Comptometer keys - making new

The project Comptometer model H got 'new' keys at its refurbishment in (probably) the 1940-ies, however these cream/green keys of a model J don't age well. Also, the keys of a model H Comptometer should be black/white instead of green/cream. These are one-piece moulded 'plastic' keys, similar to the keys of e.g. an early Oliver typewriter. As part of this model H's current 2023 refurbishment, decided to go for a full set of new keys in black and white. Replacing all keys makes it actually a lot easier, because blending in with any original keys won't be a problem.

In a 3D CAD-program (mesh-editor) new keys were designed, dimensions taken from the least-degraded remaining keys.

The dimpling was of course varied for the even and odd keys, as well as the practical depth of the lettering was worked out with some test-prints.

Also from some (failed) test-prints, worked out a way to make the pockets for the key-stem a bit flexible to get a workable press-fit on the stems. The open areas around the stem-pocket could later be filled after fitting on a stem (e.g. with glue), in case the extra strength should be needed.

Some more experimenting was done to find the best way for finishing the keys. Printing vertical gave the crispest lettering, the visible layering steps in the dimpled key-top could be sanded smooth without problems. Fine-grit sandpaper held over e.g. the ball-end of a file- or screwdriver-handle to press and smoothen properly into the dimple.

Then two layers of enamel hobby-paint to further smoothen the surface and give them an even black (or white) surface finish. How this wears will remain to be seen, but the solid material underneath is at least also black (or white-ish).

For picking-out the lettering, best results were had from filling the numbers with latex paint after lacquering the key with enamel - latex from testers; cream and black latex wall-paint (Black is surprisingly hard to find in latex wall-paint, by the way.)

A whole batch of keys was then printed in black and in white resin. All subsequently sanded, lacquered and lettered to get a full of keys set for refurbishing an 8-column Comptometer model H.

The STL models for a full column of keys for a Comptometer can be downloaded here.

Color-match for repainting Comptometer panels

Coming back to the stripped panels of the Comptometer. The side-panels came out good enough from underneath the brown refurbishment paint, won't be touched. The top-panel was however a (1940-ies?) replacement part without any copper-plating. Also the front display-panel was very worn, so these two top panels were up for repainting.

To select a fair color-match, took a panel to the auto-materials shop. Browsing the shelves with spray-cans, a decent match was found in Motip number 51350. At least, the lid of the can looked closest.

The panels were de-greased, with soapy warm water. The top-front display-panel was sanded down with fine waterproof sandpaper, to remove the rust. It shows all the layers that were worn through, from use. The cork riveted inside the top-panel was taped off.

First a grey primer was applied to both parts. Then two or three fairly thin layers of the metallic-brown lacquer was applied. The lacquer is translucent, so the finish gets darker with every layer. The front display-panel got more layers than the larger panel - and is probably the 'final' color of this brown metallic lacquer.

Metallic paint is fairly vulnerable, so a layer of clear-coat lacquer added to finish the panels. The main panel shows some color variations from thickness differences; translucent lacquer needs a lot of care to get an even finish (and me a lot more practice). Not too bad though, when viewed from a distance it isn't too disturbing.

Overall, the difference with the side-panels is probably smaller than the color differences within original panels. 

All probably too much effort to be put into a pretty common 8-column model H Comptometer, but a fun project - and it'll look nice again :)

An influx of Odhners and parts and tinkering

Because it looked very clean and with the paintwork intact, and it was the full-featured Original Odhner Model 39 version with tens-carry in the counter, back-transfer and check-register, and wanting to experience an Original Odhner descended from what started the pinwheel-type of calculators, took a chance and got it.

It was very well packed and arrived without any mishaps. Dusty on the outside, the internal mechanism was complete and really very clean - almost like new.

Not spotted on the listing images; the calculator had a wrong, stubby replacement crank-handle and a later-pattern left clearing-crank. Minor niggles, but still. Another thing was that the back-transfer did not work reliably - with larger values the back-transfer lever would snap out of its latch and not the full digit was entered into the setting-register. (Transferring a 9 from result would get a 7 in the setting.)

After some exploring, the likely cause was wear to the latch-faces made them slide off under the pull of the (strong) spring. 

The easy fix to try was replacing with a weaker spring. Here the box-of-bits from Remington Portable typewriter EV167240 again delivered! The bell-clapper spring has similar overall dimensions, but thinner gauge wire.

This must be the 6th machine that is fixed or completed with parts from this one Remington Portable :-)

With the numbers from the result-register again transferring correct to the setting-register, the stubby (uncomfortable to use) handle seemed next. 

