Step 3, Drilling the lugs.
We’re finally going to create the actual reference point – the pivot pin hole in the front lug. While this could be done in a vise, using a 1-2-3 block on the bottom of the receiver and indicating the sides (upward facing surfaces) to the same depth, I decided a fixture would be easier. The lug drilling plate (see Appendix A) aligns the lugs with the bottom of the vise or angle plate and allows for moderately quick fixturing. I recommend it if you’re going to make multiples.
Clamp you work as shown if using the lug drilling plate. Make sure both lugs rest against their surfaces and are roughly centered above the 0.375″ holes in the jig.
Locate the deck in the Y-axis, then move toward the lugs 0.250 and set a zero. Touch off on the inside of the front lug, compensate for your edge-finder diameter and move the table until your spindle is setup 0.212″ away and zero. This should be roughly in the center of the front lug. (It’s actually a little off center as the front lug is divided 0.212″ on one side and 0.219″ on the other). Spot, step drill close to .250″, then ream 0.251″. This is the (0,0) origin for all the remaining operations. Note that it won’t look centered, because the front of the lug has not yet been machined.
Move -6.375″ in the X-axis, verify that you’re over the center of the rear lug, and drill and ream to 0.251″.
The A2 upper (with carry handle) can be clamped the very same way as only low clamping force is necessary when using the lug drilling jig.
Step 4. Lathe operations
Using two 1.5″ x 0.250″ dowel pins and two 5/16″-18 x 1″ bolts, attach the upper receiver to the mounting bar. (Bar details later) The dowel pins should ensure the alignment of the forging centerline to the mounting bar.
Mount the free end of the mounting bar in the 4-jaw chuck.
Snug the chuck jaws down, and use an indicator on the compound to verify that the receiver is, indeed, parallel to the lathe ways.
Next, center the receiver (not the mounting bar). First, remove the dowel pin installed in step one. Try to do so without damaging the hole it sits in. You can now use the hole, in conjunction with a tailstock center to center up the receiver by eye. This should get you pretty close.
Using an indicator in the compound, or on the cross-slide, continue centering the pin hole as closely as possible. Less than 0.002″ TIR should be achievable if everything has been done right. I was able to get a little less than 0.001″ without much hassle.
This is an eccentric setup, so you’ll probably want to run at low-ish speeds, and if you’re using a light lathe you may need to add counterbalancing weight to the chuck.
Cut the receiver front to 0.219″ from the center of the pivot pin hole. I recommend using a high speed steel (HSS) cutter for this, as carbide hates an interrupted cut. Touch off on the tip of the mounting bar. This should be exactly 0.219 from the center of the pivot pin hole. Cut the thread boss to a diameter of 1.245″. Don’t worry about cutting the thread relief behind the thread boss yet.
It is a good idea to rotate the chuck manually before making this cut and verify that you have clearance for your tool and the compound before powering up the lathe spindle.
Now Face the front off at a distance of 0.664″ from the pivot pin.
Finish off the thread boss with a nice chamfer on the nose.
Grind a light radius on the corners on a HSS parting tool and install a thread relief 0.040″ deep going from 0.219″ from the pin (the front face just cut) to 0.309″. I had to grind a custom grooving tool to get the rigidity required – a standard parting tool has to be hung out quite a ways to provide clearance for the mounting bar and top of the receiver, so a custom cutter was just the ticket.
Thread the boss. The threads are 1-1/4″-18. This is pretty standard threading, again make sure your tool and toolpost clear the workpiece before starting the machine.
To be continued…