Machinery Repairman

Example: What is the spindle speed for a
1/2-inch cutter running at 45 fpm?
45
rpm = 0.2618 × 0.5
rpm = 343.7
To determine cutting speed when you know the
spindle speed and cutter diameter, use the following
formula:
Example: What is the cutting speed of a 2
1/4-inch end mill running at 204 rpm?
FEEDS
The rate of feed is the rate of speed at which the
workpiece travels past the cut. When selecting the
feed, consider the following factors:
Forces are exerted against the work, the cutter,
and their holding devices during the cutting
process. The force exerted varies directly with
the amount of metal being removed and can be
regulated by adjusting the feed and the depth of
cut. The feed and depth of cut are therefore
interrelated, and depend on the rigidity and
power of the machine. Machines are limited
by the power they can develop to turn the cutter
and by the amount of vibration they can
withstand during coarse feeds and deep cuts.
The feed and depth of cut also depend on the
type of cutter you are using. For example, do
7-51
not attempt deep cuts or coarse feeds with a
small diameter end mill; it will spring or break
the cutter. You can feed coarse cutters with
strong cutting teeth at a relatively high rate of
feed because the chips will be washed out
easily by the coolant.
Do not use coarse feeds and deep cuts on a frail
piece of work or on work mounted in such a
way that the holding device will spring or
bend.
The desired degree of finish affects the amount
of feed. A fast feed removes metal rapidly and
the finish will not be very smooth. However, a
slow feed and a high cutter speed will produce
a finer finish. For roughing, it is advisable to
use a comparatively low speed and a coarse
feed. You will make more mistakes if you
overspeed the cutter than if you overfeed the
work. Overspeeding is indicated by a
squeaking, scraping sound. If chattering
occurs in the milling machine during the
cutting process, reduce the speed and increase
the feed. Other common causes of chattering
are excessive cutter clearance, poorly
supported work, or a badly worn machine gear.
One procedure used to select an appropriate feed
for a milling operation is to consider the chip load of
each cutter tooth. The chip load is the thickness of the
chip that a single tooth removes from the work as it
passes over the surface. For example, when a a cutter
with 12 cutting teeth and a feed rate of 1 ipm turns at
60 rpm, the chip load of a single tooth of the cutter
will be 0.0014 inch. An increase of cutter speed to
120 rpm reduces the chip load to 0.0007 inch; an
increase of feed to 2 ipm increases chip load to 0.0028
inch. Use the following formula to calculate chip
load:
Chip load =
feed rate (ipm)
cutter speed (rpm) × number
of teeth in the cutter