Spring mandrel size

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fabricator

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Feb 7, 2020
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Location
West Michigan
Hey all, I am going to be making several .007 ID springs on the lathe from .080 music wire. Is there a general rule of thumb for calculating mandrel size?
 
Empirical results will be most useful and real. ID of coil is 7 mils? is that a typo?
 
Needed 10 springs for my Edward Radial rocker arms. Past experience making springs on my lathe have been inconsistent. Decided to make the spring winder by Dario Brisighlla in Issue #19 of Model Engine Builder. Had to experiment with the adjustments, but all ten came out perfect.
 
Spring back, the spring wound on a mandrel will upon stopping the coiling of the spring and releasing the end will uncoil a little and become loose on the mandrel, diameter increases to the final diameter. Not found a formula for determining this spring back. The bending process requires that the metal exceed the yield point. For metal the force and deflection of the steel is linear until the stress, force related to size and shape, reaches the yield point. Force flattens and the deflection from this point on is permanent which is added displacement on top of the linear displacement. Remove the force and the linear displacement removes.
stress_strain_curve.JPG
 
I recently made a few springs with 0.125" ID, I tried a variety of wire diameters from 0.021" to 0.027" the wire came from a local family owned spring making business, they had no issues selling me a few meters of each wire size, I spoke to the guy in charge of the workshop about spring back allowance, he said there is no hard and fast rule, he works on around 10%, it will vary slightly with spring diam. and wire gauge and experience here does help. He suggested I use 10% as a starting point, if the spring back is too great then go down a bit on the size of the mandrel. They use cheaper carbon steel spring wire, rather than piano wire for all of their coiled springs, the springs are all normalised/heat treated after winding.
 

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