9x20 Lathe Variable Speed DC Spindle Motor
Last updated on Tuesday, September 19, 2023 04:25:47 PM Mountain US Time Zone


DC Spindle Motor, Motor Mounting Bracket,
DC Variable Speed Controller, Dynamic Braking,
Timing Pulleys & Belt, Cutting Performance,
Safety Guards,
Control Group

It is also good shop practice to activate
the E-stop button while setting up the lathe.

DC Spindle Motor

Baldor Model CDP3440 DC motor (USA): 3/4 HP, armature 90VDC, 7.6A full load, 1750RPM, Permanent
Magnet (PM), & Totally Enclosed Fan Cooled (TEFC). This DC motor is rated for continuous duty.
 General-purpose NEMA premium-efficiency TEFC motors can be used for constant-torque loads, but
their speed range may be limited. For example, a constant-torque speed range (CTSR) expressed as
10:1 means the motor can operate from base speed to 1/10 of base speed (1750 to 175 RPM). As a rule,
lower HP general-purpose motors can operate over a wider speed range (20:1) because of their lower
temperature rise. This
motor's 20:1 constant-torque speed range spec defines the RPM range over which
the motor will not overheat while delivering a given torque.
I have run it for hours at a time using the lathe's
full RPM range with no appreciable heat buildup. Not to be confused with a metal-working machine's
constant horsepower/variable torque requirements.   Baldor CDP3440 3/4 HP 90VDC PM TEFC Motor specs.
Baldor CDP3440 Torque Curve   Baldor Installation & Instruction Manual MN605.

Double-sealed ball bearings, 5/8" x 1.86"
shaft with a 3/16" x 3/16" x 1.4" pulley key.

The TEFC design eliminates any swarf or debris from
entering the motor & it runs very cool at all speeds.

Weight 38 lb NEMA 56C frame.
Removable cast aluminum base held
on by 1/4-20 bolts with lock washers.

Motor Mounting Bracket

Baseline Jet BD-920N lathe pulley setup
The S-link reduces the idler spring tension.

Belts, pulleys, & idler were removed.
An alternate view of the
tumble reverse
plate & gears is shown.

Removed the OEM AC motor while Macie supervised.
The four motor mounting studs were removed, too.

A 1/4" thick plate was attached to the bed using flathead M6-1 bolts.
Blue tape marks original pulley location.

The motor is sitting on a piece of wood so I could
check for carriage/DRO clearances & pulley alignment.

Since this motor is 2.5" longer, it was mounted lower
to clear the carriage & the DRO Z-scale with linkages.

The mounting locations were then scribed. The bolts
are in the middle of the adjustment range of the slots.

The full leftward travel of the carriage coincides with &
stops against the top left corner of the mounting plate.

It was good that my lathe DRO design kept the
transducers & linkage positions high, thereby providing

enough clearance & enabling me to install the 2.5"
longer variable speed DC motor at the lower position.

Four, M8-1.25 flathead bolts hold the motor.
The holes were countersunk & tapped from the backside.

The bolts were then screwed into their holes & thus
will not turn as the nuts are tightened or loosened.

Here, the motor's gray bracket was rotated before
mounting as I was looking at different shaft placements.

It has now since been rotated 180 degrees,
back to the standard NEMA 56C motor geometry.

DC Variable Speed Controller

KBIC-125 DC motor controller. 
KBIC-125 Technical Overview   KBIC-125 DC Motor Speed Controller
The large, plug-in Current Limiting (CL) 0.015 ohm
resistor (top right) is matched to the motor's horsepower.

Just left of the ceramic CL resistor is a (blue/yellow)
metal-oxide varistor (MOV) for transient protection.

Five, white/blue potentiometers left to right: MAX & MIN
(RPM), ACCEL (acceleration), IR (Load Regulation), CL.

Excellent design, components, workmanship, &
engineering support; industrial grade.

KBIC barrier terminal kit with line-in & armature fuse
blocks. 110VAC Line-in (L1, L2), Armature (A1, A2),

Field (F+, F- shunt motors only), & speed control
Potentiometer (P1 low, P2 wiper, P3 high).

