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7x12 mini lathe in fetching blue!

Threading and Change Gears on a Mini-Lathe

Elsewhere on this site I have covered the subject of threading. However, one of the areas that causes a lot of confusion is how to get certain pitches of thread.

The pitch of a thread is the distance between the peaks on the thread. This can range from a few tenths of millimeter to over a centimeter depending on the application of the threaded part.

The mechanical principle at work here is simple. The threading tools is attached to the toolpost. The toolpost and carriage gets moved steadily along the bed of the lathe by means of the leadscrew. The leadscrew is mechanically linked to the rotation of the spindle. As you part turns, the tool is drawn along it and cuts a spiral: A thread!

On my metric mini-lathe the leadscrew has a pitch of 1.5mm. If the leadscrew was coupled directly to the spindle, each time the lathe chuck turns once, the tool will be moved along by 1.5mm. This would give a thread of pitch 1.5mm.

However, the leadscrew is NOT coupled directly to the spindle. It would be a bit useless if it was. Between the leadscrew and the spindle of the mini-lathe are a set of gears called "Change Gears". The purpose of the change gears is to change the speed of rotation of the leadscrew relative to the spindle.

Change gears on the mini-lathe consist of a gear on the spindle, a gear on the leadscrew, and two adjacent gears on a spindle between them. The lathe user is able to swap and change these gears as much as they like. A typical mini-lathe comes with a range of gears between 30 tooth and 180 tooth. On the front of the mini-lathe are some instructions about what gear to use to get the various pitch threads.

For example, if I put a 40 tooth gear on the top (spindle), a 60 tooth gear on the leadscrew and a sandwich of a 40 tooth and 30 tooth in the middle, I get a reduction of (40 / 40) * (30 / 60) = 0.5. As the leadscrew is of 1.5mm pitch, the actual pitch of the thread cut will be 1.5 * 0.5 = 0.75mm.

A thread of 0.75mm is one of the typical threads which is NOT indicated on the instructions. Interestingly this is one of the most common threads I cut. Normally the thread you need is on the instructions, but if it is not, it is a simple matter to work out the gears required. Just remember the pitch of the leadscrew (1.5mm) and think what reduction is needed to get from 1.5mm to the pitch you require. Then examine the range of available gears, and choose those that get you closest. You may have to think about two reductions that, multiplied together, give the required reduction. Then find gears to produce this reduction. Always think about the ratio and reduction required to modify the leadscrew pitch, not the actual pitch you are aiming for.

It is not possible to use every single combination of gears on the lathe, because there is only a certain amount of adjustment of the middle pair to make everything fit. Experiment is called for.

Thread dial indicator

The other thing to worry about is the thread dial indicator. This is a gear that rests against the leadscrew and is attached to the saddle and has a 12 position dial on top. Its purpose is to tell you when to engage the half nut on each cutting pass, so that the new cut follows the path of the previous cut. If this were not so you would get a mess instead of a thread. The gear with which the dial indicator couple to the leadscrew can be changed. My mini-lathe comes with a selection of 14, 15 and 16 tooth gears.

Let us think about our 0.75mm pitch thread again. If we use the 16 tooth gear on the dial indicator, and we make our first cut with the dial at the 12 position. Where do we start the second cutting pass? Well, gear on the dial has 16 teeth, your leadscrew has 1.5mm pitch, therefore the 12 position goes by once every 24mm. Does 0.75 fit into 24mm? Yes. 32 times. Therefore we would be safe to start again on position 12. How about halfway? Well, this goes by once every 12mm, still a multiple of 0.75mm. There are many other positions that work, but I generally choose one or two that i *know* work, and stick to those.

How about a 0.7mm thread? Well, the required reduction from 1.5mm leadscrew pitch is 0.4666. the gears (35/60) * (40/50) = 0.466. That is the change gears sorted out. Now, what of the thread dial indicator? With the 16 tooth gear, the number 12 position goes by every 24mm. Does 0.7 fit into 24mm? No it doesn't. We cannot use to 16 tooth gear. If you instead choose the 14 tooth gear, this will turn once for every 21mm. Does 0.7mm go into 21? Yes, it fits in 30 times. So if we start each cut on position 12 with the 14 turn dial gear, all will be well. You could also start at position 6 as well.

There are many websites giving tables and charts of information about change gears, but it is much better, in my mind, to think about the mechanics going on behind it - this will enable you to cut any thread you feel like with confidence.

Another thing I will always do when cutting a new thread pitch I have not tried before. I will make several passes with the cutting tool without advancing it any deeper. I will watch closely to make sure each cutting pass of the threading tool follows the groves of the previous pass. This way I gain confidence that I am not about to trash a part with some rogue threading.

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