Threading vs Rethreading Dies
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    Default Threading vs Rethreading Dies

    I inherited an Ace Super Hex tap and die set back in 1978 and have used the taps far more than the dies in the years since. Recently, however, I was trying to use the 1/2-20 hex die on an unthreaded rod and was unsuccessful. I had turned the OD down to 0.495” (major dia. minus 10% of the thread pitch) and added a substantial lead-in chamfer and the material is ordinary hot rolled steel. Research on the Internet leads me to believe ALL hex dies are actually rethreading dies and the round variety are for cutting new threads. It’s the first time I’ve ever heard of this so I have to ask: is this true?

    4thTool

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    Based on my research, both round and hex dies can be for threading or chasing threads. I have seen die manufacturer's web sites reflecting this.
    The one sure requirement for threading dies is a taper on the entry side. As it starts cutting the threads are cut shallow at the start and deepen as the die progresses. This is the same kind of taper you will see on taps.

    Thinking about this logically, why would my first tap and die set purchased in 1976 (which has hex dies) be made for tapping holes, but only chasing outside threads?

    A related note; many round dies are split which allows them to be adjusted to cut loose threads or tight threads.

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    Seems like a hex die would be problematic getting it started. What are you going to drive it with? A shallow socket, a box end wrench or worse yet a crescent wrench? I think a split round die held in in a two hand dies stock would have a better chance. That said I have never used a die to cut a thread from scratch. Made a million with self opening die heads or single pointing them.
    So what do I know.

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    There is such a thing as a rethreading die for repairing damaged male threads. They are easily recognized because they have a hexagon shape with the across flats dimension exactly the same as a standard hex nut for the thread. In other words, a 1/2-13 or 1/2-20 rethreading die will have a hex that is 3/4" across the flats and a 3/8-16 will be 9/16" across the flats. They are not sharpened like a cutting die, easily seen if compared side by side.

    The hex dies for cutting new threads will all have the same across flats dimension as the corresponding round die. Die stocks that hold both 1" round and hex dies are usually included with the sets. There are larger size sets with larger dies. In my experience, the tap and die sets with solid round or hex dies are usually carbon steel and not of industrial/machine shop quality. Which is not to say such sets are cheap, especially if they carry an expensive brand name like Snap-On or Mac. But I have found the common sets to be unpleasant to use and difficult to produce good work. My sets gather dust somewhere while I use good industrial quality HSS taps and dies for actual work. I think I had to get out a metric set a few years ago because I did not have a good tap or die of some uncommon metric size.

    Larry

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    Quote Originally Posted by L Vanice View Post
    There is such a thing as a rethreading die for repairing damaged male threads. They are easily recognized because they have a hexagon shape with the across flats dimension exactly the same as a standard hex nut for the thread. In other words, a 1/2-13 or 1/2-20 rethreading die will have a hex that is 3/4" across the flats and a 3/8-16 will be 9/16" across the flats. They are not sharpened like a cutting die, easily seen if compared side by side.

    The hex dies for cutting new threads will all have the same across flats dimension as the corresponding round die. Die stocks that hold both 1" round and hex dies are usually included with the sets.

    Larry
    Thanks for that Larry. The handful of hex dies that I own looked like they would have been good enough to thread with. Just never have seen a hex die stock. Always worked at shops that had screw cutting lathes and bought my first lathe in 1978. So these hex dies have only been used to clean up damaged threads. With retreaders being the same size as a nut hex, I have never seen one.

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    Quote Originally Posted by FredC View Post
    . . .What are you going to drive it with? . . .
    You drive it with a hex die stock which otherwise looks and functions just like the round one you describe.

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    Quote Originally Posted by Gordon Heaton View Post
    You drive it with a hex die stock which otherwise looks and functions just like the round one you describe.
    Yeah,
    Larry explained that. Like I said in the second post (#5) I have never seen one, did not know they even existed. Maybe part of the OP's problem if he was using a wrench, or not.

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    "The one sure requirement for threading dies is a taper on the entry side":

    no, not at all.

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    Normally hex shaped dies are for thread chasing ( thread repair), as they are designed to be driven with a normal wrench. Where as the round type are designed for threading unthreaded stock. Larry is correct, there are huge differences in quality out there, buyer beware! The bigger issue not yet mentioned is the relative difficulty of creating good threads with a die. It is very difficult, even for experienced people. The issue is starting the die straight. A taper certainly helps of course. We all have our own methods of creating straight, but it is difficult for sure.

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    use a guide. the only difference is the rake of the theeth, although on many threading dies its almost too small to notice by eye.

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    Quote Originally Posted by steve-l View Post
    . . . It is very difficult, even for experienced people. The issue is starting the die straight. . .
    The only sure way I know of is to use a tailstock die holder. Even when you think you got one started by hand it will probably go awol if you thread more than a nut's width. (Even then its probably off, just harder to see at that point).

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    Quote Originally Posted by steve-l View Post
    Normally hex shaped dies are for thread chasing ( thread repair)...
    This. Hex dies are generally for re-threading, not creating new threads.

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    I've cut lots of threads with hex-shaped dies. They sure have a ton of chip clearance if they're only supposed to repair threads. Lead angles look pretty damn close to my other dies too.

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    I have 2 sets of Vermont American tap & die (metric & SAE). The dies are hex and are for cutting threads with a nice lead in. I also have a set of the thread restoration tools that are also hex (and with no lead and are not much bigger than equivalent nuts)

    In the end I don't think you can expect a standard - especially across decades

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    Quote Originally Posted by 4thTool View Post
    I inherited an Ace Super Hex tap and die set back in 1978 and have used the taps far more than the dies in the years since. Recently, however, I was trying to use the 1/2-20 hex die on an unthreaded rod and was unsuccessful. I had turned the OD down to 0.495” (major dia. minus 10% of the thread pitch) and added a substantial lead-in chamfer and the material is ordinary hot rolled steel. Research on the Internet leads me to believe ALL hex dies are actually rethreading dies and the round variety are for cutting new threads. It’s the first time I’ve ever heard of this so I have to ask: is this true?

    4thTool
    As plumbers move farther and farther away from using iron pipe, I have picked up a fairly complete set of Ridgid OO-R dies including bolt dies. It's pretty easy to thread 1/2" rod if you have the Ridgid bolt die. Just be sure to buy used, new prices will give you a headache.

    metalmagpie

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    it's the hook.

    Check these two, the RH is a blue-point rethreader, and it has a neutral to negative rake. The other has a hook positive rake, and anything else with any number of sides that has the positive rake is for cutting.


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    I have machined the thread with the lathe and finished up with a die. I'm assured of a good thread this way. If I need a lathe cut thread I'll finish on the lathe.

    Roger


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