# Building a Quick Change Gear Box, need help.

Discussion in 'Machine Modifications' started by boxcarmj, Sep 30, 2010.

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1. Sep 30, 2010

### boxcarmj

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Is there a relationship between the spindle gear (40T) and the leadsrcrew (12TPI)? Is there a formula I need to work out? Thank-you Mike.

2. Oct 1, 2010

### The Artful Bodger

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The ratio between the spindle gear and the leadscrew gear is what sets the pitch of threads when you are cutting them and the speed of the carriage movement when you are turning using the feed screw for horizontal feed.

With a spindle gear of 40t and a 12TPI lead screw you could divide 480 by the required TPI to get the number of teeth required on the feed screw.

For example, for a 24tpi thread divide 480 by 24 gives 20, so 20 teeth required on the lead screw. Intermediate gears will not change the ratio provided you do not use compound gears.

Thats my understanding of it all.

3. Oct 1, 2010

### boxcarmj

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Thank-you, I'll add this to what I have.

4. Oct 1, 2010

### boxcarmj

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Artful, what happens when you use compound gears, does this change the ratio, and how? Your answer was very helpful. Thanks, Mike.

5. Oct 2, 2010

### The Artful Bodger

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A compound gear is an intermediate gear which is comprised of two gears locked together on the same bearing.

The most common compound gears are those that convert metric to imperial pitches etc, for example a compound gear set of 127T and 120T.

There is a better explanation here:
http://www.schsm.org/html/gear_ratio_calculations.html

6. Oct 2, 2010

### kf2qd

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Basically - a 1:1 ratio (Spindle:Lead screw) would be 12 TPI. a 2:1 ratio would be 24 TPI and a 1:2 would be 6 TPI. Others ratios as required for the particular TPI.

7. Oct 2, 2010

### boxcarmj

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Thank-you. I'll out the link, and this is all getting clearer. Mike

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