Re: Torque on Action Screws
Keith,
As this topic surfaces often, there's one thing that few seem to consider that I'll share with you.
I'll preface the following with this: Were in the business of splitting hairs. Meaning taking something "good" and making it "better". If that's the barometer we run by, then this should makes sense and be considered.
So here goes:
All the math regarding tensile loading/shear/friction coefficients, etc are great.
They are also useless if you don't know the material, grade, and heat treatment of the fastener you are working with.
For instance. If you make a 1/4-28 screw from 12L14 you'll find it machines awesome due to the lead/sulphur content in the steel. The threads will be bright and shiny and its commonly used in screw machines for bulk fasteners.
It's also dead soft with a tensile strength of only 78,000psi and a yield strength of 70,000psi.
So, you can make pretty screws with it, but you can very easily cause a thread to yield/stretch. One thing that must be appreciated with fasteners is the root diameter of your thread. The major diameter is only part of the deal.
Murphy's law takes the path of least resistance. The root of the thread is where your going to run into the fastener "hourglassing" due to excessive tensile loading.
For our applications, a 1/4-28 offers some advantages because the root is larger than a 20 pitch thread. The other advantage is the pitch. With a 20 pitch your on a .05"/rev pitch. A 28 has less, .0357"/rev.
Whats this mean? It means the shallower pitch (less incline) means its more tolerant to vibration- it's not going to be as inclined to buzz loose as a 20 pitch.
the trade being a 20 pitch is a stronger thread across the form which doesn't mean anything to us for this application. (example, if we were drilling/tapping aluminum we'd prolly be better with a 20 pitch because it's going to be less inclined to strip out because the thread form has more beef.
Where this is all going is I personally have seen no benefit, only issues with the std practice of torquing guns to 60-65 inch lbs. At my shop we don't exceed 40lbs.
My guard screws are made from 303 series stainless. I chose it for the corrosion tolerance and its tensile strength (85-95,000psi), which is greater than 12L14. It has a lower yield strength, but that's ok. Something has to go dreadfully wrong for us to platically deform a screw.
The material is also "sticky" compared to 12L14. Meaning it's tolerant to vibration induced movement. It's also very friendly to lathe tooling.
At 40lbs its still very, very resistant to the forces at play in a gun.
One could spend a whole bunch of money having 220,000 psi hammer forged, cold rolled threads made for gun fasteners that would allow us all to torque our guns together like a pair of connecting rods in a Pro Stock car. They'd last well into the next ice age. You'd also likely find that the threads in the receiver would start to give out and then there'd be threads all over the internet about how to helicoil a receiver.
My solution is to stop trying to solve a problem that doesn't really exist. Back off the torque load and use a fastener material that doesn't have high lubricity so that it won't attempt to buzz loose.
Various agencies insist on 65lbs for guard screws. The threads start to stretch and the gun goes bezerk on paper. Then they retorque/bed it again thinking they have some other problem. Then it starts to do it all over again.
Bed it with pillars, use a good resin system, use the right material, torque it to a tolerable level, and get on with your life.
Next:
All the screws in the world won't solve a fitment issue. Here is where you can really make a difference.
We'll start from the top. A receiver is traditionally bedded parallel to the show line of the stock. Meaning the imaginary flat surface of the stock and the imaginary line based on the OD of the receiver never cross one another.
Guard screw holes are typically machined at a right angle to this plane. They are on center as well. (least they are supposed to be)
So, if we were to draw this on paper, there'd be two lines coming off the bottom of the action 90* to the receiver.
Move now to the bottom of the stock. Depending on who you get your stock from, the draft angle runs between 2.5 and 3.1 degrees. Your floor metal is fitted to this. We all want nice clean presentation so we fit them flat to the bottom of the stock.
If the floor metal is made to a 2.5 degree draft and the stock is 3 degrees what then? You still want the clean presentation, so you fit it to run flush. What did this half a degree hurt? Hopefully nothing. In practice that's not completely true. Least from what I've seen.
Think about the countersunk head of the flat bottomed head of the screw as it purchases against the floor metal. think of how a 1/2 degree causes one side of the screw head to load more than the other. think of how this smaller contact patch now has to tolerate all the loading developed by the fastener.
What to do??
In my case when I have the stock flipped over and I'm fitting a floor metal, as a last operation I come in with either a flat bottomed endmill or a countersink and I spot the hole where the screw head goes. I remove enough material to see a cleanly machined ring around the screw head socket. This way the floor metal can sit anywhere it wants yet the screw head has a flat purchase on the bottom metal.
Does it really matter? I think it does as it encourages the gun to repeat in the stock that much better. I have more load distribution evenly applied to the head of the screw, and most importantly, I'm avoiding a binding action that could create a sort of secondary recoil lug effect.
With a round escutcheon it becomes a little more involved. A floor metal only goes in one way. A round "biskit" can rotate. If you spot it, you have to key it to repeat. Otherwise you could potentially make this "problem" worse. If it's out by 1/2 a degree and you fix it, only to install it 180* out of clock position you now have a 1* error.
My fix is to pin the part so that it can only assemble in one orientation.
Play with it once and see if you notice a difference when assembling the gun.
Hope this helped.
C.
