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184                              THE   MACHINE-SHOP

The point of a drill must be exactly central with the shank, and the lips
of equal length and angle; otherwise the work will not be shared equally, nor
will the hole be true to size, nor can it be drilled at maximum speed.
Thinning the lips in the larger drills (fig. 7) contributes to efficiency, espe-
cially as the tools wear back. Longitudinal clearance is the slight reduction
in diameter from lips to shank, which enables the drill to clear itself in its
hole. It ranges from 0-00025 to 0-0015 *n- Per *nch 'm length. Peripheral
clearance (fig. 7) is that round the circumference of the drill, starting from
the " land " a (compare with fig. 5), which backs up the cutting edge and
preserves the diameter.

Speeds and Feeds.—The performances of twist-drills vary greatly,
the controlling conditions being the quality of the drill, the degree of accuracy
of the clearances, the care exercised in grinding, the nature and the amount

A                  • B                       C   '                                   't—'
Fig. 6.—Effect of Angle on Chisel Edge or Drill Point                          Fig. 7.—End View of Drill
of the lubricant used, the build of the drilling machine, and the mass of
the work being drilled. Published tables of performances afford but a
general guide, to be accepted with caution. These performances may be
exceeded by as much as 100 per cent in exceptionally favourable conditions.
Speeds are usually stated in terms of carbon-steel drills, to be doubled
when tools of high-speed steel are used. Peripheral speeds are stated per
minute. Average speeds are: for cast steel, 20 to 30 ft. per minute; tool
steel, 30 ft.; malleable cast iron, 45 ft.; cast iron, 40 to 50 ft.; brass and
bronze, from 60 to 200 ft. per minute. Feeds of from 0-004 to °'°O7 in-
per revolution are employed for ^-in. drills, increased to from 0-005 to
0-015 in. for those of larger sizes. Generally it is better to increase speeds
than feeds.
Lubrication.—The efficiency of a drill depends on proper lubrication
more than on any other factor. Generally soda water, soapy water, or
emulsion are used, cast iron and brass being the only substances which are
drilled dry. Many recipes exist for making up an efficient lubricant, and
results are so largely dependent on an abundant supply of the lubricant
being provided, that a good many drills, especially those used in turret
work, have oil passages through which the lubricant is forced under pres-
sure to the lips. Sometimes oil grooves are formed within the body of the
drill, a more satisfactory method than letting tubes into grooves cut around
the periphery between the flutes. Such grooves provide ample room for
the escape of the chips. Drills with internal tubes are fixed in a turret,