| Backlash is a term that applies to machine tools. Backlash refers to
the "play" or excessive amount of clearance between the
machine hand wheel screw and the nut attached to the machine table or
slide (Figure1). |
Figure 1. Backlash
|
Figure 2. Screw rotation direction
maintains contact between the screw and the table nut. |
Backlash is common on manual machine tools, especially milling
machines, and is due to wear or adjustment. Backlash must be compensated
for. To compensate for backlash, you must get, and keep the slide
(table) and screw in contact with one another (Figure 2). |
To accomplish this, when positioning the table, you must always move the
table in the same direction. When positioning the table for locating a drilled
hole for example, the direction you moved the table to pick up the edge of the
part is the same direction the handle needs to be turned for positioning. If
you go past the required setting, it is necessary to go back way from the
setting at least one-fourth of a turn to eliminate the backlash and then come
back to the required dial setting.
| It is also very important to keep in mind the cut direction as it
relates to backlash. On conventional milling machines, that are not
equipped with backlash eliminators, the feed direction should always be
against the spindle rotation (Figure 3). |
Figure 3. Conventional Milling |
Figure 4. Climb Milling |
This is known as conventional or up milling. By working the cutter
rotation against the feed, the backlash is taken up. Climb milling, or
down milling, must be avoided on a machine not equipped with a backlash
compensation mechanism. In climb milling, the feed direction is into the
cutter rotation (Figure 4). |
Therefore, climb milling, as the name implies, causes the tool to want to
climb onto the part. When this happens, the workpiece gets pulled into the
cutter causing the excessive cutting pressure to seriously damage the
workpiece, cutter, and milling machine arbor.
CNC machines are equipped with ball screws and are designed to machine with
the climb milling process. Wherever possible Climb (down) milling should be
done especially with larger cutters. Before operating any milling machine,
find out if the machine is equipped with a backlash eliminator.
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