Haha, I guess I just cant speak English XD. (I actually just searched pictures of duck and dove R.I.P. my Biology)

Yeah the wedging effect of the dovetail really is a drawback... I guess a T-slot will work fine...

Design update:

Made a new model for the drum using T-slots.

http://imgur.com/59uBF1u

http://imgur.com/92OvZfv

http://imgur.com/0dkEVLv

Really dug a lot of holes on the sides to keep it within the weight limit... Still weighs 3.1 kg though.

Making that internal drum shape is going to be difficult - I don't see how you can do it without CNC and milling out a hole that deep is going need really stiff tooling to avoid chatter.
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Australian 2015 Featherweight champion
UK 2016 Gladiator champion

Yeah that's a problem...

If I can find a thick tube this might be easier, for now the T-slot requires a lot of extra thickness or otherwise its gonna be weak... Any ideas how to find thick tubes online?

Edit: Just found some thick tubes on Taobao XD, might be able to use those...

You could get approximately the drum shape you want by offsetting the tube in a 4 jaw chuck and doing two boring passes on it:



You would need a very solid boring bar and very careful set-up to keep the tube balanced but its definitely possible.
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Australian 2015 Featherweight champion
UK 2016 Gladiator champion

Hey thanks for the picture Nick.

Since I just found some pretty thick tubes on Taobao, I may as well just use them. (Or in other words, if I can start with a really thick tube instead of a solid piece of aluminium, would it be easier to machine?)

Also, I wonder if it would be easier if I do this in these three steps: cut a T-slot on the tube (the tube here would be a thin one), machine two separate piece of aluminium reinforcements that goes right under where the T-slots are, and weld the reinforcements on.

The other idea I feel like might be possible would be machining it from both ends, then I would only need 14/2=7cm long tools. But since I don't have any experience on machining (I will be taking a machining class next semester, but that's after summer school...), I would like to ask if a 7cm long tool sounds a bit more normal to you guys.

Welding in supports would be tricky - 7075 and 2024 alloys just can't be welded, while the softer 6061 welds really well. Getting a torch head & filler rod even half way down the tube is a job for an expert and the welding would likely take the temper out of the aluminium, so there would be little advantage to the extra supports.

If you were making a static part, boring from each end would be a good idea. For a rapidly rotating part with that off-set boring idea, it will take extremely careful set up to get the bores aligned at each end. If everything is not aligned, the drum will probably not be balanced and the bot will shake itself apart. It is probably more reliable to find an extra thick boring bar and slow the lathe down to avoid chattering.
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Australian 2015 Featherweight champion
UK 2016 Gladiator champion

Good points.

And... Somehow another idea popped up:

What about something like this? (using a very thick tube, but will keep its inner diameter, all machining done outside.)

http://imgur.com/E5Q0vhy

That would work but would probably need CNC with a 4th axis.
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Australian 2015 Featherweight champion
UK 2016 Gladiator champion

Just realised that dovetails allowed me to get a much shallower cut into the tube, so I can keep both the inner and outer diameter.

Guess I will go with dovetail and screws (not rendered) for now...

http://imgur.com/uEcdxoJ

So... Got this problem: the pulley seems to interfere with the front wheels, or more specifically, the shaft collars that sit on the inside of the frame.

I could have a smaller pulley, but that way I will have to weld the pulleys on, and here comes my question: Nick you said that welding will take the temper out, but I wonder if that comment is regarding welding in an enclosed tube or welding aluminium in general.

Welding generally takes the temper out of your bas metal but only in a relatively small area around the weld. Its called the heat affected zone or HAZ. Aluminium is a great heat conductor and needs a huge (relative to other metals) amount of heat input, so I figured that welding that tube support inside a thick tube would likely produce a wider HAZ that included the whole of the T slot.

With the aluminium pulleys, losing the temper isn't so much of a problem as they won't be getting huge impact shocks. I couldn't find a CAD render showing the front wheel set-up clearly - what bits are you planning to weld together?
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Australian 2015 Featherweight champion
UK 2016 Gladiator champion

http://imgur.com/LBqIDvF

Sorry I forgot to include the rendering... The front shaft collar (not rendered) will probably interfere with the pulley.

http://imgur.com/EURlvzz

For now the pulley is screwed onto the endcap and mounted onto a tube-shaped extension of the endcap. If I can weld the pulley I would only need the tube extension thing, and I can save the thickness added by the screw holes and get a smaller pulley in there.

Oh actually wait a minute...

Just realized that the gap will be big enough for a shaft collar with a set screw (those are somehow way thinner).

Maybe I don't need to weld the pulley on after all... Sorry for the confusion.