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 larg...
Screw Cutting The Square Thread-Brake rod (Part 2). Hello again, this time I intend tackling the screw cutting problem, head on. Previously I eluded to the thought of cutting an acme thread but through some useful feedback, I've learnt that a square thread is, as the name suggests (square) and not acme form. The 2nd thing I have found out is my previous thinking of a pitch 0.108" is also incorrect. 1. Why the first lathe change gear setup didn’t work I selected from the longitudinal feed section of the chart instead of the threading (metric pitch) section. These feed settings are designed for fine carriage movement per spindle revolution , not for synchronizing the carriage with the spindle to cut a thread. So the gear train I spent hours installing: 24T stud 72–18T compound 80T 60T screw was creating a very small feed ratio , which explains why: ten turns of the chuck certainly wasn’t giving 1" of carriage movement That’s expected — feed rates are ...