Problem with Threading Die

[JMI]

There are some threading dies where I work that are not adjustable (but are not thread chasing dies). What would cause the die to cut a full thread on one side of the round stock and not on the other. This was a 5/16 die NC and the stock was turned to .306. Stock was an unknown type of stainless steel.

Thanks

Jim

 

[Blogwitch]

Jim,

You have hit the age old problem of not having the die rigidly on centre and perfectly square at start off. What you are seeing, I would suspect, is not having the die on centre of the part being cut.

If you are doing it by hand, it might help if you cut the part to be threaded slightly (only a couple of thou) undersize from the recommended cutting diameter, just to relieve the cutting pressures slightly, and also turning a small feed in taper at the start to centralise the die to rod.

By cutting on the lathe using a dedicated tailstock die holder cures most of the problems. Or even using an opened out tailstock drill chuck gently fed onto the back of the tap wrench will help to keep it square.

You will most usually find that non adjustable dies are of a lower quality than split dies, and will try to remove too much material in one go if used for cutting threads rather than restoring them. The advantage of a split die is that you can creep gently to size by starting out with the die spread, and each successive cut is just a little smaller than the previous one, and you can make the thread match the tapped hole or nut.


Bogs

 

[JMI]

Hi Bogs,

Thanks for the thoughtful and thorough answer.

Jim

 

[doubletop]

I'm pretty new to this stuff and it was only recently I realized that the dies are different depending which side you start from. One face has a taper lead and the other doesn't. I think the lettering is on the tapered face (I could be wrong but it doesn't take much to check)

Pete

 

[tel]

if the material permits, I like to turn a short section to the minor diameter, then a taper to the major, you'll have a lot less trouble starting them true that way

 

[Ned Ludd]

Hi Guys,
Just a few more points when using a die on the lathe. If you are using a tailstock mounted die holder, make sure that there is no crud in the holder, stopping the die from being seated squarely. Quite often cutting oil will hold bits of swarf, so clean this out. To be sure, tap the die into the holder with a rubber hammer to seat it.

Another thing to check for is that the die is concentric, by this I mean some dies, even expensive ones, do not have the thread cutting part concentric to the outer. This does not matter if the die is turned by hand in the normal way but is does matter if the die is held in a tailstock mounted holder. It is for this reason that people have had to design (see old copies of ME, but don't ask me which issue)
tailstock die holders with oversize apertures and a fourth screw to centralize the die.

My own die holders have short handles, so that when threading under power, I can let go of the handle and it can spine freely. Cutting threads up to a shoulder is now easy, the die meets the shoulder and the handle pulls out from between finger and thumb and spins. It works rather like the screw-driving clutch on a cordless drill. Do though make sure that the saddle is clear when doing it this way, and don't try to cut too quickly, you don't want to overheat the die.
Ned