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Screw Cutting The Square Thread-Brake rod (Part 3).

 Screw Cutting The Square Thread-Brake rod (Part 3).



Hello again. (To put a face to the Blog)
 

The brake rod threading is still very much in the practice stage—we’re now on part 3. Screw cutting is proving to be a real skill to master, especially when it comes to achieving the correct thread form and overall appearance.

The latest attempt (shown above) still isn’t quite right, but it will be used as the basis for making a tap. The plan is to produce a clean, symmetrical thread, then mill flutes into it and case harden it. This tap will then be used to cut the bronze nut so it matches the thread that will be machined onto the brake shaft.

The current issue is with the pitch-to-width ratio. I used a 1.5 mm cutter for this test, but in hindsight it really needs to be closer to 1.25 mm to achieve a proper 50/50 tooth-to-gap ratio, which I wasn’t able to get here. The drawing specifies 10 TPI (2.5 mm pitch), and while I’m close at 2.55 mm, the groove width of 1.6 mm is too large.

Next step is to reduce the parting blade width to around 1.25 mm and repeat the process.

As a passing mention to my father he said "you have never done screw cutting, and is a big part of lathe work" however my previous engine I simply used a die to produce a metric thread, and being on a 1inch miniature looks fine. This time I intend to do it properly, however as he's is no longer with us Its threading by trial and error. 

The 2nd attempt 😉

 After a few scrap attempts, I'm close to tackling the actual part. I decided first to make a tap to match the brake rod thread, with the aim of producing a one-off tool to cut a bronze nut.

The tap is turned and threaded, with flats milled on the back for a spanner. It still needs the flutes cutting with a ball end cutter and case hardening, but the idea is a soft-core tool with a hardened outer layer—tough enough to avoid snapping, yet hard enough to cut effectively.

Next comes cutting the flutes, adding a slight lead-in, and hardening. With careful use, it should do the job it’s intended for: producing a single, clean internal thread. (And saves £24 on a tap).


Feel free to leave a comment !!     stephen.meredith1@gmail.com

If you have any questions while reading these blog updates—I'm always happy to reply.

If you would like to see more photo's from the archive, I have a Flickr page:  https://flic.kr/s/aHsmJ9LEH6

Until Next month, stay safe, make swarf and be the best you can be.
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