# Gear Cutters?



## SMG (Sep 6, 2012)

Dumb question,  what does the # mean or designate on different gear cutters. I understand the pitch, I just don't understand the difference between a #7 48 pitch gear cutter compaired to a #8 gear cuttter.  I"m sure its somthing really simple, but I am in the process of purchasing some gear cutters online and I need some advice.

Thanks
Steve


----------



## GailInNM (Sep 6, 2012)

Steve,
The cutter number indicated the range of the number of teeth in a gear that that cutter can cut.
Gail in NM



 *Cutter Number* *Cuts Gear from*
1             135 teeth to rack
2               55-134
3     35-54
4               26-34
5               21-25
6     17-24
7               14-16
8               12-13


----------



## cfellows (Sep 6, 2012)

To add just a bit more to what Gail said... 

Gear teeth for for a given pitch are always spaced the same, you simply add more teeth as the gear gets bigger.

However, the shape of the sides the teeth changes as the number of teeth changes.  As you decrease the number of teeth, the sides of the teeth become rounder or bulge more if you prefer... as you increase the number of teeth, the sides become flatter.  With a rack gear, the sides are completely flat, with no radius at all.  

So, you need different gear cutters to cut gears with different numbers of teeth.  Theoretically, you should have a different cutter for every size gear you want to cut.  But, since that's not practical, they make gear cutters that will cut a range of gear sizes.  Not a perfect solution, but close enough for all but the most exacting applications.

Chuck


----------



## SMG (Sep 6, 2012)

Thanks guys, That explains it really well.


----------



## rhitee93 (Sep 6, 2012)

I'm glad you asked.  I didn't know this either.  What's worse is that I didn't know that I didn't know it!


----------



## AlanHaisley (Sep 6, 2012)

Chuck,
Could you elaborate:


cfellows said:


> ... you need different gear cutters to cut gears with different numbers of teeth.  Theoretically, you should have a different cutter for every size gear you want to cut.  But, since that's not practical, they make gear cutters that will cut a range of gear sizes.  Not a perfect solution, but close enough for all but the most exacting applications.


 I assume that one of the numbers in the range is that of the number of teeth that should be "perfect" and that the other numbers of teeth in the range are off one way or another. For each number of cutter, do you know which number of teeth will be closest to perfect and which number(s) will be most off?
Alan


----------



## gmac (Sep 6, 2012)

Alan;
This may help - the lowest tooth count for a gear cutter "range" is the most "perfect", the others less so. Can't tell you if that's particularly important in our hobby application.....

http://www.precisioncuttingtools.co.uk/gearshap.html

Cheers Garry


----------



## kvom (Sep 6, 2012)

Here's a pair of mating gears with different # of teeth.  The difference in tooth shape can be seen pretty clearly.


----------



## SMG (Sep 26, 2012)

So here is another Question, Why would someone pick a 20deg pitch angle set of gears over a 14.5deg pitch angle? Does one run smothier than the other? What is the difference?

Thanks for your help.


----------



## gmac (Sep 26, 2012)

Steve;
Top of the second page, Pressure Angle (PA). 
http://www.xtek.com/pdf/wp-gear-terminology.pdf
Cheers Garry


----------



## black85vette (Sep 26, 2012)

I have this book and find it pretty useful for understanding the basics;

http://www.amazon.com/dp/0852429118/?tag=skimlinks_replacement-20


----------



## gus (Sep 26, 2012)

SMG said:


> Dumb question,  what does the # mean or designate on different gear cutters. I understand the pitch, I just don't understand the difference between a #7 48 pitch gear cutter compaired to a #8 gear cuttter.  I"m sure its somthing really simple, but I am in the process of purchasing some gear cutters online and I need some advice.
> 
> Thanks
> Steve



Hi Steve.

Gus also going into gear cutting.Been reading text books and not getting any where. Planning to go into making simple clocks.


