Prototyping Track Bike Dropouts V2

This is take two of my track bike dropout prototype. Check here for the first version. 

With this project I learned how to use a CNC mini mill (Sherline 5400)  with the program Cut3D. I had to gain an understanding of speed rates (the RPM of the spindle), feed rates (how fast the tool moves around), depth of cut/pass depth (how deep each layer is made), stepover (how far the tool moves over after each line on the same layer), and plunge rate (how fast the tool moves downward into the next layer). I also learned how to use Mach3, a program that translates g-code (the coordinates the tool follows to execute movements) for the specific machine. 

I also learned about the different toolpaths an object or outline can have. With Cut3D they are the roughing, finishing and cut out. A roughing toolpath means the object will be shaped by removing large amounts of material. A finishing toolpath basically surfaces the object to smooth it out, and a cutout toolpath follows a line to cut an object out of the material. 

I decided to do a milled piece because I was experimenting with a design that has a pocketed portion, which is like an inset (see the first photo's circular inset pocket). The mill was also one of the tools available to me that I was excited to learn, as I don't yet have access to a plasma cutter. I'm milling 3/16" steel because I can rest assured it'll be strong enough, and there are flat bars of it at the home depot. 

A few mistakes I made: I broke the first endmill (drill bit specific for milling). It was a two fluted 5/32" bit. My plunge rate was high, and my depth of cut was far too much for the tiny bit. I was also starting with a cut out toolpath, which would've prevented me from making the pocket cut. You can't cut a pocket if the piece is loose inside the rest of the material. It took 3 hours to make the piece at the settings I used the second time. I only did a rough cut, which left my piece still connected to the rest of the material. I should've combined the roughing finishing and cut out toolpath into one g-code. Then the cuts likely would've been smoother too. I messed with Mach3's configuration during operation which stopped the machine. I had to start it from what I deemed to be a close enough spot in the g-code, which led to some surface scrapes. 

I'm wondering...should I do aluminum if it is strong enough because I could mill it faster? How scalable is this mini mill? If I used a different material besides steel I could probably use our large CNC router and do a batch of dropouts. What are the limits of my bit? Can I cut down the milling speed from 3 hours? My engineering roommate thinks I could do it in 30min.