Spirograph project

[Amy]

Hello. I'm trying to make a larger version of the spirograph toy for my daughter, who has low muscle tone. I'm especially trying to figure out how to proportionately enlarge the rings and gears. Any ideas on what size(s) I can make pieces to make a functioning larger set? I just want something basic; I can embellish it later.

Any information you can help me with on this would be appreciated.

[bill.young]

Hi Amy,

I did a set of big spirograph gears (the largest ring gear was about 30") for a Christmas contest that ShopBot did a couple of years ago. You can see some pictures by following the "Holiday Contest" link at http://www.shopbottools.com/archive.htm


I created the files by measuring a set of spirograph gears and then drawing them in TurboCAD without the teeth; just rings and circles. Then I scaled them up to the size I wanted, created a "toothy" looking shape, and radial copied the tooth around the edges. I also did holes in a spiral shape in the middle of each one for the pencil holes.

I didn't know very much about gear shapes back then (still don't really) so the smallest gears didn't mesh quite right with the largest ones, especially the inside of the largest rings. They were still fun to play with, though, and looked pretty neat. I'll look around for the files, but it was a pretty long time ( and a couple of hard drives) ago.

Bill

[Amy]

Mr. Young:

Thank you for your response. I know about your spirograph, except I saw it at http://www.shopbottools.com/winners.htm. May I ask you a couple of questions?

I don't think I need my set quite as large as yours, but I do need it larger than the original. How did you figure out what size to make the inside gears compared to the outer ring?

Also, do you think the gears and ring need teeth at that size?

I apologize for all the questions; I just really want to make this for my daughter. Again, thank you for your response.

[bill.young]

Amy,

I measured the plastic gears from a Spirograph set and then kept the proportions the same when I scaled them up... if you don't have a set to measure I may have one in the shop that I can measure for you. Be as accurate as you can when you measure the originals, because any error is magnified when you scale them up.

I'm not sure they need teeth but you do need some way to keep them from slipping against each other when you use them. It would sure make them easier to cut if you didn't need to worry about teeth.

Friction might work to keep them from slipping.. how about gluing a strip of rubber or sandpaper to the edges?

Bill

[Mayo]

If I had an old spirograph set, I would just scan the pieces and then manually trace over them or vectorize them, then do some clean-up on the resulting lines and then scale them up as large as I wanted. That would almost assure the gears would all fit.

[gerald_d]

The gears are important. If you have concentrated like mad and kept the pencil in the hole for 10 revolutions, you want your daisy petals to close up smartly on the last one.


The basics of a gear tooth is a trapezium with the sides sloping at 20 degrees to the construction line drawn from the centre of the circle. Then the top half of the tooth is narrowed (sharpened) to about a third of it's original trapezium width. This sharpening is normally a radius that tangents onto the 20 degree side flanks.

You will need to watch your cutter diameter, because this will determine the smallest gear tooth that you can cut. If the cutter is too big, and doesn't clean out the bottom of the tooth-space properly, you have to "sharpen" the mating teeth even more.

A very rough guide to construct a row of "toy"-quality teeth would be:

1. Draw a circle that represents the average diameter of the gear wheel. ie, a circle that will lie halfway up the teeth.

2. Draw an array of little "square" blocks lying on the average diameter - one block for a tooth and one block for a space. Therefore, twice as many blocks as the number of teeth.

3. The sides of the square blocks are pointing directly from the centre of the circle. So you would first draw one line from the centre and then copy it around (array) to give double the number of teeth.

4. At the edge of the gear wheel, you can see how far apart the teeth will be and so you can decide the height of a tooth (about the same as the width). Half the height goes to the outside of the circle in para 1, the other half below.

5. At this stage you should be able to visualize little "square" teeth. Already they are actually slightly trapezium shaped, but in the wrong direction (tops are wider than the base). Now rotate the flanks (sides) of the teeth by 20 degrees around their own center points to give the correct shaped trapeziums.

6. It is quite important that the circle in para 1 is still divided into equal-lengthed chunks - the tooth must be as wide as the space on this circle.

7. Then sharpen the part of the tooth outside the circle - ie. the upper half as mentioned right at the top.

I'll be gone for a week, good luck!