After refurbishing a Model B Comptometer and playing with the Blickensderfer, returning to the Marchant Pony B Special calculator. The Pony B still had issues.
Though it now looks complete and mostly worked, it still occasionally blocked. It'd block hard - not just more resistance, but no way that a turn could be completed, the only way out is to reverse the handle. (That would explain why on machines the full-turn pawl is missing or the back-plate removed, not statistically significant and all that, but from looking round online this seems common on these Marchants.)
The first hypothesis was that one of the short-ish pins in a carry-lever would sometimes jam. To test/fix this, the pins of suspected levers were replaced with long pins. From having a donor-machine, there's an assortment of springs and pins to choose from :)
By driving in a rod from the right pushing the gears axle, only one lever can be carefully released (hold something over hole, to prevent pin shooting out and get lost inside the carriage). Then the lever with replaced spring and pin can be moved back in-line and the axle pushed back in place.
That did not solve the problem of randomly blocking on a carry.
Observing the blocked machine with covers removed, it turned out that the carry-lever was actually stopped by the adjacent dial of the drum.
The replacement drum-axle is of slightly different dimensions than the original and the drum is a few tenths of a mm too far to the right, relative to the carriage position. These Marchant calculators are truly a feast of interdependant adjustments. Finicky.
After trying to fix it with washers, ultimately swapped the original drum-axle back in. (Washers fixed the blocking, but created new issues with the clearing-lever. Plus the donor-axle had a broken full-turn gear.)
Replacing the main axle and keeping the donor-handle meant that the gears of drum and handle-axle were now off. To mitigate that, decided to 'cheat' and change the handle position on the rod. The pin is now merely decorative and the fixing is with cyanoacrylate. Of course, the drum needed to be taken out (and apart) and thus the right sideplate needed to come off and because the locating-pins are missing the whole machine then needs tweaking to run freely.
Having the covers off, had another look at the interlock of carriage-clearing. This does not work. Looking at the pin that is pushed out of the carriage-frame when clearing, it is hard to see how it could have worked when new.
This is the original carriage, all parts are for this machine. Yet clearing simply doesn't push out the pin anywhere far enough to reach the dimples in the extra horizontal rod on the machine base. Can't see anything that could be adjusted, this is entirely from dimensions of the parts. We'll accept this interlock doesn't work - just take care to not clear and turn at the same time :)
Because the blocking problem is solved, now daring to fit the full-turn or anti-reverse mechanism. Trying to move the parts of the 1919 donor-machine over to the 1920 machine it turns out that the screw-threads are different! Both the lock-screw and the shoulder-bolt have different threads! (Why?? I'd now really like to talk with the Marchant factory design office of 1919.)
By shortening the shoulder-bolt, a lot of careful filing of threads and an extra washer, the full-turn lever is in-place.
After this extra round of fixing (attempts) the calculator still 'mostly works'. It is ok on addition and reasonably light in operation, but it is not smooth. Some carries in subtraction need work. It also is fairly noisy; the full-turn lever makes a ratchety clinkety-click as it dances over its gear. This maybe was another reason to remove this part in the past, it adds an annoying tinkle.
The clearing-nut for the setting register started to work - partially. It also is a mystifying mechanism - can't make sense of it.
The check-wheels are geared with the setting-dials via intermediate gears. When the handle is pulled out, the intermediate gears are shifted to the left to allow the drum to rotate and check wheels to remain static. The check wheels are held by a spring-loaded ratchet in a valid 'digit' position, nevertheless an extra hard-lock is engaged when the lever is pulled. In below photo the slotted bar is holding the intermediate gears, positively blocking any rotation.
When the handle is back in the rest position, the intermediate gears are back in engagement with the setting dials and the gear teeth now run through the slots in the locking-beam - below photo.
Adjusting the positions of all these shifting axles is finicky, any wear or deforming can block things. The scratches on one check wheel suggest that it had blocked and a violent attempt was made to get it moving again. (Forcing a wheel was a bad idea - wiggling the setting level can already help. Turning the clearing nut while at the same time pulling-out the main crank will unblock the lock-bar and set all check wheels to zero. That is also the way to re-sync the drum and chek wheels if they ever get out of sync.)
The locking-bar arrangement seems a bit over-complicated and superfluous - the machine should work fine also without this extra lock, the sprung ratchets will hold the digit. Another thing that is perplexing me still is that during clearing the lock-bar is shifted to the left as well - and then the intermediate gears need a nudge to the left too. The intermediate gears however also need to remain in engagement with the setting dials. This adds to the complexity of finding all the right adjustments, so many more ways for the mechanism to fail and block!
With the machine at-rest, the intermediate gear should engage the setting dial, but must not foul the path of the pins. I.e. must be in-line, see below photo:
When clearing the drum with the wingnut via the check wheels, the intermediate gear must shift slightly to the left - just enough to remain clear of the left-shifting lock bar. But the gear must also remain in engagement with the setting dial. See below photo:
Frankly, the reasons for this complexity is not clear to me - plus that the way it is engineered makes the design 'brittle'.
After many attempts of adjusting, it now works - mostly. The column 7 and 9 do not clear, the pins on the axle must have been sheared off (again evidence that violence was applied in the past to a blocked check register). This whole mechanism is fixed with pinned collars - have not been able to remove any of the steel pins in the brass collars. Drilling out is an option, but so far that's deemed too much effort for this 'rough' machine.
The calculator assembled back together again after this round of explorations. Has to be admitted that it still looks better than that it actually works.
Even though this Marchant is not a representative experience of what these were originally like, it has given a much better understanding of how these calculators work. Also of how the Marchant factory operated and how their design evolved - the impression frankly is of two design cultures. There is the solid 'core' of the original Odher machine essentially unchanged, and then bolted on are the convoluted constructions of e.g. the carriage movement or the check wheel register.
During the 1917 to 1922-ish period Marchant would have sold every calculator they made so it may not have mattered all that much, but the impression is that they were not at 'world class' level of engineering competence for calculators. In 1920 not yet, anyways.
During the 1917 to 1922-ish period Marchant would have sold every calculator they made so it may not have mattered all that much, but the impression is that they were not at 'world class' level of engineering competence for calculators. In 1920 not yet, anyways.
This machine purchase is definitely delivering on the goal of experiencing the Marchant Pony! Oh and the Pony B still has issues :)
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