Smithy Midas 1220 XL - Manual (Page 96)

Chapter 22
Speeds and Feeds for Milling
Speeds
Milling cutting rates vary according to the machinability of the material being cut; whether
cutting fluid is used and, if so, what kind; the type, size and material of the cutter and
the coarseness of its teeth; and the amount of metal being removed. Cutting speed for
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 for a particular material (Table 22.1).
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 pi
where CS = cutting speed, mpm = meters per minute, D (mm) diameter of the cutter
in millimetres, and pi = 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 6.12
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 (Figure 22.2).
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 the best way to mill.
Down milling. Down milling usually produces good surface finishes because chips do
not sweep back into the cut. Setups are more rigid, an advantage when cutting thin
workpieces held in a vise or workpieces held in magnetic chuck. Down milling also
produces straighter cuts. We recommend down milling when using carbide cutters
because there is less wear on the cutting tool. In general, however, avoid it because of
22-1
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