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DozerMite

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A question for all the threading guru's on here.


I was given a threading chart today and was going to try some threading.
HA! The chart shows the outside dia. for a 1/4-20 thread to be .1850. This is no way going to work. I trusted the chart and turned some 6061 down to .1850 and the nut just slid right on before attempting to thread.:beat-up:

So, below that it shows 1/4-28 is .2036. Still too small.:beat-up::beat-up:

So I measured a bolt I had that fit the nut. It was .235, so I turned it down to that. No way the die was going to start. Then I reduced the dia. to .200 and got the die to start. Man was that a pain. Even after stripping the first couple threads and finally getting it threaded... the nut is very loose. It will tighten down with a wrench, but the threads are sloppy.:beat-up::beat-up::beat-up:

Can someone enlighten me as to what I'm doing wrong? I'm a tad past aggravated.:mad:
 
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The .2036 is probably the minor diameter or the diameter at the bottom of the thread. The major diameter for a 1/4" die should be about 1/4" or .250. You can taper it a little to help get it started as you have learned. Aluminum is actually one of the more difficult metals to machine. Brass is a whole lot easier to learn on. Your patience will be rewarded. Hang in there.
 
The threads are measured to the tips of the 60 degree points. As Bruce said, a 1/4" thread has those tips at exactly .250". In practice, the tips are nipped off so that friction is reduced, and the outside diameter of a bolt will be a bit less than .250". It depends on the material and the tolerance of fit as to how much is nipped off.

I find it's helpful to draw threads full scale in CAD to get a good feel for how much to remove. Another reason it's done is because taps and dies aren't perfectly pointy. The tap, and indeed the tapped part will last longer and have less stress risers, (so will be a stronger part) if the points are rounded down a bit.
 
It all depends on the material but a good place to start is 2%. Reduce the diameter 2% from nominal, in your case nominal is .250". 2% of .250" would be .005", take that away from .250" and you have .245".
Like I said though every material is different. On steel I take off more, maybe 4% or 5%. With brass it's maybe 1%, aluminum maybe 1% or 2%.

The diameter of the piece also matters. If I have two pieces of steel, one .250" and the other .500", I take off more for the larger diameter. Around 2% for the .250" and around 4% or 5% for the .500".

You need to take off a little more for fine threads than coarse. I know that sounds counter intuitive but it's correct.:wink:


One more tip, chamfer the end a little bit, it makes starting the die easier. I just use a file while it's spinning at around 500 rpm. And don't forget, a die can be put in backwards so look at it close and you'll see a taper on one side, that's the side you start with.
 
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Rick,
Great suggestion. I like the +/- 2% rule. I have to give this a try. I whacked the theread tips off real good on a tenon piece. They still thread, but look a bit goofy.
 
Thanks guys. This information is much better than what I received yesterday.
I'm going out after dinner to try it again with the use of these tips...or nipped tips.:biggrin:

Thanks again!!
 
One of two things is occurring, either your chart is incorrectly made, or you are reading the wrong lines.

.185 is for a #10 or .190 thread

.2036 is would be for a #12 or .216 thd
 
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