metric change gears [found]

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liveaboard
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metric change gears [found]

Post by liveaboard »

I was asking about metric gears here a while ago...
I needed a 30 tooth which was missing from my lathe, but had no idea about what pitch the gears are, metric or imperial.
I measured gears I do have, and compared the results with gears in online catalogues. I found that my gears are referred to as "1.5mm", which seems nuts since the teeth are 3mm-ish apart.
It turns out hat these gears are measured by the diameter of the gear; so a 100 tooth gear [pictured] is 150mm diameter to the gear tooth centers.
metric change gears.jpg
Sort of confusing, but easy to find the center to center distances of the shafts.

Anyway, I found the 30 tooth gear from a Dutch company for only a few bucks, + somewhat expensive postage.
https://www.mechanicdrive.com/

Obviously, there will be some machining to be done before it will fit the lathe.
Mr Ron
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Re: metric change gears [found]

Post by Mr Ron »

Thank you for passing that along. Every little bit of information helps.
Mr.Ron from South Mississippi
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tornitore45
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Re: metric change gears [found]

Post by tornitore45 »

Not exactly
The tooth are 3 mm deep. 1.5mm above the pitch circle and a bit more that 1.5mm below the pitch circle.

The OD of the gear is modulus X (N+2) so a 100 tooth gear 1.5 modulus is 153 mm OD, the only dimension easy to measure on even toothed gears. A thruly 1.5mm modulus gear has the teeth pie x M x (N+2)/N ~= 4.8mm apart The space between teeth at the top may look like 3mm
Mauro Gaetano
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Re: metric change gears [found]

Post by liveaboard »

As I said, measured from the tooth centers; in practice, from the bottom of the groove to the top of the opposite tooth. Although the 100 tooth gear has an even number of teeth, the diameter is big enough that the half tooth offset makes little difference to the observed measurement.

The point is that these metric gears are measured as number of teeth x pi x nominal pitch = circumference.
Teeth x pitch = working diameter [less gear lash I guess].
johnfreese
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Re: metric change gears [found]

Post by johnfreese »

Glad you found your gears. I am stuck making a 40 tooth and a 127 tooth for my Nardini.
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Re: metric change gears [found]

Post by liveaboard »

Making gears! I wish I could do that.

Are you sure you couldn't get them ready made?
What's the 127 for?

I recently spent a few days repairing a pair of stuck, bent, worn out links on my tractor. Straightened a bent lefthanded threaded rod and chased the threads, bored out the ends and pressed in new spherical joints, it's lovely.
But I forgot something important.
To check the price of new ones.
$20 each it turns out.
Oh well.
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tornitore45
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Re: metric change gears [found]

Post by tornitore45 »

The ratio between mm and inches is 25.4/1 = 254/10 = 127/5 reduced to prime numbers ratio.

The 127 tooth gear allows to cut metric threads with an imperial lead screw and vice versa.

Since 127 is prime there is no easy way to divide the circle but folks have come up with ways to index.
Mauro Gaetano
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Re: metric change gears [found]

Post by liveaboard »

Mine uses a 42 [or 84 sometimes] and an 89 tooth pair to cut metric threads [everything on the machine is metric but the lead screw].
I thought that was how they all do it.
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Re: metric change gears [found]

Post by tornitore45 »

89/42 x 12 = 25.428... an approximation within 0.11% Good enough for a short thread and loose fit.
Mauro Gaetano
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Re: metric change gears [found]

Post by liveaboard »

0.11%; that's more accurate than my work!
The wear on the lead screw probably introduces a greater error than that.

The company I mentioned above has 127 tooth gears in their catalog. 5.76 Euros.
Metric pitch though.
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