On the board, I1 & I2 are for the motor
Inhibit switch (open to run & close to stop).

KBIC-125 & barrier terminal mounted onto a 1/4"
aluminum plate that has multiple tapped holes for screws.

At the corners, the inner threads are for 100 deg
flathead screws that hold the mounting plate to the box.

The outer threads are for the cap-head bolts that
hold the entire box to the lathe's metal cabinet's side.

Plate-mounted controller fastened into an all-metal
enclosure to protect from metal debris & dissipate heat.

A heat sink is not required for the 3/4 HP application.
Uses 2, ceramic, normal-acting, 12A fuses (calculated).

Since this controller can handle up to 1.5 HP, it is
overrated & therefore robust for this particular application.

Since there are no field connections,
they were rewired & relabeled for
the motor Inhibit (I1, I2) function.

110VAC goes through the E-stop & then back out to
the DC controller box. The 90DCV comes in from the

KBIC-125 to the DPDT direction control switch & from
there, down to the DC PM motor. Also shown are the

Enable/Inhibit (ON/OFF) switch & the 5K ohm speed
control potentiometer. All high-current wire is 14AWG.

Using Inhibit, instead of main power, enables the circuit
to block the motor's reverse EMF when turned OFF.

The inhibit & speed lines are shielded & then grounded
(lower right) where the controls are mounted to

electrical noise away from the KBIC-125 motor
controller circuit board. All cables have strain reliefs.

Rear view of the 110VAC IN/OUT through the E-stop.
This heavy-duty cable strain-relief design is ideal.

110VAC IN enters at the top-left corner
then over to the Main ON/OFF switch.
The 90VDC OUT is just below it.

A red wire nut ties the three grounds together.

The two high-voltage/ampere 110VAC &
90VDC lines (top) are not bundled with the

low-voltage circuit control lines (bottom) as
that could cause electrical interference/noise.

When the AC line voltage is applied to the
PCB, its green, power ON LED illuminates.

Small, knobbed screws provide no-tool, cover attachment.
Four, 10-32 bolts screw into the back of the

base plate to hold & heat sink the box to the lathe base.
The 100 lb base acts as an effective heat sink.

Infrared thermometer temperature measurements found
only a few degrees increase after extended use.

The optional KBIC heat sink for the PCB is simply
not needed for this particular mount & application.

Added a neon 110VAC main power ON indicator at the
lower corner so it is visible from a high viewing angle.

Dynamic Braking

The (gold-colored) 10 ohm 50W resistor is part of the
dynamic braking circuit. It is bolted to the metal housing &

has ceramic polysynthetic thermal compound on its base.
The dynamic braking circuit closes the controller's

Inhibit circuit while simultaneously disconnecting power
from the armatures & connecting them to each other,

through the resistor. This, in turn, uses the power being
generated by the motor's momentum to slow itself down,

exponentially. Previously, stopping times with a mounted
6-inch, 26 lb chuck were 1s to 6s, for low (100 RPM) to
high (1050 RPM) spindle speeds, respectively. Now, full
stops for the entire speed range, are well under 1 second.

The dynamic braking circuit can be found in the
KBIC-125 DC Motor Speed Controller manual on page 20.
A DPDT switch was used instead of a relay. The controller
manufacturer recommended a 10 ohm 30W resistor for this motor.

The controller is set to a 3 sec, smooth ramp-up speed
yielding an overall, well-controlled start/stop sequence.

Main Power OFF & E-stop do not activate the dynamic
braking circuit so these stops are longer & more gradual.

However, these two switches do effectively remove all power
from the lathe motor & the associated control circuitry.

WARNING: Spindle accessories must be properly tightened
as dynamically-braked stops may cause them to spinoff.

Timing Pulleys & Belt

Tramming the 28L050 timing pulley in the six-inch 4-jaw chuck.
28 = number of teeth (t), L = 3/8" tooth pitch, &
050 = 1/2" belt width. OD = 3.312".

Spindle timing pulley bored-out to 30.02mm.

