Spent some time prepping for a new project, and realized I haven't seen the techniques I use shown, so I figured it is as good a time as any. There are likely people that could use the information, and I have a cat on my lap, so I have time.
The project I am working on (to be disclosed if I get it done to m satisfaction) requires a pretty high torque, fairly fast drive. The requirement is to move about 10mm in 100mS, position withing 0.1mm, with a force of about 50N (3/8" motion to about 0.004", with a force of about 10Lbs, for those that are trapped in the world of imperial units). Not earth shaking, but beyond the cheap hobby servo.
The cheapest, and one of the most convenient, moderate torque drive available is the battery screwdriver. Not the big gun shaped job, but the homeowner type. The good one are about $20US new, but I usually pay $0.50 to $1.00 at yard sales and swap meets. There are millions of them. Most with little to no use. People buy them for themselves or as gifts without realizing that this little bugger in not the same as the $150 Dewalt. After they sit in the drawer unused for a few months or years, I get them cheap. They have been around for at least 30 years, with 1/4" hex drive, planetary reduction, 180RPM nominal, and design voltages of 2.4 to 4.8V.
The one I dissected this time tests out at about 250Ncm at 1V, with a no load speed of about 70RPM. The design voltage is 3.6V, and it will probably stall at close to 1000Ncm (10Nm) at that. If more is needed, then I can up the voltage for short bursts. I doubt that will be the case, as at 1V, I have much more torque than I need using a 15mm arm. For the output shafts and actuator levers, I use 1/4" allen wrenches, also yard sale grabs, preferably unhardened, so the cheapest sub-harbour-freight quality is fine. If they don't stay in place themselves,a little epoxy or gel CA does it.
For this project, the first thing I did after ditching the battery end (should have been second) was mount screw terminals. There are no electronics. The switch presses contacts directly on the motor terminals, and the battery was mounted in the swivel handle. All gone, and nice big holes (12mm dia) through the body for mounting and reaction against torque.
The first and last show the parts I used and the installed terminal. Clean and fairly neat. Last shows a sampling of the drivers in my junk box. All are black and Decker. Models are LI2000 (3.6V LIon, 330g as it sits), which is a current model,. I use this one for its intended purpose. 9078 (the one being remodeled here), which is 380g as it sits without battery or handle. Heavy. Metal gearcase. Next is a 9071, about 15 years old. 2.4V and 440g. Last is a 9018,, maybe made in 1985 or so. 2.4V and 250g. Only the first cost over $1
Edit: fixed image
