# Gear Sizes



## taycat (Oct 22, 2015)

my model c 9" only came with 1 gear, want to make some more.
does anyone have list of the od on the gears?
the model eng club i belong to were left a home made gear hobber with different cutting modules and this would be good excuse to dust it off  ( they got it 7 yrs ago and been sat since as no one knows how to use it) (( can see few scrap ones coming up))  and learn a new skill.
hoping to make set of imperial and metric gears.
also what cutter will i need for it?
or can i just take my 40t in and match it to cutters there?
thanks.


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## David VanNorman (Oct 22, 2015)

You can get a set of gears for about 200. of E-bay. With them an a couple more you can do most anything.


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## taycat (Oct 22, 2015)

like hens teeth over here.
saw a 40t on there couple of weeks ago at £45.


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## Bob Korves (Oct 23, 2015)

taycat said:


> my model c 9" only came with 1 gear, want to make some more.
> does anyone have list of the od on the gears?
> the model eng club i belong to were left a home made gear hobber with different cutting modules and this would be good excuse to dust it off  ( they got it 7 yrs ago and been sat since as no one knows how to use it) (( can see few scrap ones coming up))  and learn a new skill.
> hoping to make set of imperial and metric gears.
> ...


Do you have a Machinery's Handbook (MH)?  If not, you are not ready to cut gears yet.  You need to take the one gear you have, hold it up to the full size drawings of gear teeth in MH, and see what diametral pitch they are.  You also need to find the pressure angle.  That information is probably easy to find on the web as well for a SB 9" lathe.  If you know the diametral pitch, pressure angle, and the tooth count, you just use the charts in MH and it will show you the gear blank O.D. and the depth of cut.  There are also charts of what gear cutter you need for your diametral pitch, pressure angle, and tooth count.  If all of those terms sound like a foreign language, you are not ready to cut gears yet.  Tubalcain, mrpete222 on YouTube, recently posted quite a few videos on cutting gears and how to do it right.  Check it out.


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## Heavycrimp (Oct 26, 2015)

A Model C uses 18 DP change gears.  Not sure on pressure angle.  For the OD, take the desired number of teeth, add 2 and divide by 18.  A 40 tooth would be 42/18 = 2.333" OD.  for the PITCH diameter just divide # of teeth by DP - 40/18=2.222 PD.  I have read that SB used a special pressure angle on these gears.  Most machinery in the U.S. uses 14.5 or 20 PA gears tooth forms.  I'll try and look at some of my gears tomorrow and see what I can figure out.  I have some gear gages somewhere.........  I saw a while back where a Chinese concern was selling 18 DP, 20 PA hobs at a fairly low price.  The nice thing with hobbing is you only need one hob per gear tooth profile.


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## comstock-friend (Oct 28, 2015)

The South Bend Model C uses 18 DP 14.5 deg pressure angle. The set of 14 gears for imperial threads is 16, 24, 32 (2), 36, 40, 44, 46, 48, 52, 54, 56, 60 and 80. Compound gears are 54/18 and 72/18. In "How to run a lathe" you will find a photo of the Model C chart. http://www.practicalmachinist.com/v...18344101-new-me-south-bend-9-a-b-c-charts.jpg.

John


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## comstock-friend (Oct 28, 2015)

Cutting a range of gears requires a set, each cutter is good for a small range of teeth on the small end, more as the gears get larger tooth count.

*Cutter Number* *Cuts Gear tooth range*
#1 12–13
#2 14–16
#3 17–20
#4 21–25
#5 26–34
#6 35–54
#7 55–134
#8 135 - to Rack

John


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## Heavycrimp (Oct 28, 2015)

The OP mentioned that he was planning on hobbing the gears.  For that he will only need one hob, 18 DP, 14.5 degree PA.  That's the beauty of hobbing is that you get perfectly formed teeth because the hob is basically a cylindrical rack gear with the teeth spiraling one pitch per revolution and the blank is connected through change gears to synchronize with the hob's lead.  Then voila!  You have perfectly formed involute teeth, no matter how many (within the capacity of the machine).


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