d bored. The other circle is then centered
in the same way for boring the second hole. As will be seen, the
accuracy of this method depends first, upon the accuracy with which the
circles were laid out, and second; upon the care taken in setting them
concentric. For a more accurate way of locating parts for boring, see
"Use of Center Indicator" and "Locating Work by the Button Method."

=Turning Brass, Bronze and Copper.=--When turning soft yellow brass, a
tool should be used having very little or no slope or rake on the top
surface against which the chip bears, and for plain cylindrical turning,
the point of the tool is drawn out quite thin and rounded, by grinding,
to a radius of about 1/8 or 3/16 inch. If a tool having very much top
slope is used for brass, there is danger of its gouging into the metal,
especially if the part being turned is at all flexible. The clearance
angle of a brass tool is usually about 12 or 14 degrees, which is 3 or 4
degrees greater than the clearance for steel turning tools. Most brass
is easily turned, as compared with steel, and for that reason this
increase in clearance is desirable, because it facilitates feeding the
tool into the metal, especially when the carriage and cross-slide
movements are being controlled by hand as when turning irregular shapes.

The speed for turning soft brass is much higher than for steel, being
ordinarily between 150 and 200 feet per minute. When turning phosphor,
tobin or other tough bronze compositions, the tool should be ground with
rake the same as for turning steel, and lard oil is sometimes used as a
lubricant. The cutting speed for bronzes varies from 35 or 40 to 80 feet
per minute, owing to the difference in the composition of bronze alloys.

Turning tools for copper are ground with a little more top rake than is
given steel turning tools, and the point should be slightly rounded. It
is important to have a keen edge, and a grindstone is recommended for
sharpening copper turning tools. Milk is generally considered the best
lubricant to use when turning copper. The speed can be nearly as fast as
for brass.

=Machining Aluminum.=--Tools for turning aluminum should have acute
cutting angles. After rough-grinding the tool, it is advisable to finish
sharpening the cutting edge on a grindstone or with an oilstone for fine
work, as a keen edge is very essential. High speeds and comparatively
light cuts are recommended. The principal difficulty in the mach