The flywheel finished dimensions are 3.750" diameter, and 1.125 thick at the hub. Like this:It came in slightly over the claimed 3-3/4", more like 3.8", so I cut it slightly oversized in diameter at 3.77, leaving some for future work. The raw piece was now just about 1.78" thick and as you can see from the drawing, the thickest portion at the hub is 1.125 or 1-1/8". I could part it off down to around 1.15 or 1.20 inch and save hours of facing off .010" thickness at a time.
Note that it says the material is either cast iron or cold rolled steel (CRS). So off I went to the metals dealers to find a piece of CRS that could yield this. Some shopping around, first at Online Metals and then on eBay, resulted in me finding that a 4" diameter by 2" long piece was going to cost about $55 with shipping. Then I stumbled across an eBay seller selling a slightly smaller piece, 3-3/4 by 1-3/4, but saying it was D2 Tool Steel. His price was $18.75, including shipping. I figured that the important part was that flywheel has a density more like steel or iron than aluminum and the alloy doesn't really matter. I took what's probably a better piece of steel for many uses at almost a third of the price as the 1018 CRS.
Parting on the lathe is one of those operations that is simple in concept but has lots of art embedded in the technique. The cutoff tools are narrow strips of either tool steel (so-called High Speed Steel, HSS) or a tool holding a carbide metal cutter. Left and right in this picture from that link.
The main advantage of the HSS tools is that they're easily sharpened in the shop so if one should break, it can be ground back to shape and then sharpened. I have HSS cutters in a couple of thicknesses and use a tool holder, as shown in both views above. The tool in its holder is advanced into the slowly rotating work under lots of cutting oil and cuts a groove. The tool wanders a little bit, so the groove is a couple of thousandths bigger than the tool, but there's a lot of drag.
To keep the setup rigid, it's recommended to only leave as much cutter protruding from the tool holder to take off a small amount, so you stop every quarter of an inch or so in radius and extend the tool a quarter inch farther. To cut to the center of a piece that's 3.770 in diameter, at some point I need to extend the tool 1.89". That gets scary.
The first 1/4 to 1/2" in radius went fairly quickly but that D2 steel hardens as you work on it. I'd switch to a hacksaw and extend the cut down a bit, then use the parting tool to make it the right thickness slot. My technique was to spin the work at 100 RPM (minimum for the lathe) and hold the hack saw onto it. After that first 1/2", I noticed the saw didn't seem to be cutting; metal dust wasn't piling up when I used the hacksaw. Inspection revealed the teeth were now worn down - completely gone in the middle. The D2 ruined my blade, but blades are disposable and I have a backup hacksaw. On the other hand, it's a backup because the other blade used to cut better. Now what? Time for my Ryobi reciprocating saw. Unlike the hacksaw, I can't run the lathe while cutting, but I can move the chuck by hand. I have a blade with carbide teeth for the Ryobi, so I tried that. It sorta worked except it inadvertently (and probably inescapably) turns the disk into a lumpy thing like a cam lobe (or a four leaf clover), not a smooth cylinder as gets cut on a lathe. Which means interrupted cuts and banging onto the cutting tool. Which leads to the blade binding in the cut and causing the motor to either slow down until it breaks free and suddenly races very fast, or else stalls and makes me reset its overload sensor.
With the lathe off, I cut it, used the flashlight to see progress and could see it was working. Rotated and cut, rotated and cut, until it fell off. Couldn't say exactly how long it took because of fooling around with the hacksaw blades and trying them. Once I figured the tricks, it went within five minutes. All told I spent nearly eight hours of work cutting off that thin chunk you see in my hand, below. I broke one parting tool (but had a spare).
Shakespeare said, "Parting is such sweet sorrow". I'll just take out the word "sweet". Better yet, I'll make that "parting is such a royal pain in the a**."
Now I had a problem. I had just spent about eight hours making a single cut. To turn that 1.2" thick disk into the flywheel is going to involve removing about 4-1/2 cubic inches of that steel in very thin layers. When I add in the cost of the new carbide blade for the Ryobi and a replacement parting blade, even ignoring the hacksaw blades (the one I ruined was old and they are consumables), my savings on the D2 steel is gone. One more ruined tool and I would have been better off getting the material they called for. I thought it was time to say uncle and get the 1018 steel I should have gotten in the first place. I found I could get a custom cut piece that was even cheaper, by having them cut it to 1-1/2" thick. Then I got a 20% off coupon from Online Metals in the email.
The new flywheel is in the lathe now. Completely untouched.