----------



## gus (Sep 26, 2012)

Hi Guys,
I am following and reading all the expertise.
 Did gear cutting in trade school 51 years ago.
I got good marks for cutting all 24 teeth. Some classmates goofed with the gear blank O.D. and ended up with shallow tooth height or sharp tooth. One chap did not set the compass diividers as taught and came up with 25 odd looking teeth.He manual count the no. of holes.Ha Ha Ha.
We were 18 year old kids then doing 2 years machining course.


----------



## gus (Sep 26, 2012)

black85vette said:


> I have this book and find it pretty useful for understanding the basics;
> 
> http://www.amazon.com/dp/0852429118/?tag=skimlinks_replacement-20



Will buy this book to read up.


----------



## WiKKiDWidgets (Sep 27, 2012)

rhitee93 said:


> I'm glad you asked.  I didn't know this either.  What's worse is that I didn't know that I didn't know it!



I have that problem all the time!


----------



## GailInNM (Sep 27, 2012)

> Why would someone pick a 20deg pitch angle set of gears over a 14.5deg  pitch angle? Does one run smothier than the other? What is the  difference?



Gear design/selection is a big game of trading one advantage at the expense of another.  Since there are a lot of variables that can changed on a gear it can get to be very confusing. Nothing is sacred.  Even the tooth form may be changed to suit certain applications so the involute form may not be the best in some cases.

Generally smaller pressure angle gears produce less pressure on the shaft bearings as there is not as much pressure trying to force them apart. Run a little bit quieter also as I recall. But the teeth are not as strong, particularly at low tooth count, as higher pressure angle gears so they can not transmit quite as much power.  

For model engineering most of this will not make a bit of difference.  Even though there are model engine designers who are quite capable of doing the math required to optimize gear selection in a design, I don't know any of them who actually do.  The TLAR (That Looks About Right) approach is probably the most common selection method.  And TLAR is further modified by other mundane things like what gears are available, either on hand or for purchase.  If making our own gears what tools are available.  You know the story.  32 DP gears would be ideal but 48 DP cutters are on hand and will work just fine. 

So the selection of pressure angle for model engines is mostly going to come down to practical matters.  

If you are going to purchase gears then 20 degree PA are probably more commonly available and would be my choice. In metric (MOD) sizes 20 deg PA are very predominant.   

If you are going to make your own gears you have more choices available.  If you are going to make your own cutters to make gears then it is as easy to make any pressure angle you desire.   If purchasing commercial involute cutters the situation is changed a little bit.  In recent years fewer suppliers are carrying as big a selection of involute cutters.  In particular imperial diametrical  pitches of 32 and finer are getting harder to find.  A few years ago 14.5 deg PA fine pitch cutters were 1/3 to 1/2 the price of 20 deg PA.  And this had been the case for 25 or more years.  No apperent reason -- just the way it was.  So, many of us who got into gear cutting back then went with 14.5 degree just because we were cheap.  As I have complete sets of gear cutters in the sizes I use I have not looked prices recently.  Of course the gears I make and use are because  I have cutters for those sizes.  Practical matters again. 

Gail in NM


----------



## lensman57 (Sep 27, 2012)

SMG said:


> Dumb question,  what does the # mean or designate on different gear cutters. I understand the pitch, I just don't understand the difference between a #7 48 pitch gear cutter compaired to a #8 gear cuttter.  I"m sure its somthing really simple, but I am in the process of purchasing some gear cutters online and I need some advice.
> 
> Thanks
> Steve


 
Hi,

Unless you wish to make gears for commercial purposes then I would strongly advise you to learn to make your own cutters, the best ones being a home made hobb. This will approximate the involute gear shape if multiple cuts are taken and it is a hell of fun to make. They are well documented if you Google for them. Here is one in German but the photos are self explanatory. 

http://www.metallmodellbau.de/GEAR-CUTTING.php


Regards,

A.G


----------



## ELM6061 (Sep 27, 2012)

Thanks for posting this question SMG, so many of my own have just been answered here, and to you Lensman, a great link, and as you stated "even though it is written in German". I have to follow up with; A picture paints a thousand words, and with 46 detailed photos & drawings, it is like reading a book.

One question I do have from that link, if you were going to cut your own HOB'S; What metal/steel would be best to use?

