How can a rod be threaded with square threads.

[RJSakowski]

Single point threading is the best way to insure that the thread axis is coincident with the rod axis.

To help square die cut threads on the rod, I will sometimes turn the tip of the rod to the root diameter or slightly larger for the thickness of the die. This gets the cut off to the right start. If threading in the lathe using a die, I will back the jaws on my tailstock chuck and use the face of the chuck to square the die. Also if using the lathe , you can notice if the diestock is wobbling when you first start to thread and exert a little force to correct it.

For threading using a vise, there are diestocks which have three adjustable plates which act much like a follower rest on a lathe to guide the rod in the die. My ancient Craftsman diestock had this feature.

 

[stupoty]

To help square die cut threads on the rod, I will sometimes turn the tip of the rod to the root diameter or slightly larger for the thickness of the die.

I did the tapping version of that with some M2.5 holes I was tapping by drilling it full diamitor for about 3mm , they all went in strait and I haven't broken the tap (yet)

Stu

 

[RJSakowski]

I did the tapping version of that with some M2.5 holes I was tapping by drilling it full diamitor for about 3mm , they all went in strait and I haven't broken the tap (yet)

Stu

Taps not running true is on of the biggest causes of hard tapping and tap breakage. I try to do my tapping on the mill whenever I can. I will drill the hole and follow with the tap using a spring loaded tap follower. Tap breakage went to zero since I started this practice.

Drilling a short lead at the major diameter of the thread does essentially the same thing. If tapping deep holes, remember that the shank of the tap is usually a slightly larger diameter than the thread diameter so the lead hole should be sized accordingly.

 

[stupoty]

I will drill the hole and follow with the tap using a spring loaded tap follower.

Unfortunately I don't have a tap handle that is a good size for the M2.5 tap and similar size but I have found using a small sub chuck for holding small drill bits to be good for this as it gives less mechanical advantage with the body being only about 3/4" diameter, it has knurling so i can grip it.

I have managed to brake small taps in the past with merely the weight of the tap wrench handles bending the tap too much if it slipped a bit.

I have a wide range of methods for snapping taps I'm trying to de learn them though

Stu

 

[dansawyer]

A quick update. I was trying tap 1/2 x 20 threads about 3/4 in inch deep. The tap did not have a center hole in the handle end to center it in a deed center in the tail stock to use a lathe as a jig. I tried to start the hole as best as I could manually in the lathe. The tap took hold and after 3/8 of an inch or so. I then removed it and tried to proceed manually. Alas -- the tap was not straight!! I removed it thinking I had ruined the part.
The tap had an aggressive start angle. It appeared as though it went from insert to full cut in about a 5/16 inch or about 6 threads. I compared this to a Acme 1/2 x 10 tpi tap. It had at least 3 inches of cutting threads. It did not reach full cutting depth for at least 2 inches. I decided to do an experiment and grind the leading teeth of the tap about 1/2 inch back. I did pay attention to the rake angle so the leading edge would still retain a cutting angle. The result was the tap did not reach full cutting depth for 5/8 of an inch. This created better alignment and easier entry into the work.
I then placed the tap in a chuck in the tail stock and the work in a chuck in the spindle to use the lathe as a jig. I manually turned the belt and started to feed the tap into the work. I used the dial in the spindle to keep some pressure on the tap.
This all ended up working. I was able to get the tap started sufficiently straight to finish the threading in a vise by hand. The result was clean straight threads. The experiment worked!

 

[savarin]

If your tapping a through hole I have found that spiral point or gun taps to be excellent in the small dia's to pull themselves straight. (2.5 to 8mm is the largest I've used)
If I can I start the tap in the lathe or drill press then finish in the vice.
I start all my rods now as single point threads then if required finish off with the die.

 

[homebrewed]

At the risk of stating the obvious, you can buy taps with (generally 3) different tapers on the end. The ones with the least taper are called bottoming taps because they can cut threads closest to the bottom of the hole. But they can be difficult to get started correctly, so a tap with a longer taper is used first, then you switch over to the bottoming tap. You just converted your bottoming tap to a starting (or through hole) tap.

 

[Flyinfool]

For using a die on the lathe I have used a rod turned to the minor dia of the die in the tail stock chuck with the rod tight up against the part. This will guide the die on straight to get things started.
I also have a die holder with 4 handles so that it is easier to see if it is tilted when starting.

 

[NortonDommi]

Many dwangs come with guide bushes that fit under the die so that the rod being threaded is square to the die, very easy to make if you don't have them. Basically a hat shape with the appropriate sized hole in the top and a thin brim. Fits into dwang under the die and as the rod passes through the hole it is automatically square to the die. Needs to be remove to thread close to a shoulder.