R/C tank help

[A-BOMB]

Well, I'm guessing this would go here, because its to do with building things. Well last year they were having a sale at me local Ratshack to sell of a massive over stock of old R/C cars and trucks after christmas, well seeing that I had an excess of money then I bought the biggest R/C truck they had a huge 4 wheel drive 2x 7.2v battery beohemoth. After a while its original plastic cover got old, so I ripped it off well things went down hill from there. To make a long story short, all I have left of it is the front/rear drives the 4wheels and the transmiter. So I desided to make a tank from it by putting the motor unit parallel to each other, exstending the axles and attaching the gear box at a right angle to the axle so when the power to a drive it cut it stops and locks the wheels so it acts as a brake turning the tank in that direction.

Now on to the part where I need help, for the original gear boxes I used legos gears and blocks well they quickly got chewed up, so now I need to make new gears. So I need your help, I've got the 2 gears that I need to copy, but no idea on how, I am trying to make them out of either AL/brass or epoxy (it might work its almost plastic). Now to copy them, I was thinking sand casting but that leaves the edges of to rough to use, then I though of useing a wax mold, but that one won't work with AL or brass but does work with epoxy. But to remove the epoxy gear I must tear off the wax so I need to make a new mold for every gear ,it works for the smaller gear but not for the large gear that will be connected to the wheel. And then I thought about useing pottery clay, it might take the heat of the molten AL and the epoxy for sure but still I would have too break the mold to get the gear out. So I'm stumped, I can either make the gears out of PB weld or PC-7 epoxy or resin, but those might not hold up well or I can make them out of AL, but the AL ones being much harder make. Does anybody know anything that will make a good releseing agent I've tried borax, talcum powder, chalk, graphite, any other ones that might work that I wouldn't have a hard time finding?

And BTW is the FTP haveing matainance or a down? because I was going to post a pic to help explain this post but it wouldn't load.

Please help me anyone, I really have to finish this thing so I can get all the parts of the my workbench.

<small>[ April 09, 2003, 11:54 AM: Message edited by: A-BOMB ]</small>

[zaibatsu]

Lost-wax casting. Basically you'd need to make a mold of the gear, pour hot wax in and remove the wax when it'd cooled. Then I think you can cover the wax pretty well in clay, let that harden but leave a gap near the top. Then melt up some aluminium and pour it in. It'll vapourise the wax, and result in a very accurate copy. However, I can remember fuck all about this method, but a google search will bring up LOTS of results on this. Make the mold with latex so you can easily remove the wax.

Or, sand cast a new gear and just dremel/file the gear smooth. Shouldn't be too hard as you've only got to get the teeth smooth. Graphite or chalk are supposed to be good releasing agents I think.

Putting "home foundry" into google should get you all the info you need to cast some new gears.

[A-BOMB]

I tried it with sand the teeth of the gear are too small for the AL to get fully into, then when I tried to fill it smooth I lost to much of the metal on the gear teeth that it didn't fit up with the gear correctly. Do you think gears made out of epoxy would hold up? Because they would be much easiler to make, and the fact that I 2 quarts of PC-7 epoxy to use up.

[Anthony]

Personally, I'd go into a model shop and ask about replacement gears for RC drive trains.

Casting gears, especially in such a low mod as RC cars use would be a mean feat that I'd rather now try...

[A-BOMB]

Why do you think that I'm trying to cast my own, I've been to almost every model shop in a dozen miles and the only gear that would of worked had a shaft size that was way to big(and to expensive). But do you think epoxy will make good gears?

[Tuatara]

Don't waste your time making Al gears. They will disintegrate fast, Al is too soft and sticky. You want to use brass, bronze or silicon bronze. The latter two are good for casting.

What gear ratio do you want? You could buy two cheap cordless drills and scavenge the planetary gear systems out of those - they will handle a lot of torque. The one I stripped down had a nylon set for the high speed section and sintered (something) for the low speed section. Planetary boxes are nice and compact too.

Can you get Meccano?

Failing that, go to a gear cutting shop and get them made!

Edit: If you try epoxy it will need to be loaded with something for wear resistance - iron powder? glass fibre? carbon fibre? Molybdenum disulphide?

<small>[ April 09, 2003, 05:31 PM: Message edited by: Tuatara ]</small>

[A-BOMB]

Well I don't need any ratio, I cant really explain this correctly so I'll try to put this in. You see in the drawing that I need the gears to change the direction of turn to a 90* from the drive train, and I've already tried to find a gear shop but I couldn't find one close to me. I have alot of steel shaving all over the floor in my lab and some graphite so what ratios of metal to epoxy should I use? And zaibatsu where can I get latex from other than melting down gloves?

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/|
/ |
/ |
| |
| |
]]]]]]]]]| |]]]]]]]]]]]]]]]{wheel shaft}
| |
\ |
/------\ \ |
/ \ \|
/__________\
[]
[]
[]
[]
[]
[] {shaft from drive unit}



</pre><hr /></blockquote><font size="2" face="Verdana, Arial, Helvetica">

[Anthony]

No wonder you couldn't get any from a model shop, you're after a set of bevel gears and AFAIK aren't used in models.

Have you considered using universal joints? Using two in series on each shaft, you should be able to get the 90* angle you get. These should be available from a model shop. But if you want 4wd, needing 8 of them might be a bit pricey.

