C3 Mini-Lathe Metalworking Beginners

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7x12 mini lathe in fetching blue!

Threading on a mini-lathe Introduction

If you were to list basic mini-lathe operations in order of difficulty, then it would run something along the lines of: Turning, Facing, Drilling, Boring, Parting, Threading.

Naturally, one of the first things I tried to do on my mini-lathe was work out how to do the threading! This was because I was very keen to make threaded adapters for my telescope parts.

This required me to understand how the principles of threading work. On the mini-lathe, the job is held and the chuck and is rotated by the motor. A fixed tool is held against the job to remove metal. Running along the front of the mini-lathe is a long screw. The carriage (which ultimately means the tool) can be clamped onto this long screw. The long screw is known in lathe parlance as the "Lead Screw". The little clamping system is called the halfnut or split nut or clasp nut. You should have a lever on the front of the carriage operates this lock.

If you look at the left hand end of the mini-lathe under the removable cover you will find a bunch of removable gears that connect the main rotational axis of the chuck to the lead screw. On my mini lathe there are 4 changeable gears. Due to their changeable nature, these gears are called the "Change Gears". There is also a level that engages and disengages these gears. Click on the image for some labels.

It is quite clear that the change gears could be arranged so that the lead screw rotates once for each turn of the chuck. A 1:1 gearing. Now look at the lead screw a bit more carefully. If you measure the pitch (distance between each thread) you'll probably find that it is 1.5mm. So, with a 1:1 gear ratio in the change gears, and the carriage locked to the lead screw, each turn of the chuck will move the carriage along by 1.5mm.

If we were to run our motor the tool will cut a spiral on the job - the pitch of the spiral will be 1.5mm.

However, we can't cut a deep thread (say 1mm deep) in one pass. We need to cut it a tiny bit at a time. So after each pass, we have to unlock the carriage from the leadscrew and move it back to the starting position, re-engage the half nut, adjust the tool in slightly, and make another, slightly deeper cut. And so on and so on.

The key is making sure that each pass cuts into the path of the spiral made by the previous cuts. To do this, we have to engage the carriage onto the leadscrew at the correct point. The magical mechanism to do this is the thread dial indicator. This is the little dial numbered 1 to 12 next to the half nut level.

If you disengage the leadscrew and roll the carriage back and forth you will see the dial spin around. My mini-lathe comes with 3 different gears to place into the dial indicator.

Now look again at the removable change gear cover. This cover has lots of numbers written on it. On my mini lathe they look like this:

This tells you what gears to use on each of the 4 change gears, and on the thread dial indicator, and where to engage the thread dial indicator.

The final element of thread cutting is the tool. To cut standard metric threads you need a 60 degree tool tip. If your mini-lathe does not come with one of these tools, you'll need to buy one. Actually, you'll need to buy two. One for internal threads and one for external threads. Or grind them yourself.

In part 2 I will outline a step by step guide to threading on a mini-lathe.

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