. A mixture of fuel and air is forced (by atmospheric or greater pressure) into the cylinder through the intake (inlet) port. The intake (inlet) valve (or valves) then close(s), and the compression stroke compresses the fuel–air mixture.

The air–fuel mixture is then ignited near the end of the compression stroke, usually by a spark plug (for a gasoline or Otto cycle engine) or by the heat and pressure of compression (for a Diesel cycle or compression ignition engine). The resulting pressure of burning gases pushes the piston through the power stroke. In the exhaust stroke, the piston pushes the products of combustion from the cylinder through an exhaust valve or valves.

Carnot Cycle

2009-02-20T22:41:00.003+06:00

The Carnot cycle is a particular thermodynamic cycle, modeled on the hypothetical Carnot heat engine, proposed by Nicolas Léonard Sadi Carnot in 1824 and expanded upon by Benoit Paul Émile Clapeyron in the 1830s and 40s.

A heat engine acts by transferring energy from a warm region to a cool region of space and, in the process, converting some of that energy to mechanical work. The cycle may also be reversed. The system may be worked upon by an external force, and in the process, it can transfer thermal energy from a cooler system to a warmer one, thereby acting as a heat pump rather than a heat engine.

The Carnot cycle is the most efficient cycle possible for converting a given amount of thermal energy into work or, conversely, for using a given amount of work for refrigeration purposes.

The Carnot cycle

The Carnot cycle when acting as a heat engine consists of the following steps:

Reversible isothermal expansion of the gas at the "hot" temperature, T_H (isothermal heat addition). During this step (A to B on Figure 1, 1 to 2 in Figure 2) the expanding gas causes the piston to do work on the surroundings. The gas expansion is propelled by absorption of quantity Q₁ of heat from the high temperature reservoir.
Isentropic (Reversible adiabatic) expansion of the gas. For this step (B to C on Figure 1, 2 to 3 in Figure 2) we assume the piston and cylinder are thermally insulated, so that no heat is gained or lost. The gas continues to expand, doing work on the surroundings. The gas expansion causes it to cool to the "cold" temperature, T_C.
Reversible isothermal compression of the gas at the "cold" temperature, T_C. (isothermal heat rejection) (C to D on Figure 1, 3 to 4 on Figure 2) Now the surroundings do work on the gas, causing quantity Q₂ of heat to flow out of the gas to the low temperature reservoir.
Isentropic compression of the gas. (D to A on Figure 1, 4 to 1 in Figure 2) Once again we assume the piston and cylinder are thermally insulated. During this step, the surroundings do work on the gas, compressing it and causing the temperature to rise to T_H. At this point the gas is in the same state as at the start of step 1.

Figure 1: A Carnot cycle acting as a heat engine, illustrated on a temperature-entropy diagram. The cycle takes place between a hot reservoir at temperature T_H and a cold reservoir at temperature T_C. The vertical axis is temperature, the horizontal axis is entropy.

Figure 2: A Carnot cycle acting as a heat engine, illustrated on a pressure-volume diagram to illustrate the work done.

Diesel Cycle

2009-02-20T22:08:00.002+06:00

The Diesel cycle is the thermodynamic cycle which approximates the pressure and volume of the combustion chamber of the Diesel engine, invented by Rudolph Diesel in 1897. It is assumed to have constant pressure during the first part of the "combustion" phase , v2 to v3 in the diagram .

Maximum thermal efficiency

The maximum thermal efficiency of a diesel cycle is dependent on the compression ratio and the cut-off ratio. It has the following formula:

Where

η t h

is thermal efficiency

α

is the cut-off ratio (ratio between the end and start volume for the combustion phase)

r is the compression ratio

γ

is ratio of specific heats (C_p/C_v)

The cut-off ratio can be expressed in terms of temperature as shown below:

$T 3$ can be approximated to the flame temperature of the fuel used. The flame temperature can be approximated to the adiabatic flame temperature of the fuel with corresponding air-to-fuel ratio and compression pressure, $P 3$ . $T 1$ can be approximated to the inlet air temperature.

