# 80 Tooth Idler gear



## RWanke (Mar 4, 2018)

Does anyone see me having a problem with taking an 80 tooth screw gear for a 9C and boring it out to use it as an idler gear. This is the only gear I do not have in my setup and EBay folks want $80-$90 for the idler and you can find the screw gears for about $50. I'm cheap I guess. I haven't measured the key depth but assumed I might have to overbore it and make a bushing. ???

Or does someone on here possibly have a spare laying around they might want to part with?


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## Ulma Doctor (Mar 4, 2018)

Hi RWanke,
i don't have a spare lying around,
but you certainly can bore the center out to your specifications.
just check if the pressure angle and pitch are compatible with the gearing you already have. 
you simply bore the hole to dimension and either broach, file, or cut the keyway- in a shaper or lathe


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## RWanke (Mar 4, 2018)

The idler gears don’t have a key. How hard will it be to drill out the bore of one with a keyway already in it?


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## Ulma Doctor (Mar 4, 2018)

my apologies, most transfer gears i have run across are single or double keyed.

it may be best to bore the gear rather than drill it ,
but i suppose if you step drill the bore and finish by reaming, that would be ok too


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## rgray (Mar 4, 2018)

RWanke said:


> Or does someone on here possibly have a spare laying around they might want to part with?



By 9c does that mean south bend lathe?
Is it 20DP.? I looked on ebay and most don't say. I did find one that said 20DP gears.
I have an 80 tooth gear I bought with a group of gears that I don't need , but it is a 14DP I believe.
Found this on ebay: https://www.ebay.com/itm/South-Bend...581495?hash=item28527c7af7:g:yogAAOSwvepalI~r


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## Brento (Mar 4, 2018)

I have the same issue. I need a 80 and 54 tooth gear and they are looking expensive. I wouldnt mind paying to get them 3d printed bc im sure it will be cheaper? And for the amount of time ill use the gears for metric threading, they should last.


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## Brento (Mar 4, 2018)

Srry if im hijacking this post. Ive been having issues with answers as well. How do you measure pressure angles?


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## BtoVin83 (Mar 4, 2018)

You may not have the required resources but this method looks possible
http://mikesworkshop.weebly.com/change-gear-pressure-angle.html


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## rgray (Mar 4, 2018)

Brento said:


> How do you measure pressure angles?



The measurements of the gear will be different (tooth dept) and if you have the gear in hand you can probably figure it out that way if you
get the specs for the gear in both 20 deg & 14.5 deg.
If it's a metric module gear then as far as I know they are all 20 deg.


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## Brento (Mar 4, 2018)

I dont know much about gears. So alot of it is new to me.


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## RWanke (Mar 4, 2018)

rgray said:


> By 9c does that mean south bend lathe?
> Is it 20DP.? I looked on ebay and most don't say. I did find one that said 20DP gears.
> I have an 80 tooth gear I bought with a group of gears that I don't need , but it is a 14DP I believe.
> Found this on ebay: https://www.ebay.com/itm/South-Bend...581495?hash=item28527c7af7:g:yogAAOSwvepalI~r



Yes it is a South Bend and I believe they are 20 DP. The one you posted the link to on eBay looks right but doesn't have any real info about it. I may try to contact the seller to find out more details. His price isn't too awful painful. Thanks for the help.


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## Bob Korves (Mar 5, 2018)

Know that an idler gear can have any number of teeth without causing a change in the delivered gear ratio.  The only purpose of an idler gear is to increase the reach of the other gears.  If the 80 tooth gear is also needed for a drive gear or a lead screw gear in other combinations, then you need 80 teeth.  The lack of a keyway leads me to believe it is only an idler.  Any gear might be made to work if it has the correct pitch, the correct pressure angle, and will fill the space you need to fill.


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## RWanke (Mar 5, 2018)

Bob Korves said:


> Know that an idler gear can have any number of teeth without causing a change in the delivered gear ratio.  The only purpose of an idler gear is to increase the reach of the other gears.  If the 80 tooth gear is also needed for a drive gear or a lead screw gear in other combinations, then you need 80 teeth.  The lack of a keyway leads me to believe it is only an idler.  Any gear might be made to work if it has the correct pitch, the correct pressure angle, and will fill the space you need to fill.



Thanks for this info. I'm just going by the screw thread chart on my lathe to get all the possible combinations I might need for threading. The diagram shows 4 different setups using 3 different idler gears (80 tooth, 72/18 tooth, and 54/18 tooth). I also found a formula on another site for figuring the DP of the gears I have.   

DP=(N+2)/OD
DP - Diametral Pitch
N -Number of teeth
OD - Outside Diameter of gear

I measured all my gears and came up with 18.0something on all of them so I am hopefully assuming I am correct that the gears on my South Bend are 18 DP.


