TOM build diary

[shakesc]

Hopefully the name hasn't been used previously.
So I was calling the robot TCOB but it was hard to say and really didn't make much sense to me
After Colchester the build diary is more a rebuild diary

After stripping the robot down I found a bad connection on one of the TZ85's that was causing some drive issues. Lack of ground clearance on the arena floor (especially after those heavies had chopped it up) gave control issues. The brass inserts in the GR01 were pulling out. Finally the axe stopped working because the grub screws came loose!!

The internal design caused finding these issues tricky - time for a rethink

So this is a rough layout of the new internal design
- replace the 2 GR01 driving the axe with a scooter motor
- lighten the frame for holding the axe
- put the electronics together and rewire
- mount the GR01 on plates inside that allow easier release. Drill the plates to accept GR02's for the future (though I now will have a couple of spare GR01 boxes)

[shakesc]

I've got everything to fit and all bolted down. I reprogrammed the TZ85 with the latest code so they run at 100%. The bot self rights

[Ellis]

Looks great, nice work.

Do you feel much of a difference in power in the weapon with the different motor setup?

[RogueTwoRobots]

Looking very nice

I think I might revert to using the scooter motor I've got once the RL Edinburgh event is done. As fun and easy as it was to make an axe with a drill, it's just going to be more trouble than it's worth. All I need to do is work out how the hell to get the existing sprocket off my scooter motor!

What's the ratio between the motor and axe?

[shakesc]

The motor has a 9 tooth sprocket and axe a 38 tooth running on 8mm chain

The 2 GR01's did give good torque but the problem was the pinion gears slipping on the motor shafts.
The scooter motor gives a much simpler setup and I've welded the sprocket to it so no slipping.

On the scooter motor I had the sprocket was simply held on with a nut. I had another one with a belt drive sprocket. I heated that up to break the glue down then it simply drifted off

[Bacon Wizard]

I've been wanting to ask actually: is that the right thing to do: gear it down? To my mind, speed is the key to a weapon rather than torque, if you are not trying to lift another robot into the air. I'm still not quite sure how to work my weapons system.

[maddox10]

An axe that doubles as a self righter needs torque.

[Shakey]

Bacon you have to find the balance between the two. Torque wouldn't matter if it was like a spinner where you can build the speed of the disc up but in an axe you have around 180-270 degrees to accelerate that axe head. Torque is needed to build up the speed in that time. But gear down too much and it isn't fast enough for a good axe.


And as Maddox says torque is needed for self righting. A 120W scooter motor was tried in TOM but it didn't quite work for self righting so was switched for the 300W one.

[Ellis]

In electric axes you need to gear down a lot to allow the motor to actually reach a desired speed quickly. This is why the few drill powered axes around appear to hit fairly hard. Jamie's, for example, I believe has a 5:1 reduction from the drill to the weapon. That puts the total reduction to a huge 180:1 if it's a standard drill gearbox. Allowing for load, some 100rpm on the axe shaft.

People never seem to gear very low with axes. Unless you have a dynamic ratio built into the transmission and a stonking powerhouse motor, I don't think you'd be able to accelerate to much more than, say, 250rpm on the axe. I'm sure all of this can be calculated and that's a wild guess.

If the scooter motor above is running nominal voltage I believe they typically spin around 2500rpm. With the 4.2:1 ratio that means the axe is in theory able to reach 600rpm. I'd be very surprised if it were to come close to that speed inside 180 degrees of swing. I think it's safe to say that to an extent you gain speed if you gear for less speed. If I were to ever build an axe without an accelerating mechanism I'd aim for around 250-300rpm tops, as you're getting the most out of your motor for greatest efficiency. Which is why the drill axes seem to work so well. The entirety of the useful energy put out by the tiny motor is used and so the axe comes round hard, despite the at first ridiculously high sounding gear reduction.

I... I t-think. :P

[shakesc]

Personally I've learn't a lot from building TOM. The axe won't be the greatest but few are! Scooter motors are as cheap as chips which is handy
Really looking for something with at least a 775 sized motor and an affordable gearbox. Yes Banebots do P60 gearboxes and 775's but they are pricey plus then there is import tax. I don't have the equipment to build custom gearboxes for a speed 900 (or similar) so you make your choices. All part of the fun of robot building

[RogueTwoRobots]

On the scooter motor I had the sprocket was simply held on with a nut.

The scooter motor I've got also has the sprocket held on with a nut. Tried to get it off before with no luck, but will have another crack at it soon. I'm pleased to hear that yours works and self-rights with a 4.2:1 ratio though, as one of the Aussie builders uses the same motor for his axe and I'm sure I'd seen him saying that it needed a minimum of a 7:1 ratio. But hey, if it works! Already using 8mm chain and a 9T motor sprocket so I can keep the existing setup in my robot and have it running on a 5:1 ratio.