Boring-out the 16L050 timing pulley to 0.6245" ID
for the
DC motor shaft using the four-inch 4-jaw chuck.
16 = number of teeth (t), L = 3/8" tooth
pitch, & 050 = 1/2" belt width. OD = 1.8825".


Keyways were broach cut. The small pulley has 2,
set screws, one of which clamps down the key.
The larger pulley is held on via a nut that can be locked.
The small ID is imperial & the larger ID is metric.

Since I have only one large lathe, these parts had
to be machined before it was

Laser digital tachometer measuring the
motor's RPM before
Max RPM adjustment.

The nut threads onto the spindle
then it is locked by a set screw.

Center-to-center pulley distance is 12.0". Using a web-based
calculator, belt size is 322L050 (32.2", 3/8" pitch,

1/2" width, 86 teeth). To verify, I wrapped a cloth measuring
tape flat around the pulleys & it measured 32.25".

The drive train upgrade eliminates: belt changes,
limit to six speeds, idler pulley mechanism, thin weak

V-belt, & low-speed safety clutch. The upgrade adds:
high-torque variable speeds from 50 to 1000 RPM

with no belt changing, doubled timing belt tooth shear
strength, dial-in
SFM values, & lower noise levels.

There is a broached, 2mm thick brass spacer ring
between the pulley hub & main timing gear for

clearance & proper spacing for the large, 30mm
spindle locking nut. Note the centered timing belt.

Cutting Performance

The 16t motor & 28t spindle pulleys set the ratio at 1:1.75. The calibrated maximum motor RPM was set at
1750. 1750/1.75 = 1000 max spindle RPM. L timing belt teeth have about twice the shear strength of XL teeth.
Small movements of the motor before nut tightening allows centering of the belt in the pulleys while running.
CAUTION: This is a dangerous procedure. Perform motor mount/belt adjustments at low speeds.

After centering it has not required any further adjustments. Torque calculations, comparing the original lowest
speed (145 RPM) to the lower speed range (50 to 150 RPM) of the new drive train, verified that the upgrade
has higher, low-end torque. A characteristic of PM DC motors is that they deliver full torque at 0 RPM.
As RPM increases from zero, power climbs very quickly (parabolically), peaking in the mid-speed range.
The max power output is at the parabola's peak which is at 875 RPM motor or 500 RPM spindle speeds.
When RPM goes down, torque increases until the motor stalls, delivering powerful, low-end performance.
 The power train delivers excellent linearity & torque throughout the 50 to 1000 RPM range. Cutoff operations
 from 50 to 400 RPM are no problem; plenty of torque being delivered by the motor & pulleys. No more
 changing belts to only six fixed speeds. The 50 to 1000 RPM range ideally suits my needs. The KBIC-125
motor controller has adjustable acceleration (0 to 4 sec). The default 3 sec acceleration gives a nicely
controlled, smooth, full-speed 1K RPM ramp-up even for a heavy load like the massive 26 lb six-inch,
 four-jaw chuck. When compared to the original Jet BD-920N lathe pulley & belt configuration, the overall
noise levels, especially at lower the speeds, have been greatly reduced. Cut surface quality is high. The
spindle has the same stable 50 to 1000 RPM speed range when turning in either CCW or CW directions.
Also, if for example, a 1000 RPM speed is set, then stopped & reversed, the readings are within 1 RPM.
The controller's IR Compensation circuit provides excellent motor regulation under normal operation.
As the cutting load varies for any set spindle speed, the KBIC-125 controller responds by varying the
current to keep the set RPM relatively constant & within the design limits. The robust dynamic braking
circuit brings the spindle to sub-one-second stops even with the heaviest loads turning at 1000 RPM.

Lathes, mills, & drill presses have constant horsepower/variable torque requirements. On applications
requiring constant horsepower, the torque requirement is greatest at the lowest speed & diminishes at
higher speeds. For example, drilling a large hole requires low speed & high torque while a small hole
requires high speed & low torque. Not to be confused with the motor's 20:1 constant
-torque speed range
Read the excellent Motor Basics reference paper (page 35) for additional design information.

