I would like to share with you details of a CNC conversion I did for a
South Bend SB1001 lathe. This conversion is interesting because the parts were 3D
printed and the controller is an Arduino. It is also small enough that I could get it
down into my basement. My dad and I were able to lift the lid onto the bench
without any tools. This is what we're going for, two motors mounted on the lathe, one
controlling cross feed and one controlling the carriage.
No attempt was made to control spindle speed although it would be pretty
straightforward by replacing the speed potentiometer with a DAC. After the
conversion we'll be able to easily make parts like this, parts that have multiple
radii and ball ends. This lathe was selected partially because after a
conversion there would be few parts that would be discarded. Depending upon the
accuracy we'd would like to achieve, a great deal of the lathe could be used as is. For
higher accuracy, we'd replace the lead screws with ball screws. In the 3D
printed conversion, to keep costs under control,
I didn't use a ball screw but I did replace the lead screw with a new Acme
screw. Replacing the cross feed leadscrew is challenging because of the small amount
of clearance that's available.
Another reason that I 3D printed the conversion parts is I had spent a lot of
time on this model. That may be a drawback of CAD. It's easy to make new
versions, so it's easy to never be satisfied, and you just keep iterating. At
some point you just have to say that you're done,
and that's the point that I had reached when I 3d printed the parts. I didn't
have access to a mill. Doing this also helped me to explore some of the
advantages and limitations of 3D printing for mechanical design. The
motors that I used were on loan from a friend.
They are manufactured by Oriental Motor and they're closed loop stepper motor.
They're directly attached AC mains without a need for power supply.
They have a built-in resolver for feedback so there's never lost steps.
The motors are very undersized with respect to what is normally used for
a conversion. Most likely motors are chosen they're done so with a large amount of
margin.
This is a chance to explore the suitability of smaller motors. These
motors also have a nice speed-torque curve. There isn't a lot of torque lost with
high RPM and they may have to do with the higher operating voltage. Here the
Arduino sends pulses to the motor driver and we're measuring the maximum
RPM with a stroboscope. The speed-torque curve suggests the maximum
speed is 4000 RPM. I was never able to achieve that high of a speed. That likely
has more to do with the Arduino than it does with the motor. The easiest way to
use a stroboscope is to start with the highest speed and decrease it until you see
only one stationary copy of the shaft flat. As you pass through multiples of
this shaft speed, you'll see copies of the flat.
Once you reach the true speed of the shaft will see only one copy.
However, if you continue to decrease the flash rate, you'll see other speeds at
which the shaft appears stationary.
These are not true speeds of the shaft but instead fractions of the actual
speed. An Arduino Mega was used to generate step and direction pulses.
Custom software is written for the Arduino which took a function to
generate the longitudinal and cross-feed motions. G-code was not used. And while the
shapes that can be generated from this program are limited,
the emphasis was on development time, so the ideas of the conversion can be
tested. Here the motors are being exercised by the controller.
The motor that is turning the most is the longitudinal axis motor. The motor is
turning the least is the cross-feed motor. If you watch carefully you might
recognize retractions and rapid motions. The Oriental Motor drives have nice terminal
breakouts and make wiring easy.
Unfortunately, that wasn't enough to make my wiring job look nice.
If you notice, there is an unusual sound to the motors. I'm not sure what caused
this noise.
I thought maybe it was in the coupler but I don't know for sure. The Arduino
is connected to a laptop, but it's only for programming the Arduino. There's no
communication otherwise between the laptop and Arduino. The Oriental motor drives
are a little larger than other drives, but they don't require a separate power
supply so you actually save space.
Now we're going to begin the actual conversion and the first method that
we're going to use is replacing the hand wheels.
We simply take the handles off and bolt the motors in place. This allows one to
switch between CNC and manual operation with minimal effort.
The CNC conversion is not permanent. First, we'll replace the cross-feed hand
wheel. The adapter is in two parts. The opaque part is the motor adapter. The
same motor adapter will be used for both axes. The transparent part mounts against
the cross-feed slide. By printing in two parts, it's easier to maintain the
parallel surfaces between the motor
and the cross-feed slide. Both of these surfaces are flat with the print bed.
If we attempted to print this as one object, there would be overhang to deal with and
maintaining that parallelism becomes harder.
The two parts are held together with a couple of screws. For all of the printed
parts, one of the original MakerBots was used. I have since replaced the MakerBot
with the Taz5 but I believe the MakerBot produced better quality prints.
All the parts are printed in ABS.
You may be able to see a little bit of delamination in this part near the top. Likely,
those layers are where the screw holes are at. Here we're making a coupler to attach
the motor to cross feed lead screw.
First mount the adapter in place. The original screw holes are used. The
coupler from Misumi installed.
This shows the relation of the coupler
to the motor adapter. Again, this same motor adapter is used on both axes. The
slits near the base of the adaptor provides access points for allen wrenches to the
coupler. All holes are printed undersized and drilled out the final size on a drill
press.
And here the motor has been slid onto the coupler and the screw is being
tightened.
Now the cross feed axis is complete.
An improvement was made to the cross feed adapter which retained access to the
dial indicators.
The MakerBot was able to span this overhang well. With 3d printing it's easy
to produce parts that would be difficult by traditional machining methods.
There's a little bit of delamination at the layer of the overhang. A little bit
of Super Glue is added to the crack just to strengthen the part. And finally, the
improve crossfade adapter is installed. The carriage is moved with the hand wheel
via a rack-and-pinion system.
This is the design of the adapter that will replace the carriage handle. Again
the motor adapter design is identical to the one used on the cross-feed. The
orientation of the print has an effect on its strength.
