|Lathe geared for 12 TPI|
|Threading Gear string not engaged|
|Gears to go on Compound|
|Compound gears mounted|
|Option gearing for feed rod|
|Re geared for 4 TPI|
These section will give attention to the process of regearing a 1900 vintage American Tool Works Lathe from 12 TPI to 4 TPI. The top photograph shows the lathe geared for 12 TPI. Very early in the 20th century a feature called "Quick Change" was patented and universally adopted for lathes which made manually regearing lathes unnecessary. The quick change featured a sliding idler gear that drops on one of several gears in a conical stack making an very compact transmission consisting of many speeds. For lathes made prior to about 1905 it was customary for the machinist to configure them for threading by changing some of the gears on the head end. These lathes were sold with a pile of loose gears, and in order to thread you have to adjust the ratio of the speed of the lead screw to the spindle. The lead screw on this particular lathe is a 2 threads per inch (TPI) design. You will find many different thread pitches on lathes, but the old ones tended to be course because they did not have thread gauges, and with a 2 TPI lead screw you can engage the half nuts on any even numbered TPI thread without being out of sync. The rule is that you can engage without 'syncing' any thread that is divisible by the number of TPI on the lead screw. Thus if your lead screw is 4 TPI, you will have a sync issue on any thread not divisible by 4.
In this case the lathe is set up in the beginning for 12 TPI because that is the last type of thread I cut with it. Now I am going to cut a 4 TPI thread and must change the gearing to make that possible. If you expand the first picture, the numbers near the gears are the tooth count of the respective gears. In looking at a gear train like this the only gears that affect the ultimate speed are the first gear and the last gear and any compound gears. The other gears are idler gears and regardless of size they receive and deliver at the same speed. You will notice near the middle of the sequence of gears there are 2 gears side by side that are locked together. Power comes in on the big gear and goes out on the little gear. In this case the ultimate speed of the lead screw relative to the spindle is a function of both gear combinations combined. there is a 40 tooth gear on the spindle (visible in the 3rd picture) feeds to a 60 tooth on the idler. this effectively slows things down to 2/3rds of the spindle speed. (4/6ths = 2/3rds). The second set of gears include the outer compound a 20 in this case and the lead screw gear which is an 80 tooth gear. Here the 80 tooth gear will turn once for every 4 turns of the 20 tooth gear providing a ratio of 1:4 or a reduction to 1/4th of the input speed. Now multiply the two speed reductions together (2/3rds times 1/4th) and you get 1/6th which is exactly what you want. It tells you that the spindle will have to turn over 6 times for every time the lead screw rotates once. Now multiply the gear reduction by the number of threads per inch on the lead screw (6 x 2 = 12).
Stated differently, if the spindle and the lead screw turned at the same speed you would get 2 TPI, but want 12 TPI, so if you make the spindle spin 6 times while the lead screw turns once you will end up with the 12 TPI you want.
The best news is that lathes with gear changes like this typically have a brass tag on them that tells you which gear to put where to get the desired TPI. The second photo down shows the brass tag that goes with this lathe. Read down to 12 TPI and it gives you the position of each gear and the number of teeth needed on the gear in that position. Actually you don't have to be able to count either because the number of teeth on each gear is stamped on the gear, but at a minimum you must be able to read the brass tag.
In this case I want to change the lathe so I can make a 4 TPI thread. That's an easy one. I should be able to easily figure out that if I have a 2 TPI screw already I just need to slow the screw down by 1/2 and I'll get 4 TPI.
Now lets look at the tag and see if I'm right. The tag says that the first pair of gears should be a 40 and a 40. Wow, that is easy--- no speed change at all. If you have a 40 tooth gear turning a 40 tooth gear it isn't rocket science to figure that they will turn the same. Next read the rest of the tag and it says a 30 and a 60 should be used. No surprise here 30/60= 1/2 exactly what I expected.
Now if you have an enquiring mind you may wonder why the tax says 30 and 60 instead of 20 and 40 or 40 and 80. Actually either of these combinations would ALSO produce the desired 4 TPI if you had the gears. Now the gear on the spindle is fixed at 40 and cannot be changed, and you are provided a pile of gears, but every gear is different. You have used your 40 tooth gear in the first combination (40 on a 40) so unless you have an extra 40 tooth gear up your sleeve, you need to use something else to make the ratio. The 30 and 60 work, and aren't already used.
If you look at the two middle photos of the 2 gears you can see how the compound gears are locked together. Both gears simply slide on the little bushing with a key so they have to turn together.
The other thing useful in understanding these pictures is the distinction between a 'lead screw' and a 'feed rod'. The lead screw is used only in screw making. The feed rod does everything else. Do understand, however, that 'screw making' is the common language machinist used a 100 years ago (and the British still use) for 'threading'. As mentioned above the 'lead screw' is a long threaded rod within the lathe that draws the carriage and therefore the cutter along the carriage at a speed coordinated by the gearing to produce the desired number of Threads per inch (TPI). On the other hand, the "Feed Rod" is typically a long keyed shaft which turns through the apron. Inside the apron, a gear sliding on a key takes the power from the feed rod and drives the carriage or cross slide (or both) for ordinary metal cutting.
It was common in 19th century lathes (like this one) to offer the user a choice of a belt drive or a gear drive for the feed rod. If you use a belt drive, you can disconnect all the gears as shown in the 3rd photo leaving all that rotating machinery stopped and just turn the feed rod. In this mode you can do everything that the lathe does EXCEPT cut threads. This feature explains why this lathe which is well over 100 years old still has gears in excellent condition as thread cutting makes up only a small (but important) part of the work a lathe does. Even though however there were rare occasions when a gear driven feed rod was required. One such occasion would be if you were cutting threads with the cross slide as in making a scroll plate, as the lead screw only moves the carriage, and the power source for the cross slide is the feed rod.
I have identified on the photos the forward and reverse tumbler. They are a pair of small gears that feed each other so one turns one way and the other turns the other direction. As shown in both photos the bottom gear of the pair is engaged to both the gear on the left and the gear on the right (as well as the reverse gear above it. The reverse gear, however, is not engaged to anything so it just turns without purpose. That pair of gears however are mounted on an arm that pivots from the idler gear on the left. If you loosen the lock bolt the two tumber gears will drop down a few inches while swinging on the radius of the left hand gear. As a result the lower gear of the pair will stay engaged with the left gear exactly as it is now, but will drop out of contact with the right hand gear, but alas, the top gear will engage with the right hand gear, and since it is turning the other direction, will cause the lead screw to reverse rotation. This, is how you cut left hand threads.