Question Home-made Recoil (and other) Spring Weight Measurer

[Bobster]

Home-made Recoil (and other) Spring Weight Measurer

My question is at the bottom...

I'm getting ready to "tune" the Macatto so I thought I'd see what all I'd need including springs. Ideally, I'll have it tuned by next weekend to test it in the Action Pistol at my range so I wanted to get parts on order, if needed, before I took it apart. I already received a single-side unobtrusive "nub" thumb safety to go on.



I had an old 1911 spring "kit" from Wilson's I wasn't sure if I had used or installed as it had been over 20 years. The MAC 9 apparently comes with a 14 or 16lb recoil spring and I wanted to lighten that a little bit. Also shown are a couple "luggage" scales picked up from TEMU for $5-$6 each.



Not being quite sure if the marked spring weights were actual/accurate and curious about what weight was in the Macatto, I looked into a spring weight scale. The one shown is about $45 and not in stock anywhere. Notice the scale they used and the scale I have above--they are identical. This got my brain wheels spinning...



So I took the scale apart to see what I was dealing with. I figured I would need a better anchor point than a strap so I fabbed a rod out of 1/8" 70S and tapped 6-32. I used the little bender I picked up from Duluth Trading BF a year or two ago. It was like $5... Taped up the exposed battery tabs just in case.



At my home shop, "resources" are limited, sort of. Had a piece of 1-1/2"x3/16" flat and a little 1"x1"x1/8" angle piece. Pop-riveted together because it is easier to drill a rivet out than to grind welds off should I need to modify it. Also used an old GM starter housing bolt (I think) and welded a washer to a wingnut. Used a piece of fuel tubing cut at 1.625" (1-5/8") to determine the compressed measuring distance for a full-size recoil spring and to help space the spring as it wound up. Commander is 1.125" and Officer's is .7" The angle piece (more scrap from the pile) is pulled down in the direction of the arrow until the tube is bottomed then reading is measured. I used a cheap HF pump clamp to hold the rig in place initially, but it became a casualty of the operation. I then went to a couple screw clamps. For Rev. 1.0 I drilled three #7 holes for mounting. I can always tap them 1/4-20 if I need to...

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So the weights I got were the spring marked 18# was actually about 15lbs, the spring marked 10# was actually about 20lbs! The random "light" spring for SWCs in the bag measured at about 11lbs. The spring in the Macatto was actually only around 12lbs so it might stay although I DID order a 10lb-er from Wilsons.

Conveniently, I can also check hammer and firing pin springs. Install the spring over the rod, put a ruler down, and pull on the hook to compress the spring. Unfortunately I do not know the distance I should compress them to. The FP spring bottomed at 3lbs. DOES ANYONE KNOW THE COMPRESSION DIMENSIONS FOR THESE SPRINGS?

 

[clm2112]

Neat project.

The springs are linear, not progressively wound, so if you add a ruler to you project you could graph the relationship between length and force, which should be a straight line on the graph for these springs.

Typically, the advertised weight of a spring is at coil bind (the point where all the loops are touching each other.) Though you pretty much demonstrated that the weights on the packages don't match what you measured. But the numbers are really just for comparing one spring against another.

Unfortunately, I can't tell you what the length of each spring is going to be when install, because it really is unique to the actual pistol's parts, but it is pretty easy for you to measure on the pistol you are installing them in.

For example, take the hammer spring... First measure the depth of the hole in the hammer spring housing and subtract the length if the plunger that is on top of the spring. The pre-loaded length of the spring is that dimension (the minimum force on the hammer.) To measure how much the spring is going to be compressed, do a little geometry experiment. Just take the hammer out of actual pistol and measure the distance between the pivot point of the hammer and the leg that is going to compress the hammer spring as the hammer rotates around the pivot point. The dimension is how much the hammer spring is going to be compressed at full cock. Now you have the two spring lengths to measure to:

Min force length = length of hammer spring pocket - plunger
Max force length = Min force length - distance between pivot point and leg on the hammer.

If the pistol is like a 1911, then there is a little extra pre-load subtracted from the minimum force length... just because the hammer's leg compresses the spring a little more when assembled to keep the plunger from banging into it's retaining pin every time the pistol cycles. If you want to measure that, just take out the pin holding the hammer spring housing in the frame and measure how far the housing extends out beyond the frame... that distance is the little bit of extra pre-load of the hammer spring Browning built into the design to minimize wear.

In fact, while you are at it, cock the pistols hammer while the hammer spring housing's retaining pin it out and measure how much further the housing gets shoved out of the frame... that's the same length that the hammer spring is going to be compressed when the pistol is fully cocked.

You can do the same to every spring in the system if you like to figure out where on the graph of force to length of each spring to start and stop. (Good engineering exercise to show what the design process of the gun is) but for the sake of tuning the pistol, measuring the force of the spring at coil bind is good enough for comparing spring against spring

Hope that helps

 

[Bongo Lewi]

I just keep changing the springs until I find one that works.