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## Bob Korves (Mar 5, 2018)

If the 80 tooth gear is only spacing the gears and reversing the direction, then the tooth count does not matter beyond influencing diameter.  The double count gears you mentioned are the same, but only if you are just using one of the gears, leaving the other unused.  If you are using both sides of a double gear, then you are doing compound gearing, and then there is a calculation for each tooth count, and everything affects the end result.


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## Kernbigo (Mar 5, 2018)

why use a 80 tooth i use a 72, no keyway


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## RWanke (Mar 5, 2018)

I can see using the 72 tooth in a single idler setup when threading 8 through 30 TPI (at least if it will reach). But when threading 4 through 7 1/2 or 32 through 120 TPI I need a combination of the 72/18 double gear and the 80 tooth at least according to the gear chart. There may be other possibilities but that would involve me doing lots of math.


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## brino (Mar 6, 2018)

rgray said:


> If it's a metric module gear then as far as I know they are all 20 deg.



That would be great if true!
I have been asked to 3D-print a gear for a friend. I have gear tooth gauges for Diametral Pitch gears, but not for Module Pitch gears. I won't know what it is until he removes the old, stripped gear from the equipment.

However, I was looking for gear tooth gauges just the other day, and this site shows 14.5 and 20 degree pressure angle gauges for both diametral and module pitch gears:
http://www.fujitool.co.jp/eng/products/measuring_tools/000092.html

-brino


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## brino (Mar 6, 2018)

Brento said:


> How do you measure pressure angles?



I picked up a set of gear tooth gauges from Boston Gear years ago. It is a series of flat almost rectangular "leaves" on a key ring. Each leaf is stamped with the diametral pitch at the end, and has teeth cut down each long-side; one side is 14.5 degrees, the other is 20 degrees. Unfortunately I cannot find it on their site now.

I am also certain that I saw some full-size gear tooth silhouettes printed in a text book that could be used to help determine pressure angle. I'll try to dig up that reference, but my collection of machining book is large, and my memory is weak.

The pressure angle difference should show-up as a slightly thinner or fatter tooth.
It can be difficult to be certain.

-brino


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## BtoVin83 (Mar 6, 2018)

When I worked for my dad we had cabinets full of gear cutters and hobbs, usually we would check the gear against one of the cutters. Most hobby machinists don't have that resource, and I don't anymore either.


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## rgray (Mar 7, 2018)

One simple method I've seen was to roll the gear in clay, then cut that so it looks like a rack gear and use a protractor to measure the angle
of 14.5 or 20 deg.
Never tried it myself though.
I usually have a known gear to compare to the one in question. The tooth thickness is visibly evident when comparing the two together.

I also have a set of the leaves in DP and I was trying them in the 80 tooth I have and one is very close but not quite right. The answer was right
on the gear it was stamped 80 m=2   it is a #2 module 80 tooth gear.


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## rgray (Mar 7, 2018)

rgray said:


> If it's a metric module gear then as far as I know they are all 20 deg.





brino said:


> That would be great if true!



I was way off on that one. Guess I just never ran into a 14.5 degree module gear yet. I see there are plenty of gauges for them.
They sure are proud of those gear gauges. Maybe they just aren't popular enough to build enough to make the price come down.


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## brino (Mar 7, 2018)

rgray said:


> Guess I just never ran into a 14.5 degree module gear yet.



Me neither, but I have not yet had to deal with module pitch gears at all......



rgray said:


> I see there are plenty of gauges for them.
> They sure are proud of those gear gauges. Maybe they just aren't popular enough to build enough to make the price come down.



It just struck me this morning that since Fusion-360 is great at designing gears, maybe I could figure a way to 3D print a set of Module Pitch gear gauges in both 14.5 and 20 degree pressure angles. The set would likely end up looking like those "Play-School" plastic key sets for young children, but it just might work.

-brino


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## ericc (Mar 7, 2018)

I just repaired a tooth on my broken bull gear.  I used that water mix strong putty (the one with the strongman on the can).  A couple of tips: use mold release agent (else it will stick and break), wait until long after it sets (else it is not strong enough), shave off one face so there are no bits getting in the way of a clean axial slide, and only slide it off axially (radial yanking will crack it).  As you can tell, it took a few tries.  When using it to measure, only slide it axially.  Measure the widening gap at the new tooth as you are filing to fit.  It is good for about 2 or 3 checks, then the tooth breaks off.  Clean it up and use the next tooth.  When you get close, fit it so that the new tooth is in the middle of the arc.  If it has a clean slide, you can go to the real gear fit with bluing for high spots.  Do not force anything.


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## rgray (Mar 7, 2018)

brino said:


> maybe I could figure a way to 3D print a set of Module Pitch gear gauges in both 14.5 and 20 degree pressure angles



I think you're onto something.


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