Wednesday, May 30, 2012

Poor man's lathe

There are times when you need to modify a part in such a way that you need the use of a lathe or CNC machine, but the cost of such a job or the wait time may not be worth it. I like to spend the day in the garage and get things done right away, but I have only the basic hand held tools, not a lathe or even a drill press. So what is a garage hacker to do when a part needs to be turned down on a lathe and you lack such a tool or the patience to wait for the shop to do it for you? Simple - make an impromptu lathe from your hand held drill!

I was working on this massive autonomous robot project and needed to adapt the axles from a pickup truck rear differential to take a set of bearings and sprockets. Sure, I could have sent them to the shop and paid $150 to have them machined on a lathe, but I did not want to fork out the dough any more than I wanted to wait a month to have this done, so I decided to pull a "MacGyver" and machine down the axles without needing a lathe. Impossible do to the precision needed? Heck no! I even added a keyway to each axle using an angle grinder and the end result was a perfect fit.

Let me show you how I adapted the axle shafts shown here to fit onto the 1 inch diameter bearings and sprockets.

A lathe in its simplest terms is a machine that spins a part so that a cutting bit can remove metal a little bit at a time. So, I knew if I could get the axle to spin, then I could just use my grinder to carefully remove the metal around the end of the shaft until it was exactly 1 inch in diameter. At this point, the axles were about 1.25 inches in diameter and tapered. To spin the axle, I welded a bolt to the center of the hub flange and then placed it in the chuck of my hand drill. The axle was then placed into a simple wooden jig and greased so that it would spin freely.

I found the drill spun the axle a bit too fast for my liking, so I needed a way to reduce the speed of the drill to about 120 RPM. Trying to tie wrap the variable trigger in place did not really work out so well, and then I remembered something about the series wound motors in drills and saws - they work with both AC current and DC current. My idea was to just reduce the voltage to the drill motor.

Going from 120 volts AC to 24 volts DC slowed the drill down to the perfect speed, yet gave it enough torque to spin the axles as I held the grinder disc to them. To power the drill from 24 volts DC, I just took two of the four robot batteries (marine batteries) and wired them in series with the drill power cord completing the circuit. I didn't even use wires for the plug; it was just sandwiched between the two battery terminals and held there by friction.

My machining process required shortening the axles and then turning them down to exactly 1 inch in diameter at the last 2 inches on each end. Cutting the axle short was easy; I just spun up the drill and then held the zip disc on the axle until it cut all the way through the axle. I did not need the splined end piece, so it was tossed in the scrap bin.

To machine down the axles, I ran my grinding disc back and forth along the 2 inch section while the drill spun the axles at about 120 RPM. I did not push hard on the grinder, and tried to keep a constant pace as I moved it back and forth along the area to be reduced. After about 50 strokes, I would stop the drill and check the axle thickness using the bearing I intended to install.

Once the axle was just slightly larger than necessary, I switched from the grinder disc to a sanding disc so the final machining could be done more accurately.