Comet A34 Scratch build

[tomhugill]

Double post

[wibblywobbly]

There is a guy on the Armourtek site that has built a 1/6 scale one from scratch, he has published some dimensions and interesting info, but nothing about the rear sprocket diameter. At least I now know the idler diameter.

I tweaked the printer and managed to get decent print quality, so did a test print of the sprocket. It 'looks' about the right size, but as the diameter is pretty much determined by the size of the teeth and the pitch of the track links, it can't really be any other size to accommodate the correct number of teeth. I literally ran the filament down to the last inch, it got about 80% through the print over about 3.5 hours, so at least I could test it. The driveshaft fits, but the teeth need to be slimmed down so that they slide easily in and out of the holes in the tracks. I also need a groove in the hub to take the guides on the tracks.

I don't reckon the new filament will arrive until late next week, so I will just have to be patient and work on other parts in the meantime.

[silversurfer1947]

A small observation - the teeth on a Comet sprocket look as though they have a taper, rather than being square ended, presumably to improve the meshing with the tracks.

[tomhugill]

Look forward to seeing how those printed sprockets work out! I can ask Gary to get the sprocket dimentions from mine

[wibblywobbly]

A small observation - the teeth on a Comet sprocket look as though they have a taper, rather than being square ended, presumably to improve the meshing with the tracks.

They do indeed, but the PIII has square teeth that fit the PIII tracks. For some obscure reason, Openscad will not let me design a pointed sprocket tooth and then stick 18 of them around the rim, or more precisely it probably would if I could be bothered to go and find out how to, it's just syntax. The other issue is that small pointy things are not something that a 3D printer copes with very well, as it can't print anything to a point, the nozzle is 0.4mm diameter, so the smallest it can really go is 1mm, and then it just creates a mess.

It was just easier and neater to go with square teeth. Ludwig took the same route on his flashy metal ones.

PLA isn't very durable, these sprockets are prototypes more than anything, I am pretty certain the teeth will round off. If they do, and they pass muster with everything being the correct size to fit the shafts, space from the hull, and fitting the tracks smoothly, I will think about getting some printed in ABS by professionals.

[AlwynTurner]

Hi Rob, re the pointed sprocket thing Warren did some pointed sprockets for my Universal Carrier so maybe you should pm him for tips/advice? Loving the bild and watching with great interest.

Alwyn

[wibblywobbly]

I seem to recall that he has an extruder that will take smaller nozzles, and he uses ABS, so he can probably print sharp teeth that won't snap off. Using 2mm thick PLA to print them would make them inherently weak, so for now squared off teeth seems the way to go. They are hidden behind the track guards anyway, so its not the end of the world.

[wibblywobbly]

Plodding on. I completed the idler, the Comets had two designs, the earlier ones were spoked and collected mud, so they switched to this design for the B model.

I found a way of getting the diameters of the sprocket and idler, and more by good luck than judgement the sprocket is pretty much accurate, and the idler is too. Once I have some filament it is going to take days to print the roadwheels, suspension arms, sprockets and idlers.... ...but at least they will look like the real ones do.

[AlwynTurner]

Hi Rob, have you considered taking a panzer 3 sprocket and 'modifying' it? The panzer 3 sprocket also has squared off teeth. You could maybe cut off the outer portion of the sprocket and give it a new inner?

Cheers
Alwyn

[wibblywobbly]

My objective is to try and build a complete tank, using as few off the shelf parts as possible, if the PIII sprocket isn't around 48mm diameter then it would be the wrong size. There is a deep dish on the Comet sprocket, so I would have to carve up a PIII one quite a bit.