Making Go/No Go Headspace Gauges

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chrestme

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I just built my first AR308/DPMS pattern rifle in 6.5CM and was looking at Go/NoGo headspace gauges. At first glance they seemed simple enough to machine, until I realized the critical length references a diameter on the shoulder (.400" for 6.5CM according to SAAMI) and the tolerances are only a few thousandths between go and no go.

Looking through other gunsmithing forums, they all seem to advise just buying a set of gauges from a reputable source, which is understandable when you're dealing with explosives sitting a few inches from your face. However, this being a machinist forum, I was hoping I could at least start an academic discussion on the process.

From my amateurish understanding, the rough process would be to start with a piece of drill rod close to final diameter, turned down to the case max diameter (.473 for 6.5CM). Then turn a taper (30 degrees) to simulate the case shoulder. Then cut the part a little longer than final length.

Then to reference the shoulder datum (.400), you could drill, bore, and ream a bushing to exactly the datum diameter. Then face the bushing, mic the length, and keep that number as reference. Then the shoulder of the go/nogo gauge can be placed into the bushing and measured with a mic or calipers and further faced down to the final length (1.5438 -.0070).

The part I get hung up on is the right angle on the bushing between the face and .400 bore. Straight off the reamer, I imagine it could still have a bur on the edge, which could affect your measurement. But if you de-bur/chamfer the hole, you either change that critical .400 dimension, or the length of the bushing. I suppose you could lap the face of the bushing, and then the bore. That way any burs would be removed and the change to critical dimensions would be minimal.

Does that make sense? Am I missing something? Making it more complicated than it needs to be?
 
You can chamfer or better yet radius the end of the bore as long as you have the chamfer or radius in your design. This way the bottom of the chamfer is the actual gaging point yet the chamfer will do the deburring. A sharp edge is also not good for gauging as it will either damage the surface it is trying to gauge or the surface will damage the sharp edge of the gauge.
 
Flyinfool said:
You can chamfer or better yet radius the end of the bore as long as you have the chamfer or radius in your design. This way the bottom of the chamfer is the actual gaging point yet the chamfer will do the deburring. A sharp edge is also not good for gauging as it will either damage the surface it is trying to gauge or the surface will damage the sharp edge of the gauge.
Click to expand...
This is what I was thinking, but then I started to confuse myself with how to measure the Z component of the chamfer. Touching off first before taking a cut doesn't seem accurate enough if your end product only has a tolerance of a few thousandths. It seems, if you can cut a chamfer with high-precision, you could do the trig to figure out the Z depth.

This spurred me to find this video from Joe Pieczynski on measuring conical features.
 
I have wondered about how to make headspace gauges too as a purely intellectual exercise. I doubt that I will ever become proficient enough at machining to make one. Trimming the gauge to the proper length seems to be the most difficult part of making the gauge. Where do you measure from and how do you take the measurement? Do you need to taper the body of the gauge? Probably not.

If I was to try to make one for a 6.5CM this is how I would do it. The min max differential is .010. 1.541" to 1.551." The thickness of the rim is .055

Set up a round bar between centers.
Turn the bar to .473 OD.
Turn one end to a 30* taper.
Turn a portion of the bar at the taper end to .400 OD.
From the intersection of the taper and the .400 OD turn the bar to .400 OD for 1.486" from the intersection.
Part the work off 1.541" from the intersection.

You would end up with this.

go gauge.jpg

If the body taper is necessary to have you would end up with this.

go gauge with body taper.jpg
From a practical standpoint it only costs $8 to rent the go/no go gauges if your are also going to rent the chamber reamer.
 
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chrestme said:
This is what I was thinking, but then I started to confuse myself with how to measure the Z component of the chamfer. Touching off first before taking a cut doesn't seem accurate enough if your end product only has a tolerance of a few thousandths. It seems, if you can cut a chamfer with high-precision, you could do the trig to figure out the Z depth.

This spurred me to find this video from Joe Pieczynski on measuring conical features.
Click to expand...
I have used a similar technique for specifying and measuring conical features. An easier way which avoids the math calculations is to use a parametric CAD program like SolidWorks or Fusion 360 to handle the math. I draw out the conical feature and the ball, putting in the constraints and add the driven dimensions.

One thing which complicates this method is the reliance on knowing the exact angle of the cone. Using a second ball of different diameter can find both the cone angle and the depth of the cone. The math becomes more involved but using SolidWorks makes it a piece of cake.
 
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