Screw cutting Square Thread - Brake rod (Part 2).

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 typically thousandths per revolution, not millimetres per revolution.

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2. What you actually need for threading

When cutting threads, the relationship must be:

$$\text{Leadscrew rotation per spindle rev} = \frac{\text{desired pitch}}{\text{leadscrew pitch}}$$

My Boxford BUD machine:

• Leadscrew pitch = 3 mm

Desired thread:

• 2.5 mm

So:

$$\frac{2.5}{3} = 0.8333$$

Meaning:

the leadscrew must turn 0.8333 times per spindle revolution

3. Checking the new gear ratio

Proposed gears from the chart after much studying:

• Stud gear = 40T

• Idler = 80T

• Screw gear = 48T

Idlers do not affect ratio, they only reverse direction.

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4. Why the chart showed 32T / 32T for 3 mm

That makes perfect sense:    32/32=1  

So:

$$3 \text{ mm} \times 1 = 3 \text{ mm pitch}$$

Exactly matching the leadscrew.     

So firstly, sorry for the long maths but finding this information out is not easy and a certain amount of trial and error was used but this may serve useful to someone else with a similar machine, and about to cut a metric square thread. 

The additional nugget of information:  When cutting a thread on a lathe, once the half-nut lever is engaged and the carriage is locked to the lead screw, it must remain engaged until the thread is complete.

At the end of each pass, stop the lathe, withdraw the tool to clear the thread, and run the lathe in reverse to return to the start position. Advance the tool for the next cut and repeat the process until the required depth is reached.

When finished, make a final light pass and clean the threads with fine emery cloth or a wire brush. Below is my first practice go all looked well until the 2nd pass. (To be explained).

 

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

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