Marking out or scribing metals may be
performed for laying out the guidelines over the job which might exactly
represent the workshop drawing intended for the particular job work. This is a
lesson plan on marking out tools for metal.
While carrying out various
engineering processes like cutting, measuring, drilling, etc., marking out
accurate guidance lines through scribing over the job becomes very important so
that the desired operations are implemented flawlessly.
Various specifically designed precision marking tools are
employed for the above purpose.
The following lesson plan on some of
the important marking-out tools for metals provides a comprehensive study
regarding the subject, let’s study them one
by one:
The Scribing Block
It’s a precision instrument used for
accurately marking out lines over metals with respect to a standard parallel
true surface. The true surface is formed by a “marking
off table" which becomes an integral part of the whole system and is used
in conjunction with a scribing block.
As shown in the figure a
scribing block typically consists of a firm pillar holding a scriber, the
system being supported by a machine finished
precise bottom base.
The base of the tool carries a
“Vee" groove intentionally dimensioned in the respective shape for
facilitating concentric guidance over cylindrical jobs and thus becomes
suitable for checking the accuracy of cylindrical tube bases.
The sharp straight and bent ends of
the scriber are used for drawing horizontal
lines and internal marking respectively over vertical surfaces.
The marker piece at the pillar’s
bottom end of the tool is used for drawing
lines over rough surfaces.
The pillar adjuster shaft carries a
small hole for adjusting the scriber directly into while the bigger hole is
included for associating a dial test Indicator.
A sliding dowel mechanism at
the base is used to adjust the position of
the scriber over the edge of the tool, thus enabling scribing parallel to the
edges.
Scribing blocks are classified according to the height of the
incorporated pillars.
The base of scriber tools is made up of case hardened low carbon steel, hardened
and tempered cast iron, or high carbon steel. Double terminated pointed scriber
ends and dowels are made by using hardened
and tempered high carbon steel. Other parts like rocking brackets, pillar, and
fine adjustment screw are made up of low
carbon steel.
How to Use a Scribing Block
Generally the operating procedure of the tool
involves the following steps:
The pillar must
be kept almost horizontal.
The bent end of the scriber
point should be positioned downward.
Keeping the scriber as close as
possible to the pillar, its angle should be held almost
horizontal.
The operator should always position
himself on the left side of the pillar and the scriber height adjusters.
The fine adjusting screw is kept at a neutral level.
It must be
ensured that while operating the scriber its point follows the base
of the unit.
Key Seat Rule
Also referred to as “Box Square",
these tools are used for marking out surfaces of circular bars and tubular
metals with lines parallel to their axis.
Key seat rules are also made up of hardened and tempered high carbon
steel and are classified with their lengths.
The tools accuracy
can be tested by first marking out a line through one of the edges of the tool over a parallel
bar and then turning the device to see whether the line exactly coincides with
the previous marking. If it does, the key seat rule is a true one.
Surface Plates
These are used for checking the
precision of flat surfaces and also for
marking out a supported small job in order to form a base from where further
measurements can be taken.
The tools are
made up of packed grain cast iron, are machine finished and
surfaced for perfection. The edges are made into
perfect right angles and the top flat.
How to Use Surface Plates
For checking the accuracy of flat
surface of a particular job, the flat top of the surface plate tool over which
the job is to be tested is first coated with
a sort of color film (“Marking"). Once
placed over it, the color gets transferred over the job’s surface, either
spreading uniformly (if it’s true) or indicating the high spots (in case it’s
incorrect).
Surface plates are portable units and
operated over bench tops. When not used for a longer periods
of time, its surface must be protected by applying oil over it and by keeping
it covered under its wooden enclosure.
The accuracy of surface plates is verified through another true surface called “Master
Surface". If a master surface is not accessible, three different surface
plates may be checked against each other for
comparing inaccuracies.
Vee Blocks
These devices come into play when
circular jobs are involved over surface plate tools, Vee Blocks are used for holding and supporting them in place.
They are also used in conjunction with
scribing blocks for supporting and setting the scriber to correct heights.
Vee blocks are
also made up of packed grain cast iron or case hardened mild steel.
They come in identical pairs, the pairs being marked with
identical numbers or symbol. The surfaces are accurately set with “Vee" at
right angles through machine finishing.
How Vee Blocks are Used
The adjustable parallel strips over
Vee Blocks allow its height to be raised by
bolting them over angle plates. Some clearance at the center permit the Vee
Blocks for accommodating rough edges or burrs generally associated with
rectangular or square jobs, thus ensuring right seating.
A pair of Vee Blocks may be checked
for accuracy by laying a parallel test bar in the “Vees" and verifying
that it’s precisely parallel with the surface table using a dial test
indicator. An affirmative result indicates that the pair is of equal heights.