ABSTRACT
This presentation introduces the machinist’s trade, and
further deals on the essential sector in the machine shop. A brief description
of a machine shop equipment, processes and safety is emphasized.
INTRODUCTION
The
machinist’s trade has made possible what we today call the atomic age, the
technological revolution or the electronic age. Without the machinist there
would be no engines or dynamos to generate power or any machinery to produce anything.
The
machinist's trade is a great trade in its vital necessity, great in its
ever-increasing interest and great in the opportunity it offers for
improvement, advancement and self-actualization.
Many great
inventors in Europe and America started as machinists in their youth. In the wider
engineering world, the machinist’s trade is known as Machine Shop Engineering
and also as Production engineering. The machine tool principle remains the same
irrespective of the thousands of machines built and assembled.
THE MACHINIST (The Machine Operator)
This is the
operator of a certain machine for doing a certain class of work in a machine
shop or in a factory. The machine operator must be skilled in the use of all
the attachments of the special machine. He is skilled in the operation of one
machine.
The machinist
is skilled in the operation of most of the machines in the machine shop. To acquire the status of a
good machinist, it involves hard work, serious study and sacrifice of money at
the start. For the engineering student, knowledge of machine shop practice is
absolutely necessary. To enable the engineer to design and aid in the
manufacturing of machine tools, he/she
must know what these tools are able to do.
If he/she aspires to be a production engineer,
one who decides which machine tools will do certain machining operations, he
must have a thorough knowledge of machine shop practice.
WHAT IS A MACHINE SHOP
It is place
where metal parts are cut to the required shape and size required and put
together to form mechanical units or machines. The machines so made are used
directly or indirectly in the production of the necessities and luxuries of
civilization. Machine workshop is the basis or foundation of all mechanical
production.
MACHINE SHOP EQUIPMENT
These consist
of certain standard machine tools, the size, the number and kind of machine
tools depending upon the product of the shop. They include the tools used at
the bench and on the floor, the measuring and adjusting tools, the work-holding
and tool-holding accessories and the small tools used on the machines shop.
STANDARD MACHINE TOOLS
The standard
machine tools include:
•
Centre
lathes and its vastly improved models
•
Milling
machines
•
Shaping
machines
•
Drilling
machines
•
Power
saws
•
Grinding
machine
•
Boring
machines
•
Planing
machines
•
Threading
machines
•
Jig
mils
SEQUENCE OF OPERATION ON CENTRE LATHE
Sequence of
operation is the systematical and orderly arrangement of all undertaking
operations needed for the production of a work piece.
EXAMPLE: The
work piece in figure 1 below, is to be turned on the centre lathe machine,
prepare an operational sequence for the machining of the component
LATHE TURNING TOOLS (TOOL ANGLES)
•
CLEARANCE
ANGLES: The
clearance angle allows the tool to bite into the work piece and reduces
friction so that the tool lasts longer. Lathe tools are ground with side
clearance angle limited to between 5˚
and 10˚. If it is too small the tool rubs and wears out quickly or does not cut
at all. If it is too large the tool is widened leading to short tool life and
tends to ‘’dig in ‘’ and ``chatter’’ producing a poor finish on the work piece.
Clearances ensure contact between the actual cutting edge of the tool and the
work.
•
RAKE
ANGLE: The rake is an important angle which
influences chip formation. It affects the type of chips, the cutting force,
tool wear and the roughness of the finished surface. Increasing this angle
makes cutting easier for ductile and low-strength materials but the strength of
the cutting edge is reduced.
•
TOOL
OR WEDGE ANGLE:
This is the actual angle of the tool wedge. Increasing it makes the tool
stronger but increase the cutting force. Increasing it also backs up the
cutting edge with greater mass of metal which conducts away the heat cutting
more quickly and increases tool life and vice versa.
BENCH WORK AND FLOOR WORK
Bench work in
a machine shop company involves laying or making out, assembling and the final
fitting of parts. When the same operations are performed on the floor, floor
work applies. The machine shop produces parts machined from stock material,
finished castings, forging etc requiring machine surfaces. Cylindrical and
conical surfaces are machined on the
lathe. Flat or plane surfaces are machined on a planer, shaper, milling,
broaching machine or in some cases (facing) on a lathe.
Holes are
drilled, reamed, counter bored and counter-sunk on a drill press or lathe
machine, for exact work, grinding machines with wheels of abrasive material are
used. Grinders are also coming into greatly increased use for operation
formerly made with cutting tools. In
quantity production, many special machine tools and automatic machines are in
use. The special tools, jigs and fixtures made for the machine parts, are held
in the tool room (store) ready for machine shop.
All machining operations remove metal either to make a
smoother and more accurate surface, as by planing, facing, milling e t c, or to
produce a surface not previously
existing, as by drilling , punching etc. The metal is removed by a hardened steel,
carbide or diamond cutting tool (machining) or an abrasive wheel(grinding).
All machining methods are classified according to the
operating principle of the machine performing the work.
MACHINE TOOL HAZARDS:
Machine tool and machine shop hazards comes from various
sources:
•
Improper
Guards: improper guarding of the mechanical drive (transmission)
system-pulleys, belts, gears, coupling etc.
•
Faulty
wiring.
•
Individual
motor drive- electric problem caused by breaking in the insulation on the
electrical controls.
•
All
machine tools have these points in common
•
They
use driving power of one sort or other.
•Their tools have sharp cutting edges
•They have dangerous moving parts
•They throw off flying chips or swarf
5. Method of handling materials
6.Falling objects
7. Difficulties chargeable to the work and not to the machine
8.Insufficient training
9. Carelessness
10. Overconfidence
11. Use of improper tools or worn out tools
CONCLUSION
With the ideas
shared in this paper, we have seen the machinist’s trade relevance and it’s
usefulness in bringing into reality the conceptual designs created by the
design engineer. Therefore, the need to appreciate the trade more arises.
THE PRODUCTION OF ACCURATE
GEOMETRICAL SURFACE
•
The
surface may be generated by moving the work with respect to the cutting tool or
the tool with respect to the work, following the geometric laws for the
production of the surface.
•
The
surface may be formed with a special
shape cutting tool, moving either work or tool while the other is stationary.
•
The forming method is general less
accurate than the generating method, in as much as any irregularities in the
cutter are produced on the work. In some cases
a combination of the two methods is used e.g. Thread cutting on the
lathe.
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