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Showing posts from October, 2024

detents

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Babbage was not a fan of springs, which at the time were not uniform and were prone to failure. One of his 1837 "Principles of the Engine" was "to have no springs to do work, only retaining springs". But he certainly made liberal use of spring for detents, which impose one or more preferred locations for an otherwise continuous movement. It is, as Tim Robinson observes, a "soft" version of the locking that he uses when parts should not be moving at all. Here are examples, some of which include the option of a hard lock. So how to easily implement detents, with fewer parts -- and in particular fewer vertical shafts? My first attempt was to use a "spring plunger" from McMaster-Carr, which is an acetal plastic nose in a steel 8-32 threaded case, with an embedded spring that provides 1 to 3 pounds of force. I used it screwed into a 3/8" vertical shaft and had it engage with notches of the kind Babbage showed. It works, but the small 1.7mm tip si

fixing problems with subtraction

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Solving the interference problem describe in the last blog post was fun and gratifying, but ultimately pointless because there was a more fundamental design problem that was revealed by hand-operating the mechanism for subtraction.  Using the carry sector lifters for four different functions was clever, but one of them failed: forcing a "warn" on carry warning arms that were raised by movable wires (so that the raised sector lifters could be forced down) allowed the wire to fall before it should if it had been in the "detect 0" position for subtraction. So there needed to be a different way to return the sector lifters. I thought that would require an additional articulated mechanism: another stepper motor in the prototype, or another cam follower in the eventual all-mechanical version. But it turns out that all it takes is a scythe-like keeper above the carry warning arm that moves vertically with the arm but is fixed in rotational position.  This is a great simpli

debugging the anticipating carriage

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In playing with the manual test jig for the anticipating carriage, there was one configuration that caused interference between two parts. It became clear that I needed to do a systematic analysis of all combinations of vertical motions. There are four basic vertical configurations which have to accommodate various part motion:  the rest case where all warning arms and sector lifters are down the non-carry case, where the arm is "not warned" and is raised, and the sector lifter is not raised the direct carry case, where the arm is "warned" and raised, and it raised the sector lifter the propagated carry case, where the arm is "not warned" and raised, but the sector lifter has been raised with the movable wire The analysis showed that it was case 2 that caused interference, when the sector lifters were rotated to the position which forced the arms to "warned", which is needed so that case 4 sectors will be engaged and positively lowered when the w

the "Anticipating Carriage"

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Of the many clever mechanisms in the Analytic Engine, the one of which Babbage was most proud was his "Anticipating Carriage", which accomplished the ripple carries during addition through an arbitrary long string of 9's (or borrows during subtraction through a string of 0's) in a single time unit. He spoke of it in almost mystical terms as "teaching the Engine to forsee". From "Passages from the Life of a Philosopher", 1864 But Babbage didn't invent it just once. In  his draft monumental book-length technical history of the development of the AE, Tim Robinson catalogs 88 different versions created over 35 years, most of them incompletely described, and of course none of them proven to work by having been built.  The underlying principle is the creation of a chain of fixed and movable "wires" (which are actually stiff rods) that link together consecutive 9's that must be turned into 0's during addition. For a simplified explan

mechanical clearances: good or bad?

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To investigate why the lock on the fixed long pinion was so important in making number transfers reliable, I slowed down and single-stepped the copy operation, and watched carefully. What I discovered was that in "giving off" the first non-zero digit, the source digit wheel was not turning a full digit position -- it was only moving about 80% of the way.  Why? Because there was too much free play between the finger on the axis and the nib on the digit wheel. It turns out to have been intentional, because the design provided clearance to allow for the elevation of the finger to the level of the nib without any possibility of interference. The digits are spaced on the wheel by 18 o . The finger width was 8 o and the nib width 8 o , for a total of 16 o , which provided 2 o of clearance. In this case clearance is a bad thing because it produces a dead zone of movement. I changed it to instead have negative clearance of 0.5 o by increasing the finger width to 10.5 o , and usin

the virtues of locking

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I implemented Tim's suggestion to do Babbage-style consecutive locking from source to destination, instead of engaging all the locks at the same time.  Each locking takes half a time unit, so doing all three locks takes1.5 time units. That conforms to what Babbage seems to have indicated in his timing diagrams. Nothing changed in the operation, though, so it's not clear under what circumstances it makes a difference. Maybe it allows more misalignment to be corrected. My new little tester continues to copy numbers with 100% reliability. I decided to do an experiment to see if the new lock on the fixed long pinion had anything to do with it, so I pulled the plug to its stepper motor. The machine failed immediately. The problem was a jam when the small pinions tried to mesh with the digit wheels. I think the issue is not that the new lock corrects small misalignments after the operation, but that it prevents derangement when the four-gear pinion train (small to fixed long to mov