Thread with steep helix angle?

Hi all,

I quite like the idea of using a tap to create a rope knurl: **LINK**

Which got me wondering which thread has the steepest helix angle "built in", (that I can buy a tap of), hopefully the collective has some suggestions!

Simple, for any given diameter choose the smallest TPI, or largest pitch. Realistically any single start thread is unlikely to have a helix angle of more than a few degrees. If a higher helix angle is needed then it would be better to make a cutter and cut the knurls by indexing. Free hobbing with a tap is hit and miss at the best of times.

Andrew

Threads are mainly intended to allow things to be tightened and remain tight. Friction holds things together, and the slope of the thread is low so that vibration (etc) cannot shake things loose.

But, there are threads for other purposes - I recall the both-ways threads on the spindle of a push screwdriver common in my youth*. But I guess that the threads (more likely to be called a scroll in this sort of case) were milled, rather than die-cut. Even if you could get good results with the first thread, the second is going to be difficult as you would be cutting across gaps in the metal and the die would jam.

*but - I guess - now obsolete because of cheap battery-electric drivers everywhere.

Sorry !

Tim

There is a compendium of thread data here in a spreadsheet form:

https://journeymans-workshop.uk/downloads.php

You can use it to calculate (an approximation*) of the helix angle (ha) of the threads.

ha = arctan(pitch/(2*major dia)

Note that it is easier to use the metric values of major dia. and pitch from the table. If you use imperial values, you have to convert tpi to pitch.

Sort the spreadsheet by calculated helix angle.

To me, the results were surprising: the lowest helix angle is something like 6" BSP - understandable because BSP is a constant pitch series above 1".

The steepest helix angle is 1/8-26 brass, which we normally think of as a fine pitch thread. Above that one is some very high number (greater than 20) BA threads.

Before I did this, my money would have been on 1/4 BSW as it is a coarse pitch for its diameter, but I was nowhere near.

* I believe helix angle is more correctly calculated at the pitch diameter, but for this exercise, we are only doing a comparison.

Here's another way to make a rope knurl

http://www.frets.com/HomeShopTech/Projects/RopeKnurl/ropeknurl.html

Dave
The Emerald Isle

The pics linked to in the OP look like standard UNC taps were used. Looks like somewhere round 3/8 or 1/2" diameter but hard to tell with nothing in the pic to give an exact scale. BSW would give you the same pitch in most sizes as UNC.

EDIT: PS he says in a later post that the knurl wheel is made from 3/4" diameter drill rod so scaling off that in the one pic of a knurl wheel up against a tap, it looks like the tap could be as small as 1/4 or 3/8". Almost certainly UNC on that side of the pond. Looks too coarse for UNF.

You won't get much coarser threads than that in V form. High helix threads are more common in Acme or Square threads, eg the 1/2" diameter x 10tpi feedscrew on the topslide of many British lathes. Now, that's high helix angle. But you wont find it in V form threads unless maybe its a two or three start thead.

Edited By Hopper on 13/04/2022 11:31:20

I should have said in my opening post that I don't have an indexing/dividing head or rotary table, having a go at the tap method is an affordable experiment.

Just what I was hoping to find out, thank you! :D

Thanks for all the suggestions

Glass bottle threads are probably the coarsest "popular" thread especially the ancient ones. After that, Eddison light bulb threads. Nowhere near a rope twist though!

Nice working out, thanks

Yes, I was googling leadscrew threads the other day which lead me down this train of thought, taps (when available) are a bit pricy for an experiment.

When I made the worm wheel for my traction engine governor I used a 7/16" UNC spiral flute tap, quite coarse for the size at 14tpi:

Tap was bought secondhand on Ebay for a few pounds.

Andrew

Great photo Andrew! Thanks

For rope knurls a look at Clickspring will help.

A possibility?

For a coarse adjustment, I cut a 4 mm pitch thread with a semi circular form.

Would a multiple start version produce the sort of thing that you want?

Howard