Jan 29, 2012 Most people shooting AR-15's are using the M-16 bolt carrier group as it is a little heavier and slows down the cycle time a bit. This is perfectly legal, even in Kalifornia, as it does nothing to make the firearm into something other than a normal semi-automatic rifle, or carbine. Perfect for you to build your Viet Nam era XM16E1 or M16A1 rifle. This is a true 'A1' type without the case deflector. This is also the correct type of upper for most of the 'Retro' carbine builds you might want to do. They are cut for a.250' front push pin, and recessed (left side) for the 601 type front takedown pin.
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When you look at an AR-15 lower receiver it doesn't seem too awful complicated. Maybe this will change your mind. This is the drawing that the 1970s era M16 lowers were inspected to by the Army. It is a redraw of a Colt drawing, and would have been used by receiving inspectors and government source inspectors to determine conformity to Mil-Spec for the lowers. If anyone has questions about any specific dimensions or notations, I'll do my best to answer. This one is right up my alley as a Dimensional Inspector / Quality Engineer.
Seeing all of the dimensions that the manufacturing engineers and machinists have to hold should give new respect for the people who make these guns, as well as the people who designed them. The vast majority of the dimensions on the drawing are toleranced, just in different ways.
There are some different types of dimensions that mean different things, and I'll try to explain them. There are explicitly toleranced dimensions, title block toleranced dimensions, basic dimensions, and reference dimensions.Toleranced dimensions are the ones that have an explicit ± tolerance listed right next to the dimensions on the drawing.
They are the easiest to understand, obviously. Engineers often try not to use too many of these, because they tend to clutter up the face of the drawing, and make it harder to read.Then there are title block toleranced dimensions. Any dimensions that are not explicitly toleranced, and that are not labeled as basic or REF (reference) are toleranced by the title block. On the first page of the drawing, down on the lower right hand corner you'll see the title block. At the bottom middle of the title block you'll see a statement that says 'MAKE FROM LOWER RECEIVER FORGING F8448641.'
Right above that statement are the title block tolerances. They say that any 3 place dimensions are ±.005, and any 2 place dimensions are ±.01. That is so that the engineers don't have to tolerance every single dimension, and clutter up the face of the drawing. This drawing actually has an inordinate number of explicitly toleranced dimensions, compared to most standard drawing practices. There is nothing wrong with that, but it makes the face of the drawing more cluttered, because instead of.250 DIA THRU for a hole size it is.250 ±.001 DIA THRU. Even though that's just a little more text, it all adds up, and clutters up the drawing.Basic dimensions are there to either define a nominal that is then controlled with another dimension, or to give the location where a particular toleranced dimension applies.
A perfect example can be seen on Sheet 2 of the drawing, at zone D-7. It is the definition of the threads that hold the buttstock to the lower. They are defined as threads, then there is a dimension below that in a box. That box is called a feature control frame, and the feature that it's controlling is the buttstock thread axis. The basic dimensions locate the tolerance zone, and those dimensions are found on Sheet 1 at location C-1. They are.25 BASIC from Datum -AH- to the axis of the threaded bore, and.529 BASIC from Datum -S- to the axis of the threaded bore.
So here's how the true position tolerance on Sheet 2 works. You take those two dimensions, and they control the location of the nominal axis of the bore. Then you imagine a circle that is.008' in diameter drawn with the center of that circle on those two dimensions, then extrude that circle out into a cylinder, because the axis of the threaded bore has length, so the tolerance zone is cylindrical. The axis of the threaded bore must be inside of that.008' diameter cylinder over its whole length. This is a true position callout, and is probably the easiest of the Geometric Dimensioning and Tolerancing dimensions that are found on this drawing.
It takes most engineers, machinists, and inspectors years to master this entire system of dimensioning, but without it, the engineers could never accurately convey what the parts have to look like, and what the tolerances really are. Another use of basic dimensions is to locate a place where a dimension applies. Let's say we have a tapered bore that will have something that goes down inside it. Since it is tapered you can't just say that it has a diameter of.200, but you can say that it has a diameter of.200 at a certain location along the taper.