[A-BOMB]

What are "universal joints"? its sounds like weed. I was going to use right angle drill attachments that allow you to drill in close spaces, because they already made but being to go forward all the shafts are turning right 2 of the wheels would be going forward and 2 back. But still any idea on how to melt latex or rubber like that make a mold? I made a casting of the gear in wax this morning and filled it with PC-7 epoxy, I'm waiting for it to dry so I can try it out.

[Tuatara]

I'm a bit puzzled as to why you want the 90 deg bend. The bevel gear system wont lock the wheels as described in your first post. Only a worm drive will do that, but the gear ratio will be so high your tank will crawl, though you could probably tow a house with it :D

Tanks drive left wheels as a set and right wheels as a set, so why not link them with a belt and pulley system. Then put your motors across the vehicle (opposite orientations) and drive the belt with another pulley on each motor. If you use toothed belts/pulleys you should get a good result.

BTW isn't the explosives and weapons forum kinda the wrong place to discuss model tanks? Unless you're building a robot suicide bomber...

[kingspaz]

once he has it working i'm sure he'll have a few shotguns on it :D
what make is the model car?
try a search on the net for the company website. you could order some spares.

[zaibatsu]

Latex is available from GRP suppliers, such as <a href="http://www.cfsnet.co.uk/acatalog/CFS_Catalogue_Latex_Dipping_Rubber_163.html" target="_blank">this one</a>

Also, if you cannot find one of these (which you should be able to) I think it is used in theatrical productions to simulate skin.

[Anthony]

This is a universal joint:

<img src="http://www.huco.com/images/huco-pol_universal.jpg" alt=" - " />

It allows a drive shaft to deviate off at an angle.

Considering the low torque of your application, a piece of strong flexible hose could be used as a universal joint. This method of coupling drive shafts is quite common. I once disassembled a tumble dryer that used a short length of hose secured with jubilee clips to transmit the drive from its motor.

I'd be interested in how the gear casting goes.

[Energy84]

Go back to your model car shop and ask for a set of STOCK differential gears for a Kyosho Sandmaster ST-2 Stadium Truck. I had this model and it had a set of 4 beveled gears just like you described in the differential. Make sure it's the stock diff though because most aftermarket diffs are ball differentials. It should only be a few dollars, they're not too expensive, and IIRC, they are about the same size as the lego bevel gears.
I just that you can still get parts for it though because that model has been discontinued last year I think...

Edit: I just checked my parts catalogue for the part numbers. Prices are in USD from Tower Hobbies (1999).
KYOC2336 SM-06 Bevel Gear A $3.89
KYOC2337 SM-07 Bevel Gear B $1.45

I can't remember which is which, but they're cheap enough to just get both and figure out which one is best. IIRC, they come in packs or either 2 or 4.
BTW, the Sandmaster ST-2 is a .12 nitro truck, so I'm sure the gears will be plenty strong for you.

<small>[ April 10, 2003, 07:14 PM: Message edited by: Energy84 ]</small>

[A-BOMB]

Energy I see about those gears, Tuatara, the gears themself won't lock but the shafts on the drive unit lock when there is no power applied. And I already though of spring as "universal joints" but that still wouldn't work because with all the shafts turning in the same direction 2 wheels would turn forward and 2 back. And the make of the car I Radio Shack brand it was named something dumb like country destoryer or atomic tank.

[A-BOMB]

Holy shit! Energy84 those gear where expensive, the first gear is now $6.53 and the second is $4.21, plus the store would have to special order them because there discontinued and I would have to pay shipping. So $6.53x4 and $4.21x4(plus 4x of each for replacements) equals way to much. I think I'll stick with making my own. Well I got some liquid electrical tape a few days ago and have slowly build up a 1/16" layer of rubber on the gear so now I have a mold to make wax gears with now. And I have tried making a epoxy gear, and it looks good but a few air pockets on the teeth so next time I need to thin out the epoxy some to alow all the air to rise to the surface. Sure it will take longer but there will be no air pockets now which will ruin the gear. I'll post a picture of the hole setup in a minute. But I still have no Idea no how to cast the bigger gear. Its to big to make a rubber mold of, and to big to make a wax too, so I'm thinking i'll have to cut the teeth out manually with a model knife out of some balsa or wax and use that to make a mold :rolleyes:

[Energy84]

Ouch, that sucks that the gears are so expensive now. Maybe you could ask around some other RC message boards to find out if there are any other models that use bevel gears somewhere in the drivetrain.
For some reason, something tells me that you might have luck among model railroad enthusiasts...

[Tuatara]

If your want to get the bubbles out of your epoxy, put a bell jar, or similar, over the mould and pull a moderate vacuum. You'd probably get sufficent vac with a domestic vacuum cleaner. No need for a fancy pump, you'd only end up boiling all the volatiles out!

[A-BOMB]

Well forget me and my epoxy gears I just found some that will work in my junk room where we keep all the old toys, national geographics, taxes, forms, and shit we aren't useing. Well I was in there looking for the window screens for my front porch windows, and I found this old contruction set that I had when I was a kid called a 'Gearopolis' well I found some perfect gears there a bit big and a hideous purpleish color nothing a can of spray paint wont fix. And if you can find one of these kit the plastic snap together belts for the gears make great tank treads.