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http://www.conradhoffman.com/metricthreading.htm
Metric Threading on the Lathe
Metric threading using a lathe with an inch lead screw isn't difficult, but instructions that cover the finer points are few and far between. It's a metric world and I now cut more metric threads than anything else. Chances are, if you're not cutting metric threads now, you will be sooner or later. My guess is sooner. With just a few extra gears you can cut any standard metric pitch on your small Logan, Southbend or other quality lathe. The following explanation may seem a bit tedious and repetitive, but a solid understanding of the basics will help you solve any problems that might arise, for any lathe, gear train or pitch.
Metric Transposing Gears & How They Work
The most common lead screw is 8 TPI, a pitch of 0.125". That's 3.175 mm, not a terribly convenient number from which to derive the standard metric pitches. The purpose of metric transposing gears is simply to scale that pitch into a more useful value that can be divided or multiplied, using common gear ratios, to get the standard metric pitches.
The traditional pair of transposing gears have 127 and 100 teeth. The lead screw pitch, 3.175 mm, divided by the ratio of those two gears, 1.27, gives us 2.5 mm, a much easier number to further divide or multiply into standard metric pitches. The important thing to remember is that using metric transposing gears allows you to treat your inch lead screw as a metric lead screw. The rest of the gear train simply multiplies or divides that new pitch value to get to the desired pitch. If you think of the transposing gears and lead screw together it avoids having to include the specifics of the conversion in every gear calculation.
A simple example will reinforce the concept. You've installed the metric transposing gears and now think in terms of having a 2.5 mm pitch lead screw. You want to cut a 1.0 mm pitch thread so you need some gears in the path between the transposing gears and the lead screw (or spindle, depending on where the transposing gears are installed) with a ratio of 1:2.5. A 16 and 40 tooth pair would do the job (as would various others having the same ratio). If you have a lathe with a quick change gearbox, the same rules apply. You still need an overall 1:2.5 ratio, using some combination of external gears and the ratios available within the gear box. I'll cover QC gearboxes in more detail later, but for the moment we'll assume a simple change gear machine with no gear box.
Big Transposing Gears
The traditional 127 and 100 tooth transposing gears are big and expensive. Worse, they usually won't allow the gear cover to be closed, increasing the chance of damage from metal chips and exposing people to the hazards of an open gear train. Make no mistake, open gear trains are extremely dangerous. They will pull things in, destroying both object and gears. Think injuries and amputations. Open un-guarded gear trains are never allowed in industry and a company would be heavily fined by OSHA for allowing such. If you use a gear train that doesn't allow the cover to be used, fabricate a suitable guard to protect yourself and others. It may seem overly cautious, but you should also unplug the lathe whenever you're working on the gear train.
The next issue is the banjo, the frame that holds the gears and allows their relationship to be adjusted. With the large transposing gears you may need an alternate or even custom banjo. Everything is lathe dependent. Some lathes accommodate these gear trains with little trouble. Others, typically the smaller machines used by hobbyists, require some ingenuity to convert. One could, of course, obtain a metric lead screw, but it's far easier to swap gears than lead screws.
The 127 and 100 tooth gears are the smallest gears that offer an exact metric conversion. If you were making lead screws or micrometer screws, those would be the gears you'd choose. Obviously you'd also need a perfect lead screw with no wear. The reality is that some slight error is tolerable; in fact it's unavoidable. There are other gear combinations that offer a more convenient metric conversion, albeit with a tiny error.
Small Transposing Gears
A common metric transposing pair is 47 and 37 teeth. Compared to the perfect ratio of 1.27, these gears will give you 1.27027027... That's an error of 0.021%, which is only 0.0026" per foot. For any normal length thread you'd be hard pressed to measure the error, much less notice any functional difference. In return for this small compromise you get a pair of gears only a few inches in diameter. They'll fit within the factory gear cover and the cost will usually be lower.
The smaller pair of gears also lend themselves to DIY fabrication if you have a mill and dividing head. Unless you're cutting very coarse threads, the forces on the gear train are low. Because of this, I make my transposing gears, and any other change gears I might need, out of Delrin or a generic equivalent. It's easy to cut, the cutters last forever and I've never seen any significant wear in the finished gears. My guess is that they'd even stand up to moderate production use with little trouble.
Another possible pair is 80 and 63 teeth. The ratio is 1.26984, an excellent match, but the gears are still quite large and may not fit inside the gear case. They may be a good choice in some situations, but as the gears get larger the exact match of the 127 and 100 tooth pair makes more sense.
Practical Gearing With Simple Ratios
If you limit yourself to the transposing gears and the standard change gears that come with most lathes, there are quite a few metric pitches that can be cut using simple ratios. They are 0.75, 1.00, 1.25, 1.50, 2.00, 2.25, 2.50 and 2.75 mm. Simple ratios also allow a variety of coarser threads but you'll rarely need them. For finer threads and for in-between pitches you'll need compound gearing- another pair of gears to increase the division ratio. Here's a photo of a simple ratio set up to cut 2.00 mm pitch threads. Notice that spacers have been used under both the idler gear, which can be any number of teeth, and the screw gear. These are one gear thickness, so a small spare gear can also be used. The gears are thus in what I refer to as the outboard position. Move your mouse over the individual gears for more details.
