High-Performance Drill Motor Modification
● by Bryan Ruddy
PHOTO 1. The 36-volt De Walt DC900KL
contains a gearmotor rated for 750 watts of
power and is capable of a bone-crushing 200
foot-pounds of stall torque in low gear. On
paper, this gearmotor should be capable of
moving a 200 pound robot at about five miles
per hour in a 4WD configuration.
Cordless drills are among the
richest sources of affordable
gearmotors for the robotics hobbyist;
from inexpensive, low-power imports
(such as the drills offered by Harbor
Freight) to high-performance, name-brand drills (particularly De Walt).
Unfortunately, drill gearmotors lack
convenient mounting features, often
have minimal bearings, and generally
contain slip clutches, making them
unsuitable for use in robotic drive
and mobility applications without
modifications. While there are
kits for some drill gearmotors, most
motors must be modified by the
hobbyist. The photos here document
my modifications to
De Walt’s newest and
most powerful drill
PHOTO 4. As shown on the left, the first-stage
ring gear (part #628002-00) is free to rotate as
part of the clutch mechanism. To lock this gear
in place, we can machine it down to a square,
as shown on the right. The modified gear
presses into the motor mount.
PHOTO 6. The stock gearbox
output has an overrunning
clutch to prevent back-driving.
Since this can cause the output to
lock up under some conditions,
it must be disabled by removing
the five pins and outer ring.
PHOTO 2. The gearbox (part #629059-00) and motor (part
#639521-00) are shown without the drill casing. They are
mounted to the casing by the small plastic tabs where the
motor and gearbox meet. To mount the gearmotor securely
to a robot, we will make a metal replacement for the stock
PHOTO 5. The DC900KL uses a three-speed
gearbox — for robotics use, the gears must
be secured for a single speed. The stock
gearcase (top) uses a set of molded-in teeth
to keep the ring gears (bottom) from turning
when engaged; these have been duplicated in
the magnesium gearcase (right).
PHOTO 3. The original motor mount is shown on the left, with
its twist-lock interface to the gearbox. An alternative mount
— machined from a block of magnesium — is shown on the
right. Long bolts will be used to assemble the motor mount
to the new gearcase.
PHOTO 7. The gearbox output (left)
uses a simple double-D shaft
geometry, duplicated in the hardened,
high-strength shaft on the right. The
step in shaft diameter allows the new
gearbox to be protected from impacts
by a bronze thrust bearing.
PHOTO 8. The completed gearcase, with
motor and gearbox parts installed, is
shown here. The parts were made on CNC
equipment, but could be made manually
with slight simplifications. The motor,
gearbox, and spare ring gears are
available from www.dewaltservicenet.
com, at a total cost of $110 per
30 SERVO 07.2008