Sunday, August 5, 2012

Semi RPD Build [Part 3] - Lower, FCG

Part 3.

In this post we will cover the modifications to the lower and FCG. The FCG was purchased from Project Guns.


Firstly, strip all the wood from the lower. Knock out the trigger pin first, and remove the safety and the safety spring.


The lower will need to be cut to length.

Here is a blueprint that we will use for the next few steps. Refer back to it for dimensions as you read further.

First, mark the location of the trigger bar channel cuts. Then some material was taken out from the trigger bar channel using a dremel, so as to have less material to mill out. Be extremely careful when doing this. There is nothing wrong with milling out the entire area either, it just takes a little longer. 

Using a machinist square, mark the length of the sear pocket and the overall lower length. You can see the final markup in the second picture (the front has already been sawed off).

Now, we are ready to do some mill work - clean up the front of the lower, mill out the sear pocket, mill out trigger bar channels, mill out the round trigger bar relief. Hold the lower as shown below, and align the z-values along the x-axis, also double check consistency of y-values as x-axis is traversed.

The trigger bar channels are cut using a 1/8 carbide end mill. To start the cut - touch off on the inside of front cavity (the longer narrower one) and cut (in several passes) towards the stock. 0.125" is a sufficient width for the channel and further widening is not necessary. 

The sear pocket is milled out using a 1/4 end mill. For trimming the front, we used a 3/8 mill, but any large-enough cutter will suffice.

The following picture, shows the (almost) finished product. There is still one more cut remaining - it will be done shortly.

We can now start fitting the lower to the receiver. Using a small file, round the front of the lower as shown in the picture.

Additionally, these areas of the receiver will need to be rounded to mate with the lower.

The charging handle also needs a little filing. The areas that need to be filed are circled in red in the following picture. Also, give the flat area on the top a little work with some sandpaper.

Here is the fitted assembly.

Having fitted the lower, we can now drill the pin holes for the FCG. Here is a blueprint that we used. It is based on the blueprint was originally included with the FCG, however some values have been updated to fit the included parts. Refer to this picture for dimensions as you read the description below.

Here is a picture of the FCG itself, which includes: new rear pin, new barrel pin, screws for installing the drum bracket, trigger spring, striker, striker spring, FCG pins (there are 4, though you will only need 3), trigger bar (contains sear and disconnect), trigger bar spring, firing pin extension and firing pin extension cross-pin.

If you install the trigger bar, you will notice that it rubs against the lower in the area shown.

To accommodate this, we will mill out a relief cut in the lower. This is an inexact cut, as it is difficult to fit the ball end mill into this particular area of the lower. Using a 1/2 ball end mill, cut as long of a cut as you can manage, down the centerline of the lower. The total depth of the cut is 0.015".

Here is a picture of the trigger bar installed. Make sure it is free to move back and forth.

The trigger needs to be modified, in order to accept a trigger spring. This is accomplished by making two cuts using a 1/4 end mill. The first cut, 0.050" deep, starts from the center of the existing trigger pin hole and proceeds in a direction that is perpendicular to the nearest edge. The second cut, 0.100" deep, also starts from the center of the pin hole but proceeds at a 45 degree angle to the first. You can see both cutting directions marked in this setup picture. Center on the hole and proceed in light cuts (~ 0.010" per pass) from the center outwards.

The first cut is done.

We now, realign the trigger along the next cutting axis. Note the original mark is no longer visible, so we draw a new guide.

Center on the hole and proceed in small increments, as before.

Here is the finished trigger after some cleanup with a small file.

There is one more operation that needs to be applied to the lower - we need to drill out the return rod hole to 3/8. The setup is shown in the picture below. We use a machinist square rather than a test indicator for alignment due to the difficulty of getting the latter into the area of the measurement. The entire setup is extremely vibration-prone, since we're holding the lower by a (thick) piece of sheetmetal. For this reason it is very important to drill the hole in increments and at a small feed rate. We drilled first to 11/32 and then to 3/8.

Finally, a chamfer is applied using a scraper - a ground down triangle file.

Installation of the trigger and its spring reveals that the spring is rather long and will need to be cut to length.

Mark the area where the spring touches the lower, and using a dremel cut-off wheel make a groove just deep enough to hide the spring wire. Cut the spring to length -- careful here, cut it purposefully long and then trim to fit.

Without going into chronological detail as to how these locations were identified, I will simply show you which areas of the trigger bar were making too much contact with the trigger and its spring. You will need to file these areas just enough to clear the binding. (You will also need to file the trigger itself, so don't focus on the trigger bar alone.)

In the following picture we can see the areas of the trigger nose that will need to be fitted.

To fit the trigger, you will need to remove a significant amount of material from the top of its nose, as well as the sides of the nose (see pic below). Additionally, you will need to polish these areas to a shine, in order to ensure a smooth function of the disconnect. The entire thing must operate smoothly, no matter how hard you pull the trigger.

Here is a picture of the finished trigger.

Here is a picture of the fully-assembled FCG.

And here is a video of the FCG in action (sorry about the blur :) Stay tuned for the next post, where we will put the rifle together and do some test firing.

PS. In case of Bubba, break glass -- a short story.

Without going into details as to how this occurred, other than to say -- no it wasn't me, the lower which I had been painstakingly working on for so long was scarred by Bubba's favorite weapon - the dremel.

This was very upsetting, as the whole thing was almost finished, only to receive this horrible mark. I did not want to weld, as the location of the gauge was very close to the FCG pin hole... but how else to fill this void? And then it hit me -- soft solder. The location of the gauge is on the bottom of the lower (not a critical area) and it will be finished by parking followed by dura-coating, so there really should be no difference in the finish once the paint is applied. Here is what to do. Apply a generous drop of soft-solder to the damaged area.

Using a MAPP torch heat the solder from below, otherwise the gas pressure ends up blowing away bits of the solder. Once it melts, let go of the heat, and check for air bubbles and small voids in the filler. If necessary apply another layer of solder and melt it again. Once satisfied, take a hand scraper (like the ground triangle file from before) and scrape the bulk of the solder off. Follow by a sandpaper polish, and you will have something that looks like this:

Damage undone. Nice!

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