Threading 7/16-20 on a lathe - what size TCMT will work ?

Well, a worthy male shall grind his own threading tools :), but since I have an indexable tool bit that takes TCMT with 1/4 IC, what "nose radius"
do I need:
21.51 ? 21.50.5 ?
I can not figure out the root "radius" for this particular type thread. I tried 21.52 and it seems to be too wide at the nose .
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wrote:

The root radius of an external thread is .14434 x pitch maximum material condition. Minimum is .10825 x pitch. This is for UNR threads. However, I have seen this spec applied to UN threads. The difference between UN and UNR is that the UN thread has a flat at the root. Ah, here it is. Now, even though a UN thread still specs a flat root a root radius is optional. And because this radius makes for a longer lasting threading tool and a stronger bolt it is almost universal in use. I guess when the radius wears off of the tool and it becomes flat it still meets UN spec and saves someone's butt because they didn't have to scrap the last two tons of screws the machine spit out. ERS
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I threaded that screw using TCMT 2151 and it came out great. The screw is replacement crossslide leadscrew for my 9x20, LH 7/16x20.
Eric R Snow wrote:

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rashid111 wrote:

Tcmt is not a threading insert its a triangle 120 deg. its just a turning insert. You want 60 deg insert made for threading use a tpmc .
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