Tuesday, January 3, 2023

The massive Monroe model G adding calculator

This is the Monroe model G calculator.

This is a mechanical calculator with a full keyboard and 8x8x16 capacity. This machine is massive; both in that it is quite large, and also in that it is constructed with all the subtlety of a steam-roller. The base of the machine resembles a strongbox - with wall-thickness to match.

The weight of the machine combined with ludicrously ineffective padding unfortunately caused much damage during shipping, but the otherwise solid build ensured that the core of the machine survived. Bent handles are generally recoverable, more concerning was that 12 of the 16 numeral-wheels had shattered. Equally bad; that spring-part amongst the shards is the positioning stud of the carriage that got ripped-out.


All fragments were carefully recovered from the box, filtering all the loose buffering chips and crumpled newspaper. More fragments were later recovered from inside the machine. Some gear-wheel teeth had been bent sideways, but luckily none were broken off.

Most of the mechanism was also glued solid with old oil - as is common - so it would need to be completely taken apart and cleaned anyways. Oil was applied to all visible screws and the mechanism and then the machine was left sealed in a bag for several months. To let the oil seep in, but equally to put out-of-sight the disappointment of such transport damage - and to take time to think if or how it could be repaired.

How to take apart these machines is well documented online, additionally the model G is mostly identical in construction to the predecessor model F and the model D. After watching several of these useful guides, the machine was easily taken apart to first get at the shattered numeral-wheels. Removing the carriage is trivial, then the three 'frame-plates' of the carriage need to be unscrewed from its curved cover and the whole mechanism drops free.

The Monroe company unfortunately at that time used quite a few riveted parts and, just as bad, tapered pins to mount parts on axles. To take off the numeral-wheels, to e.g. replace the numbered rings with new 3D-printed rings, two such pins have to be hammered out. This is not easy and requires a solid support and 'some violence'. Supporting the axle is important; it is not hardened and at risk of bending from the force of the blows!

With some filing and more hammer-blows, the two pins at one end were successfully removed. Then it turned out that all the risk and effort had been useless - the distance-bushes that are keyed onto the axle between all the wheels were glued in-place solid with the old grease. Aggressive solvents would risk attacking the remaining numeral-wheels. Applying force was out of the question, because the bushes are of soft material that will damage before budging. Ergo, on to 'plan B'. 

The careful collecting of all bits paid off, because with a morning of puzzling with glue, all 12 numeral wheels could be re-constituted from the many little fragments. A bit like an archeologist reconstructing iron-age pottery :-)

With cushioned pliers over the gears all the wheels were forced to move a bit, fracturing the solid-grease surface. Subsequent 'washing' with light oil made everything move smoothly again very quickly, including the little detent-pins.

With the parts all cleaned-up and touched-up where needed, the carriage could be put together again. The upper- and lower-dial axles in position plus the clearing-mechanism and frame-plates in place. With a bit of fiddling, the whole assembly screws back into the carriage-cover.

The stud with its spring-dampened was 'crimped' back into the re-flattened hole (top-centre in image above). The tens-carry pin of the numeral wheels passes right over the mounting, making it impractical to simply mount it with a bolt. Cyanoacrylate glue was applied to prevent it working loose again too easily; hoping this fix lasts a while.

The base of the machine was taken apart in a similar manner; all gummed-up parts were slowly worked loose and new light oil worked into all the moving bits. The stepped-gear wheels (like base-5 Leibnitz wheels) were still freely moving on their axle. (Not so the other parts, but oil, time and the occasional application of force made all other parts free too.) 

The large driving gear-train has helpful markers for assembling it correctly again.

This gear-train also shows how the G is different from its predecessors; in the earlier models the stepped-gears are driven for only part of the cycle (with an abrupt start-stop jolt). In the model G, the stepped gears are driven continuously, with a 2-1 step-up. The 27-teeth gear of the stepped-gears-axle is driven by the 54-teeth linking-gear. To only count the numbers once, the stepped-gears-axle is mounted on a rocker and moved out of engagement for half the cycle by a cam on the tens-carry-drum axle. More complex construction, but should be smoother in operation.

Anyways -after a bit of puzzling it all went together again and worked smoothly. Fastening the outer covers needed some shimming however, to prevent the mechanism from binding.

This specimen came with raised rear-legs, to increase the banking-angle. This is a bit unusual, not seen legs on any other old Monroe - the machine seems very heavy for such thin legs. Note the slot in the rear-plate; this gives access to a screw for adjusting the friction of the clamp that causes the tens-carry drum adjusting for addition or subtraction.

Also without raised rear-legs, the keyboard of the old Monroe's is banked - the key-stems are longer further up the keyboard. The travel is the same for all keys though.

In-character with the overall 'robust' construction-style, the springs of the keyboard are not subtle either. The keys require a firm push to depress, and the clearing key is probably meant to be pressed with the palm of the hand. (It really is by design of the springs, not from any friction, rusty surfaces or gummed-up oil - the whole keyboard was taken apart and is squeaky clean.)

With serial number G35641, this Monroe calculator was most likely made in (mid?) 1920. It does not yet have the automatic lifting of the carriage when clearing the lower-dials, but one decal does already note that Monroe is a registered trademark. The carriage-lifting patent was applied for in August 1920, machines with this feature would not have been sold before that filing. And according to the Smithsonian, the Monroe name was registered as a trademark in 1920. 

From the early Monroe serial-numbering that is available, it is possible (plausible?) that they started at 20,000 with the model G in 1919 and then incremented to 30,000 for 1920 and 40,000 for any made in 1921. The model K was then started at 50,000 in 1921. These old Monroe machines are not rare, but also not quite so common as to expect more than 20,000 to have been made.

One missing keytop was re-manufactured using polymer-clay. Using an original to create press-molds and several tries, a reasonable facsimile could be made. Not perfect, but when viewed from a distance it blends in nicely.

The little Monroe L calculator behind the model G shows by the way just how massive this model G calculator really is - or how tiny the model L is, of course. In a picture of an office interior of ~1925 at the early office museum web-site several of these large Monroe adding-calculators can be seen in use.

After all the repairs and a thorough cleaning; again a massively impressive calculator that works beautifully!

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