This formula only gives the ideal thermal efficiency. The actual thermal efficiency will be significantly lower due to heat and friction losses. The formula is more complex than the Otto cycle (petrol/gasoline engine) relation that has the following formula;

The additional complexity for the diesel formula comes around since the heat addition is at constant pressure and the heat rejection is at constant volume. The Otto cycle by comparison has both the heat addition and rejection at constant volume.

Comparing the two formulae it can be seen that for a given compression ratio (r), the ideal Otto cycle will be more efficient. However, a diesel engine will be more efficient overall since it will have the ability to operate at higher compression ratios. If a petrol engine were to have the same compression ratio, then knocking (self-ignition) would occur and this would severely reduce the efficiency, whereas in a diesel engine, the self ignition is the desired behavior. Additionally, both of these cycles are only idealizations, and the actual behavior does not divide as clearly or sharply. And the ideal Otto cycle formula stated above does not include throttling losses, which do not apply to diesel engines.

The Diesel cycle is a combustion process of a reciprocating internal combustion engine. In it, fuel is ignited by heat generated by compressing air in the combustion chamber, into which fuel is injected. This is in contrast to igniting it with a spark plug as in the Otto cycle (four-stroke/petrol) engine. Diesel engines (heat engines using the Diesel cycle) are used in automobiles, power generation, diesel-electric locomotives, and submarines.

Bending

2009-02-18T21:37:00.005+06:00

Bending is a common metalworking technique to process sheet metal. It is usually done by hand on a box and pan brake, or industrially on a brake press or machine brake. Typical products that are made like this are boxes such as electrical enclosures, rectangular ductwork, and some firearm parts .

Usually bending has to overcome both tensile stresses as well as compressive stresses. When bending is done, the residual stresses make it spring back towards its original position, so we have to overbend the sheet metal keeping in mind the residual stresses.

Sheet Gauge Measurment

2009-02-18T13:29:00.002+06:00

It is very important for a Mechanical Engineer who work in the field of Sheet Steel, to know about the measurement of SWG

Standard Wire Gauge (SWG) Dimensions

SWG	inches	mm
7/0	0.500	12.700
6/0	0.464	11.786
5/0	0.432	10.973
4/0	0.400	10.160
3/0	0.372	9.449
2/0	0.348	8.839
1/0	0.324	8.236
1	0.300	7.620
2	0.276	7.010
3	0.252	6.401
4	0.232	5.893
5	0.212	5.385
6	0.192	4.877
7	0.176	4.470
8	0.160	4.064
9	0.144	3.658
10	0.128	3.251
11	0.116	2.946
12	0.104	2.642

SWG	inches	mm
13	0.092	2.337
14	0.080	2.032
15	0.072	1.829
16	0.064	1.626
17	0.056	1.422
18	0.048	1.219
19	0.040	1.016
20	0.036	0.914
21	0.032	0.813
22	0.028	0.711
23	0.024	0.610
24	0.022	0.559
25	0.020	0.508
26	0.018	0.457
27	0.0164	0.417
28	0.0148	0.376
29	0.0136	0.345
30	0.0124	0.315
31	0.0116	0.295

SWG	inches	mm
32	0.0108	0.274
33	0.0100	0.254
34	0.0092	0.234
35	0.0084	0.213
36	0.0076	0.193
37	0.0068	0.173
38	0.006	0.152
39	0.0052	0.132
40	0.0048	0.122
41	0.0044	0.112
42	0.004	0.102
43	0.0036	0.091
44	0.0032	0.081
45	0.0028	0.071
46	0.0024	0.061
47	0.002	0.051
48	0.0016	0.041
49	0.0012	0.030
50	0.001	0.025

Basic Of Steel Rolling Part 5

2009-01-20T21:21:00.000+06:00

Biting condition & Roll bite friction coefficient

Roll bite friction coefficient = tangential or frictional force/normal force

Biting

In the roll gap are two forces :

