Sharp EL-7050

I recently bought one of these calculators as it is a calculator of a type I didn't know existed until recently. It is a simple non-programmable with the added feature of a small ALPS four colour pen plotter. It can print out bargraphs, pie charts and tables of data that you enter into the calculator. I knew the ALPS mechanism was used by many manufacturers and many years ago I had a printer that used the mechanism.

It's a well known problem with these printers that a couple of small gears split with age and the mechanism dies. Unfortunately this had happened to the mechanism in my calculator as well. This stops the pen carriage from moving left and right and also the paper from feeding.

Some people have successfully fixed their mechanisms using small gears they have bought and others have 3D printed replacements. I thought of that but then I have a workshop for making things, so maybe I'll have a go at making some gears. I ordered some 4mm rod in various materials and went to set up the CNC 3020 ready to machine gears. Unfortunately the computer that drives it failed to boot properly and as a double whammy the spindle PSU has failed.

So, I decided to set up on the mill and do the gear cutting manually. I used the rotary table:

The cutter is a 0.2mm D bit that is one of the cutters I use on the 3020 for PCB milling. There's 13 teeth on the wheel, it has a 4mm OD and a 1.4mm hole for a push fit on the motor spindle. I worked out the 13 angles needed and cut a gear to a depth of 0.4mm or so. The dimensions are a bit rough as it's tricky measuring exact sizes with the manual calipers I have. I could use the spindle camera I suppose...

Anyway the gear was cut and I drilled a hole (1.2mm but enlarged manually, I had no 1.4mm drills, surprisingly as I have lots of drills of different sizes).
I managed to get the gear onto the carriage motor and it seemed to work, so I cut the second gear for the paper feed. Trying it out the papaer feed gear just didn't turn freely. I gave up and cut another one, this second one was much better:

I'll maybe cut it down to size later, it seems to work fine with the extra length, so I'll leave it for now. The carriage gear is here:

There's some oil from the mill on the wheel, it's actually white acetal.

The mechanism runs well enough to print out pretty well, here's the printer test (only two pens are working, I'll have to get some more somehow).

So the calculator now prints almost perfectly ( the carriage seems a bit too loose and rotates as it is drawing, not sure what th eproblem is there), and it's just as mesmerising as the plotter I had way back.

One problem that is left is that the LCD display has bad bleeding:

I have an ambitious possible solution for this, but it'll take a while to get to it I think.