Like smaller spade bits. Reducing the rake angle decreases the depth of cut and the power absorbed.
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"The twist drill bit was invented by Steven A. Morse of East Bridgewater, Massachusetts in 1861.The original method of manufacture was to cut two grooves in opposite sides of a round bar, then to twist the bar (giving the tool its name) to produce the helical flutes. Nowadays, the drill bit is usually made by rotating the bar while moving it past a grinding wheel to cut the flutes in the same manner as cutting helical gears."
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See Fig. 19. "English and American Tool Builders" claims on page 196 that the first one (Fig 16) was made to machine twist drills. The rotary index head is geared to the table feed and the table swivels on its circular base, allowing the mill to cut a spiral groove as the slowly rotating work passes under the cutter. See fig 29.
The text states but the engraving doesn't clearly show that the table of Fig 16 swivels.
Here's an indexing head like Fig 19's by itself, minus the table feed connection:
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I normally leave the mill set up for drilling and use the tops of the vise jaws as the table.
--jsw