# Sieg SC2/SC3 what involute cutter for gears?



## addertooth (Nov 21, 2020)

I want to replace my tumbler gears with metal ones.  I bought the "metal gear set" for my Sieg SC2 (family) lathe, but it did not include the tumbler gears.
I am an experienced with lathes, so I am not concerned about needing a "weak plastic gear" in the gear train for "protection".

What cutters are used to make these gears (14.5/20) degree?  What "M" size are they? Etc..

Could anyone provide a link to the proper sized cutters?  I could do some measurements and probably guess them, but I am certain
that someone out there has already cut gears for these very common 7 X 10/12/14/16 lathes.


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## Ulma Doctor (Nov 21, 2020)

The mini lathe will have metric 1.0 modulus gearing


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## addertooth (Nov 21, 2020)

Is this also true of the lathes which do both metric and standard, and require a 21 tooth change gear to do metric?


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## Ulma Doctor (Nov 21, 2020)

there are many different gear combinations to get metric threads.
on larger lathes they use 100/127 transposing gear 


these may be useful:
http://conradhoffman.com/metricthre...text=The purpose of metric transposing,have%2

and









						Change Gears for Threading
					

This calculator determines the correct change gears for any thread pitch. Works for all mini lathes and HiTorque Bench Lathes.




					littlemachineshop.com


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## hman (Nov 21, 2020)

Thanks!  That Conrad Hoffman article is a real gem ... packed with good information!

Just for completeness, here's a table of possible transposing gear ratios I've gathered from several sources:





I hadn't thought if it previously, but Conrad Hoffman makes a good point about gear sizes.  If (as he says) 80/63 is nearly as impractical as 100/127, then 91/86 would even more impractical.  I can only speculate that gear sizes like 91, 86, 80 and 63 gain practicality because the tooth counts are NOT prime numbers.


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## addertooth (Nov 22, 2020)

More to the point. are they 14.5 degree or 20 degree teeth?


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## Downunder Bob (Nov 22, 2020)

My lathe came equipped with 120/127 gears I fail to see what is impractical, they are the smallest gears that will give the perfect pitch for metric threads, all others are only approximations.
.


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## Ulma Doctor (Nov 22, 2020)

addertooth said:


> More to the point. are they 14.5 degree or 20 degree teeth?


they use both 14.5 and 20° pressure angles in modulus 1.0 gearing
generally speaking the smaller the gear, the larger the pressure angle

if you are to examine your gearing already present, you'll see that if the teeth are like triangles, you'll most likely have 20° PA gearing
if the teeth are more parallel, you may have 14.5° pressure angle gearing


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## addertooth (Nov 22, 2020)

Ulma Doctor, I will take a peek at some of my spare change gears.


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## Jim F (Nov 22, 2020)

Here is a calculator that I use.



			Ride The Gear Train software by Evan Lewis


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## Jim F (Nov 24, 2020)

PSA,  a 127/100 transposing gear is 7" dia., the thing is huge, for a SB 9.


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## royesses (Nov 24, 2020)

Mini Lathe is Module 1, 20° PA. 

Roy


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## addertooth (Nov 24, 2020)

Thanks Royesses.   I appreciate that answer.  I didn't want to start forming an involute cutter collection.


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## Downunder Bob (Nov 25, 2020)

Jim F said:


> PSA,  a 127/100 transposing gear is 7" dia., the thing is huge, for a SB 9.



Yes a 127 gear is rather large for smaller lathes, but it is the smallest gear that will give you an exact metric pitch. There are various other combinations that can be and are used un smaller machines,many of these combinations give threads that are quite usable for nuts and bolts. however they are not suitable for long threads such as leads crews as the error does accumulate.

There are many calculators around, both on HM and other sites that will give you all the info you need.

Also I have seen photos and drawings of ways to fit a127T gear onto a smaller machine if the accuracy is desirable.


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## Jim F (Nov 25, 2020)

Downunder Bob said:


> Yes a 127 gear is rather large for smaller lathes, but it is the smallest gear that will give you an exact metric pitch. There are various other combinations that can be and are used un smaller machines,many of these combinations give threads that are quite usable for nuts and bolts. however they are not suitable for long threads such as leads crews as the error does accumulate.
> 
> There are many calculators around, both on HM and other sites that will give you all the info you need.
> 
> Also I have seen photos and drawings of ways to fit a127T gear onto a smaller machine if the accuracy is desirable.


I had one 3Dprinted, it just barely fit his machine, I will be doing threads that need to be right on, not just close enough.


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