1911 American machinist handbook

[Mr Ron]

I was browsing the American machinist handbook of 1911 and came across a table of chords. With it one could layout equally spaced holes along any given radius. The spacing then could be stepped off with a divider. Sounds right, but the chords are listed to 4 decimal places. Was there or is there a device that can step off dimensions to 4 places along a circle? I'm guessing that it can't be done and the table only lists an "approximate" value. I know it can be done with a DRO on a mill, but can it be done manually with dividers?

[pete]

I've got an old book titled Tool and Gage Work by Goodrich and Stanley that's part of the American Machinist Library and first written in 1907 that might help to answer your question. They mention extremely precise work being done on master gauges and I think that was before jig borers were even invented. Chord work like you mention was just part of what they were doing. But they do describe using shop made hardened and ground toolmaker buttons, a shop made DTI that's quite different than what we think of today as a DTI and it could indicate well under 10ths for accuracy. And then using a very good micrometer for measuring across the tool maker buttons. Doing it that way then 4 decimal place accuracy could be used. I don't know of anything that was available at the time that would allow a hand layout to that level of accuracy so my guess what I mentioned is what those types of chord tables were for. They still would have done a fairly close hand layout, but drilling and tapping for the buttons and shifting them until they were correct by micrometer measurment, and then dialing in the spindle to the buttons was likely how the very accurate work was done. Really slow to set up of course, but if the work justified that level of accuracy it was the only real method they had.

Later on P & W and a few others developed the first jig borers and super precision feed screws were the large problem. They finally went to machine mounted 1" travel indicators and high precision rods in 1" increments much like micrometer setting rods so they had an independant method of high precision X,Y table movements that didn't rely on perfect unworn feed screws. But those tool maker buttons were still in common use as well. The above book I mentioned does talk about precision "space blocks" that must have been very common in tool and die shops at that period and shops must have had hundreds if not thousands of them since the usual Johannson gage blocks weren't in common use in America at the time. Those space blocks were precision made to micrometer accuracy and used to move the work a set distance with an X,Y fence and the space blocks. The same book towards the end does go into detail about what must have been the very first set of Johannson's gage blocks in America and what they might be used for. At that time they conclusively proved it was much more than air pressure holding the gage blocks together. Yet even today some still think that's the only thing that is holding them from seperating. I don't think all that many today realise that very high precision work was being done as far back as it was. Maudslay invented a millioths reading micrometer in the early 1800's.

[Mr Ron]

I thought about how I could use the chord method with pretty good accuracy, maybe ±.003? With the chord dimension, you could set a vernier caliper; lay it down on a flat sheet of aluminum, or copper and scribe a line with a razor blade. I could then set my dividers to the scribed lines with the aid of a magnifier; then starting on the scribed circle, make a shallow punch mark and step off the dimensions. I think I could get pretty close with even spacing.

[pete]

Yep I think that could work and maybe even a few thou better than that at least on the layout itself. I've read a few mentions about the general accuracy of hand layouts and center punching and a few have said .002" - .005" with someone very experienced that had a lot of practice. I'm nowhere near that good so don't bother to center punch much of anything unless it's going to be drilled with the usual hand drill.

If you read about the history of machine tools starting around the time of the industrial revolution they accomplished some amazing hand work just to get the first early designs of machine tools built and then use to build the next generation that were more accurate until we have what we do today. There were some more than skilled and clever people who developed simple but accurate methods of getting what they wanted done and starting with almost nothing. Hand cutting and filing out the very first leadscrews to then machine the next ones is more than impresive to me.