# Involute gear cutter ?'s



## kcmillin

Hello everyone, do any of you guy's (or gals) know where i can get a good all around set of involute gear cutters. I know there is a set a parameters for them when you order them, but thats all greek toi me. I have had limited success in the past with homemade gear cutting bits from hss, but i never used any sceince, I would just do it by eye. Including the number of teeth. nothing was certain. 
i dont want to get to technicle with it 
i just want a set of cutters i can play with and figure it out. anything from 1/4 inch diameter gear with 5-10 teeth up to 2-3 inches in diameter.

Please help, i am incredibly jealous of the most of you :bow:, and would like top try my worth by making amazing projects like yours.

thanks,
kel


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## steamer

http://www.victornet.com/results.html

Their prices are reasonable....MSC has them also

www.mscdirect.com

A full set will cost you some coin...better to settle on a specific set of gears and maybe 1 or 2 cutters.....


Dave


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## Deanofid

Hi Kel;



			
				kcmillin  said:
			
		

> I have had limited success in the past with homemade gear cutting bits from hss, but i never used any sceince, I would just do it by eye. Including the number of teeth. nothing was certain.
> 
> kel



The first thing that really jumps out here is this; "I would just do it by eye. *Including the number of teeth*."
Simply put, you will never get a decent gear by cutting them by eye. Gear teeth must be accurately and precisely spaced, and there are no exceptions to that rule that I know of.
This is why you see people using an indexing fixture of some sort for these chores, whether it be a simple spindle with index plates, a dividing head, or RT, or even using another gear as an indexer.

You can buy the best set of cutters in the world and your gears will not come out any better if you think you are going to space them "by eye". Sorry.

To the cutters, you need to prepare yourself to spend some money if you want cut the range of gears you mention. There are a number of cutters in a set that will cut only one pitch on a given set of gear sizes. Those sizes are specific, and you generally cannot just pick a random size and just start cutting teeth on them. If you want to cut a different gear pitch, it means a whole new set of cutters.

I know you said you don't want to get technical about it, but there are some basics that simply cannot be ignored. Before we lay down a load of stuff you don't want to hear, do you wish to continue?

Dean


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## kcmillin

I do have a pretty accurate rotary table and the spacing is always the same, as long as i eyed the first one i knew the steps. I made a crude clock doing this, along with some aluminum gears for a v-4 steam engine, it spins over 4500 rpm no prob. 

But yes 
I am interested in furthering my knowlede in the matter, i figure if i had success doing this haphazerdly, i should be able to pick up a few basics.

So to answer your question; 
Yes, lay it on me!
 thanks,
kel


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## hammers-n-nails

get the book "gears and gear cutting" by ivan law. its not very long and will tell you how to make gears that will work 100x better than guessing. its $7.69US at amazon. it even tells how to make your own gear cutters.


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## Deanofid

Okay Kel. 
Get the book mentioned in the previous post. I've never seen it, but it seems to be the one people like, judging by how often it's mentioned.

These are the basics I use for my gearcutting. 

Diametral Pitch, what we usually just call "pitch". You need to now this in order to size your gears for your projects. Different pitches produce different sized gears for a given tooth count. All gears in a train have to be the same pitch to mesh. Common pitches are 16, 24, 32, 48, 64, and there are lots of others. The smaller the pitch number, the larger the teeth on the gear will be. 

The pitch of a gear is a constant, so you can't just pick some random diameter and decide to put a certain number of teeth on it. The pitch and number of teeth you need for a given gear will dictate the diameter of the gear. What that means in real life is that when you design an engine, you don't get to pick the distance between things like a camshaft and a crankshaft. The gears will determine that distance, and you design around it.

To get a smaller or larger gear for a given amount of teeth, you choose a pitch that fits. You don't always get to pick your exact diameter. You pick the pitch that most closely matches the diameter you need. All other gears in that train have to be the same pitch.

Some examples;
Say you want two gears, one about 1/2" diameter with 20 teeth, and one about 1" diameter with 40 teeth.
If you choose 24 pitch for your gears, you won't even get close. 
A 24 pitch gear with 20 teeth will be .916", and you can't change it. A 24 pitch gear with 40 teeth will be 1.75".

So, to get closer to the size you want, to go to a higher pitch, which will make the teeth smaller, reducing the size of the gear for the same amount of teeth.
If you choose a 32 pitch, the 20 tooth gear will now be .687" diameter and the 40 tooth gear will be 1.312".  The gears are getting smaller, but still have the same amount of teeth because you are changing the pitch.

There are other pitch numbers you might try, to see if they give you the exact diameter of gear you want, but it usually doesn't work out that way. You just have to pick what is close, and build your project around those measurements where they are concerned.

To figure out what the diameter of a gear will be for a given pitch and fixed tooth count, use this simple equation.

Diameter = Number of teeth, plus two, divided by the diametral pitch (DP, or pitch)
For a 40 tooth gear with a 24 pitch it looks like this:
40T+2/24DP = 1.75"

You can use that to find out how large your gear blank needs to be so when you cut the appropriate number of teeth on it, they will come out spaced exactly right. No left over spaces, or half teeth, etc.

