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Old 9th Jul 2019, 8:51 am   #21
crackle
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Default Re: Plastic cog needed

Looking at the dimensions the thickness of the "body" of the cog where it snapped, it can only be 1mm or less. The D shape acted like a lever to split the cog apart.. It was either designed badly or something caused extra strain on the D drive shaft and the cog just gave up.

So is printing a new one out of the question?

Mike
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Old 9th Jul 2019, 10:36 am   #22
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Default Re: Plastic cog needed

Maybe this?

ZG0.4_16 or ZG0.4-17 type ZPG from HPC gears

https://www.hpcgears.com/n/products/...spur_gears.php

The gear itself would seem to be a MOD 0.4 (pitch size) type, which might help with further searches.
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Old 9th Jul 2019, 12:01 pm   #23
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Default Re: Plastic cog needed

Thanks Chris I’ll check it out
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Old 9th Jul 2019, 3:41 pm   #24
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Default Re: Plastic cog needed

Is there another supplier Chris of these cogs as they want £9.40 to post it !
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Old 9th Jul 2019, 11:46 pm   #25
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Default Re: Plastic cog needed

Maybe here:


https://shop.kkpmo.com/index.php?cat...h=21_32&page=2


I can't really do any more than just search (Google) for "0.4 MOD gear"


This company was mentioned in a forum post that was about three pages down in such a search.


I have no first hand knowledge of any source in the UK.


You might get lucky with Indian or Chinese suppliers but I've no idea who.


Might be worth a flyer on one of the cheap assortments you can find on Ebay.
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Old 10th Jul 2019, 4:05 pm   #26
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Default Re: Plastic cog needed

Thanks Chris
May I ask why . 4 mod ?
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Old 10th Jul 2019, 4:17 pm   #27
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Default Re: Plastic cog needed

I guess I could go to 15 or 16 teeth to get the diameter right ?
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Old 10th Jul 2019, 7:41 pm   #28
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Default Re: Plastic cog needed

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Originally Posted by Goldieoldie View Post
Thanks Chris
May I ask why . 4 mod ?
It's the only metric size that fits 16 or 17 teeth on a cog of about 7.5mm OD.

If you can figure out the imperial equivalent, that might give you further search options.
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Old 10th Jul 2019, 9:08 pm   #29
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Default Re: Plastic cog needed

Thanks for your help Chris
I've had another look and it is 16 teeth
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Old 27th Sep 2019, 9:11 am   #30
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Default Re: Plastic cog needed

Hi
I have found a suitable cog ( thanks to Chris )
My challenge is to make a D shape in the middle for the drive
Any tips on how to do this.I can get the cog in brass or steel ,would brass be easier
To shape ( I'm thinking at the moment of using a needle file as I have no access to lathe etc )
Cheers
Pete
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Old 27th Sep 2019, 4:32 pm   #31
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Default Re: Plastic cog needed

I can imagine it is going to be very difficult to file a round hole larger but keep a flat on one side, and keep it all centred.
Can you drill a smallish hole the through the teeth and shaft and fit a pin, so long as the pin is flush with the bottom of the teeth it wont effect operation.
I would suggest getting more than 1 steel cog and using one as a sacrificial guide to start the hole through the shaft, which may be very hard steel.
Mike

Last edited by crackle; 27th Sep 2019 at 4:39 pm.
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Old 27th Sep 2019, 6:45 pm   #32
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Default Re: Plastic cog needed

What size hole does your new gear come with?

I guess your shaft is 3.5mm OD with a flat on it. Then I would drill your new gear (brass or steel) to a 3.5mm hole, which will stay concentric with whatever hole it comes with. Then file a flat into the ID which makes it *bigger* than 3.5mm across, align this flat to that on the shaft, and file a small steel / brass / ali peg to fill the gap. Hold it together with epoxy if you think it might move.
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Old 27th Sep 2019, 6:54 pm   #33
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Default Re: Plastic cog needed

That sounds like it would work, in effect you have made a keyway.
A good idea.
Mike
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Old 28th Sep 2019, 4:32 pm   #34
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Default Re: Plastic cog needed

Thanks for your suggestions
The key idea sounds good
Should I order brass or steel ,the cogs it engages to are nylon plastic
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Old 28th Sep 2019, 5:41 pm   #35
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Default Re: Plastic cog needed

If I'm understanding this right, the idea is to drill the hole out to fit snugly on the shaft, leaving a space between the flat and the edge of the hole; then file away the bottom half of a square which bounds the circle. Now the gear must be running true on the shaft because the round edges are in contact with the circumference of the hole (which was already central before you enlarged it). Finally a packing piece is inserted between the flat on the shaft and the flat edge of the aperture to ensure the whole ensemble rotates.

Effectively, you are subtracting from an aperture!

If the shaft is steel, then the packing piece should also be steel. If it is made of a softer metal, then it could deform and come loose. The forces at play near the centre of a large cog are magnified: you effectively have a type 2 lever, with the fulcrum at the centre, the effort at the teeth and the load where the shaft meets the packing piece.
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Old 28th Sep 2019, 6:04 pm   #36
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Default Re: Plastic cog needed

I've had similar problems, mostly with nylon gears.
I have always managed to find something online, various model suppliers are out there, although it sometimes means buying a small quantity.

I usually end up modifying something that's not exactly what I require. The last time I needed a similar cog, I found a spur gear with the correct number of teeth, & removed the outside section.


David.
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Old 28th Sep 2019, 6:10 pm   #37
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Default Re: Plastic cog needed

My goodness, plastic gears, gears in general working out the modulus of a gear. I took pot luck on a well known auction site and bought a huge bag of assorted gears, I did have a calculator for working out the modulus, I'll see if I can find it.
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Old 29th Sep 2019, 9:10 am   #38
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Default Re: Plastic cog needed

Should you be able to identify the material used for the part, 3D plastic printing suppliers, who specialise in 1 off or low volume production, might be the answer.

On eBay, there is a 3D printed name place supplier and I wonder if they might be interested in helping you out.

Chris
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Old 29th Sep 2019, 10:03 am   #39
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Default Re: Plastic cog needed

Quote:
Originally Posted by julie_m View Post
If I'm understanding this right, the idea is to drill the hole out to fit snugly on the shaft, leaving a space between the flat and the edge of the hole; then file away the bottom half of a square which bounds the circle. Now the gear must be running true on the shaft because the round edges are in contact with the circumference of the hole (which was already central before you enlarged it). Finally a packing piece is inserted between the flat on the shaft and the flat edge of the aperture to ensure the whole ensemble rotates.
That's a more eloquent expression of the idea I was trying to put across - thanks!
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Old 15th Oct 2019, 8:27 pm   #40
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Default Re: Plastic cog needed

An update
I received the steel cog with a 2 mm hole
Drilled it out as suggested to 3.5mm then soldered a small lump inside the hole and filed
To form a D shape .
I know solder is not very hard but it seems stop the shaft from turning
Thanks for all your help
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