In my last post, outlined some ideas and goals for talking the slop out of the fit between the upper and lower receivers on the AR-MPR rifle. The first step is to remove the gaps between the sides of the upper receiver lugs, and the channels at the front and back of the lower receiver. With approximately .017″ of total slop (front gap + rear gap), that would equate to approximately 8″ of hypothetical variation in point of impact (if the receiver could twist fully in each direction). .
While I contemplated bedding the lugs with the receiver with epoxy, I decided that since the lugs are bearing surfaces when the upper and lower are opened/closed, this could cause wear with a “softer” material like epoxy. I am looking for a more long-term fix.
Based on the goals, and the tools that I have on hand, I decided to remove the hard coating and weld the lug holes shut, leaving excess aluminum “raised up” that could be milled down to the proper dimensions. While not as ideal as hard coating, the aluminum should prove reasonably resistant to wear and tear.
Step 1: Prep the Surfaces
I did two things to remove the hardcoat (anodizing) from the lug surfaces where welding would take place. First, I filed down the sides of the lugs. I was amazed at just how hard the hardcoat treatment is. It felt more difficult than filing mild steel by far! Second, I drilled out the holes to remove the hardcoat from the insides of the holes. Again, I was amazed at how hard the harcoat treatment was to drill – in fact, I had to sharpen the HSS drill bit a couple times to get the job done. Drill Doctor to the rescue!
Rear lug prepped for welding - Image Copyright 2011 Ultimate Reloader
Step 2: Weld the Lugs
I’ll admit- using my Lincoln Powermig 200 for this job was a bit of overkill – in fact, I would have preferred using a TIG welder (something I’ve yet to try) – but I thought this would be worth a try.
The welder - A customized Lincoln Powermig 200 - Image Copyright 2011 Ultimate Reloader
MIG welding aluminum? Don’t forget your spool gun! I used 100% argon gas and my spoolgun for this welding job. I don’t weld aluminum frequently, so I had to experiment on a test piece, and put on a split tip. Once I got things setup, it was time to go.
Aluminum welding spoolgun - Image Copyright 2011 Ultimate Reloader
Welding aluminum is very exacting- it’s not like welding steel where you have a high degree of flexibility on temperature, and can see the temperature by examining the color of the weld puddle (Red = Cold, Yellow = Hot). With aluminum, you have a response more like microwaving butter. All solid one second, melted through the next second. You really have to watch the shininess of the edges of the puddle, and practice a lot.
Overall, the welding went well using quick “blasts”. As I expected, the corners rounded off a bit due to the high intensity of heat.
- Lug after welding – Image Copyright 2011 Ultimate Reloader
The result was not perfect, but given the tools at hand, and the experimental nature of this project, I felt the outcome was acceptable. Next, we’ll cover milling the sides of the lugs, and re-drilling the holes.