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Repair MSX-2 VG-8235 floppy drive belt

Nov 14 2015
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I've owned the VG-8235 MSX-2 home computer of the '80-ies. I've spend countless hours playing on it, but eventually sold it to buy an Amiga. Fast forward many years and I bought it in a second hand shop the same model. After inspection of the machine it turned out the floppy drive motor spins up but the floppies couldn't be read.

 

This is a perfect opportunity to test the capabilities of my 3D printer!

In the image below you can see the drive belt is hanging slack:

I've taken the measurements as follows

Measurements of the belt pulleys:
Distance from pulley center to center: 91.1mm
diam. motor pulley: 5.5mm
diam. flywheel: 39.9mm

I've made a drawing in Illustrator to get an idea what the length needs to be for the belt since the old one is too slack to get a good measurement:

I've based my calculations on two half circles and two straight lines. The line in the center represents the distance between the pulley centers.

Circumference of a circle is piD. where D is the diameter of the circle.

flywheel pulley: pi x 39.9mm = 125.3mm

motor pulley: pi x 5.5mm = 17.28mm

Pulley top to top: 92.48 mm

Dividing the circumference by two and taking the straight line

Belt calculated length: (125.3/2) + (17.28/2) + (92.48 x 2)  = 256.3mm
Belt diam. 82mm
Belt height: 3.5mm

To make it a bit tighter I've chosen a diameter of 80mm. I've created an stl object with an inner diameter of 80mm, 1mm wide, 3.5mm high. At the time I've printed is my printer wasn't well calibrated, I could now easily print 0.5mm without problems.

I've printed using TPC flex 45, which is poly urethane. 0.4mm nozzle, extrusion width of 0.5, layer height 0.3mm, print speed 30mm/s. I don't print any faster because the filament tends to cut corners, the diameter of the circle will be smaller, and can make the object pull in on itself, lifting the corners and warping thin walls.

The result was good.

Since I was using a bowden tube and retractions weren't well calibrated I had some blobs on layer changes. I've been able to sand the blobs away and it was nice and straight. There's no guide on the flywheel pulley to guide the belt, it really had to be very precise or the belt would slip from the pulley.

Here's the completed print of the belt:

The blob visible due to retractions taking too long and the distance too small (1:1 macro shot):

After sanding down the blob, I've installed it successfully on the drive. Here's the drive with the belt in place:

 Here's a short video of the printing process:

Last changed: Nov 15 2015 at 7:10 PM

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