I do have an electric kiln & quenching oil, though apart from Tech school 30 odd years ago, with an oxy torch, I have never tried to heat/soak/temper/harden.

Eddie


----------



## Maryak (Sep 28, 2012)

Annealed, harden and temper after machining.

US - Drill Rod
UK - Silver Steel

Best Regards
Bob


----------



## lensman57 (Sep 28, 2012)

ELM6061 said:


> Thanks for posting this question SMG, so many of my own have just been answered here, and to you Lensman, a great link, and as you stated "even though it is written in German". I have to follow up with; A picture paints a thousand words, and with 46 detailed photos & drawings, it is like reading a book.
> 
> One question I do have from that link, if you were going to cut your own HOB'S; What metal/steel would be best to use?
> 
> ...


 
Hi,

As Maryak has already mentioned use either silver steel and or drill rod , once the teeth have been cut heat to cooked carrot colour and quench and then heat again to about 250 C to temper , I just wack the oven up to its max and leave the tool there for about an hour as I find it difficult to bring the whole tool to straw colour at once. You may have to do a bit of maths to get the top land dimension for the gear and hence the cutter you have to make to cut the silver steel into a hob. Here is another link for making cutters per specific module or DP.

http://www.metalwebnews.com/howto/gear/gear1.html

Regards,

A.G


----------



## gus (Sep 28, 2012)

ELM6061 said:


> Thanks for posting this question SMG, so many of my own have just been answered here, and to you Lensman, a great link, and as you stated "even though it is written in German". I have to follow up with; A picture paints a thousand words, and with 46 detailed photos & drawings, it is like reading a book.
> 
> One question I do have from that link, if you were going to cut your own HOB'S; What metal/steel would be best to use?
> 
> ...




Tubal Cain wrote a 124 page book on heat treatment-------------------
"HARDENING,TEMPERING&HEAT TREATMENT.Page 24 has a very useful para
on on holding time .
Found it very useful.Bought a electric furnace to do heat treatment of die tools. Made and heat treated my die tools for department use.
Tempering can be tough w/o a tempering furnace but some guys found out domestic ovens can used for tempering.A good tempering furnace cost "an arm and leg".But it is worth money spent.


----------



## ELM6061 (Sep 29, 2012)

G'day and thanks for the replies. The little electric kiln is capable 1800C, so getting hot enough should not be an issue. How even it will be is a different kettle of fish????? Bought it to melt metal in, trouble with being square, there are no heating elements in the door which creates a cold spot and cost me a graphite crucible.
I have read a few different sites about heat/quench tempering and probably the only reason I have held onto the kiln.

I will have to do some more reading in regard to making the cutters to shape a hob.

Eddie


----------



## thayer (Sep 29, 2012)

lensman57 said:


> Here is one in German but the photos are self explanatory.
> http://www.metallmodellbau.de/GEAR-CUTTING.php



Google translate does a reasonable job with this page too. While not perfect, I found it much easier to understand.

Thayer


----------



## Maryak (Sep 29, 2012)

Here's the start of the section where I made a gear hob.

http://www.homemodelenginemachinist.com/f31/hit-me-miss-me-4414/index37.html#post94403

Hope this helps

Best Regards
Bob


----------



## Cogsy (Sep 29, 2012)

Maryak said:


> Here's the start of the section where I made a gear hob.
> 
> http://www.homemodelenginemachinist.com/f31/hit-me-miss-me-4414/index37.html#post94403
> 
> ...


 
Am I right in thinking that a hob like this will cut any number of teeth, so I would only need one hob per DP? I'm liking the sound of that, rather than buying sets of 8 cutters... Thanks.


----------



## Herbiev (Sep 29, 2012)

G'day Eddie. Can you tell us a bit about the kiln. 1800 deg should be good for melting cast iron I think


----------



## Maryak (Sep 29, 2012)

Cogsy said:


> Am I right in thinking that a hob like this will cut any number of teeth, so I would only need one hob per DP? I'm liking the sound of that, rather than buying sets of 8 cutters... Thanks.



Yes that's correct but the tooth form is a series of small flats rather than a true involute.

