Cutting Male Threads In Both Directions

[TonyL]

I have watched several videos which recommend cutting male threads (external threads using a die) in in both directions. To clarify, the demonstrators recommend cutting the threads with the larger side of the die first, then rotating the die (flipping it over) and cutting the threads again to remove the "starter" threads. This is supposed to created more/deeper threads along the whole length of the tenon.

I can barely spell threads, but I always thought that threads were angled/slanted giving them a direction. If they are angled,aren't you ruining the first set of threads and changing the direction of the threads (aka cross-threading)?

I have tried this several times and it seems to work, but I can't help but think that something doesn't make sense.


See this around the 9 minute mark: https://www.youtube.com/watch?v=vMvVgGivocw&t=683s

 

[rholiday]

Think of it as putting a nut on a bolt. Doesn't matter which side of the nut is threaded on.

 

[cjester]

Think of the die as a sharp nut. When you put a nut on a bolt it doesn't matter which side goes first.

Aside from being sharp, the "front" threads in the die are also shallower. Flipping it means you can get an even depth all the way to the end.

 

[hokie]

I saw this technique of his as well and while I understand what he's trying to address, I don't think it's actually as big of a problem as he's making it out to be. As suggested in another thread of yours, removing the first few female threads of the body into which the section screws in should address the issue of shallow threads in the section. I have done the parting tool technique of removing the section threads next to the shoulder, but I don't like how much material needs to be taken away and how thin the walls can get.
I am not sure about the angle of threads, but on the taps/dies I use, they *seem* to be something like a perfect 60 degree angle and even on both "slopes" of the threads, so flipping the die shouldn't matter in theory, but it's a hassle for sure (to me at least).

 

[monophoto]

Tony


Your observation is correct. Dies used to cut male threads are slightly tapered, with the opening a bit larger on the side that is engraved to show the size.

The reason for that in normal use when threading metal, that causes the male thread to be tapered, with the end having a slightly smaller diameter than further back. As a result, it's easier to start the threaded rod into a threaded hole or nut.

But that's undesirable in pens because it causes additional stress on thin plastic components. Instead, you want the male thread to have a uniform diameter along its length. Hence, the recommendation is to run the piece through the die twice, first entering from the side with the engraving, and then reverse the die and run the piece through again to produce uniform diameter along the full length of the thread.

Taps used to thread female holes are typically also tapered - but here things get complicated. Taps come in three varieties determined by the degree of taper. On so-called 'tapered' taps (which are the most common), the 6-8 threads nearest the end are significantly tapered, and that taper is usually readily visible. If you are threading a blind hole, the diameter of the threads at the bottom of the hole will be smaller than the diameter near the opening. Obviously, if the hole is not 'blind' (ie, it goes all the way through the piece), then the tap can be run in much deeper and potentially can be used to cut full diameter threads over the full depth of the hole.

Then there are 'bottoming taps' where the first, and perhaps second thread are slightly undersized, but the remaining threads are full diameter. Bottoming taps are iused to complete the threads on blind holes.

Finally, there are intermediate taps that are somewhere between tapered and bottoming.

If you buy a tap at the hardware store, it will usually be a tapered tap. You also can buy taps in sets of three (one of each), and you can buy them individually from specialty suppliers. Bottoming and intermediate taps are typically more expensive because the demand for them is less.

 

[Dalecamino]

It's just a little more assurance that your parts will thread together properly the first time. If you are cutting a relief on the female part, it shouldn't be necessary. But, some use this step, just to be certain that, the part is indeed threaded all the way to the end of tenon.

While we're talking about this, bevel the end of the tenon where you are starting to cut your male threads. Then when you flip your die around, the die threads will catch your tenon threads, and not cross thread.

 

[bmachin]

Tony,

As the Nike ads used to say,"Just do it!". You need to reverse the die to make sure that the threads have the same root diameter for their full length. If they don't, your cap or section will start to get tight before it gets fully screwed down.

Bill

 

[Penultimate]

I double thread to get the die with the full size thread cutting teeth up against a shoulder. It it cuts full threads to the end. You can avoid tapered threads by cutting a relief diameter at the shoulder. I use a parting tool.
Good luck, happy threading.

 

[TonyL]

A machinist provided me the answer. Thanks all!

 

[Curly]

A machinist provided me the answer. Thanks all!

And he said?

 

[randyrls]

Actually that may be a good idea to leave the threads a little tight at the very end. Many years ago Baron fountain pens would tend to unscrew in the pocket. Messy! People tried all sorts of methods to increase the tightness of the threads.

 

[TonyL]

He said to think of the die's threads as a equilateral triangle: no matter which way it is flipped (as long as the point of the triangle is upwards) it will cut at the same angle/orientation.

 

[mredburn]

The issue is when your threads are only .160- .200 (4-5MM) long. Thats not long enough to get the theads all the way through the die. Since the die threads are tapered inside and only give the full depth as they exit the die, you use the start side to form the initial threads, flip it around and run the back side so that it cuts all the threads but the last couple to full depth. The dies do not cut threads the full width of the die so even then you end up with threads that are not fuily formed next to the shoulder where the threads stop. I have even ground the dies thinner to get the teeth as close to the shoulder as I can.