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How To Blow Up A Mandrel - Powerwinding Safely
Article © MAIL User: Namshaske

Abusing a mandrel, and how not to make it explode:

Recently, I had a mandrel snap while I was powerwinding at extreme speeds (4,000+ rpm), using my wire-feeder jig, and I then began to wonder how to solve this problem, as well as wind safely in future. So, I began to do a series of tests, deliberately breaking mandrels by both hanging weights on them while winding, and exposing them to undue torsional forces. I found the following results were pretty consistent-- any force on the TIP of a long mandrel, off-center, will eventually break it. So, here are a few cautionary signs, and a few methods to prevent this from happening:

Firstly, I use two foot cold-rolled steel mandrels of varying sizes, and found it impossible to break anything larger than 5/16" with less than thirty pounds of applied force at greater than 2,000 rpm. Most drills won't reach 4,000 RPM, but some high-speed drills will hit 8,000. I use an older corded high-speed shop drill in a stand-type wire-feeder winding apparatus, BUT the end is insecure. DO NOT leave the end of your mandrel unsecured while winding; That's a world of hurt waiting to happen when it breaks.

Now, for the breakage tests;

I tested a variety of mandrels from 3/16" to 1/2", all cold-rolled steel, for their various breaking strengths. Each test was repeated twice, and produced consistent results.


I didn't even have to stress this one to break it. With the end unsecured, it snapped clean at 1,500 RPM, no load whatsoever. GO SLOW if you use small winders, especially if it's a 1/8" brass winder (I've broken dozens of those).


The 1/4" winder fared a little better. The first one broke with no load at 3,000 rpm (But to be fair, the hole was a little off-center on it). The second one broke under ten pounds of load at 2,000 RPM.


This one was a lot harder to break. It did not break even at maximum rotational speeds of 8,000 RPM, but showed noticeable stressing after two minutes at that speed. The second one was loaded with 10 pounds at 2,000 RPM, and half snapped, partially breaking through at the threading hole.


It took a lot to break these. Spinning it at the maximum velocity of 8,000 rpm began to bend it, but did not visibly stress the metal (It did not receive the full two minutes, as it was beginning to stress the drill used). Loading weight on it showed no effect at 2,000 RPM, until the weight loaded reached 30 pounds, at which point it snapped like a twig. Note that I estimate the forces from winding 16g steel is about equal to ten pounds, and all weights were loaded 3/4 the way to the tip of the mandrel, about where the end of a coil would stop.


Again, spinning it at full rotation did not damage it, and no longer bent the metal. Took a total of fifty pounds to finally do it in, in both cases, and there is now a twenty inch length of one of these somewhere in my light fixture in the basement. DO NOT try this without proper precautions in place.


Full rotation in one case totally destroyed the mandrel, as it was slightly bent, resulting in a very large dent in the workbench the vise and drill were attached to. The other rod was not damaged by the fifty pound weight-load, and at that point, I had run out of things to load it with. Therefore, it survived the testing with only slight warping.

Now, how to prevent this? The best way you can do so is by constructing yourself a modified hand-winding rig, and rather than using a crook in the end of the mandrel to turn it by hand, use the drill to spin it, with BOTH ends secured. Even if the rod breaks this way, it will not be able to flail around with the trademark "VOOMVOOMVOOM" sound an unsecured length of 5/16" steel makes when whipping around at almost a hundred miles per hour. Wear very thick leather gloves, or better yet, a mail glove over top of leather, to avoid wire-cuts, and BE CAREFUL with the wire-end when done coiling-- it has a LOT of springback, and will lay open any flesh it hits, if let go carelessly.

Well, hope it's been informative. Next time, I'll be testing other breakable things!
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