A Beginner Lathe Project for the Z3 Coupe

In my last post, I told of my new "mini machine shop" equipment, and of how I needed to make a big buy of tooling to complete it. Well, that didn't take long! After a few nights understanding the offerings on the Little Machine Shop website, I pulled the trigger and bought two "kits" of tooling, one for the mill and the other for the lathe. It was a pile of stuff:

I wanted to find an initial project that was relatively easy, and had absolutely no negative repercussions if it failed miserably. I settled on one for the least important part of the car - the license plate frame. Actually, there was nothing wrong with the BMW M frame that I pulled from my 128i convertible before trading it away. The problem was with the plastic caps for the screws, and specifically, for the deep plastic washers that the caps fit on. See how the clear washers have torn and degraded?

I decided I'd make metal washers to replace them. I started by chucking a 5/8" aluminum rod in the lathe. I turned a section to a tight fit in the black cap, and then drilled it (starting with a stiff center drill, and then with a standard twist bit) to allow the screw to pass through:

Next, I needed a recess for the screw head. The Internet suggested that an end mill would be a good thing to try. After a bit of fiddling and some very ugly noises from the lathe, I settled on drilling a starter hole to 7/16" diameter, just 1/16" deep (which means that the hole was actually cut with just the tapered tip of the bit). That made clearance for the 1/2" end mill to work, sort of. Lots of chatter, but running at a very slow 100 RPM (but see EDIT below pic!) and feeding the bit slowly worked acceptably well. The pic below shows the basic setup for the end mill.

IMPORTANT EDIT: In case somebody finds this post later, I need you to know that my guess of slowing down the lathe was not right!  There's a formula that I learned later, that relates Surface Feet Per Minute (SFPM) of cutting speed to the material and diameter of the cutter, or of the stock being turned. It works for drilling too. It uses these SFPM constants for various materials: mild steel: 100, tool steel: 70, cast iron: 60, aluminum: 250, brass: 300.

Thus, with a mild steel bolt and a half-inch cutter, I should have been turning (4 X 100) / .5 = 800 RPM. The tool was chattering because I was going too slow, not too fast!

I made a chart with Excel so I wouldn't have to do the calculations over and over. The numbers get kind of crazy for aluminum and brass with small diameters, since the lathe and mill top out at about 2500 RPM. Just read those as "as fast as possible."

END EDIT 18 Nov 2013

Once I had the recess drilled, I used a very thin cutoff blade to "part" the washer from the rod. Repeat the process three more times, and I had a set of washers, sized to fit the cap, and counterbored to allow clearance for the screw head:

AND.... here it is, on the Z3 Coupe! This narrative sounds so linear, just as if I knew what I was doing, but there was a whole lot of messing around to get these things made. In fact, the first one was about twice too tall. I used the lathe to remove some of the material, and then belt sanded the rest. In the course of that exercise I was impatient to remove the part, and touched it while it was quite hot. Yow! That may well be a permanent scar on my fingertip...