3. Preparation for shaping of surfaces standing at an angle to each other and stepped surfaces
Prophylactic health and fire protection and labour safety are an
essential requirement for carrying out machining of material.
Therefore, the following requirements are to be met:
- Keep your workplace always clean and in order. So,
among other things, you will not lose any time in looking for things you need
for work.
- Use only such working tools which are in an unobjectionable
condition.
- According to the job use appropriate protection for your body
(safety goggles in case of flying chips, protective gloves against sharp-edged
workpieces).
- For cleaning, checking and clamping place the machine out of
operation. Do not make safety devices ineffective.
- Use appropriate aids for chip removal (brush, broom etc.).
- In case of defects which affect the operational reliability of
the machine do not start the machine and eliminate the defects.
- During shaping great lateral compressive forces (shaping) or
support pressures (slotting) often occur. Make use of the work-holding equipment
in such a way that the workpiece will not be permanently deformed.
When is it necessary to wear safety
goggles?
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Prepare the sequence of operations in such a manner that all
necessary working tools and objects of work can immediately be used without
losing any time.
This includes above all:
- Receive the work order and the workpieces to be
machined. Check the workpieces for completeness and accuracy to size, i.e. check
the premachining quality.
- Read the technical drawing (working drawing) and the work
accompanying sheets.
- Lay out and check the necessary working means (tools,
measuring and testing means, clamping and auxiliary equipment) according to the
work order.
- Determine the cutting values.
- Fix the sequence of operations.
Clamping the workpieces
When machining is made on shaping machines, work-holding
equipment has the following functions:
1. Positioning - fixing of the workpiece position to
the tool
2. Clamping - counteraction to the cutting forces occurring during
the machining process
The clamping elements to be used must bring the workpiece into
the necessary position to the tool and neutralize the cutting forces.
Therefore, each workpiece clamping must
- be firm, safe and definite in position and
-
ensure the clamping reliability without resulting in permanent deformations of
the workpiece.
For this reason greatest importance shall be attached to a
job-related, firm workpiece clamping according to labour safety.
The choice of the respective clamping equipment depends on
- the shape, size and number of workpieces to be
machined,
- the cutting values to be applied,
- the workpiece position,
its direction and quality.
Proper use of the clamping means requires to know exactly what
cutting forces occur.
The following main clamping means are used:
- machine vices
for chucking small workpieces
with parallel bearing surfaces (most frequently used kind of clamping
workpieces) Positioning of the machine vice is made by means of
tongues.
Figure 15. Machine vice
- clamps
for clamping large workpieces on the
machine table or thin-walled workpieces which would bend in the machine
vice
Figure 16. Clamp (1 clamp, 2
workpiece, 3 worktable, 4 base)
- clamping bolts
for
mounting further clamping means or workpieces with bore holes
Figure 17. Clamping bolt
- clamping dies
for clamping workpieces which
cannot be clamped by means of clamps by making use of the wedge
effect
Figure 18. Clamping dies in use (1
workpiece, 2 fixed clamping die, 3 movable clamping die, 4 workpiece clamping
area)
- angle-plates
as rotatable or tiltable clamping
possibility with the help of T-slot bolts
Figure 19. Angle-plate
- electromagnetic chucks
for chucking workpieces
which are machined with small cutting forces (finish machining and fine
finishing)
Figure 20. Electromagnetic chuck
(1 plate, 2 pole strips)
- clamping supports
for height adjustment when
clamps are used
Figure 21. Clamping support
- fixtures
They are used for clamping workpieces which due to their
geometrical shape cannot or only with considerable effort be clamped with usual
clamping means such as vice, stops and clamps. This is the safest kind of
clamping ensuring high quality. It is, however, dependent on the specific
workpiece and construction of the fixture.
- clamping the workpiece on the machine table under
compression.
Here the workpiece is clamped from both sides by means of
clamps.
