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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 chuck. 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 drill. 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. |