IMHO for the gears we use in 95% of model applications this is more than adequate and the gears can always be bedded in with a run in valve grinding compound.

Best Regards
Bob


----------



## lensman57 (Sep 29, 2012)

Cogsy said:


> Am I right in thinking that a hob like this will cut any number of teeth, so I would only need one hob per DP? I'm liking the sound of that, rather than buying sets of 8 cutters... Thanks.


 
Hi,

That is true so long as you understand that the hob will only approximate  the involute curve, but in most cases that is more than acceptable.
The trick is to cut the gear in multiple cuts not in one go.

A.G


----------



## ELM6061 (Oct 2, 2012)

Herbiev said:


> G'day Eddie. Can you tell us a bit about the kiln. 1800 deg should be good for melting cast iron I think


Sorry Herbiev, bit of a miss print and not sure what I was thinking when writing 1800c, it is supposed to be 1200C.
The kiln is a small (square) 240v B&L Tetlow potters kiln, where the door swings up and it has elements on the rear and two side walls.
I originally bought it from a hobby glass slumper, so I could start casting with aluminum. However it cracked my graphite crucible first go and while I had melted aluminum on three sides of the crucible, in the front where the crack appeared, the aluminum had not begun to melt. I immediately put the crack down to uneven heating, as there are no elements in the front (door). I could probably use and get away with using a steel crucible, however I have not bothered to try, and plan to build myself a new furnace, which I will heat using an oil burner.
Electricity is just way to expensive now and by the looks, only going to get worse. Our last bill went up over $200 and we went with out heating for the most part of winter, got rid of the second fridge, led lights for our main lights, power savers in all the main power sockets and I hardly did any work out in the shed.

Eddie


----------



## lensman57 (Oct 2, 2012)

Hi Eddie,

I am sure that you know that you'd only need a propane torch to bring the tool to cooked carrot colour, I just use the domestic oven heated to about 250C to temper it. As far as hardening goes you only need to harden the teeth and not the whole length of the tool.


Regards,

A.G


----------



## seadragonfoundry (Oct 2, 2012)

An excellent reference is "Gear Cutting Practice" by Colvin and Stanley dated 1937, reprinted by Lindsay Publications.
There is an invaluable article in the Model Engineer by D.J. Unwin, " Simple form relieved Milling Cutters" in issues 21st August 1970,1st October 1971 and 15th October 1971. I have used this method on many occasions to produce my own cutters from Silver steel. If the gear teeth are of a larger D.P. to preclude or make difficult their manufacture at home, e.g. 6 D.P, Tracy Tools in the U.K. supply gear cutters of all D.P. at a very cheap price, some I have purchased were ex WW2, unused, still in the original preservative.


----------



## gus (Oct 3, 2012)

lensman57 said:


> Hi Eddie,
> 
> I am sure that you know that you'd only need a propane torch to bring the tool to cooked carrot colour, I just use the domestic oven heated to about 250C to temper it. As far as hardening goes you only need to harden the teeth and not the whole length of the tool.
> 
> ...



Hi A.G.

Heat Treatment of gear cutters. Your expert advice is sought.
I am wondering would it be the same as heat treatment for gears where only the gear tooth/teeth is hardened using high frequiency coils. Back in the Painted Post and Ahmedabad plants we us this process to case harden piston rods so that they could survive the oil scrapers and metallic packings.

Youtube has a video of a DIY High Frequency Induction Coil Heaters.


----------



## parisharma (Mar 16, 2013)

If you want to get information regarding different types of tool then http://www.supercuttingtools.com/ helpful this site  provide different types of tools.


----------



## gus (Mar 29, 2013)

SMG said:


> Dumb question,  what does the # mean or designate on different gear cutters. I understand the pitch, I just don't understand the difference between a #7 48 pitch gear cutter compaired to a #8 gear cuttter.  I"m sure its somthing really simple, but I am in the process of purchasing some gear cutters online and I need some advice.
> 
> Thanks
> Steve



Hi Steve,
Please include Gus too.
Just got myself educated thru brousing Internet suppliers.
Gear cutting books were not very helpful. Have no idea one cuttter will not cut the entire O.D. Range .Ha Ha. :wall::wall:
 My gear cutting experience was cutting one 4 " spur gear using involute gear cutter as specified on print. Using the dividing head was kind of mysterious to a seond year trade school student. Workshop instructors were not best. Some of my mates had half tooth.Ha Ha:wall::wall::wall:
This was way back in 1961/2.