1. Friction FR : tries to pull through the stock

2. Pressure of rolls FN : tries to push back the stock

Biting condition

Biting condition : friction angle bigger than roll angle

Pass through condition : friction angle more than double of roll angle

Biting condition (example):

too large stock - large roll angle

- small friction angle

* resulting force pushes backwards

* no biting

Influences on roll bite friction coefficient

Influences on roll angle

1. If friction coefficient is fix, biting conditions can be improved by decreasing roll angle.

2. For fix height reduction :

by bigger roll diameter and / or smaller initial billet size

3. For fix roll diameter : by smaller height reduction

Law of continuity

Cross section • stock speed = constant

A0 = Cross section of entrance

A1 = Cross section of exit

v0 = Entrance speed of stock

v1 = Exit speed of stock

Law of constancy of volume

Distribution of the velocity in the roll gap and constitution of the roll Neutral point

Neutral point :

point where roll (at circumference) and working piece have the same speed

*Zone of backward slip : here the main deformation is done

speed of roll > speed of working piece

*Zone of forward slip :determines real exit speed.

number of revolutions of following stands

control of flying shears

speed of coilers

*The slip,i.e.the difference of speed of rolls and working piece before and after the neutral point,is the main reason for wear of rolls.

Rolling speeds

Law of volume constancy and law of continuity.

v1 : Rolling speed in the first stand

A1 : Material cross-sectional area in the first stand

v2 : Rolling speed in the second stand

A2 : Material cross-sectional area in the second stand

Rolling torque

In general,torque = force • lever arm

Rolling power

Basic Of Steel Rolling Part4

2009-01-20T20:26:00.000+06:00

Stress-strain-curves2

Compressing (upsetting) of a rectangular body not considering friction

Deformation resistance

Deformation resistance = yield stress + losses by external friction at compacting surface + losses by internal friction in the metal itself .

Internal friction ki= friction in the deformed material between atoms

External friction kr= friction between tool and material during deformation

Rolling force

Rolling force = deformation resistance x area of contact

Roll gap & deformation area

Arc of contact= rootover (roll diameter x height difference)

Area of contact = Arc of contact x average width

Basic of Steel Rolling Part3

2009-01-18T19:50:00.000+06:00

Definitions of Stresses

In general : Stress - Resistance

Resistance = force per area from material`s point of view

Yield stress
It defined as the stress where plastic deformation starts

Influences on yield stress

Yield stress determines forces required for determination

Degree of deformation

Rate of deformation

Stress-strain-curve (tensile test schematic)

Basic Of Steel Rolling Part2

2009-01-17T21:29:00.000+06:00
Non-wanted constituents in steel

Sulphur S: breaks/cracks at hot forming
Phosphorus P: embrittlement, especially at low temperatures
Copper Cu: cracks at hot forming
Gases (Hydrogen H, Nitrogen N ,Oxygen O ):
holes in steel (gas bubbles), additionally H -flakes, N - aging
What is Rolling ?
Continuous shaping by plastic deformation = continuous forging leading to improved mechanical properties.

Atomic structure of steel

Elastic deformation

Plastic deformation

Basic Of Steel Rolling Part1

2009-01-16T20:38:00.000+06:00

What makes rolling high productive?

Contents

• Introduction
• Constituents of steel,atomic structure,elastic/plastic deformation
• Basics of deformation
yield stress
deformation resistance
• Basics of rolling :
· conditions in roll gap
· material flow in roll gap
· biting
· spread
· rolling force, moment, power
· rolling speeds
· tension control
• Slit Rolling
• Material defects
• Summary

What is steel ?

• iron with up to 2 % carbon :
• shaping possible by plastic deformation, i.e.forging,rolling
• cast iron /pig iron
• iron with more than 2 % carbon :
• shaping only by casting

