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Welsh Dragon Models

Comet A34 Scratch build

This section is for builds that are not strictly Tamiya or Heng Long. For instance, replacing the electronics from a WSN or Matorro, or even a scratch-build.

Postby wibblywobbly » Wed Dec 28, 2016 2:22 pm

For no other reason than I have sheets of styrene and a 3D printer, I have decided to have a go at scratch building a Comet A34. Despite the box shape, it will be neither easy or quick, so don't expect rapid progress, quite apart from which I am clean out of printer filament, and need new nozzles from Australia. :|

I have cut out the hull side panels so that's one job done, and have another piece done for the hull base. A test fit of HL gearboxes miraculously allows them to be bolted to the hull floor and the shafts will line up with where the sprockets should be.

The road wheels are my current challenge. I could use Bulldog/Pershing/Leopard wheels, but they are 800mm on the real thing, which makes them the same size as Tiger roadwheels, so quite a bit bigger. I am currently trying to design wheels as one piece two wheel setups, with recesses for bolt heads and a roller bearing. If I could find a way to obtain rubber rims that would fit that would be the icing on the cake, maybe Tamiya Tiger ones might fit? If that doesn't work, the cheap and cheerful option might be to get hold of Tiger wheels and print inserts to turn them into the Comet design. I could then use Tiger suspension arms and save the hassle of printing those. :clap:

A couple of pics, ignore the print quality, I am using up the dregs of some old filament that is never going to give me a good finish in a million years.
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Postby tomhugill » Wed Dec 28, 2016 2:57 pm

Hey Rob, my Cromwell uses Taigen metal suspension arms with some small modification for the suspension Spring. The wheels are looking excellent! The tyres on mine are what looks like a rubber tube of correct diameter cut down to thickness. Are you planning on using pz iv tracks? If so the gap between the wheel halves looks on the narrow side for the guide horns.

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Postby wibblywobbly » Wed Dec 28, 2016 3:58 pm

I have a couple of PzIII track links so tested the setup using one of those. 3.5mm between the wheels seems about right. The gap in the pics is just a rough mock up, there is no spacer in there, and the wheels are seperate items, so they are closer than they will be.

These pics are what I am aiming to print when I have got the printer, filament, bearings, and bolts.
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Postby tomhugill » Wed Dec 28, 2016 4:50 pm

Excellent! They look pretty identical to the ones chris sells which are on my Cromwell, they seem to work well so should be no issue with yours

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Postby AlwynTurner » Wed Dec 28, 2016 6:50 pm

Hi Rob, looking forward to this, following along.
Alwyn :thumbup: :wave:
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Postby Ad Lav » Thu Dec 29, 2016 9:08 am

Nevermind
Last edited by Ad Lav on Sat Dec 31, 2016 4:55 pm, edited 1 time in total.
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Postby wibblywobbly » Fri Dec 30, 2016 10:45 am

I had a quick whizzover on the wheel design. Looking at the pics of the real ones, mine were nowhere near right. This is pretty close, the one on the left is the later version, which is what I need.

I may have a go at trying a drive sprocket, though I know nothing about gear teeth size/spacing etc. The idler and return rollers should be fairly straightforward though. Suspension arms are fairly simple, I have an idea for spring mounts that should work, hidden inside the hull...we shall see... 8O

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Postby wibblywobbly » Fri Dec 30, 2016 12:35 pm

Does anyone know the diameter of a Comet idler wheel, and drive sprocket??

It seems that a PIII roadwheel is almost identical to a Comet idler, if I take that route it will save me some hassle. Idiot that I am I sold off a box of crap that included metal idlers, and a full set of metal return rollers and mounts months ago...kicking myself now.

It might seem a daft question, but I have never owned a P IV, are the tracks exactly the same as the PIII, it seems wise to ask before I order two sets...one pair won't be long enough.
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Postby HERMAN BIX » Fri Dec 30, 2016 1:05 pm

H/L PzIII tracks definitely fit a H/L PzIV running gear.
Any other brand of track/drive sprocket combo I cannot say for certain.
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Postby tomhugill » Fri Dec 30, 2016 4:25 pm

wibblywobbly wrote:Does anyone know the diameter of a Comet idler wheel, and drive sprocket??

It seems that a PIII roadwheel is almost identical to a Comet idler, if I take that route it will save me some hassle. Idiot that I am I sold off a box of crap that included metal idlers, and a full set of metal return rollers and mounts months ago...kicking myself now.

It might seem a daft question, but I have never owned a P IV, are the tracks exactly the same as the PIII, it seems wise to ask before I order two sets...one pair won't be long enough.


Mine uses a panzer iv road wheel modified to look like a Cromwell idlers

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Postby tomhugill » Fri Dec 30, 2016 4:26 pm

Double post

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Postby wibblywobbly » Sat Dec 31, 2016 4:07 pm

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.
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Postby silversurfer1947 » Sat Dec 31, 2016 4:31 pm

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.
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Postby tomhugill » Sat Dec 31, 2016 4:49 pm

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

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Postby wibblywobbly » Sat Dec 31, 2016 5:15 pm

silversurfer1947 wrote: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.
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Postby AlwynTurner » Sat Dec 31, 2016 5:33 pm

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.

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Postby wibblywobbly » Sat Dec 31, 2016 6:43 pm

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. :shh:
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Postby wibblywobbly » Sun Jan 01, 2017 9:37 pm

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.... 8O...but at least they will look like the real ones do.
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Postby AlwynTurner » Mon Jan 02, 2017 10:20 pm

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
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Postby wibblywobbly » Mon Jan 02, 2017 11:16 pm

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.
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