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Vmc Maintenance; Cabinet Fans; Lubrication; Machining Practices - Siemens SINUMERIK 840D Maintenance Manual

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5.6 VMC



The fans in the cabinets must be functioning properly. The purpose of the fans in the
cabinets is to cool the electronics and to move the heat out the vents. An excessive
amount of heat in the cabinets, from clogged holes or nonfunctional fans, will transfer to
the column and cause it to expand, as well as damaging the electronics. On a regular
basis, remove and clean the vent hole grates, as the vents occasionally become
clogged with dust or other particles, and inspect the fans to see that they are working.
Lubrication is essential for the free motion of the table, saddle, and head, as well as for
the nut on each ball screw. Each way must be lubricated, and the ball screws greased,
to minimize both the wear on the machine and the excess heat generated by friction.
The lubrication system must be inspected and serviced regularly. Inspect all of the ways
to confirm that each way is getting lubricated. The lubrication system may need to be
flushed by a qualified service person to remove any contaminants on the inside of the
tubes and joints.
Warm Up
The first step in combating thermal expansion effects is also the simplest. Run the
machine through a series of moves, at feeds and speeds equivalent to what will be
encountered in the production run, long enough to reach an equilibrium; that is, where
the amount of heat being generated is balanced by the amount of heat being removed
(via convection, evaporation, or absorption through expansion of material). After that
point is reached, the effect of thermal expansion in the VMC stabilizes, and the only
corrections needed are for the expansion of the part material as each successive part
feature is machined.
Locate fixture offsets and establish tool length offsets after the machine has attained
the optimum operating temperature. Avoid using moves or spindle speeds that exceed
those used in the part program.
In addition to running a warm up routine at the beginning of the production shift, the
same kind of routine can be run during breaks. This will maintain the VMC at operating
temperature, especially the two subsystems that heat up or cool down the most (the
spindle and the ball screws).
Rough Cut / Cool Down / Finish Cut
A complement to the warm up of the VMC is the use of a rough cut to remove most of
the material from the part. Although the part material heats up, the material can then be
cooled to a stable temperature. Then, a series of finish cuts can complete the part in a
short time, so that the material doesn't have time to expand beyond acceptable


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