How to Measure Strength of Small Springs?

I am attempting to lighten the ejector & extractor springs in a couple of
AR-15 based target rifles. My wife and I shoot in a league in May, and
it's all standing (offhand), one shot at a time. I would prefer A) not
to bounce hot brass off adjacent competitors, and B) not have to hunt for
brass flung all over the place.
One guy on our team has gone so far as to remove his ejector completely.
He ends up having to fish hot brass out of his action, which I view as a
good way to cook your trigger finger.
I have about 6 different brands of extractor springs to compare. These
are a bit special, and are conical & have to fit properly. The ejector
spring is a simple straight compression spring. I measured one, and it
is 0.103" in diameter, and just under an inch long. It's is very stiff.
For the extractor, I was thinking of comparing their strength by stacking
two of them on an 2-56 screw with a washer in between. I can use a nut
to provide some arbitrary amount of compression, and then compare how
much each one compressed. This should give me some sort of relative
strength numbers. I'm mostly interested in finding the feeblest one, but
it would nice to be a bit more quantitative.
The big problem is the ejector spring. 0.103" appears to be an odd
diameter for a spring, and it only gets compressed about a 10th of an
inch, so shortening it doesn't do much. I really need to get a softer
spring. It's awfully long & skinny to try to wind my own. Assuming I
can find someplace that will sell me small quantities in the right
diameter, I need to specify a spring constant, i.e. pounds per inch.
Does anyone have any neat tricks for measuring spring constants, and/or
suggestions for comparing the strength of small springs? I'd also like
suggestions for spring vendors who sell small quantities. I've checked
McMaster Carr, and they don't have anything that small. Brownells has
0.088" diameter, and I'm a bit concerned that is small enough to kink in
the hole. Alternatively, how can I wind a spring that small?
Thanks!
Doug White
Reply to
Doug White
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"Doug White" wrote: (clip) For the extractor, I was thinking of comparing their strength by stacking
^^^^^^^^^^^^^^^^^^^^ That method should work, to give you comparative values. If you want real spring constants, you could measure their length with a pair of calipers while hanging a weight from one end. The spring constant should be the same for extension as compression.
To get a spring that does the job, you could rotate it lightly against a belt sander to reduce the cross section of the wire, and measure your progress with the "Doug White Screw Compression Method. Having a spring "in series" with it that remains unaltered, and which is compressed the same amount every time will enable you to see how the spring changes as you lightly sand off metal.
Reply to
Leo Lichtman
This is one type of device used to test springs. It wouldn't be too difficult to scab together something functionally equivalent in a drill press or small arbor press.
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These places have a huge selection of springs, but quite expensive compared to McMaster.
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You can wind serviceable springs with a little fussing on a lathe.
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Machinery's Handbook or any machine design text for spring design formulas.
Reply to
Ned Simmons
I don't have the articles handy, but if someone else from the group can check the May and July 1987 issues of the Home Shop Machinist, there was a detailed article on coil spring making. I would not be surprised if there was a section on measuring the results, but I don't have the articles readily at hand to check.
RWL
Reply to
GeoLane at PTD dot NET
I did something a whole lot simpler: I cobbled up two chopsticks, one fixed, one (above the other) hinged. I put one spring between the sticks at a marked point and hung a little weight off the end, and measured the deflection. Then I put the other spring in and did the same thing. I never measured the spring constant, but I knew for sure which spring was stiffer.
GWE
Ned Simm> >
Reply to
Grant Erwin
Hi Doug
It can be easy to measure spring constant with a weight measuring scale and a distance measuring ruler. You could probably get good enough accuracy of the "weight needed" to compress your springs some "given distance" by placing the spring on a postage scale of the appropriate sensitivity and push down on the spring . First, force the spring to move some small distance and record a weight and a length reading. Then add some force to compress the spring a little more. Those force and length numbers provide all the data needed to identify "Pounds per Inch" of each spring.
Jerry .
Jerry
Reply to
Jerry
"Jerry" wrote: (clip) You could probably get good enough
^^^^^^^^^^^^^^^^^^^^^^^ Theoretically. What will actuaqlly happen is that there will be too much vibration. Both the length and the force will be bouncing around so much you won't get good readings.
Reply to
Leo Lichtman
why not put a valve on your gas port?
My FN-49 has an adjustable gas port, and I can park the brass at my feet, or hurl them into the sun.
Gunner
Reply to
Gunner
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Or stick on a brass deflector. They are legal under NRA rules
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Gunner, jonesing for an M1A, really really bad. And they are legal here in Kalifornia.
Anyone want to trade one for machine tools?
Reply to
Gunner
Hi Leo
You are probably right, or why would you write about vibration. But, I have measured alot-alot of automotuve suspension springs exactly as I tried to outline.
Jerry
Reply to
Jerry
On Sat, 12 Jan 2008 08:19:08 GMT, with neither quill nor qualm, "Leo Lichtman" quickly quoth:
You'd get bounce on a balance scale, but not on a digital, pressure-sensing type. Use ACME/SAE screw holddowns vs. your hand for providing the down-pressure and it'll be clean. Whooee, ah gare-on-tee.
--- Chaos, panic, and disorder--my work here is done.
Reply to
Larry Jaques
I was thinking about this, but in one case the spring is too short to work with easily, and in the other, it's too long and flexible to keep straight. I like the idea of adding a weight & measuring extension if I can figure out a good way to hang on to each end.
Thanks to everyone for all the good ideas!
Doug White
Reply to
Doug White
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I thought about that, and there are aftermarket gas tubes with valves built in. That would require a fair amount of disassembly to retrofit. I've also heard that the gas tube valves can get clogged up & are a royal pian to clean. I figured I'd fuss with the springs first & see how it works out. It only takes a couple minutes to pull the bolt & disassemble it completely so it's an easy thing to do.
Doug White
Reply to
Doug White
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Both rifles are flat tops, and have side cocking handles that will probably require a custom deflector design. The side cocker is needed because the adjustable butt stock has a raised cheek piece that interferes with the standard charging handle:
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I've already started thinking about how to build a deflector. Even if I get the springs nice & light, a deflector will tend to bounce the brass forward, which is an OK direction as long as they don't go too far. The range is built on a swamp, and the firing line is a raised berm. Bouncing the brass more than a few feet forward gets them in the tall grass at the edge of the swamp. My hope is to come up with a soft deflector that will drop them neatly on my shooting stool. if I can get them into a cartridge box, I'll be really set...
Doug White
Reply to
Doug White
Side cocking handle? You just had to be different huh...
Gunner
Reply to
Gunner

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