Disassembling a Curta Calculator

What's Inside?

By Greg A. Saville

Taking apart your prized Curta Calculator isn't advised for everyone. If you don't feel confident enough of your mechanical skills, then just enjoy the pictures here looking inside someone else's Curta. On the other hand, if you've got the right tools and feel comfortable using them, you may enjoy opening yours up for a closer look to better appreciate the inner workings of this amazing mechanical wonder.

It actually isn't that difficult, as long as you have the right tools, take your time and know when to stop. Taking the covers off or removing the carriage won't unleash spring loaded parts flying across the room, but there are a few hints I'll share along the way that'll help make sure you can get everything back together again.

You can click on each photo for a larger view if desired.



Bottom baseplate cover
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Start at the bottom of the calculator where you'll see the baseplate cover and find two screws holding it on.

 




Removing the bottom baseplate
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It's very important to use a properly fitting high quality screwdriver when disassembling an instrument like this. You don't want to damage or mar the appearance in any way. Use a screwdriver that fits the screw slot properly and carefully remove the two screws. Use good tools. The $1.98 variety of jeweler's screwdrivers often aren't correctly shaped and aren't tempered properly, so they won't fit and may deform in use and deface your screw heads. This set of Vigor watchmaker's screwdrivers is imported from France. They cost more than $50.00, but are well worth it.

 




Inside the bottom
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Once the two screws are removed, the bottom cover comes free and you can look inside. Take a close look at the inside bottom cover, you may find some dates there. I've found some Curtas with a printed date inside, presumably the date of manufacture. I've also seen some with several hand written dates inside which are probably dates of maintenance and servicing.

 




Ratchet pawl
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A closer look at the ratchet pawl which prevents the crank from turning in the wrong direction. Turn the crank a few times and you can observe how the pawl allows it to be turned clockwise, but prevents it from going the opposite direction.

 




Detent roller
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Watch the action of the spring loaded roller detent while turning the crank to see how the handle locks into the rest position after each rotation.

 




Main housing and base ring
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Start sliding off the barrel cover main housing and base ring. Don't worry, nothing will spring loose or fly out when you do this.

 




Removing main housing and base ring
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Once the main housing cover is free, you can get a good look at more of the inner workings of the calculator.

 




Input setting shafts
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Now operate the input sliders and watch how the linear sliding motion is converted into a rotary motion of the setting shafts. Also notice how the detents hold the desired input value in position. Imagine how complicated the machining operations are to create these spiral grooves and detents for these shafts!

 




Input setting shafts
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A closer look at the top end of the input setting shafts showing the numeral dials.

 




Stepped drum
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Although it's hard to see the stepped drum through all the shafts and pillars in this photograph, it's much easier to see in person and fascinating to watch the drum in action as you turn the crank.

 




Crank pin
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The crank handle is held in place with a tapered pin which can be removed by tapping it out with a small pin punch. Make sure you tap it out in the right direction! Take a close look at both ends of the pin and you'll see one end is flat while the other end is rounded. This view shows the rounded head of the taper pin which is the larger end.

 




Crank pin
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Here's the other end of the taper pin showing the flat side which is the smaller diameter end of the pin and is the side you want to tap on to drive the pin out.

 




Pin punch and block
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You need to support the crank handle on a block while tapping the taper pin out. This photo shows a simple hardwood block with a hole drilled to allow the taper pin to be removed and a pin punch used to drive the pin free.

 




Loosen the taper pin
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Support the crank on the hardwood block with the pin centered over the hole, insert the pin punch and drive the pin out with light taps of a small hammer. It should come out easily. If not, recheck to make sure you're tapping the smaller (flat faced) end.

 




Remove taper pin
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Take note of which end you're removing the taper pin from so you can reinstall it back the same way. On this calculator, the taper pin is inserted in the same general direction as the arrow on the handle.

 




Main shaft
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With the taper pin removed, you can lift the crank handle off and view the top of the main shaft where you'll find two circlips (snap rings) holding the carriage in place.

 




Remove circlip
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To release the top carriage, you must remove the larger of the two circlips with snap ring pliers. I've never found snap ring pliers with small enough tips for this so have had to make my own. The hole diameters in the snap ring are approximately .031 inches, so I lathe turned two new .029 inch diameter tips out of drill rod and fitted them to the pliers. Use caution here as circlips have a habit of flying across the room never to be seen again.

 




Remove bush & spring
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With the circlip removed, carefully lift the carriage, but keep your hand over the top of the calculator to catch the brass bushing and spring in case they spring free suddenly. You may need to lightly tap the shaft while you're holding the carriage up to free the retaining bush. Lift out the brass carriage retaining bush and then the carriage pressure spring and set them safely aside.

 




Remove brass washer
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Lastly, lift out the brass carriage pressure ring (washer). You might want to make a sketch of the parts as you disassemble them to be sure you reinstall them in the right order. When reinstalling, remember first goes the thin brass washer, next the spring, then the brass bushing with the larger opening end down and lastly, the snap ring.

 




Remove the carriage
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With the internal shaft parts removed, you can now lift off the carriage unit, but before you do anything else, see the next photo.

 




Carriage locking ball
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Before you go any further, be sure to remove the tiny carriage locking ball and set it aside for safekeeping. This ball bearing is what allows you to lift and turn the carriage or turn the crank handle or lift the crank for subtraction - but only one of these operations at a time.

 




Carriage underside
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With the ball bearing now safely set aside, you can take a close look and study the underside of the carriage unit.

 




Carriage close up
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A closer view of the pinions, digit wheels, and carry pins for the carriage digits register. Try turning the digit wheels and then turn the clearing lever to watch how the digits are all zeroed. I don't recommend any further disassembly of the carriage unit by novices.

 




Tens' carry levers
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Here's a view showing the levers that implement the tens' carry function between digits.

 




Removing top cover
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Three screws hold the top cover in place and are easily removed with a small screwdriver.

 




Lift off top cover
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With the three screws removed, lift off the top cover to expose the workings of the top section of the calculator. You may want to temporarily reinstall the crank handle at this point so you can operate it and view the action.

 




Top section close up
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Here's a closer view of the top section, showing the tens' carry levers. Notice the tiny wire springs on each lever that hold them in the set or clear position. I don't recommend any further disassembly of the main barrel by novices.

 




Reassembly

Reassembly should present no problem, basically the reverse of what I've shown here. I haven't shown further disassembly of the carriage or main barrel units here nor do I recommend it for the casual hobbyist. Ernie Jorgensen can supply photocopies of the original Curta service manuals and are highly recommended if you have further interest in more disassembly and servicing details. Please contact him for ordering info.

 




Clearing lever
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Alert readers may have noticed the silver colored clearing lever on this Curta. It is not original. I received this calculator with the clearing lever broken off so I fabricated a replacement out of steel stock, but didn't bother to paint it to the original color. Installing it requires disassembly of the carriage unit, drilling out the old rivet pin and re-riveting the new replacement lever in place.

 



Please send any corrections or comments to me at gregsa@teleport.com.
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