X-Axis Powerfeed for Mini Mills Part 1
I decided I wanted a powerfeed for the x-axis on my Harbor Freight
minimill. I found a couple of examples on the Web and they both looked
promising but I realized that it was really pretty simple so I started looking
for a motor to design my own around. Surplus center has a couple that I
was considering but one day when I was in my local Harbor Freight store looking
around I noticed what may be the perfect choice for a motor! They had a
14.4v rechargeable drill on sale for $9.99! I'm thinking, it has a motor
with a gearbox that gives it plenty of torque, it has a built-in speed control,
and it even has a clutch that I may be able to use to keep from damaging
anything at the end of travel!
So I bought 2. One to tear into and one to have as a spare
or in case I ever wanted to do the Y axis too. One had a keyed chuck
because I thought I might actually end up really cheaping out
and using the chuck to couple it to the mill and the other one had a keyless
When I got it home I tore into the one with the keyed chuck.
I'd been into a 12V Black & Decker drill once before so I had some idea
of what to expect and this thing really wasn't very different. In fact the
main difference I noticed is that the 14.4V battery pack for the HF drill weighs
about half what the 12V B&D pack weighs.
With the chuck removed the motor/gearbox/clutch assembly seemed
perfect for my needs!
Now all I needed to do is figure out what to
use for a speed control. I thought about it for awhile and
looked at a few simple circuits I could build but in keeping with the whole
"cheap" concept I pulled the trigger apart and started poking around inside
it. What I found was a circuit board and a few heavy duty contacts that
work something like this:
The negative terminal from the battery is always connected to one
terminal on the motor and in the off position the motor leads are connected
together. I'm guessing this helps with braking since I can't think of any
other reason, but I could be wrong. As you pull the trigger the motor
terminal that is not connected to the battery is switched to the drain of the
MOSFET and the circuitry on the board provides a width modulated pulse at full
voltage. The on time of the pulse increases as you push
the trigger further in but as you reach max travel the motor is switched from
the pulsed output to full battery voltage. This gives you the fastest
possible speed and highest torque. Now all I needed to do was figure
out how to make all this happen with a pot and a couple of switches on the front
of my power feed box and it actually turned out to be fairly simple!
The first thing I did is draw out a quick schematic but it was
just on a Post-it and I can't find it at the moment. If I run across it
later I'll post it here.
The speed controller from the trigger is shown here:
I soldered 3 wires to it to bypass the pot on the board and
replace it with my own. You can't see it in the picture but I used a
Dremel tool with a small pointed burr to cut the carbon of the potentiometer
(the black band between the black and red wires) so that it didn't interfere
with my pot. The metal tabs at the top are contacts that are accessed
through holes in the trigger housing shown below:
The labels didn't show up so well in the image but they're labeled
for the gate drain and source of the MOSFET, the battery connections and the
motor connections. Rather than have to come up with connectors I just used
these same holes and the original wires from the drill wherever I could.
I measured the speed control potentiometer as 500KOhms. I
had a bit of trouble finding one locally and was in too much of a hurry to see
it work to wait for mail order so I used a 100K pot from Radio Shack and a 330K
resistor in series with it. Not exactly right but near enough to be
safe. With a bit of experimentation I found that the speed was really
slow this way and I was concerned it may be too slow so I bypassed the
330K resistor and it ran quite a bit faster. In the end I added a switch
to bypass the resistor for 2 speed ranges.
I also added a rapid traverse pushbutton that connects full
voltage to the motor to move the table faster than you would while
machining. This is like pushing the trigger all the way in on the original
I was a bit concerned about the current the motor would draw
and how I would power the whole thing. I read something about a DeWalt 18V
drill motor drawing something outrageous like 50 amps at full load and there was
no way I wanted to deal with something like that! I finally decided that
the load would be quite a bit less in this application because I planned to
adjust the clutch to slip fairly easily. In the end I settled on 5 amps
for my power supply and max motor current. The switches I used for the
normal on/off and the rapid traverse were a bit whimpy for that so I had them
turn on a relay that switched the current to the motor.
For a power supply I lucked up and found a 12V
supply sold for Coleman electric coolers at a local Goodwill store, new in
the box for $4. It's a nice little switching supply that's probably pretty
expensive from retail stores and it works great for this. I also found a few
laptop supplies that would probably have worked but the Coleman seemed the best
choice. I chose 12V over the drill's 14.4V because 12V supplies are more
readily available. Even a 12V/13.8V automotive supply would work fine.