Machining an AR-15 upper receiver forging – part II

Setup 2, the deck

Next we’ll machine the deck flat and cut the lugs to size.  In doing so, we’ll establish a reference to the position of the upper along the line established in the first setup.  The real reference will be the pivot pin hole once we have machined enough that we can drill it accurately.

We’ll use the convention that the X dimension goes more positive as the table moves from left to right (as if the cutter were moving right to left) and that the Y dimension increases as the table moves away from you.

Setup the part, using the clamping plate, with the deck up and the thread boss pointed left. Make sure the deck is about 1/8″ above the top of the angle plate, so you don’t cut into it. Indicate the deck to be as horizontal as possible.  It’s still a raw forging at this point, so the surface won’t be perfect – just get it as close as possible.  Less than 0.010″ across the space between the lugs is good.  Dont indicate along the parting line, use the smooth surface to one side or another.

Indicating the deck to horizontal.

Indicating the deck to horizontal.

We’ll make five important passes, bringing the lugs to dimension, except the front surface of the front lug, which will be addressed later.

Find the centerline of the forging using an edge finder against the dowel pin that we inserted in step 1.  Move over half the diameter of the pin (0.125″) plus half the diameter of the edge-finder tip. Zero your Y-axis. Sanity check your setup to see that the spindle is over the center of the pin.

Zeroing the Y-axis using the center pin.

Zeroing the Y-axis using the center pin.

Now find the inside edges of the lugs.  Split the distance in two, move to that position, and zero the X-axis.  This sets up a temporary X-axis reference until the forging has been refined enough to install the ‘real’ reference.

Finding the space between the lugs to establish an X-axis reference.

Finding the space between the lugs to establish an X-axis reference.

X,Y = ( 0,0 ) should be about here:

The point between the lugs, this this our temporary X-axis reference.

The point between the lugs, this this our temporary X-axis reference.

Install a 1/2″ end mill and touch off on the dowel pin, then move the table up half the thickness of the dowel pin (0.125″) and set your Z-axis to zero.  Dial the table down 0.625″ from there, and we’re at the level of the deck-to-be.

Using a piece of cigarette rolling paper to find the dowel pin in order to establish a Z-axis reference.

Using a piece of cigarette rolling paper to find the dowel pin in order to establish a Z-axis reference.

The cuts are each made along a single axis.  The holding plate is sacrificial, you will be milling part of it away, but it should still work for other forgings if you plan on doing multiples.  Go slow so you don’t disturb the setup.

1. Start at perhaps 0.550″ in the Y-axis and work your way in to 0.497″ in successive passes.  Lock the axis and make a final pass from left to right for the full length of the part along the X-axis.

First cut along the deck.  One side of the lugs is now cut to dimension.

First cut along the deck. One side of the lugs is now cut to dimension.

2. Repeat on the other side, working in to -0.497″.

The opposite side is cut and the lugs are now at finished width.

The opposite side is cut and the lugs are now at finished width.

3. Set the X-axis at 2.650″ and work in to 2.719″ to cut the back of the front lug.

The back of the front lug is cut to dimension.

The back of the front lug is cut to dimension.

4. Move the X-axis to -2.650 and make Y-axis passes working in to -2.719, to cut the front of the rear lug.

Cutting the back of the front lug.

Cutting the back of the front lug.

5. Move the X-axis to 3.669, lock it, and make a pass behind the rear lug.

Cutting across the back of the rear lug.

Cutting across the back of the rear lug.

At this point the only material remaining above the level of the deck should be the lugs and an island in between the lugs.  Keeping your Z-axis setting, mill away the island.

Removing the island of remaining material.

Removing the island of remaining material.

Finally, we’ll cut the lugs to height.  Zero the Z-axis and move down 0.468″.  machine the tops, err, bottoms of both lugs flat.

Machining the bottoms of the lugs flat.

Machining the bottoms of the lugs flat.

To be continued…

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