Aligning threads

I wish to install a blower tube through the barrel of a boiler which is 2" in diameter and 6 1/2 " long. At the front end of the boiler I intend to s/solder a bush with a 1/4" x 40 thread. That is the easy bit. What thread can I cut in the corresponding bush in the backhead which will guarantee that both ends of the blower tube will engage with its corresponding thread without jamming up? Or am I pushing lady luck just a little bit too far?

Mick

It's not the thread, it's the distance between the bushes and their angular alignment. With the thread starts aligned the threads must be an integer number of thread pitches apart.
I assume that the blower tube is threaded along it's entire length so you can screw it in from one end. A possible solution is to machine both boses and their mounting holes. S.Solder one bush in place. Screw the tube into position on that bush. Run the other bush up the tube until it is in contact with the boiler wall. It should be tight enough to stay in place but allow you to rotate the tube wile holding the bush. Mark the angular position of the bush. Remove tube (note whichway it went in). Put the push in ths same angular position and s.solder up. Allow to cool You should now be able to screw the tube in. As long as the boiler has not distorted of course. If tight warm / cool the boiler a bit.

Robert.

It's not usual to thread full length, you just have threads both ends the same pitch. You get a lot of screeching when you screw the blower pipe in. All very unsatisfactory, but it has worked for a long time.

The norm is to have a bigger thread in the backhead with a double threaded male/female blower valve. 

Edited By duncan webster on 21/06/2023 17:42:24

Edited By duncan webster on 21/06/2023 17:44:25

1) Do you need to remove the blower pipe - if not, silver solder the pipe in place?

2) On my 5" SPEEDY loco, on one end, there is the blower valve which is attached to the boiler via a flange with 4 screws, the other end passes through the smokebox tube plate. The blower tube is threaded 5/32 x 40tpi. The tube plate bush is threaded 1/4" x 40tpi and a fitting is made with a 5/32" internal thread and 1/4" external thread. The tube itself has a couple of small bends in it to allow a bit of a spring allowing a bit of tolerance in misaligned threads. Once the tube has been machined, anneal it so that when the bolted flange is tightened up, the blower tube will find its "adjusted" length.

I can send drawings if you Pm me.

Bob

I am struggling to understand why that is important.

End-to-end reversing a threaded rod does not change the linear or angular relationship between any thread crests (buttress threads excepted).

It is just a precaution incase there is any variation in the threads at the two ends.

I have just had a similar problem, and it makes no difference how far apart the nuts are. I have been making an improved cross slide for the Atlas 12 x 24 using a new leadscrew and a new genuine Atlas nut, plus the nut that came with the leadscrew. The original nut is fixed and the new one has a small ammount of adjustability for antibacklash. The way to get the spacing correct is to screw the nuts on with a gap between them and measure their fixing points. Those dimensions were transfered to the cross slide. The nuts both had to be marked as only keeping the original directions would ensure that the threads would still line up when they were fixed in place. If the adjustable nut had a lot of axial movement that would not matter, but I only wanted the minimum of allowed movement. It would be possible to adjust nuts length so that the start of the thread was matched both ends, relative to their fixing points, but that is more bother than simply marking their direction, as Robert advises.

Edited By old mart on 23/06/2023 16:34:08

If the threaded bushes are brass you could consider making a long tap with a long pilot on the end to cut the threads in both bushes. As long as it is sharp steel it probably will not need hardening for a one-off job. You can then single point cut the tube threads on a lathe to ensure they are pitched correctly. You can do the same pitch and different diameters if that is what is needed.

Martin C