Steel Structure
Variation of Properties of Steel

Common Codes

2008-12-15T20:36:00.000+06:00

G-codes are also called preparatory codes, and are any word in a CNC program that begins with the letter 'G'. Generally it is a code telling the machine tool what type of action to perform, such as:
rapid move
controlled feed move in a straight line or arc
series of controlled feed moves that would result in a hole being bored, a workpiece cut (routed) to a specific dimension, or a decorative profile shape added to the edge of a workpiece.
change a pallet
set tool information such as offset.
There are other codes; the type codes can be thought of like registers in a computer
X absolute position
Y absolute position
Z absolute position
A position (rotary around X)
B position (rotary around Y)
C position (rotary around Z)
U Relative axis parallel to X
V Relative axis parallel to Y
W Relative axis parallel to Z
M code (another "action" register or Machine code(*)) (otherwise referred to as a "Miscellaneous" function")
F feed rate
S spindle speed
N line number
R Arc radius or optional word passed to a subprogram/canned cycle
P Dwell time or optional word passed to a subprogram/canned cycle
T Tool selection
I Arc data X axis
J Arc data Y axis.
K Arc data Z axis, or optional word passed to a subprogram/canned cycle
D Cutter diameter/radius offset
H Tool length offset
(*) M codes control the overall machine, causing it to stop, start, turn on coolant, etc., whereas other codes pertain to the path traversed by cutting tools. Different machine tools may use the same code to perform different functions; even machines that use the same CNC control.
Partial list of M-Codes
M0=Program Stop (non-optional)
M1=Optional Stop, machine will only stop if operator selects this option
M2=End of Program
M3=Spindle on (CW rotation)
M4=Spindle on (CCW rotation)
M5=Spindle off
M6=Tool Change
M7=Coolant on (flood)
M8=Coolant on (mist)
M9=Coolant off
M10=Pallet clamp
M11=Pallet un-clamp
M30=End of program/rewind tape (may still be required for older CNC machines)

Common FANUC G Codes for Mill
Code Description
G00 Rapid positioning
G01 Linear interpolation
G02 CW circular interpolation
G03 CCW circular interpolation
G04 Dwell
G05.1 Q1. Ai Nano contour control
G05 P10000 HPCC
G10/G11 Programmable Data input/Data write cancel
G17 X-Y plane selection
G18 X-Z plane selection
G19 Y-Z plane selection
G20 Programming in inches
G21 Programming in mm
G28 Return to home position
G30 2nd reference point return
G31 Skip function (used for probes and tool length measurement systems)
G33 Constant pitch threading
G34 Variable pitch threading
G40 Tool radius compensation off
G41 Tool radius compensation left
G42 Tool radius compensation right
G43 Tool offset compensation positive
G44 Tool offset compensation negative
G45 Axis offset single increase
G46 Axis offset single decrease
G47 Axis offset double increase
G48 Axis offset double decrease
G49 Tool offset compensation cancel
G53 Machine coordinate system
G54 to G59 Work coordinate systems
G54.1 P1 to P48 Extended work coordinate systems
G73 High speed drilling canned cycle
G74 Left hand tapping canned cycle
G76 Fine boring canned cycle
G80 Cancel canned cycle
G81 Simple drilling cycle
G82 Drilling cycle with dwell
G83 Peck drilling cycle
G84 Tapping cycle
G84.2 Direct right hand tapping canned cycle
G90 Absolute programming (type B and C systems)
G91 Incremental programming (type B and C systems)
G92 Programming of absolute zero point
G94/G95 Inch per minute/Inch per revolution feed (type A system)
G98/G99 Inch per minute/Inch per revolution feed (type B and C systems)
G96/G97 Constant cutting speed (Constant surface speed)/Constant rotation speed (constant RPM)
A standardized version of G-code known as BCL is used, but only on very few machines.
G-code files may be generated by CAM software. Those applications typically use translators called post-processors to output code optimized for a particular machine type or family. Post-processors are often user-editable to enable further customization, if necessary. G-code is also output by specialized CAD systems used to design printed circuit boards. Such software must be customized for each type of machine tool that it will be used to program. Some G-code is written by hand for volume production jobs. In this environment, the inherent inefficiency of CAM-generated G-code is unacceptable.
Some CNC machines use "conversational" programming, which is a wizard-like programming mode that either hides G-code or completely bypasses the use of G-code. Some popular examples are Southwestern Industries' ProtoTRAK, Mazak's Mazatrol, Hurco's Ultimax and Mori Seiki's CAPS conversational software.

