J. Kelly's Mill/Drill PowerFeed...updated 26 January 2002
I decided I needed a powerfeed on my mill/drill. Really,
it had nothing to do with the fact that my friend has a powerfeed on his Bridgeport
(he's one of those guys you hear about who bought a used Bridgeport in nearly
new condition for a ridiculously low price). I really wanted the powerfeed to
improve the finish I could get on finish cuts. A powerfeed ought to be able to
control the speed better than I can by hand and result in a very even finish.
I could have bought a powerfeed (but there's no challenge in that). Harbor
Freight has a powerfeed for mill/drills that costs about $200 and by all accounts
works well. But, I decided to build one instead. Steve Bedair has posted a very
nice explanation of building a powerfeed (for his mini-mill) using an automobile
electric window motor.
So, I went to the junkyard. Came home with this electric window motor for an '86 Ford pickup.
According to the guys at the junkyard, all Ford window motors since the mid-80's are similar in design (worm-gear drive) and they let me have this one
cheap.
I used the mill to build a block (aluminum) that would mount the motor to the end of the table. Actually it screws to a couple of holes drilled and tapped
into the casting that holds the left end of the X-axis leadscrew.
The original drive for the motor uses a brass worm to drive a nylon gear. Power is transmitted from this nylon gear through a small steel gear that
fits into the nylon gear with some nice elastomer bushings. Unfortunately I couldn't quite figure out how to use this gear (also, the flange on the back
of the gear is just pot metal and if you try to anneal the gear so you can cut it the flange that fits into the nylon gear just melts and falls off).
So I machined a new part, triangular on one end to fit into the nylon gear, rectangular on the other to fit a collar that slides on the end of the feed screw.
The collar is machined from tool steel and fits on a pin through the end of the feedscrew.
Sliding the steel collar back and forth on the end of the feedscrew engages and disengages the powerfeed.
Having dealt with the mechanical questions, I needed to be able to control the power to the motor.
Of course I needed variable speed and I decided that a PWM (pulse width modulation) controller was the answer. A battery charger wasn't adequate
for a power supply (mine is only a half-wave rectifier) so I also needed to build a power supply. I found a schematic for the PWM controller on the
web, and cobbled together a 24V DC power supply from various parts. At right are the components before being boxed and cleaned up. The power
supply is at the back (in the box for an old computer power supply) and the PWM controller is in the front (just soldered together on a board). It looks
better than this with everything covered up. I'm not going to give detailed descriptions of the wiring (I really don't need the lawsuit if you fry yourself).
Design information is available on the web. PWM is definitely the way to go if you need variable speed from a DC motor. It gives very nearly constant
torque.
I built a box to hold the PWM control board (OK, it's not a real pretty box) and mounted the controls
on the left of the mill/drill head just below the power switch. (I'm going to have to find a better location because this interferes with the quill lock, but it
was a good idea.) There is a fan in the box (stolen from an old cpu chip) to keep everything cool. The upper toggle switch controls the powerfeed
through the speed control (i.e. variable speed). The lower switch bypasses the speed control and gives full power to the motor (poor-man's rapid
traverse). In this position the controls are very convenient and easy to use.
The powerfeed motor looks good mounted on the end of the table. It's quite easy to engage
and disengage the powerfeed. Generally I'm quite pleased with the way the home-brew powerfeed works. I was concerned that the motor wouldn't
have enough torque. This shouldn't be a big problem if the purpose is just for finish cuts, but it might limit its utility for general milling. However, I've
been taking cuts up to 0.200" deep and 1/2" wide (in aluminum) using the powerfeed and it doesn't complain at all. I do need to finalize the location
for the control box. The logical and comfortable spot is below the powerswitch. Maybe I'll modify the quill lock handle so that it works. It probably isn't really a practical way to come up with a powerfeed for your mill. Sending a check to Harbor Freight would be a lot easier. This took
quite a bit of time (I had to learn a lot more electrical engineering than I meant to). It was a lot cheaper than buying a factory powerfeed. It was also
very amusing and very satisfying to make it work (that's the real purpose here and the advantage to doing this as a hobby).
Cheers,
Kelly
I am not a professional machinist or engineer. I'm not giving advice, just my opinions. I'll endorse no use of this
information, nor accept any liability for its use or misuse.
I used the mill to build a block (aluminum) that would mount the motor to the end of the table. Actually it screws to a couple of holes drilled and tapped
into the casting that holds the left end of the X-axis leadscrew.
The original drive for the motor uses a brass worm to drive a nylon gear. Power is transmitted from this nylon gear through a small steel gear that
fits into the nylon gear with some nice elastomer bushings. Unfortunately I couldn't quite figure out how to use this gear (also, the flange on the back
of the gear is just pot metal and if you try to anneal the gear so you can cut it the flange that fits into the nylon gear just melts and falls off). 
Having dealt with the mechanical questions, I needed to be able to control the power to the motor.
Of course I needed variable speed and I decided that a PWM (pulse width modulation) controller was the answer. A battery charger wasn't adequate
for a power supply (mine is only a half-wave rectifier) so I also needed to build a power supply. I found a schematic for the PWM controller on the
web, and cobbled together a 24V DC power supply from various parts. At right are the components before being boxed and cleaned up. The power
supply is at the back (in the box for an old computer power supply) and the PWM controller is in the front (just soldered together on a board). It looks
better than this with everything covered up. I'm not going to give detailed descriptions of the wiring (I really don't need the lawsuit if you fry yourself).
Design information is available on the web. PWM is definitely the way to go if you need variable speed from a DC motor. It gives very nearly constant
torque.
I built a box to hold the PWM control board (OK, it's not a real pretty box) and mounted the controls
on the left of the mill/drill head just below the power switch. (I'm going to have to find a better location because this interferes with the quill lock, but it
was a good idea.) There is a fan in the box (stolen from an old cpu chip) to keep everything cool. The upper toggle switch controls the powerfeed
through the speed control (i.e. variable speed). The lower switch bypasses the speed control and gives full power to the motor (poor-man's rapid
traverse). In this position the controls are very convenient and easy to use. 
The powerfeed motor looks good mounted on the end of the table. It's quite easy to engage
and disengage the powerfeed. Generally I'm quite pleased with the way the home-brew powerfeed works. I was concerned that the motor wouldn't
have enough torque. This shouldn't be a big problem if the purpose is just for finish cuts, but it might limit its utility for general milling. However, I've
been taking cuts up to 0.200" deep and 1/2" wide (in aluminum) using the powerfeed and it doesn't complain at all. I do need to finalize the location
for the control box. The logical and comfortable spot is below the powerswitch. Maybe I'll modify the quill lock handle so that it works.