Just then, a clearly broken Odhner was listed on the local classifieds site - just the thing to use as a parts-donor. Contacting the seller, it turned out that more parts were available - clearing-out of a halted restoration or demo-machine project. Net result; one Odhner purchase leading to another (with correct handles included).

It all arrived in a box in many bags and little boxes - approximately 1.8 calculator :-)

A 3D puzzle - these machines are fascinating puzzles anyways. The amount of interlocks and general ingenuity is impressive. The simple handle of the main crank is even a neat trick to assemble, easy when you know it.

Loosening the screw at the bottom allows the pin to be pushed all the way in. Then the C-clip emerges from the recess in the grip and drops off the pin. The grip can then be slid off the tube on the handle. Then the pin can be retrieved the other way and the spring comes out too. To reduce play of the grip on the handle, regular thin brass M3 washers can be used under the C-clip.

This handle-pin screw was a special pac-man screw - came out fine though :)

Being distracted by so many Odhner parts, there seemed to be enough to re-build a complete model 9, dating from around 1936. Another puzzle and no qualms about experimenting to fix things.

Some shimming of the left frame for clearances and replacing the right side-frame (one was shattered) made it rotate fine and even calculate correct. The button-mechanism for the quick-clearing of the setting-levers did not work and more concerningly, the left-most wheel of the counter register did not clear. This means a broken clearing pin - as it indeed was.

Taking out the register means removing the crank. Cranks are fitted with tapered pins (awful) and require targeted violence with a hammer to remove (scary). This is well described in the section on tapered pins on John Wolff's page on repairs. This pin fortunately came out fine and the register could be taken out, numeral wheels slid off the axle and indeed one broken pin (red arrow). Someone must've really forced the handle when this numeral wheel got caught on the notch-tip, Forcing the handle round, the notch was sheared off and the machine broken - instead of a wiggle with finger-tip of the numeral wheel that would've unblocked it. Oh well.

To repair: the spot drilled out 1.6mm hole with a hand-drill and a nail driven into this hole. The notch only needs to twirl round numeral wheel, so cyanoacrylate glue should be good enough to keep it from working loose.

The nail is then sawed off and the stub filed down to shape. The exact shape and accuracy isn't even that critical; important is that the sharp, chisel-edge tip is at the correct pitch - so 7mm tip-to-tip for this one. After several repeats of re-assembling the register, testing, dis-assembling and filing a bit more - the counter-register works beautifully again on all digits. Fix.

The machine's carriage was also 'blocked'. By the 1930s Odhner was using an alloy for the carriage base that can warp or swell a little over time. This wedges the carriage stuck in its guide, and makes the machine 'broken'. With fine-grade sandpaper, the bearing-sides can however be smoothed and made to fit in the guides of the cast-iron base. After some sanding and new grease, the carriage slides without effort.

The quick-clearing on this specimen probably hadn't worked for ages. This mechanism only works with the cover mounted, the clearing-bar that pushes against the large knob is attached to the cover. A small spring-loaded stud in the drum travels over a curved bracket, to catch it and push the knob-bracket from its latch on the way back. Simple, once you've seen it work. The knob-bracket needed a lot of forming (bending) and some filing to make it clear properly and pop-back.

The counter-direction occasionally does not fully re-set but hangs (red circle); the cause is known (wear) and a solution too (file a slight extra chamfer on cam), but for now this will be left as is. Anyways, giving the crank a little wiggle before a next turn also works to prevent a hang.

As a finishing touch, a comma-bar was mounted back on the cover. Instead of rivets, now fixed with M1.2 screws and tiny hex-nuts (carefully tightened with tweezers).

Not using the quick-clearing function matches the wear of the machine - the paint is worn away exactly where the user's thumb would push back all levers, with the hand resting also on the machine back and on the crank-support of the machine. The serial number 131568 on the back-cover places this calculator somewhere around 1936.

After this tinkering detour, an extra Original Odhner. This one a model 9 pinwheel calculator that looks well used. The worn look actually suits it, so probably this machine won't be repainted or touched-up at all. It again works ok and actually runs remarkably light. 

After this distraction, got back to the model 39 and finally replaced the grip with the correct pattern from the remaining parts. On this calculator the quick-clearing must've worked fine, because there is very little wear to the paint around the setting levers and on the back.

The wear of the lever tips on the right-columns however shows that it was well-used too. It still (again) works fine now. Not quite as light-running as the worn model 9, but very shiny!

Great puzzles to explore, and amazing feats of human ingenuity :)