Building The Steven's Favorite

[TomG]

Tonight was breech block night. I sawed out a piece of the O1, squared it up and located the three cross holes. Two of the holes were temporarily threaded to screw the block to a sub-plate for the rotary table work. It was then set on a sine bar at 12º, the slot milled and the firing pin hole added. Next it will be mounted to an aluminum sub-plate containing locating holes for the centers of all the arcs. As with the receiver, that will make the rotary table work a lot easier and faster.



Set on 12º to mill the slot.



The survival of long, skinny end mills is always a concern. This one is 3/16" diameter by 1 1/2" long. The secret to making them long lived is to plunge cut first and side mill to finish. The slot was cut as deep as possible with a standard length end mill before resorting to the long one. This cut was over an inch deep and made in one pass using .02" steps. The same technique applies when milling tool steel like this or when milling aluminum. A lube such as WD-40 is necessary for deep slots in aluminum to keep the cutter from loading up.



Gauge pins are handy for locating features from cross drilled holes. Just place a pin in the hole and touch it off with the end mill or drill using a piece of paper between them as a witness. Here, the block is set up to drill and ream the hole for the firing pin. To set the 12º angle, I just put a pin in the chuck and held the bottom of the slot against the side of it as the vise was tightened.



This next series of photographs shows the method I use to set up and cut radii on the rotary table. The sub-plate has been made large enough to include the centers of all the radii and a hole was drilled and reamed for each one. To locate them, the rotary table is centered on the spindle, then a pin is put in the spindle and inserted into the hole at the center of the radius to be cut. The sub-plate is then clamped down, the table offset by the radius plus half the diameter of the cutter, and the radius milled.











In this case, where the radius blends with another, the part needs to be indicated at set-up and coordinated with the graduations on the rotary table. Then by watching the graduations, you can calculate where the radii end.