Now planning to gear up to cutting spur gears. Will post.


----------



## Mainer (Mar 29, 2013)

The shape of gear teeth change according to how many there are. TO visualize this, compare the teeth on a 12-tooth gear (very rounded flanks) with the teeth on a rack (straight flanks). Strictly speaking, any given gear cutter is correct for cutting one tooth count.  A cutter will be approximately correct, however, for a range of tooth counts near that one "exact" tooth count. I don't remember the numbers offhand, except that #8 is 135 teeth to a rack. #1 is something like 12 to 14. 

There are, I believe, gear cutters available in sets of 15 to give better approximations of the correct tooth form.


----------



## gus (Mar 29, 2013)

Mainer said:


> The shape of gear teeth change according to how many there are. TO visualize this, compare the teeth on a 12-tooth gear (very rounded flanks) with the teeth on a rack (straight flanks). Strictly speaking, any given gear cutter is correct for cutting one tooth count.  A cutter will be approximately correct, however, for a range of tooth counts near that one "exact" tooth count. I don't remember the numbers offhand, except that #8 is 135 teeth to a rack. #1 is something like 12 to 14.
> 
> There are, I believe, gear cutters available in sets of 15 to give better approximations of the correct tooth form.



Hi Mainer,

Thanks for the expert advice. Gear cutting will happen in the later half 2013 when I get a simple dividing head done from Harold Hall's prints and the first complete set of cutters from UK.Same cutters are Chinese Made. Hope they last.

Way back 1981 ,went to the Leblonde Singapore Lathe Plant to make pre-delivery inspection of the 18" Swing x 8" Precision Lathe. Was shown how they cut precision gears and lapping.Plus "running in" gear box. Opportunity missed. The Gear Cutting Module Supervisor was a Canadian with many years of gear cutting experience. Same would have been my best "Guru".

Gus Teng


----------



## Tin Falcon (Mar 29, 2013)

pdf book that can be downloaded
Gear Cutting in Theory and Practice Joseph Gregory Horner

http://books.google.com/books?id=U7ZKAAAAMAAJ&oe=UTF-8
Tin


----------



## Mainer (Mar 29, 2013)

I think I may have gotten the numbering of the cutters reversed (what I said was #8 is really #1, etc.) so don't let that confuse you.


----------



## kf2qd (Mar 29, 2013)

With a universal mill you can use 1 cutter to cut all gears. The cutter is called a hob and is a spiral rack profile and the spindle and the indexer arbor have to be mechanically synchronized. Cutter is both more complex, and simpler, but the setup is more complex.

Though, in this day of stepper motors and computers it is really quite simple. Either mechanical gearing (the old way, needs lots of gears) or electronic gearing (the new way. all the gears are in software...).

Here's an example.
[ame]http://www.youtube.com/watch?v=ZhICrb0Tbn4[/ame]


----------



## Mainer (Mar 30, 2013)

This link http://homepage.ntlworld.com/peter_harrison/workshop/gearcutting/index.htm describes what might be called an incremental hobbing method of cutting spur gears.


----------



## stew (Apr 18, 2013)

HI,
Thanks for asking the Question SMG, I was thinking about this the other day and not really understanding it. I actually ordered The gear & Gear Cutting Book No. 17 of eBay a few days ago. Hopefully it will give Me a better understanding still.


----------



## chrsbrbnk (Apr 18, 2013)

I have problems with the module   thing  trying to relate it back to regular cutter numbers


----------



## Fluffy (Apr 19, 2013)

To bring module gear sizes to DP equivalents divide 25.4 by the module size, eg. 25.4/0.6 = 42 DP approx.
I hope this helps.
Don.


----------



## parisharma1 (Apr 19, 2013)

Very Informative post I appreciate your post.


----------

