Wednesday, July 6, 2011

Coupling Nuts

Pictured below are the four sizes[1] of common plated steel coupling nuts that my local Home Depot routinely stocks. (I think they may have at least one larger size as well, but I'm not certain of that.)

These things are useful. They're normally intended for joining lengths of threaded rod end-to-end, but I'm always finding other uses, and would never be without several of these on hand in the shop. Many more sizes can be had, along with metric and stainless steel versions, but you'd likely have to go to an industrial supply house like Spaenaur for them.

Here's one example of an unorthodox application. (Scroll way down.) I quite like that one.

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I have noticed a little quirk about them that can complicate things a bit at times, though. I'll try to explain it clearly here.

Looking at a coupling nut, one would be inclined to assume that the axis of the nut's threaded bore will be perfectly coaxial with the axis of the nut's hexagonal body. I've found that it ain't necessarily so, especially with the small size pictured (1/4"-20).

Where that can cause you some trouble is when you need to bore through the thread for some purpose with a nut chucked in the lathe. The nut's threaded bore may not present perfectly coaxially to the drill chucked in the tailstock, and some undesirable results may attend.

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I had occasion recently to put two 1/4"-20 coupling nuts to use as joining sleeves for two different sizes of steel rod; 9/32" diameter and 8mm diameter. I first bored completely through the two nuts with a 9/32" drill. Then I bored half way through each nut with an 8mm drill.

The 9/32" portion of the bores was fine. The 8mm portions turned out to be oversize. The way I had gone about it was fallacious. It had assumed the nuts' two axes to be perfectly coaxial, and they weren't.

What I should have done was this:

1) Chuck a nut and bore through it 9/32". LEAVE IT IN THE CHUCK.

2) Bore half way though 8mm, then take it out of the chuck.

3) Repeat for the second nut/sleeve.

By not unchucking each nut between the two different drill sizes, I would have greatly improved the likelihood of a good outcome. Unchucking and rechucking each nut between drill sizes had the effect of reorienting each nut's bore axis slightly differently for the 8mm drill, from what it had been for the 9/32" drill. That led to an oversize result from the 8mm drill. That's a great example of the sort of subtle consideration that can arise in even the simplest of machine shop work. Next time I'll know better.

Anyway, here's the whole story surrounding the aforementioned joining sleeves. They turned out ok, but the oversize 8mm bores did complicate things a little bit.

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[1] For whatever it's worth, following are the specs for the four sizes of coupling nuts shown in the photograph:
  • 1/4"-20; 7/16" A/F; 7/8" long.
  • 5/16"-18; 1/2" A/F; 1 3/4" long.
  • 3/8"-16; 9/16" A/F; 1 3/4" long.
  • 1/2"-13; 3/4" A/F; 1 3/4" long.
Note: The length dimensions are nominal. These are not gauge-precision items.

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