18 West Washington Street, Newnan, Georgia
I was born and raised in a machine shop and we were around some complicated and nice jobs, including making a blank for gears and getting the boys at the cotton mill to cut the teeth for us; once in a while, making gears.
Back in my early days the mills must have made hundreds of gears a month as it took a different set of gears to make every kind of thread. They had different machines by the dozens, besides all of the ones that stripped and wore out so we never did think we were ever a match for any of the mill boys where gears were concerned.
My folks usually had gear scales and pattern rules that had lines of 10, 12, 14,, and 16, but we were never able to get a scale that 18ths and all of the other lines up to 32 like the mills had. Some said the Cotton Mill Machinery people had the rules made by the hundred and then resold them to people who bought the mills. I never saw but 3 in my whole life like I always wanted and now you cannot even buy one that has 18ths on it anywhere. This small one used to have a lot of useful information about a circle and figuring gears.
My Dad left me a 301 Starrett 10, 12, 14, 16ths and all subdivided, but no 18th and a South Bend lathe has 18 pitch gears. We use a 5/16 tap if we do not use a mike when on 18ths.I knew a Mr. Brasch, a master mechanic of a large mill at Grantville, who was a graduate of the Brown and Sharpe Trade School. Mr. Brasch was noted for his secret trick gears, a gear that could go behind or replace another gear with a smaller or larger number of teeth and keep the same adjustment. This would save the company lots when a minor change was necessary on the finished product. Mr. Brasch carried his dozens of secrets to his grave but I believe he used the same method that Ford did in the Model T rear end. He had 2 different pinions to fit the same set-up. No doubt Mr. Brasch studied under the same men who helped Ford with his gears.
More about the Marsh valve gear -replacing worn gears. Some of the early Advances were equipped with a casted cast iron gear with 21 teeth and the circular pitch was 2/3 of an inch. This is the measurement taken at the pitch line from the center of one tooth to the center of the next tooth. This way and style for measuring gears was the most practical way to make gears prior to 1900 or so because gears were casted in a foundry and this was the easiest way to make the wooden patterns.
Around 1900 Brown and Sharpe perfected the cut gear and the invulute system along with the better way to measure a gear in whole fractions by expressing the distances from one center to the other at the center of the wheel circles or from one shaft center to the other in whole fractions. This was such an improvement over the old way that companies almost changed over night to the new way.
The Brown and Sharpe Company also perfected a 12' rule 2' wide that had an inch divided in lines starting from 18 lines to the inch and going up to 32 lines to the inch. This included 18, 20, 22 , 24, 26, 28,, 30, 32, 34, 36, and it might have had 16 to start with. Every 10 lines were counted and some of the small division lines counted to and above 300, keeping up with the count every 10 lines.
In the old system a fellow had to be an expert pattern maker and also he had to be up on his geometry where 31/7 or 3.1416 were concerned. Back 50 years ago, this type of workman was hard to find so the gear business was handled by very few shops. But with the new system, a fellow did not have to know how to read or write very well but only had to learn to count and read a few thousandths on the dial of the milling machine and count on his new rule that came with the gear cutting machine. These boys were cooking with gas in no time and if they had a lot of gears to cut on the old circular pitch system, the Brown and Sharpe Company could make on special order a cutter which was an odd pitch. Most of the mills around here solved the problem by simply readjusting the centers on the gears to fit the new size gear wheels; but there were lots of expensive machines made in England that had gears and the shalves could not be adjusted and the office would not talk about spending more money for special cutters, which could amount to thousands of dollars even in those days as there were more pitches represented by the circular whole system than there were to start with on the diametrical layout. There are about 8 cutters to a pitch.
This is where the Ole Daddy Darlings with the handlebar mustaches (Machine Shop Professors) came into the limelight. Remember some of them could not write their names but give them a lathe and the treasured gear scale and they could make you most any kind of gear. Lots of them had been carpenters and had built steps. As you know, very few places can be reached by steps made exactly by a certain pitch. So the ole boys either made the rise or tread a little oversize or undersize to make the steps come out even. On the gears they did practically the same way. They selected a standard gear to fit the nearest size and count and to this they either subtracted or added the difference of what the pitch distance should be. To this they added 2 teeth to get the blank and this was the gear they needed if they installed all in the train the new pitch.
Let's talk a little more about the gear scale they had that cannot be had today thanks to Mikes. It is impossible to buy a new B & S old gear scale. In the case of the Advance Marsh gear,, we find the pitch diameter is 4 5/11 inches. This is quite a fraction today if you do not have Mikes. If you do not watch you still have bad decimal equivalent at this is hard to find in any book, but it happens to be about four inches and four hundred and fifty-five thousandths. A 6 pitch would be better and heavier than the old pitch, but a 7 pitch seems to work out nearest to the 4.455' centers better than any other heavy pitch.
The pitch diameter for a 7 pitch gear with 30 teeth is 4.428' and our needed distance is 4.455' so we just add the 27 thousandths to the standard 7 pitch gear and again we get approximately 4.455' To all of this we will add two teeth to find the blank size of the new gear which is really the pitch diameter of a gear the next 2 teeth larger than the 30 teeth gear. This gives us a size of 4 4/7 or a decimal of 4.571 and to this we add the .027', making the total size of the blank 4.598' (oversize blank). Look on the cutter and find the depth and cut as you would any standard gear.
One sure way to find the center distance, which in this case is the same as the pitch diameter of either gear, is to turn up two dummie blanks and make them fit the shafts and turn the outside diameters the same so that the two blanks will just have a paper clearance. This is about the only way one can be sure of finding the distance or the shafts on the bracket on the Advance because the whole thing could have rebabbitted several times.
If your sizes are way off from the ones mentioned, you can find different sizes to suit your needs. Try to stay to 6 pitch as near as possible so as to keep the gears strong and if you rebabbitt the bracket, I would go to 6 pitch as this is what my Advance is and on standard centers. There is no need to rebabbitt if you are careful and follow the 50 year old practice and time proven. Don't let anyone kid you that this will not work; I have seen it work 3 times in my life (up to .065' oversize). It does not matter how many teeth you use, just so they are the same on each gear. Cut the big hole gear in half and braize to old hub on main shaft to save work.