Step 11 – Milling the Lug Slots ___________________________________________________________________________________
This is one of the more intricate steps in making the receiver. First we had to find an end mill wide enough to cut the width of the lugs. It also had to be small enough to clear the main threads.
A four flute 3/8” end mill was dug up but the shank was too large in diameter. In other words there was a lip that wouldn’t clear the front of the action and/or the threads. Our fix was to grind it just under 3/8”:
Using a centering device we positioned the action nose forward in a Hartford Super Spacer:
A depth gauge was set so the milling would go just beyond the 3/4” long raceway:
Initial cut on one edge:
The final pass, left side. Note our digital readout at 2:00. You can go off the mill dials but to hold to ten-thousandths these are a must:
Step 13 – The Lug Ramps _________________________________________________________________________________________
In order for the bolt to cam over the cartridge rim and ride into the lug seats there has to be a transition; or as I call it a ramp.
Like the passages we made in step 11 this is done in a vertical mill. First the head is canted to 10 degrees:
A small end mill is positioned and the depth stop is locked at 0.003”:
The cut is very mild. Starting at the outer edge the table is indexed until the end mill reaches the center hole. It touches but doesn’t extend into the shaft recess; if it were to we’d lose concentricity.
Here’s a picture of the transition, right side. Note the depth appears much deeper than the actual pass. Again we only took a couple of thousandths.
You’ll also notice we counter bored the main threads. Here are two photos of that process:
Recessing the Threads - Explained ____________________________________________________________________
I thought I’d touch on why we counter-bored the main threads. While seems like a logical step the reason may not be so obvious.
When you internally thread an action there’s a transition to the edge of the shoulder. The outer most thread actually becomes weak as the cutter transverses the opening. What you’re left with is what I called “ringing”. See below:
The thread is so thinned between 5:00 and 8:00 it’s pulled out by the tool bit on exit. If you run your finger across the face you’ll feel it protrude a few hundredths. And at 10:00 you’ll see the entry point of the previous turn, or 0.0625” cut. Combined these generate ~0.08” in delta from 5:00 to 11:00. How much that affects barrel harmonics is a source of debate. What I can positively say on the matter is this. If it’s left the exposed lip will be wedged causing pitch deformation.
The other concern is shank crush where the action and shoulder engage. Ideally you want full contact throughout the threads. This load should be evenly applied at the bearing surface but not the upper and lower ends. Touch between the top of the barrel threads and bottom of the action threads should not occur. By lathing a 30 degree lead over 0.10” the barrel can evenly shoulder on the action. The tricky part is ending the recess right at the lead of a thread; cutting across one spawns the same problem just farther into the receiver.
Post by Lee Martin on Dec 17, 2013 15:22:10 GMT -5
Step 14 – Laying Out the Port ______________________________________
We made more progress last night....
Before hogging the port we honed the tail end of the receiver:
The port position is set off the back of the raceway. Some quick math:
Barrel shank = 1.5” Bolt passage = 0.75” Port set-back = 0.25” Total from the main ring = 2.50”
So 2.50” from the receiver face is where the port begins. 6 PPC brass is 1.50”, OAL is 2.10” when loaded with 65 – 68 grain bullets. We settled on 2.25” length by 0.810” height. That port provides plenty of room to load and remove shells without being overly generous (rigidity is important on a precision action).
Layout was done with red marking paint:
With a measuring scribe the upper edge was lined at 2.50”. The rear edge was marked at 4.75”:
The two were connected 0.810” apart; the bottom was set 15 degrees off the X-axis centerline: