Repair of Machine Tools
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Repair of Machine Tools
Repair of Machine Tools - Machine tool parts wear and tear due to their use and with the passage of time owing to neglected maintenance or environmental effects. Even with the most perfect attention to cleanliness and lubrication, wear and deterioration are bound to occur, but these may be evened up and minimized by the observation of certain precautionary principles.
Many machine tools are used over long periods for a certain very narrow range of work. This results in wear occurring at much localized points. A lathe, for example, employed continuously on a short, turning job which always occupies the same position between the centres will eventually develop wear on the bed over the short portion traversed by the saddle, and to correct this, a major overhaul would necessitate a complete resurfacing of the bed.
Although
it
might
not
be
economic
from
the
operational
aspect
to
vary
the
class
of
work
allocated
to
machines,
it
would
certainly
result
in
evening
out,
and
prevent
localization
of
wear,
resulting
in
longer
periods
before
the
loss
of
accuracy
becomes
sufficient
to
necessitate
a
major
overhaul.
The same principle applies to the leadscrews employed for actuating slides where, if the slide is always used in one place, the threads of the screw will be subjected to a localized thinning, and even with a new nut, backlash will be present unless the whole length of screw is corrected.
Deterioration and fretting take place where looseness is allowed to develop, and a very flagrant example of this once occurred with the fit of a shaft in the inner race of a ball bearing. It was true that the grease in the bearing had dried up long before the trouble asserted itself, but when the unit was dismantled.
The
end
of
the
shaft,
where
it
fitted
in
the
race,
was
about
half
its
normal
diameter,
the
remainder
of
the
metal
being
present
as
dust.
Owing
to
an
initial
loose
fit,
a
process
of
wear
and
fretting
had
taken
place
between
the
shaft
and
the
bore
of
the
ring
terminating
in
the
result
explained.
Looseness in adjustments often results in uneven wear taking place, examples of which are liable to occur in slides. If the adjusting strip does not keep the slide in good contact over its whole length a certain amount of side oscillation will accompany the longitudinal movements of the slide, so that when wear at the ends of the Ways due to the slackness of the adjusting twisting effect strip is taken up, there may not be a straight face left upon which the slide can bear.
| A - Regions of Wear | B - Slack | C - Slide |
Sometimes uneven wear is promoted by some inherent weakness or defect in the design of a machine. A good (or bad!) example of this occurs with the carriage unit of an ordinary lathe. The tool pressure occurs at some point about midway between the slide guiding surfaces, but the propelling force of the feed or leadscrew is applied at a point some distance off set, at the front of the machine bed.
Normal Arrangement
| 1 - Cutting pressure | 2 - Feed Shaft | 3 - Feeding Pressure |
Central Feed Shaft
| 1 - Centre Feed Shaft | 2 - Feeding Pressure | 3 - Cutting Pressure |
Twist in Lathe Carriage with normal and centrally placed Feed Shaft subject the saddle to a twisting moment which promotes uneven pressure and wear on the slides. On one or two lathes the saddle feeding arrangements are arranged in the centre of the bed, and this makes for a closer approach to the ideal reactions of force as shown at (b).
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