Metric Threading on the Lathe
Metric threading using a lathe with an inch lead screw isn't difficult, but instructions that cover the finer points are few and far between. It's a metric world and I now cut more metric threads than anything else. Chances are, if you're not cutting metric threads now, you will be sooner or later. My guess is sooner. With just a few extra gears you can cut any standard metric pitch on your small Logan, Southbend or other quality lathe. The following explanation may seem a bit tedious and repetitive, but a solid understanding of the basics will help you solve any problems that might arise, for any lathe, gear train or pitch.
Metric Transposing Gears & How They Work
The most common lead screw is 8 TPI, a pitch of 0.125". That's 3.175 mm, not a terribly convenient number from which to derive the standard metric pitches. The purpose of metric transposing gears is simply to scale that pitch into a more useful value that can be divided or multiplied, using common gear ratios, to get the standard metric pitches.
The traditional pair of transposing gears have 127 and 100 teeth. The lead screw pitch, 3.175 mm, divided by the ratio of those two gears, 1.27, gives us 2.5 mm, a much easier number to further divide or multiply into standard metric pitches. The important thing to remember is that using metric transposing gears allows you to treat your inch lead screw as a metric lead screw. The rest of the gear train simply multiplies or divides that new pitch value to get to the desired pitch. If you think of the transposing gears and lead screw together it avoids having to include the specifics of the conversion in every gear calculation.
A simple example will reinforce the concept. You've installed the metric transposing gears and now think in terms of having a 2.5 mm pitch lead screw. You want to cut a 1.0 mm pitch thread so you need some gears in the path between the transposing gears and the lead screw (or spindle, depending on where the transposing gears are installed) with a ratio of 1:2.5. A 16 and 40 tooth pair would do the job (as would various others having the same ratio). If you have a lathe with a quick change gearbox, the same rules apply. You still need an overall 1:2.5 ratio, using some combination of external gears and the ratios available within the gear box. I'll cover QC gearboxes in more detail later, but for the moment we'll assume a simple change gear machine with no gear box.
Big Transposing Gears
The traditional 127 and 100 tooth transposing gears are big and expensive. Worse, they usually won't allow the gear cover to be closed, increasing the chance of damage from metal chips and exposing people to the hazards of an open gear train. Make no mistake, open gear trains are extremely dangerous. They will pull things in, destroying both object and gears. Think injuries and amputations. Open un-guarded gear trains are never allowed in industry and a company would be heavily fined by OSHA for allowing such. If you use a gear train that doesn't allow the cover to be used, fabricate a suitable guard to protect yourself and others. It may seem overly cautious, but you should also unplug the lathe whenever you're working on the gear train.
The next issue is the banjo, the frame that holds the gears and allows their relationship to be adjusted. With the large transposing gears you may need an alternate or even custom banjo. Everything is lathe dependent. Some lathes accommodate these gear trains with little trouble. Others, typically the smaller machines used by hobbyists, require some ingenuity to convert. One could, of course, obtain a metric lead screw, but it's far easier to swap gears than lead screws.
The 127 and 100 tooth gears are the smallest gears that offer an exact metric conversion. If you were making lead screws or micrometer screws, those would be the gears you'd choose. Obviously you'd also need a perfect lead screw with no wear. The reality is that some slight error is tolerable; in fact it's unavoidable. There are other gear combinations that offer a more convenient metric conversion, albeit with a tiny error.
Small Transposing Gears
A common metric transposing pair is 47 and 37 teeth. Compared to the perfect ratio of 1.27, these gears will give you 1.27027027... That's an error of 0.021%, which is only 0.0026" per foot. For any normal length thread you'd be hard pressed to measure the error, much less notice any functional difference. In return for this small compromise you get a pair of gears only a few inches in diameter. They'll fit within the factory gear cover and the cost will usually be lower.
The smaller pair of gears also lend themselves to DIY fabrication if you have a mill and dividing head. Unless you're cutting very coarse threads, the forces on the gear train are low. Because of this, I make my transposing gears, and any other change gears I might need, out of Delrin or a generic equivalent. It's easy to cut, the cutters last forever and I've never seen any significant wear in the finished gears. My guess is that they'd even stand up to moderate production use with little trouble.
Another possible pair is 80 and 63 teeth. The ratio is 1.26984, an excellent match, but the gears are still quite large and may not fit inside the gear case. They may be a good choice in some situations, but as the gears get larger the exact match of the 127 and 100 tooth pair makes more sense.
Practical Gearing With Simple Ratios
If you limit yourself to the transposing gears and the standard change gears that come with most lathes, there are quite a few metric pitches that can be cut using simple ratios. They are 0.75, 1.00, 1.25, 1.50, 2.00, 2.25, 2.50 and 2.75 mm. Simple ratios also allow a variety of coarser threads but you'll rarely need them. For finer threads and for in-between pitches you'll need compound gearing- another pair of gears to increase the division ratio. Here's a photo of a simple ratio set up to cut 2.00 mm pitch threads. Notice that spacers have been used under both the idler gear, which can be any number of teeth, and the screw gear. These are one gear thickness, so a small spare gear can also be used. The gears are thus in what I refer to as the outboard position. Move your mouse over the individual gears for more details.
