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Anyone who's spent much time in a machine shop will be familiar with the hardware commonly used for work-holding and fixtures on a machine tool. In particular, the flange nut is a common item used with strap clamps and other work-holding devices. TeCo is a well-known manufacturer of these items.

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I just happened to be shopping for some of these flange nuts for a fixture and noticed that some of these are specified as being made from 12L14 (leaded steel). This detail is not buried in the data sheet, but appears where I would interpret it as a feature, meaning a purchaser might have reasons for choosing one in this material, as opposed to one in a stronger steel for example.

My question is why 12L14 might be a preferred material in this application. I've understood leaded steels to be free-machining, which would make manufacturing easier (although making the part non-weldable), but I can't imagine that would be a driving factor in high-volume parts like this and in any case wouldn't rise to the level of a feature to be advertised if that were all it was.

I have two theories:

  1. The lead in 12L14 somehow increases the lubricity of the fastener, making it less likely to bind (or perhaps gall) in a frequently used fastener like one in a manufacturing fixture.
  2. Lower tensile strength (although not that much lower) is a plus, because if over-tensioned, it's better for the fastener to give than the t-slots machined into an expensive machine table.

Can anyone enlighten me?

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    $\begingroup$ They are used when some second op machining is required - for example turning a conical seat on the nut. The material is quite malleable and deforms when the nut is used on an uneven surface, so refinishing is sometimes required.12L14 cuts cleanly and leaves a smooth finish. $\endgroup$ – Donald Gibson Jun 25 '18 at 22:20
  • $\begingroup$ @DonaldGibson GisMoFix mentions (quite rightly, now that I think of it) that these are case-hardened. Wouldn't that argue against an "easily customized to your particular application" type of claim? $\endgroup$ – scanny Jun 27 '18 at 19:24
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    $\begingroup$ So I took one of the Teco nuts I have and turned it easily with nice, even swarf. Definitely not case hardened. Perhaps some varieties are hardened which would make sense for a heavy duty fastener, but not the ones I have. $\endgroup$ – Donald Gibson Jun 28 '18 at 14:56
  • $\begingroup$ Excellent! Really good to know, thanks Donald! I can think of a number of reasons why I might want to tune one of those in for a particular fixture :) Thanks! $\endgroup$ – scanny Jun 28 '18 at 17:55
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It is likely made from 12L14 for machine-ability of the threaded/tapped hole in the nut. Manufacturers can make them faster and probably have better tool life. Reduced cost. Simply that.

Those nuts should be case hardened. That protects from wear from friction and compression when spinning the nut down when clamping.

I’m going to disagree with your theories. There’s no galling or anti-friction happening from the leaded steel. Lead primarily works as a lubricant during cutting. This works because of lead’s lower melting temperature and heat generated at the cutting contact point. Liquid Lead acts as a lubricant to the cut.

Your machine table is likely some heat treated cast iron. It will likely take a few dings and dents from impacts, but your clamps will almost always yield before your table does. The t slots have much more stiffness and material than your t-slot clamping System. If your clamps damage the t-slot, something else is wrong.

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The lead improves machineability with some sacrifice in toughness and ductility. It does nothing to improve serviceability. The lead may be present as stringers which are undesirable , the mill tries to have it as isolated globs. The shape and size of the mill product determine the lead. Industrial applications do not permit free machining ( lead or sulfur) in fasteners. I think it is OK for consumer fasteners as they generally have lower stresses.

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