A basic dimension would be used to locate that diameter callout along the taper. These dimensions are also noted as GAGE instead of BASIC on some drawings, but they fulfill the exact same purpose.Reference dimensions are there purely for information purposes. Usually they either reference another explicit dimension on the face of the drawing, such as the.500 ±.004 dimension that defines the opening where the lug from the upper fits down into the lower for the front takedown pin. That dimension is explicitly defined and toleranced on Sheet 1 at E-3. That same dimension is referenced elsewhere on the drawing, such as on Sheet 1 at B-1. You can't have a double dimensioned feature, so they call it a reference dimension. They want it in the view where it is shown, for information purposes, but the dimension is already toleranced somewhere else, so they can't do it again.
There are other uses of Reference dimensions, but that is the most common, and it is used extensively on this drawing. They can also be used just to give information, such as the results of math between two toleranced dimensions.
Say you have a magazine opening that is 2.398 inches long, such as what this drawing shows. That dimension is important, because it defines where the magazine has to fit. Also important is the metal thickness at each end of the opening, so let's say that we define that as.100 inch at the front and.300 inch at the back. These are numbers I made up for this illustration, they're not on the drawing. By adding up the nominals of those three dimensions, we know that the total length of the magazine opening and the front and back walls would be 2.798 inches, if everything were held to nominal. That dimension isn't important, so isn't toleranced, but since it is the result of the addition of those three dimensions it could be shown on the face of the drawing as 2.798 REF.
It wouldn't have tolerance, and wouldn't need to be measured, or worried about, but it would be there for information. That is another common use of reference dimensions.Sorry for the novel, hope this added to your understanding of the way the drawing is drawn, and what some dimensions mean. Yeah, I've got a copy of Mil-Std-8 right in front of me on my desk, right next to ANSI Y14.5M 1973 and 1982, ASME Y14.5M 1994 and 2009, and a couple of other odd customer standards. Although Mil-Std 8 is an obsolete drawing standard for all new design activity, it is still in use for interpretation of drawings that reference it, so it's not really obsolete yet. Fun stuff, these standards that don't supersede.
I like normal standards, when one is superseded, you file it in the round file and get the newer version. These are capitalist blueprints, not imperial blueprints. It would be possible no matter what units the dimensions were in. Simply convert them as you program. It would take longer, but certainly isn't impossible.I've seen old guys who can't think in metric go through a whole drawing with a calculator and a pencil, and convert every dimension and tolerance before starting a machining project. Drives me nuts, because I'm fluent in either metric or SAE, with no preference either way. I can talk in either, think in either, and measure in either with no problems.
Many people can't though, so they have to deal with it in their own way. Absolutely, page that describes the process of the change. The value of the inch was and is still actually derived from the conversion of a meter to a yard, and the American yard was changed in 1959, as was the Imperial yard. The value of the inch comes from that conversion.On another note, the machine that the nutty professor looking guy in the upper right hand corner is running is the predecessor to the machine that I run today. I have a better probing system, better software, and probably better deliverable accuracy, although his machine is most likely more mechanically accurate (mine is comped through software to achieve better deliverable accuracy with less mechanical accuracy). It's fun to talk about what I do on Gunnit.
Yeah, but in the industrial world, most of the younger generation of engineers, machinists, and inspectors are using metric right now. New designs are almost all done in metric, and as the older generations retire we'll start to see almost everything transition to metric. I've been working the same job, with the same customers, for 13 years now, and have seen the change myself, even over that time.I'm lucky enough to be able to think equally in either system, but most people can't, and newer technical types are often more comfortable with metric. That's a good thing in my eyes. We saw it change in automobiles, we're seeing it change now in aerospace, and we'll see it change in everything before too long.
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