- Joined
- Dec 9, 2021
- Messages
- 731
Breaking this down to separate questions.
I measured my lead screw, and confirmed that it is indeed 8 TPI. I had measured the cross feed, and it moved exactly .125" for one turn of the screw, also 8 TPI. I checked the rack, and it seems to be 6 TPI. When I get a chance, I'll put a dial indicator on the carriage and see how far it moves for 10 turns of the FEED bar.
The tumbler gears (moved by the "forward/reverse" lever between the spindle and the gearbox operate differently in forward than they do in reverse. In the forward position, the 26 tooth gear engages the 50 tooth spindle gear, and also the 50 tooth tumbler gear on the shaft the tumbler assembly pivots on. The 26 tooth gear acts only as an idler, so it does not affect the reduction ratio of the gear train, and the tumbler gear rotates at the same speed as the spindle. 1:1.
When the lever is in the "reverse" position, the 21 tooth gear engages the spindle gear and also the 26 tooth gear. The 26 tooth gear engages the 50 tooth tumbler gear. The ratio should be 50/21*21/26*26/50, which is also 1:1, but makes the tumbler gear rotate in the opposite direction.
The tumbler gear is keyed to the 40 tooth "top" change gear, so that gear makes one rotation with each rotation of the spindle. It seems to me that the first gear that needs to be considered in analyzing the gear system it that "top" change gear. All the gears before the "top" change gear can be disregarded.
As long as the "top" and "bottom change gears have the same number of teeth, and both engage only the 127 tooth gear which is acting as an idler. the input ratio from the spindle to the gearbox is also 1:1. Hence, the input shaft of the gearbox rotates at exactly the same speed as the spindle.
My gearbox with the Norton 2-lever system is quite different from the later Jet and PM units. In practice, it works the same way, but the controls are different. Separate from that, my system is laid out with slightly different gearing in order to cut 11-1/2 TPI for American NPT pipes. I have not determined a use for any of the other threads available in the #5 column. In doing this, they gave up having the 4.75, 9.5, 19, 38, 76 thread options. I believe that PM (or more likely the OEM) decided that it was more important to have the 4.75 thread multiples available, than have the ability to cut 11-1/2 NPT threads. David's excellent PDF of the PM1340 thread combinations does not show any use of the #3 column of options to cut a metric thread, and they do not show up in British Standard or Whitworth, so perhaps they are used somewhere else? I am beginning to wonder it this is a holdover from someone (perhaps dyslexic, but more likely having a poor understanding of the English language) confusing "4-1/2" with "5-1/2" in the original Taiwan cloning process. Since they were patterning on English machines, and the British Standard Pipe threads are 11 TPI instead of the 11-1/2 of NPT, the lack of that option may have been overlooked.
Interestingly, the Atlas 10" and 12" QCGB's have a 9th column, with threads of 7.5, 15, 30, 60, and 120!
At any rate, I discovered that the Metal Max metric thread chart shows that, using 40/127*120/40 gear train, setting C-6 cuts a 1.0MM thread. C-6 is the 24 TPI setting. This is identical to David's settings for the PM1340. This makes sense since they both have 8 TPI lead screws.
I analyzed my feed box a little differently than you did. I saw that position B-1 was 8 TPI, which matched my lead screw. Your lead screw being 4 TPI made using A-1 logical for you. I made a chart of the ratio of each position in relationship to the "root" setting. I started converting it to revolutions of the lead screw per revolutions of the input shaft of the gearbox. I call it "QCGB Analysis for Dummies" because it takes out the need to analyze each gear inside the gear box. I assume it isn't precise, since the gears were chosen to be as close a practical from a manufacturing standpoint, but if it was close enough for a manufacturer, it should be close enough for us in the hinterlands. by working backwards from the lead in inches of a metric thread, I should be able to find the input ratio needed to match a gear position.
It will be a week before I can continue this.
I measured my lead screw, and confirmed that it is indeed 8 TPI. I had measured the cross feed, and it moved exactly .125" for one turn of the screw, also 8 TPI. I checked the rack, and it seems to be 6 TPI. When I get a chance, I'll put a dial indicator on the carriage and see how far it moves for 10 turns of the FEED bar.
The tumbler gears (moved by the "forward/reverse" lever between the spindle and the gearbox operate differently in forward than they do in reverse. In the forward position, the 26 tooth gear engages the 50 tooth spindle gear, and also the 50 tooth tumbler gear on the shaft the tumbler assembly pivots on. The 26 tooth gear acts only as an idler, so it does not affect the reduction ratio of the gear train, and the tumbler gear rotates at the same speed as the spindle. 1:1.
When the lever is in the "reverse" position, the 21 tooth gear engages the spindle gear and also the 26 tooth gear. The 26 tooth gear engages the 50 tooth tumbler gear. The ratio should be 50/21*21/26*26/50, which is also 1:1, but makes the tumbler gear rotate in the opposite direction.
The tumbler gear is keyed to the 40 tooth "top" change gear, so that gear makes one rotation with each rotation of the spindle. It seems to me that the first gear that needs to be considered in analyzing the gear system it that "top" change gear. All the gears before the "top" change gear can be disregarded.
As long as the "top" and "bottom change gears have the same number of teeth, and both engage only the 127 tooth gear which is acting as an idler. the input ratio from the spindle to the gearbox is also 1:1. Hence, the input shaft of the gearbox rotates at exactly the same speed as the spindle.
My gearbox with the Norton 2-lever system is quite different from the later Jet and PM units. In practice, it works the same way, but the controls are different. Separate from that, my system is laid out with slightly different gearing in order to cut 11-1/2 TPI for American NPT pipes. I have not determined a use for any of the other threads available in the #5 column. In doing this, they gave up having the 4.75, 9.5, 19, 38, 76 thread options. I believe that PM (or more likely the OEM) decided that it was more important to have the 4.75 thread multiples available, than have the ability to cut 11-1/2 NPT threads. David's excellent PDF of the PM1340 thread combinations does not show any use of the #3 column of options to cut a metric thread, and they do not show up in British Standard or Whitworth, so perhaps they are used somewhere else? I am beginning to wonder it this is a holdover from someone (perhaps dyslexic, but more likely having a poor understanding of the English language) confusing "4-1/2" with "5-1/2" in the original Taiwan cloning process. Since they were patterning on English machines, and the British Standard Pipe threads are 11 TPI instead of the 11-1/2 of NPT, the lack of that option may have been overlooked.
Interestingly, the Atlas 10" and 12" QCGB's have a 9th column, with threads of 7.5, 15, 30, 60, and 120!
At any rate, I discovered that the Metal Max metric thread chart shows that, using 40/127*120/40 gear train, setting C-6 cuts a 1.0MM thread. C-6 is the 24 TPI setting. This is identical to David's settings for the PM1340. This makes sense since they both have 8 TPI lead screws.
I analyzed my feed box a little differently than you did. I saw that position B-1 was 8 TPI, which matched my lead screw. Your lead screw being 4 TPI made using A-1 logical for you. I made a chart of the ratio of each position in relationship to the "root" setting. I started converting it to revolutions of the lead screw per revolutions of the input shaft of the gearbox. I call it "QCGB Analysis for Dummies" because it takes out the need to analyze each gear inside the gear box. I assume it isn't precise, since the gears were chosen to be as close a practical from a manufacturing standpoint, but if it was close enough for a manufacturer, it should be close enough for us in the hinterlands. by working backwards from the lead in inches of a metric thread, I should be able to find the input ratio needed to match a gear position.
It will be a week before I can continue this.
