Check twice, cut once

For the lower receiver on my M16A1 build I'm using a Cerro forged 80% from Right To Bear.  I got it on sale as a "blem" for $30, and for the life of me, I can't find anything wrong with it that would make it a blem.  I'm not going to reprofile it into a "correct" A1 style lower(maybe a future project), but since the rest of this is all Colt M16A1, I decided to engrave the side of my lower to match.  Colt used several styles of markings on their lowers through the years.  With none being more "correct" than others, I decided to just make what I liked based on what I'd seen.  I couldn't find a good CAD drawing of the Colt horse logo that they used, so I ended up drawing my own from scratch.  I got it all drawn up and programmed, and headed to the mill...and that's when the trouble started.

This engraving is the whole reason I upgraded to ball screws on my CNC.  I actually started this project back in January, and only now with the mill done was able to get back to it.  I almost always run my programs on a test piece first, and it's a good thing I did because it didn't work...at all.  I started with a chunk of blue foam, and you can see the results on this test piece here(painted silver for more contrast).

It's a darn good thing I ran a test piece first, or I would have effed up my lower because both the lettering and the logo were very messed up.  The first two attempts were just...wrong...and I couldn't figure out why.  The CAD file was fine, the programming was fine, the toolpath preview on Mach3 was fine, but it wasn't machining correctly. The only think I could figure was that the stepper motors were skipping steps.  I upgraded to ball screws thinking that the more coarse screw pitch would need fewer steps, so it would be less likely to skip.  I was wrong on that too, it wasn't any better with the new screws(although still definitely worth the upgrade for many other reasons).  After much head scratching and internet searching, it turns out all I had to do was increase the pulse width going from my PC to the stepper driver.  My CNC PC is pretty old, and borderline fast enough to run my mill.  It just couldn't output a clean pulse with Mach3's default pulse width, causing the motors to miss steps.  Increasing the pulse width allowed the stepper driver to read the signals better.  Think of it like the difference between yelling "Ah!" and "Ahhhhhh!"  Same message but your brain can process the second one better because it's longer.

With the mill upgraded and test pieces finally looking good, I could do the real thing.  I had to get a little creative to hold it in my vice.  I have a solid block of iron where the trigger would be so that the lower is firmly held in place without the vice touching the thin areas of the mag well and trigger guard.  It's hard to see here, but the engraving turned out perfect.  I left off the Colt manufacturer info block by the selector since this isn't, in fact, a Colt made part and I'm not really trying to fool anyone who takes a close look.  After engraving and while on it's side and all indicated in, I CNC drilled the FCG holes too.

The fire control pocket was next.  I had to get creative with the clamping again, because these things are a weird shape.  As much as I like "hit go and let it run" machining, I had to babysit this thing the whole time.  My coolant mist system doesn't have the air pressure needed to blow the chips out of a deep pocket like this, so I had to pause it and clear the chips out frequently.

It took a while, but that's a pretty nice looking pocket if I do say so myself.  I also left a bridge of material between the the new FCG pocket and existing takedown pin pocket, so even if I hadn't crushed my autosear in a vice, it wouldn't physically fit into the lower.  It's not really necessary and most ARs have this whole area open, but when building a former machine gun with a lower receiver marked like a machine gun, I want my intent to be very clear.