Machining AR-15 upper receiver forgings part I – redux

Machining the upper receiver of an AR-15 is an intermediate to advanced machining project.  It is more difficult to make, on manual machines, than the lower receiver, and should only be undertaken by those who already possess a firm grasp of machining fundamentals.  This document is based on the format of the “Ray-Vin” (Ray Brandes) document which describes the machining of a lower receiver forging.

Because of the complexity, a grasp of machining techniques and terminology are assumed.  This tutorial is not intended as a machining lesson for the beginner.

I freely admit that I am not a professional machinist and that there are likely better ways to perform certain operations.  The setups shown reflect my preferences and the tooling on hand in my shop.  There’s more than one way to skin a dog, and you should feel free to modify the instructions to suit your equipment and preferences.

If you have never machined from a forging or casting before, things are a little different than machining from billet. The AR upper receiver forging has no truly flat or truly round surfaces to use as a starting reference, so you must establish references as early in the machining process as possible and then use them throughout.  You have not created complete references until you have points you can measure from to achieve consistent location in the X, Y, and Z planes, as well as theta (rotation).  Don’t worry too much if that doesn’t make sense to you – we’ll fill in the blanks as we go along.

Setup 1, Establishing theta.

In this first setup, we begin the process of establishing our references.  The first reference point we want to create is the centerline of the bore.   This will give us the centerline of the forging.  The best way to create this reference is to use the thread boss at the front of the receiver.  The thread boss is the round ‘nose’ of the forging.

Dial in an angle plate to be parallel to the X-axis on the mill.

Mount the forging vertically using the receiver mounting plate.  Using the quill to run up and down in the Z-axis, adjust the forging to be vertical.  Note that the forging doesn’t have an accurate edge, so you are adjusting for minimum deflection of the indicator, no zero deflection.  Up to 0.010″ is normal, and nothing to be concerned with.  Using a machinist’s square can help speed this up.

Dialing the receiver in to vertical.  The surface isn't perfectly flat, but it's pretty close

Dialing the receiver in to vertical. The surface isn’t perfectly flat, but it’s pretty close

With the forging dialed in and securely clamped, with two clamps, use an edge finder to find the center of the thread boss.  Locate on the front, zero, then locate on the back.  Divide the result in two, move to the Y-axis centerline and set your Y-axis zero.  In the X-axis, you do the same thing, but need to do it off of the center, so you’re not finding the edge of the parting line, which is too variable for our needs.  Again, find an edge, zero, find the opposite edge (without moving the other axis) and split the distance to find center.  At this point you should have 0,0 set at right over the center of the thread boss.  Chuck up a center drill (combination drill and counter-sink) and verify by eyeball that you are indeed centered over the workpiece.

Centered over the forging, we spot the virtual bore.

Centered over the forging, we spot the virtual bore.

Using a center drill, drill a spotting hole.  This is important as a normal drill may try to wander over the uneven surface of the forging.  Follow up with a drill ~1/8″ diameter to a depth of ~0.5″.  Then drill to something very close to 0.250″.  Finally, chuck up a 0.250″ reamer, put the mill in back gear, and ream to 0.250″.

Reaming the 1/4" hole for a dowel pin that we'll use to locate from until the bore is cut.

Reaming the 1/4″ hole for a dowel pin that we’ll use to locate from until the bore is cut.

Break the setup and drive a 1/4″ (0.250″) dowel pin into the newly formed hole.  The dowel pin will create an accurate reference that can be used to locate the centerline in the next few setups, but it will have to be removed when we bore the centerline.

Dowel pin installed along centerline of bore.

Dowel pin installed along centerline of bore.

To be continued…

This entry was posted in Gunsmithing, Shop work. Bookmark the permalink.

Leave a Reply

Your email address will not be published. Required fields are marked *

You may use these HTML tags and attributes: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <strike> <strong>