Cad drawing a tiny sprocket

Hi I have a need for some tiny sprockets, but I cannot draw them as not sure how to draw the teeth!

The chain roller is only 1.67mm in diameter, the roller pitch is 3.18mm 28 teeth; 6mm center bore.Problem is when it is rotated there seems to be error... sprocket measures 30.22mm od over teeth... the small sprocket is 12 teeth! Any help please! Thanks vic

Hi, Vic.

Note that the roller pitch is a straight line, not a circular pitch like the distance between gear teeth. So the 'pitch circle' is a polygon with the same number of sides as the number of teeth, not a circle.

As each link 'unrolls' from the sprocket/chainwheel, the centre of the 'unrolling' roller generates a radius with respect to the roller still on the chainwheel. As the next link unrolls, the radius changes, but I think this is not important, as the rollers are held in the root of the sprocket by the tension. No doubt someone will come up with a specification.

It might be useful to note that the chain pitch is 1/8 inch.

Bill

Have a look here if you need some ideas on how to draw a sprocket Vic...

http://www.gizmology.net/sprockets.htm

Regards,

IanT

Here is a recent patent that shows the relationship between sprocket and roller.

Standard sprocket for chainwheel (2007)

I note that bycycle sprockets seem to have a different 'tooth' form compared with motor cycle sprockets, presumably each are optimised for the power transmission vs friction of the two applications.

I hope this helps a bit.

Bill

.

Excellent link, Ian

Thanks [and apologies for the inevitable pun]

MichaelG.

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Edit: This may also help:

https://mechanicalengblog.com/how-to-design-a-chain-sprocket-by-nx-expressions/

... and there’s a video : https://youtu.be/tm59xwUMxwM

Edited By Michael Gilligan on 05/01/2020 20:15:46

Easy way is to import a cad file from a sprocket supplier and if they don't do 1/8" pitch then scale to suit.

this is a 28T 1/4" pitch Macmaster Carr sproket scaled 50% and I have added the two 1.67mm pins and measured the distance to show it works.

Edited By JasonB on 05/01/2020 20:08:29

A big thankyou to Bill, Ian, Michael and especially Jason, 👍. I really appreciate your answers, I had no idea. I look forward to experiment...

kindest regards

vic

Hi Vic,

Can I ask you where you got the chain from?

I've been looking for that size for some time now.

Thanks

Steve

Hi Steve, yes these tiny chains and sprockets are used on the radio control 1:8 scale motorbikes! By kyosho. If you go to ebay hobbies / radio control parts, they do come up for sale. The rear 28 tooth sorockets are made aftermarket, instead of nylon ally or steel. However the standard nylon take alot of punishment! Eg the motor runs st 30,000 rpm reckon , reduction gear box is about 14:1 so its high speed! If you e me here, I can send you more info !/ pics

kind regards vic

ps Sprocketeer is a free software which works out your g code for you once you enter your values! Looks good

Does the software only use a small diameter cutter or does it drill out the internal fillets and then mill away the rest as you would want quite a small cutter into those internal radii.

.

Noted that one for future reference ... Thanks

**LINK**

http://www.idleamusements.com/?page_id=54

MichaelG.

Thank you Vic. I've ordered one already!

Steve

Hi Jason, that is a good question! Mmm well yes you are right , worryingly it does not have a box to enter the cutter diameter... it does have a tick box for tool offset.. It seemed to calc my sprocket g code to my specs above ie 28 tooth, 1.67 mm roller, and 3.2mm pitch, I can only try it and see! But yes that is a concern no cutter diam... I might send the site a email! And let you know...

Thanks vic

I would assume that offset is half the diameter of the cutter. But it would need to know number of flutes, feed rate and speed as well as any roughing cuts, finishing cuts, etc as you would not want to try and cut full depth and width with a 1.5mm cutter in one pass!

I did generate the code last night in alibre doing a facing pass over the top, drilled each at 1.6mm then adaptive clearing with a 4mm cutter and finally the contour with multiple passes so as not to overload the small 1.5mm cutter.

Watch that 3.2mm pitch, should be 1/8" or 3.175mm

Edited By JasonB on 07/01/2020 20:40:07