Film Prop

[Dom2012]

Hi,

I'm building a small set for a film and I need to try and source cog wheels as cheaply as possible. The problem is that these cog wheels are to be four feet and three feet in diameter (two of each)!

If anyone has any ideas where I could find these or get them mde really cheap, please elt me know.

BTW - we also need to make them turn - to be motorised. So I also need to find motors, cog wheels, pulleys, etc., for peanuts.

Any help or advice would be really appreciated.

Thanks,

Dom

[Lady Chrystal]

MDF and paint?

[von Corax]

One option would be to make them yourself. Since this is prop work, plywood would do the job; you can use the woodgears.ca template generator not only to get the correct shape for each, but also to make sure they mesh properly. Just remember that since the ratio of sizes is 2:3 the ratio of tooth counts must also be 2:3.

Hope that helps, and hope to see photos of your work.

[Capt. H. Viscount]

Foam? I don't know if this will help much but depending on what you're going to be using them for you could try making them yourself out of some sort of foam. That way they'd be fairly cheap, easy to make and you wouldn't need much power to turn them; in fact it they're light enough you might even be able to turn them using a standard electric drill.

Good luck.

[bicyclebuilder]

Downside of using plywood or MDF is that you need decent thickness of the board to prevent wobbling. If your prop only have to turn and doesn't have to do anything else, you might concider cardboard. Or even Polystyrene with a coat of paper maché.
You could make them turn with old bicycle parts. Have someone off camera turn the wheels. The problem with motors is that they rotate fast. You need to find a way to slow things down. You can also use bicycle wheels as a base for your cogs. They already have the construction, axle and almost the right diameter. Cover the wheel with cardboard, modify a cheap bicycle and your done.
Thank you von Corax for posting the woodgears link. It's been a while since I've seen that.

O yes, we here on Brass Goggles LOVE to see DIY pictures, as well as the end product.

[Capt James Salt]

I think most of the people on this section of the forum could whip this up for you in a couple of hours with a couple of sheets of plywood, a jigsaw and a pencil.

You need to find someone in your area that has some stage craft experience.

[Uncle Arthur]

Pink Styrofoam sheet and careful painting.

[robotmastern]

The probelm with the wood gears link is printing out a 4ft piece of paper

[Mr. Boltneck]

http://blog.makezine.com/2012/04/04/easy-to-make-mdf-and-timing-belt-gears/
Is this of any interest?

[Damnd of Hell]

chicken wire frame with paper mache exterior or foam, or even auto body bondo.

[Narsil]

The probelm with the wood gears link is printing out a 4ft piece of paper

You don't really need to print out the whole gear though. If you can print out an arc containing a reasonable number of teeth then you can mark out the base circle diameter directly onto your board and use your tooth profile template to marke the teeth around the circumerence, by overlapping teh template wthe the previous set of teeth by say 50% you should be able to kep it pretty concistent and accurate.

[robotmastern]

Now why didn't I think of that

[Uncle Arthur]

To paraphrase Sherlock Holmes. The answer is always simple when someone explains it.  It's easy too look so closely at a problem that the obvious soloution just slips past.

[robotmastern]

as i was writing my post i was searching for that exact quote.

[Uncle Arthur]

Great minds tend to think alike. (That and I just re read the Complete Sherlock Holmes)

[Narsil]

Something else to bear in mind is that meshing spur gears should all have the same tooth profile and the number of teeth is proportional to the radius of the gear. This means that you can't just draw one gear and scale the whole thing up or down to make bigger or smaller ones since they will all have the same number of teeth and the teeth will be different sizes.

This also creates a practical lower limit to the smallest gear you can make with a given tooth  profile since it's impractical to have a spur gear with less than about 8 to 10 teeth.

For cutting gears form board (plywood, MDF etc) I would suggest using a drill or hole saw (a Forstner bit is probably the best thing) of an appropriate diameter to cut out the gear roots and then cut the rest of the profile with a jigsaw, cutting the outer diameter first and then cutting into the holes.

In terms of making them turn I would agree that, if at all possible, having them turned by someone out of shot would probably end up being the best option.

Simple bearings can be made by drilling a hole slightly larger than the axle through a block of wood, it may make things easier if you saw them in half and bolt them back together for assembly. You need 2 bearings per gear, ideally one either side.

A bit of extra effort with the bearings and mounting should considerably reduce any visible wobble and go a long way to making them look convincing.

To attach the axles (which can probably be made from broom handles) you can build up the thickness of the centre of the gear by gluing or screwing additional circles of plywood etc to form a boss. Cut or sand a small flat on the axle where it passes through the gear with a corresponding notch in the bore of gear itself and use a shallow wooden wedge to hold them together. A bit like a Woodruff key http://en.wikipedia.org/wiki/Key_(engineering)

[Otto Von Pifka]

cut your cog sides from thin plywood and sandwich the pink celotex foam between them to make them seem massive while still weighing next to nothing. using a hot wire to cut the foam by simply following the edges of the wood will leave a stiffer melted surface that will take latex paint and not need any messy sanding. you can also build up the axle areas the same way. a layer of plywood over a thicker layer of foam. you can use large plain coat buttons as the ends of massive dowel pins on the cog faces, once painted to look like wood, they blend in fine. I made some large rivet domes by pouring plaster of paris in the dished bottoms of some plastic drinking cups. the edges don't need to be perfect, in fact I sanded them more rounded so they looked more 3D when glued in place.

your work only has to be as good as the filming medium, if you are using HD then you have a lot of work ahead of you.
only thing I would add is don't skimp on the framework for the cogs, I always cringe when I see soem big honking structure with skinny litttle axles and 2x4 framing.

cedar 4x4 posts would be the minimum I would think for that hefty look, you could double them up too. I go through the pile of posts at the big hardware stores and pick out the lightest posts. they can be almost as light as balsa and most people pass over the light ones thinking they wouldn't be strong fence posts. the prices aren't usually too bad on them too.

[Achrist]

This, only bigger:

Terrakids.mov

Thinking of wooden cogs, windmill cogs came in mind and I googled it (pictures) It opened a whole new perspective on cogs.
Maybe something to think about.