Introduction to Basic Manufacturing Processes and ... - always yours

328 Introduction to Basic Manufacturing Processes and Workshop Technology
smoke, fumes and gases, excessive shock or vibrations, steam and corrosive
atmosphere, etc.
32. One should not work on the wiring of an arc welding machine unless qualified to
do so.
33. Welding equipment used in the open and shall be protected from weather conditions.
If it has been wetted it shall be thoroughly dried before being used.
34. Proper terminals should be used on the arc welding machines for the power line
voltage connection.
35. Neither terminal of the welding generator shall be bonded to the frame of the
welding machine.
36. Periodically clear out the accumulated dust from the welding machine with suction
cleaner as this will not blow dust into other parts of the machine.
37. Over greasing may foul the commutators of DC generator.
38. Check and, if necessary, clean commutators of DC generator periodically, using fine
sand paper.
39. Excessive sparking may result in a worn commutator of DC generator which may
be cured by skimming in a lathe.
40. Brushes should move freely and have adequate spring tension. This can be tested
by lifting and releasing them. Brushes should snap be firmly against the commutator
of DC generator.
41. Greasing points need attention periodically.
42. Switch contacts should be cleaned periodically.
17.7 RESISTANCE WELDING
In resistance welding the metal parts to be joined are heated by their resistance to the flow
of an electrical current. Usually this is the only source of heat, but a few of the welding
operations combine resistance heating with arc heating, and possibly with combustion of
metal in the arc. The process applies to practically all metals and most combinations of pure
metals and those alloys, which have only a limited plastic range, are welded by heating the
parts to fusion (melting). Some alloys, however, may welded without fusion; instead, the parts
are heated to a plastic state at which the applied pressure causes their crystalline structures
to grow together. The welding of dissimilar metals may be accomplished by melting both
metals frequently only the metal with the lower melting point is melted, and an alloy bond
is formed at the surface of the unmelted metal.
In resistance welding processes no fluxes are employed, the filler metal is rarely used
and the joints are usually of the lap type. The amount of heat generated in the workpiece
depend on the following factors:
(1) Magnitude of the current,
(2) Resistance of the current conducting path, and
Mathematically, H = IVt
= I(IR)t
= I 2 Rt