Change Gears on the Lathe

Side view of the change gears.

When I finished the first book in Dave Gingery's series, The Metal Lathe, the leadscrew was driven by an assembly of belts and pulleys. This was a good way to transfer power, but its uses are limited. But at that stage of development there aren't many alternatives.

Lathes have some capabilities beyond turning, boring, and facing parts connected to the spindle. These include cutting internal and external threads, turning and boring with fine feeds, and cutting in the reverse direction. To accomplish these the lathe has to have change gears installed. The picture above shows my lathe with the gears installed.

I'll take a moment to show how the belt and pulley system works before I show how the change gears work.

Belt and Pulley System

Drawing of the belt and pulley system.

Here is a drawing of the belt system used on the lathe. The 1" pulley on the spindle drives the 4" pulley on the left (the idler pulley). Since the power is going from a small pulley to a larger one, there is a reduction in speed. The reduction with this part of the assembly is 4:1, so when the spindle is turning at 768 revolutions per minute (RPM), the idler pulley is turning at 192 RPM. (The second lowest speed on the lathe is approximately 768 RPM.)

The 1" pulley on the left is a part of the idler pulley, and it drives the 4" pulley on the right (the leadscrew pulley). This results in another 4:1 reduction. So the leadscrew pulley will be turning at 48 RPM. The combination of these two reductions gives a spindle to leadscrew ratio of 16:1. This means that no matter how fast the spindle is turning, the leadscrew will make one turn for each 16 that the spindle makes.

The lathe is fitted with a piece of 3/8"-16 allthread rod. This rod is 3/8" in diameter, and it has 16 threads per inch. When the leadscrew makes one turn it pulls the carriage 1/16", or 0.0625". And since the spindle rotates 16 times while the carriage moves, the feed is approximately 0.004" (0.0625"/16 ≅ 0.004"). This is a decent feed, but it can't be changed.

Change Gear System

Drawing of the change gear system

Here is a drawing of the change gear system. As you can see, it's a lot more complicated. The spindle gear is mounted to the spindle and drives the system. The next gears in the system are the tumbler gears, which are mounted on the tumbler plate. As pictured the leadscrew will turn in the same direction as the spindle. This is because all the odd-numbered gears will turn the same direction, and there are five gears in the gear train. The even-numbered gears will turn in the opposite direction.

When the tumbler plate is rotated counterclockwise, the forward tumbler gear is moved into the train. That makes the leadscrew turn in the opposite direction as the spindle since the spindle is odd-numbered and the leadscrew is even. The tumbler plate also has a setting between these two, and in this position neither of the gears are in mesh with the spindle, so the leadscrew will not turn at all. This setting can be used when facing off a piece and the carriage does not have to turn.

The next gears in the train are the compound gears. The large gear (the one with 64 teeth) is always driven, and the smaller one is attached to the large one. The compound gears will just transfer power if the next gear (the idler gear) is attached to the 64 tooth gear, or will make a 2:1 speed reduction if attached to the 32 tooth gear. This is because of the difference in the number of teeth on the driven/driver combination. (To make that clearer, envision one turn of the compound gears. There will always be 64 "teeth" going into that one turn. If the idler is attached to the 64 tooth gear, there are 64 "teeth" out, so it's a 1:1 ratio. But if the idler is attached to the 32 tooth gear there are only 32 "teeth" out, so that gives a 2:1 reduction.)

The gears up to this point are installed permanently. All of the change is done in the last two gears. The idler gear, as pictured, is just transfering power. But two gears can be attached and installed on the idler bracket to increase or decrease the speed of rotation. In the drawing the idler is a 44 tooth gear, and has a spacer in front of it because it is attached to the 64 tooth gear. If the idler is attached to the 32 tooth gear there is a spacer behind it.

The final gear is the change gear, and it is the one that is attached to the leadscrew. In the drawing the change gear is a 40 tooth gear. This setup is used to cut a 20 pitch thread, or 20 threads per inch. This is the same pitch that is on a 1/4"-20 bolt.

All together there are 21 different gears in the set that I made, with another one that will show up farther down on this page. The 16 that are not permanently installed are combined to give a wide range of spindle to leadscrew ratios. They can cut many different threads, with piches of 8 to 80 threads per inch, and also fine feeds of 0.0025" to 0.005" per spindle revolution.

Back to My Lathe

Side view of the change gears

This is another view of my change gears. The idler has two gears combined, a 96 tooth and a 24 tooth. Since they are attached, the idler causes a 4:1 speed reduction. The change gear is a 100 tooth gear, and it's the biggest one in the set. With this setup the lathe has the finest feed of 0.0025" per spindle revolution. The white and black spots on the gears are grease. It concerns me to have aluminum gears running together without any lubrication.

View of the front of the lathe.

And here's a view of the front of the lathe. In case nobody's noticed, I like showing it off.

The Threading Indicator

Picture of the threading indicator

This picture shows the threading indicator. It is used when cutting threads, hence the name. I can explain how it works, which I'll do next, but I'm a little fuzzy on why it works.

Picture of the side of the threading indicator

The indicator gear in the picture is made of brass, and it has 32 teeth. It's attached to one end of a shaft, and a shaft collar is attached to the other end. The shaft collar is divided into quarters, and each is numbered from 0 to 3. Since the indicator gear has 32 teeth and the leadscrew has 16 threads per inch, the indicator will rotate once for every 32 revolutions of the leadscrew.

To cut threads, the carriage has to be engaged at a certain point in the leadscrew's rotation. These points are indicated by the marks on the indicator. To cut even-numbered threads (8, 18, 24, etc., threads per inch), the carriage is engaged when any mark on the indicator is at the top. For odd-numbered threads, the carriage is engaged on the 1 or 3 only. And for half-threads, the number 1 mark is the only one used.

Picture two complete sets of change gears.

Here are two sets of change gears. I am planning on building another lathe, so I made both sets at once. This saved a lot of setup time. It took me about 3 minutes to index and cut each tooth. There are a total of 967 teeth in the set, so it would take about 48-1/2 hours just to cut the teeth. (Actually, it's to cut the tooth spaces.) While I probably didn't cut the time in half, I know that I saved a great deal of time by making them all at once.

And this completes my lathe, as well as all of the other projects in the Gingery series. I started this lathe in July of 1994, and finished it in August 2003. Since there were 4-1/2 years that I couldn't work on it, it took a total of 4-1/2 years.

Updated August 12, 2003

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