October 28, 2011
By George Spafford
George Spafford is building a replica of the Yugo M70B rifle using a stub parts kit. A new installment will appear here every Friday
Fabricating the Trunnion - Part 2
The smooth-sided milled receiver lies nice and flat. It's easier to drill it before shaping the sides and making it irregular. Center punch the hole locations, use good cobalt bits and plenty of cutting oil. The big hole is where the barrel pin goes.
A screw pattern similar to a Hungarian was center punched. The top rivets followed the centerline of the barrel to absorb recoil. I purposely relocated the lower back holes up in the trunnion so they went through thicker parts of the trunnion wall, allowing for more threads and a more secure screw.
With the decision to use screws made, I proceeding with the build. The two front holes on each side were drilled using a #10 drill for 10-32 button-head alloy screws. The rear lower hole on each side was drilled using a #3 drill to accommodate º-28 button head alloy screws. Larger screws were use to accommodate the torque expected during recoil. I did not tap the holes yet, as the receiver will need its holes drilled and the easiest way is to do that with the receiver on the trunnion. Rather than risk the threads being damaged, they will be tapped after the receiver is drilled.
Also, spend some money and buy quality drill bits and taps. The quality in cheap imports can vary dramatically at your expense. I wait for them to go on sale at http://www.use-enco.com and then load up. Shopping at industrial supply stores/websites can save you a bundle while also delivering quality.
In case you are wondering, I drilled the holes at this point because the receiver is flat sided and easier to support compared to after the shaping is done and the sides become irregular. For me, when I can think of it, simpler is better.