G-code

2008-12-15T00:37:00.000+06:00

G-Code, or preparatory code or function, are functions in the Numerical control programming language. The G-codes are the codes that position the tool and do the actual work, as opposed to M-codes, that manages the machine; T for tool-related codes. S and F are tool-Speed and tool-Feed, and finally D-codes for tool compensation.
The programming language of Numerical Control (NC) is sometimes informally called G-code. But in actuality, G-codes are only a part of the NC-programming language that controls NC and CNC machine tools. The term Numerical Control was coined at the MIT Servomechanisms Laboratory, and several versions of NC were and are still developed independently by CNC-machine manufacturers. The main standardized version used in the United States was settled by the Electronic Industries Alliance in the early 1960s. A final revision was approved in February 1980 as RS274D. In Europe, the ISO standard DIN 66025 is often used instead.

Due to the lack of further development, the immense variety of machine tool configurations, and little demand for interoperability, few machine tool controllers (CNCs) adhere to this standard. Extensions and variations have been added independently by manufacturers, and operators of a specific controller must be aware of differences of each manufacturers' product. When initially introduced, CAM systems were limited in the configurations of tools supported.
Today, the main manufacturers of CNC control systems are GE Fanuc Automation (joint venture of General Electric and Fanuc), Siemens,Mitsubishi, and Heidenhain, but there still exist many smaller and/or older controller systems.

Some CNC machine manufacturers attempted to overcome compatibility difficulties by standardizing on a machine tool controller built by Fanuc. Unfortunately, Fanuc does not remain consistent with RS-274 or its own previous versions, and has been slow at adding new features, as well as exploiting increases in computing power. For example, they changed G70/G71 to G20/G21; they used parentheses for comments which caused difficulty when they introduced mathematical calculations so they use square parentheses for macro calculations; they now have nano technology recently in 32-bit mode but in the Fanuc 15MB control they introduced HPCC (high-precision contour control) which uses a 64-bit RISC (reduced instruction set computer) processor and this now has a 500 block buffer for look-ahead for correct shape contouring and surfacing of small block programs and 5-axis continuous machining.

This is also used for NURBS to be able to work closely with industrial designers and the systems that are used to design flowing surfaces. The NURBS has its origins from the ship building industry and is described by using a knot and a weight as for bending steamed wooden planks and beams.

Basic of Electrical Engineering

2008-12-04T18:05:00.000+06:00

CNC Machine

2008-12-03T01:05:00.000+06:00
Computer numerical control (CNC) is a computer "controller" that reads G-code and M-code commands and drives a machine tool, a powered mechanical device typically used to fabricate components by the selective removal of material. CNC does numerically directed interpolation of a cutting tool in the work envelope of a machine. The operating parameters of the CNC can be altered via the Master Control Unit (MCU) of the machine.

Code	Description
G00	Rapid positioning
G01	Linear interpolation
G02	CW circular interpolation
G03	CCW circular interpolation
G04	Dwell
G05.1 Q1.	Ai Nano contour control
G05 P10000	HPCC
G10/G11	Programmable Data input/Data write cancel
G17	X-Y plane selection
G18	X-Z plane selection
G19	Y-Z plane selection
G20	Programming in inches
G21	Programming in mm
G28	Return to home position
G30	2nd reference point return
G31	Skip function (used for probes and tool length measurement systems)
G33	Constant pitch threading
G34	Variable pitch threading
G40	Tool radius compensation off
G41	Tool radius compensation left
G42	Tool radius compensation right
G43	Tool offset compensation positive
G44	Tool offset compensation negative
G45	Axis offset single increase
G46	Axis offset single decrease
G47	Axis offset double increase
G48	Axis offset double decrease
G49	Tool offset compensation cancel
G53	Machine coordinate system
G54 to G59	Work coordinate systems
G54.1 P1 to P48	Extended work coordinate systems
G73	High speed drilling canned cycle
G74	Left hand tapping canned cycle
G76	Fine boring canned cycle
G80	Cancel canned cycle
G81	Simple drilling cycle
G82	Drilling cycle with dwell
G83	Peck drilling cycle
G84	Tapping cycle
G84.2	Direct right hand tapping canned cycle
G90	Absolute programming (type B and C systems)
G91	Incremental programming (type B and C systems)
G92	Programming of absolute zero point
G94/G95	Inch per minute/Inch per revolution feed (type A system)
G98/G99	Inch per minute/Inch per revolution feed (type B and C systems)
G96/G97	Constant cutting speed (Constant surface speed)/Constant rotation speed (constant RPM)