Figure 22. Clamping the workpiece
on the machine table under compression
- clamping the workpiece on the machine
table
Clamping is reached by lateral clamping forces.
Figure 23. Clamping the workpiece
on the machine table (1 stop, 2 workpiece, 3 work supporting block, 4 clamping
piece with clamping nut)
On principle, when clamping workpieces, take care to ensure that
- an exact positioning of the workpieces to the
primary motion is reached (adjust clamping means or workpieces)
- parallel-ground bases are used
- clamps are resting horizontally
- those points on the workpiece which are sensitive to
distortion are protected (bases, wedges etc.)
- the clamping elements are located as close to the workpiece as
possible and are arranged in such a manner that the cutting forces act against
the fixed stops
- impurities between locating and supporting surfaces are
removed
- the clamping forces act against fixed back rests (stops,
supports).
Clamping of tools
As tool clamping device the tool holder is used which in the
case of shaping is also called tool post. The tool is fixed by means of clamping
bolts. The tool holder is arranged to swivel around its axis. This makes
machining of acute-angled surfaces possible.
Figure 24. Tool holder (1 tool, 2
clamping bolt, 3 tool post, 4 clapper, 5 clapper box, 6 fulcrum pin)
When clamping the tools, make sure that
- the tool inserted is stable enough to neutralize
the cutting forces occurring
- the tool is clamped as short as possible to
prevent bouncing or twisting phenomena
- the clamping surfaces are free from
burrs and impurities
- point-like clamping forces in hard tool material
(super high-speed steel and high-speed steel) are distributed to larger surfaces
by means of supports.
What is the fundamental function of work-holding
equipment?
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Mention three important clamping means for carrying out the
technique of
shaping.
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What are the advantages and disadvantages of using
fixtures?
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Choice of cutting values
When determining the cutting values, pay attention to the
following hints:
- As a rule, the shaping and slotting tools consist
of high-speed steel (HSS) or super high-speed steel (SHSS) or they are compound
tools with carbide tipping. This is necessary because the impact load during
starting of the cut does not allow the use of other superhard cutting materials.
- High-speed steel cutting materials are more break-proof than
those made of hard metal, however, they do not allow high cutting speeds.
- Liquid coolants are relatively seldom used for shaping, as the
tool is not constantly engaged. If, nevertheless, coolant is used, the tool life
(mainly when high-speed steel is used or the cuts are long) and the surface
finish of the workpiece are increased and improved.
- When steel is machined with carbide-tipped shaper tools, the
necessary high cutting speeds are often not reached for reasons resulting from
the machine. Thus, built-up cutting edges and a considerably higher load of the
tool cutting edge occur. Lower cutting speeds with the use of high-speed steel
prove to be more favourable in this case.
- The grind of the tool is of decisive influence on the quality
of machining and the life of the tool. The angle of inclination of the cutting
edge, which due to the favourable chip disposal is chosen to be mostly negative,
is of special influence.
Figure 25. Shaper tool (1 angle of
inclination at shaper tool)
In the following machining recommendations for v (cutting speed)
are given:
When the technique of shaping end faces is used, vm is to be
halved once more due to the unfavourable cutting conditions.
What mean cutting speed (vm) is chosen for shaping an end face
with a tool made of high-speed
steel?
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A workpiece made of St 38 with a length of 80 mm shall be
machined by face-shaping. Determine the number of double
strokes.
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When determining the return stroke, choose the return speed to
be as high as possible (time-saving). If the return speed is fixed by the
construction, no choice is possible.
Take into consideration the accelerating and braking forces of
the table or ram in case of small strokes.
Choice and application of measuring and testing means as well as
coolants and lubricants
The use of necessary coolants and lubricants as well as other
auxiliary equipment and utilities (brushes, compressed air etc.) shall also be
taken into account.
In most cases the use of coolants and lubricants is not
necessary, as the interrupted cut with sufficient chip removal prevents
generation of much
heat.
is recommended for simplicity.