This 5/8" (15.875mm) wide, negative 5 deg
carbide tool bit used to put high stress
on this machine but now that it has been upgraded, it easily makes no-chatter cuts.
The design's efficacy is thus demonstrated by the
highly improved low-
speed cutting performance.

At low RPM, drilling a 1-1/2" hole through one-inch thick
aluminum block held in the 26 lb, 6",
4-jaw chuck.

Smooth, high-torque cuts on larger
parts are a motor upgrade benefit.

This photo shows the lathe's Sound Pressure Level (SPL)
being measured with a digital meter (lower right).

The measured SPL range is from a minimum of 69 dB(C)
at 50 RPM to a maximum of 78 dB(C) at 1000 RPM.

 This graph shows the recorded sound pressure levels
for the variable speed drive. The meter is on a tripod.
The DC
motor is much quieter than the 110VAC OEM motor at all speeds.

C-weighted Sound Pressure Level (dB) plotted
as a function of lathe Spindle Speed (RPM).

The built-in tachometer displayed by the DRO
(60 RPM) integrates with the variable-speed drive.

The DPU-550 DRO also calculates SFM.
I can now dial-in speed to obtain a specific SFM (e.g., 200)

which varies instantaneously with diameter changes.
The DRO's Function 7 toggles & assigns which

X, Y, Z line for Tach or SFM are to be displayed on.
Here, SFM was assigned to be displayed on Y.

Safety Guards

Belt safety cover back on. Notched-out a section using
the band saw then I broke it off along the spot-welds.

The cover is held closed using a cabinet magnet.
The paint was scraped away at the contact area.

A small thumb hold is part of the mount.
Corners were removed & stoned for safety & comfort.

Added two shield extensions to cover the exposed
timing belt & pulley. They are fastened using blind rivets.

The small patch of retro-reflective tape on the pulley is
for the external calibration,
tachometer RPM readings.

Added an inner shield that is attached to the motor's flange.
The lower edge's 2-1/4" curve's radius was matched to the

flange. Affords extra protection when adjusting
while running. There is a small gap at the top edge
between plates so small
, anti-vibration pads are behind each
aps around the edges allow cooling air to flow
though this sealed DC motor never gets very hot.

A shield to increase safety & to keep
most swarf out of the belt area.

The bottom edge has an inward 45 degree bend to
channel cooling air from the end-mounted motor fan.

Control Group

Red E-stop, ON/OFF (Inhibit), variable speed
potentiometer, & CCW/CW spindle rotation direction.

The RPM vs. speed dial setting graph shows that
the spindle/motor & dial relationship is nearly linear.

This is a good E-stop location for me as this is
where I rest my left hand to manipulate the controls.

I also feel that when one is working close to the
lathe & the red button is located at the front of the

machine, it will be harder to hit stop due to the
crowding of one's arms & torso in that area. It is easier

to extend an arm & swat a button down than to
retract & push inward. These are personal preferences.

A circular-shaped switch guard for motor reverse. The three, small flat-head screws are 3-48.
The switch shroud has been carefully deburred so all edges are very smooth to the touch.
A 9/16" deep socket easily fits down into the shield for tightening. The tactile feel is different
enough from the start/stop switch making it reflexively harder to mistake & throw into reverse.
The reversing switch is also mounted slightly higher than the start/stop switch on the left.
The switch lever slants toward the direction of rotation where front is CCW & back is CW.
These distinctive physical switch attributes follow established ergonomic design principles.
CAUTION: For the KBIC-125 motor controller, do not change rotational direction while running.
I have tested reverse at moderate speeds with no damage, but the factory does not recommend it.
KB Electronics has a model, which is twice as expensive, that is designed for dynamic reversing.
This would certainly speed specific types of operations, for example, tapping threads.

Thread locker (green) penetrating liquid was
applied to all of the switch & potentiometer nuts.

A 9/16" deep socket was used to tighten
the reversing & ON/OFF switch nuts.

Click the photo to view a video
of the spindle turning at 80 RPM.

DC Spindle Motor, Motor Mounting Bracket,
DC Variable Speed Controller, Dynamic Braking,
Timing Pulleys & Belt, Cutting Performance,
Safety Guards,
Control Group