Smithy Midas 1220 LTD - Manual (Page 87)

milling is the distance the cutting edge of a tooth travels in one minute. If cutting speed
is too high, the cutter overheats and dulls. If it's too low, production is inefficient and
rough.
There is no exact right cutting speed for milling a particular material. Machinists usually
start with an average speed, then increase or decrease it as appropriate. For light cuts,
use the upper end. Use the lower end for heavy cuts and when you don't use cutting fluid.
Determining rpm. To set the spindle speed, you have to know the cutter rpm (revolutions
per minute). For inch measurement, use the following formula:
rpm = 12 x CS (fpm) / D" x it
where:
CS = cutting speed
fpm = feet per minute
D" = diameter of the cutter in inches, and
it = 3.14
For metric measurement, use this formula:
rpm = CS (mpm) x 1000 / D (mm) x it
where:
CS = cutting speed
mpm = meters per minute
D (mm) = diameter of the cutter in millimeters,
it = 3.14
You can use an rpm chart for selected diameters of cutting tools at different cutting
speeds.
To change speeds, set the belts according to Figure 5.3.
Feeds
Set the direction of feed before you begin milling. Up milling, or conventional milling, is
when the direction of feed is opposite to the direction of cutter rotation. Down milling, or
climb milling, is when the direction of feed is the same as the direction of cutter rotation.
Up milling
In up milling, forces on the workpiece tend to pull it out of the vise or fixture holding it,
so fasten it securely. These forces also push the workpiece away from the cutter, which
eliminates backlash. Up milling is advised for milling cast iron, softer steels, and other
ductile materials. In general, it's how you should perform milling operations.
17: Milling
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