And here the assembly is ready to be mounted onto the apron.
In the side of the apron two holes are drilled and tapped.
The carriage adapter is installed. Our motor will use the existing gear
reduction in the rack and pinion system of the carriage.
And finally the carriage motor is installed.
A lathe operator mainly uses the carriage and cross feed handles.
By replacing these with motors, we now have a machine that can replicate the
same motions.
However, under computer control more complicated shapes can be easily made.
Coordinated motion between the carriage and cross feed could eliminate the
compound providing additional swing over the cross feed slide.
This is a test of the basic motions and to verify the motors have sufficient torque.
These motors are closed loop and cannot lose steps. When I create
resistance and stop the carriage, the motor does not lose the steps.
Eventually, it catches back up. Ideally, when resistance is encountered on one axis
the second axis would slow down to accommodate. Perhaps also the spindle
should slow down. The motor has just barely enough torque. In order to move
the carriage using the rack and pinion system
there is a split in the rack that takes a little bit extra power get pass.
Occasionally, I assist the motor by pushing the carriage pass the split in
the rack.
Here we see the servo behavior in action. Steps that would have otherwise
been lost are recaptured. In a normal conversion, the carriage would not be driven
using the rack and pinion system.
There's too much slopped in it. Instead a ball screw would be used to drive the
carriage. Instead of the expense of a ball screw, I'm going to use the existing
leadscrew. Initially, I had tried driving the carriage using the existing leadscrew
half-nut. This had far too much friction an drequired more torque than what
the motors were able to provide. A new smoother running ACME nut was installed.
Also tested was a zero backlash ACME nut. The zero backlash ACME nut was made from
Delrin and had a spring that expanded to compensate for wear. Unfortunately, it
had more resistance than the half nuts and could not be used. Instead a new
ACME nut was installed. This is the power end of the lathe.
The shaft that's coming through this tension arm is the lead screw. Onto the leadscrew
mounts a series of gears that are used to synchronize the spindle to
lead screw speed.
This is necessary for threading operations. In place of the gears, we'll
mount the motor and drive the lead screw with it.
This is the design of the motor and lead screw adapter. The handle will mount onto
the tensioning arm.
There's only that single point of attachment, but attachment is straightforward.
Here we see the adapter installed.
The coupler is slid into place and the motor and motor adapter are installed.
A clamp is installed to hold the lead screw in place. This clamp is also made
from Delrin and care must be taken and tightening this type of clamp as are
easy to break. We'lll attach the lead screw to the carriage using a new ACME nut and an
adapter. The horseshoe boss is increased the strength of the part. It's probably not
necessary but it was easy to add with 3d printing. The threads for the ACME nut
are printed.
I did not use a tap to true them.
And here we see the ACME leadscrew nut and its adapter installed on the
carriage.
We're still driving the cross feed with the previous system, but we're driving
the carriage with a lead screw that has less backlash than the previous
rack-and-pinion system. The backlash of the cross feed is minimal compared to
the rack and pinion of the carriage. With a more sophisticated controller that can
accept G-code such as LlinuxCNC this machine could already produce basic parts. While
not of high-precision, it could still be useful for ornamental turnings. For all the
tests I use wood. It's less expensive and will be more forgiving if there is a
problem. With the first lead screw base system.
I hadn't installed support at the far end. Because the motors have low torque,
any reduction in friction is useful. We'll try to stabilize the lead screw by
installing the support at the far end.
This new adapter had better rigidity than the previous adapter that was
attached with only one screw.
This overhang
was too large to print without support. The support material comes out easily
although there are a few burrs left. The burrs are quickly cleaned up with a Dremel
sander mounted a drill press.
This is the new leadscrew adapter mounted in place of the original bearing
support.
Because we have shifted the leadscrew, we can shorten the support and and make
it more rigid.
We again make use of the existing holes to mount the support. To this point, the lead
screw has been supported by the plastic parts, and while the friction isn't bad,
the lead screw is rubbing against and wearing the plastic parts. To further
reduce friction and to improve longevity, we'll add thrust and ball bearings to the
power and support ends. This is the support end assembly. Another collar
clamp is used which presses against thrust bearings pushing against the ball
bearing into the support assembly. A small amount of support material is used.
Here the support end is with the ball bearing inserted. The power and adapter
is a bit more complex. Ball bearings are installed on either side of the adapter,
pressed and held in place with thrust bearings and collar clamps. To
accommodate the bearings, the entire adapter had to be increased in size and a small
cutout was necessary for the screws. The lead screw was turned down slightly to
fit the ball bearing on the support end. With the exception of the motor and the
ACME lead screw these are all the parts that were installed for the final CNC carriage.
The controller is pretty simple,
the motion is pretty slow, I didn't program it with acceleration. The top
speed of the motor is limited by what is able to achieve from a standstill before
stalling.
It may look like the part is changing the rotational speed but it's just an
artifact of the camera and lighting.
I don't know if I would recommend this approach for making a CNC lathe to
anyone, but it was a fun project. It only took a weekend and it was low cost and
it gave me something tangible to show for all the time that I put into
building the model of the lathe.
All the files for the lathe model and the CNC conversion are available.
The links are in the description. I hope there's at least some ideas here that
you've benefited from. If nothing else this is a pretty interesting CNC
conversion using 3D printed parts on a metalworking lathe controlled by an
Arduino to turn wood parts.
And no 3D printing project will be complete without a few disasters.
I have no idea what these parts of supposed to be.
Thank you for watching. I hope you enjoyed this video. I had a lot of fun
doing this conversion. if you like the video please hit the like button.
If you have an interest in CNC conversions, electronics, or programming,
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