Heat Sources; Friction; Ambient Temperature; Machining Practices - Siemens SINUMERIK 840D Maintenance Manual

5.2.5 HEAT SOURCES

5.2.6 FRICTION

5.2.7 AMBIENT TEM-
PERATURE
5.2.8 MACHINING
PRACTICES
OCTOBER 2005
It comes as no surprise that the primary source of heat in machining is friction; however,
some of the sources of friction, and all of the areas affected by a given friction source,
may not be readily apparent. Additionally, the effects on the ambient temperature from
various heat sources in the machine shop are often overlooked.
The most obvious heat source from friction is the cutting of material itself. Heat is
transferred to the chips, which then can transfer their heat to the table. Heat is also
transferred to the tool, and then to the spindle, and then to the head. Finally, heat is
transferred to the part material itself, and from there to any fixture or sub plate holding
the blank.
Movement of the table, and of the head, also generates heat from the friction. The ball
screws turning produces friction between the nut and the screw, heating the screw and
causing expansion, with the Y axis of the machine experiencing more thermal
expansion than the X axis because the Y axis is moving more weight than the X axis.
The movement of the table on the ways produces friction which can increase the
expansion of the ball screws.
The general temperature of the shop environment will affect the machining process, but
the most significant effect will be seen when temperature differences are created.
Sunlight on a VMC will cause the side of the machine in the sun to expand at a different
rate than the side in the shade. Radiant heat sources, such as ovens or hydraulic
pumps, will heat the side of the VMC closest to them significantly more than the side
furthest from the radiant heat source, causing uneven expansion. Cooling vents without
diffusion gratings can blow cooler air onto one area of the VMC than another area, once
again causing uneven expansion.
The goal is to surround everything with an even temperature. A shop at 90 degrees will
experience more thermal expansion than a shop at 70 degrees, but a shop at 70
degrees with an air conditioning vent blowing directly on the VMC will experience more
problems with uneven thermal expansion.
There are a few machining practices that can aggravate thermal expansion, or its
perceived effects, and deserve special mention. One is the use of rapid moves in a
CNC program, which can vastly increase the friction, and therefore the thermal
expansion, on the ball screws. The obvious drawback of reducing rapid moves is a
slower production rate, however, this may be compensated for by increased accuracy
and reduced scrap rates.
Another practice is using ceramic (or other) cutters without coolant and allowing the
chips to sit on the table. The chips absorb the majority of the heat from the cutting
process, and then transfer this heat to the table, causing the table to expand
independently from any other part of the VMC.
A third practice is the use of cutting oil in place of water-soluble coolant. Although
cutting oil does reduce friction at the surface of the cut, coolant does a much better job
GENERAL INFORMATION
SIEMENS MAINTENANCE MANUAL
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