Tap and die question


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Old 07-22-21, 05:13 PM
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Tap and die question

I've got a little project where I have a 5/16" diameter rod and I just need to use a die to cut about 1" worth of 5/16-24 threads on just one end of the rod.

However I'm finding that even after tapering the end of the rod, the rod is acting too big for the die. So I'm assuming that I need to grind the rod down slightly so that the die fits over it easier. Correct?

In other words, the rod needs to be slightly under 5/16" in order to cut 5/16" threads on it?
 
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Old 07-23-21, 05:03 PM
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I have no say in the matter... in a small town you get the only 5/16" rod they have. And be glad they even had it. Bottom line is it threaded better after reducing the diameter.
 
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Old 07-22-21, 09:08 PM
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Interesting question, i never thought about that. I found a calculator here:
https://littlemachineshop.com/mobile/die_threading.php
...that says for 5/16 reduce the stock from nominal 0.3125" to 0.3083". That seems to corroborate post #15 in this thread,
https://www.practicalmachinist.com/v...eading-175196/
...that says to reduce stock by 1%.

Whatch makin? Any way you could weld or braze a 5/16-24 bolt onto a shaft of the right length? Or do you have access to a lathe to skim that 1% off?
 
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Old 07-22-21, 09:11 PM
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No, it needs to be 5/16.
Are you using real thread cutting oil?
Are you making a full turn then backing it off 1/2 a turn to clear the chips?
Using a real, quality die, not just a cheap thread chasing die?
100% sure your starting the die on the tapered side?
 
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Old 07-23-21, 05:34 AM
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I agree with joecaption. No need to reduce OD (outside diameter) before male threading if the thread OD designation and the material OD are same. If you use a quality adjustable die with cutting oil, you make the threads using multiple passes, adjusting the die smaller each pass until the thread crests become close to zero width. You can't do this with a thread chaser die.
 
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Old 07-23-21, 06:06 AM
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I usually take the rod to a grinder or sander and cut a bevel in the end of the rod. It makes starting the die much easier.
 
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Old 07-23-21, 03:02 PM
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Ok thanks. Already knew most of that. CycleZen, I found a different chart that recommended .3114 (.0011 under) and went with that. Just made threading a little easier than using the stock dimension.
 
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Old 07-23-21, 03:35 PM
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I will often grind down the diameter when cutting threads into something difficult like stainless. Luckily you only loose a bit off the top of the threads so no big loss of strength.
 
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Old 07-23-21, 04:09 PM
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Glad you got it squared away! .0011 was enough to get the die moving, a thou and a tenth, interesting. I put a micrometer on the threads of a Home Depot bolt in my inventory and got .3079, .0046 under nominal.

Do you mind sharing the chart you found?

DaveO
 
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Old 07-23-21, 04:24 PM
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Ugh I didn't save it... I just typed in rod major diameter die 5/16-24 and then clicked on Google images. There were oodles of them.
 
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Old 07-23-21, 04:45 PM
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The tolerance on non machined rod diameter is greater than the 0.001 inch you say should be removed from a 0.3125 rod before threading( ASTM diameter tolerance for 5/16 bar is +/- 0.005 inch). Save yourself the need to machine by selecting a rod with the diameter on the low side.
 
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Old 07-24-21, 06:55 AM
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I worked as a machinist for 5+ years. When threads were specified as SAE class 2/3 we used 3 wires to measure correct thread depth (diameter) for the class specified. The rod OD was not involved since the machining (lathe) of the thread would reduce the rod OD (if oversize) as the thread depth increased.
 
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Old 07-24-21, 07:07 AM
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Thats great that you had a lathe. My grandpa had one and I about cried when they hauled it off for scrap iron. No one would buy it.
 
 

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