Screw cutting Acme Thread - Brake rod.

Welcome to this month’s traction engine build update. With the weather not helping motivation, I decided to tackle some straightforward lathe work — the brake rod.

The drawing specifies a 3/8" × 10 TPI left-hand square thread. While anticlockwise operation to apply the brake would justify a left-hand thread (as seen on some full-size engines such as those by Aveling & Porter and Burrell), cutting an internal LH square thread at this size would be challenging. A right-hand thread will be perfectly adequate for the model.

My plan is to screw cut a trial thread in silver steel, mill flutes to create a tap, harden it, and use it to thread the bronze nut. I’ll then cut the matching screw so both parts mesh correctly. Since the nut is bronze, machining should be manageable.

Although the Boxford metric lathe is the easiest machine to set up for screw cutting, producing true 10 TPI would require imperial conversion gearing. Instead, cutting a 2.5 mm pitch thread (close to 2.54 mm / 10 TPI) is a simpler and entirely acceptable solution for this application I feel. The actual screw cutting has stopped at a blank ready for the first test go with a 60deg threading tip before I purchase a proper 26deg Acme tip to produce the square thread. 

Top Right:  The brake arrangement requiring acme thread. Left the threading data plate on Boxford. 

Boring the Brake Shaft Casting

This month, the fixed steady was put to proper use while boring the brake shaft casting. The main challenge was ensuring the bore finished central and square through the part.

To maintain accuracy, the hole was machined from both ends. For the second operation, the casting was mounted on a piece of silver steel to act as an alignment dowel. A spare hole in the collet chuck provided drive, allowing the part to be removed, checked, and refitted in exactly the same position. This ensured the second bore met the first cleanly.

The cast iron machined beautifully with sharp drills, producing a clean and accurate result.

I’ve chosen to retain the original cast finish on the outside surfaces. Apart from a light rub with emery cloth, no additional machining has been done. If a part is cast, I believe it should largely remain that way — with only the functional areas machined.

A careful setup, a straightforward method, and a satisfying outcome. (Above Right!)     

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

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