To figure how deep to run your cutting tool into the gear blank when cutting teeth, use the constant of 2.157. Divide the gear pitch into that number and it will give you the depth of cut for gears of that particular pitch.

Learning to make your own cutters may well prove worth your while, once you've seen what follows.

Each pitch of gear takes a different size of cutter, of course, since different pitch means different sized teeth. The big deal about this comes from the fact that a cutter is not only unable to cut any pitch other than what it was designed for, but that it will only cut a certain tooth count range on a gear. To be able to cut all the gear sizes in one gear pitch, such as 24DP, you need eight cutters. If you want to cut 32 pitch gears, none of your 24DP cutters will work, and you will need another set of eight cutters to cut 32DP. It can add up quick.

The cutters are made to cut a certain range of tooth numbers. A 24DP no. 8 cutter will only cut 12 or 13 tooth gears. If you want to cut 14 to 16 teeth of that same pitch, you have to buy another cutter. If you buy a whole set of eight, you can cut from just a few teeth, (12, I think) to as many as you want. But only in that one pitch, unless you buy another set.

You have an RT, so you can divide a circle into any number of teeth you want, for cutting your gears. Make sure you know before hand what all the different settings on the RT should be for all the tooth locations on the gear, before you start. Work it out on paper, and keep a list of subsequent settings near the mill as you cut.

Make sure your gear cutter is set dead center to the gear blank. If you don't have it right, it will show up in the gear having teeth that appear to lean to one side, and they will mesh tight, or not at all.

If you need to know more about this, ask. The few basic things here will tell you how big to make your gear blank, how deep to cut the teeth, and how to figure different sized gears for a given pitch.

Read up on it some, and don't get freaked out by the stuff you don't really need to know. There seems to be more math theory to it than actual machine shop practice. You don't need to know the same stuff as an engineer who gets his jollys drawing out involute curves, but many books include that type of thing.  Learn how to calculate the proper diameter for a given gear pitch/tooth count, and the correct depth of cut, and you will get along.

Dean


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## kcmillin

;D ;D
thank you VERRY much for this info> I was totally lost before. 
I am so glad I found this forum. 
 you know I did not even know what a metal lathe was until I bought one at a Cummins tool show. and for 5 years I used a drill press with an x-y table on it. I finally bought a used mill/drill this fall from a local hobby shop owner. and WOW WOW WOW I did not know what a mill actually could do. I never seen one in action till I bought it. and then a month ago I finally broke down and got the internet, its an incredible wealth of knowledge that I wish I had a long time ago. 

Once again THANK YOU Dean for the great info, and I will definatly check out that book. 

Thank you my internet pals,
kel


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## Deanofid

You bet, Kel. 
There are others here that know a lot more about this stuff than I. If you have more questions, a good place to put them is in the "Questions and Answers" section nearer the top of the forum home page. I think more people read that part of the forum, so you might get more answers.

You've got the enthusiasm for the job!

Dean


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## 1hand

Dean, You Hobb any gears yet? That video I got shows how to make them and use. 

Matt


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## Blogwitch

I know this post is about involute cutters, but I thought I would just mention another method, using gear hobs.

Rather than having a 'set' of cutters to do all sizes, a hob uses just one cutter to do the whole range. They work out cheaper buying just one cutter rather than the whole 'set'.

When eventually I can get a round tuit, I will be experimenting with these hobs to cut gears very easily on the mill. I have the system worked out, and if theory is to be believed, then these hobs will cut any size gear in a matter of minutes.

This is a hob of the correct size to cut any size of change wheel on my lathe.







And how they are held on the mill. It isn't the hob that is the difficult bit, it is getting it to the correct cutting angle and holding the blank. But if my calculations are correct, it will be a very easy method of cutting gears.






Hopefully later on in the year, I might be able to shed a little more light on it.

This is where I obtained my hobs from.

http://www.arceurotrade.co.uk/Catalogue/Cutting-Tools/Gear-Hobs

I will be using module sizes (metric), but some normally used DP (imperial) sizes are available as well.

I got my ideas from watching the Jose Rodriguez's DVD, obtainable from here

http://littlemachineshop.com/products/product_view.php?ProductID=2057&category=

And a box of matches to hold my eyelids open while watching it. He has a very 'put you to sleep' drawl. I would suggest watching it in stages.


Blogs


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## Majorstrain

Taking this thread a little further off the garden path,  




			
				Blogwitch said:
			
		

> And how they are held on the mill. It isn't the hob that is the difficult bit, it is getting it to the correct cutting angle and holding the blank. But if my calculations are correct, it will be a very easy method of cutting gears.
> 
> Hopefully later on in the year, I might be able to shed a little more light on it.



I look forward to your insight Blogs, 

What has my mind on a bender is that when we make our own hobs the teeth are in line, but a lot of the hobs I see on the net have the teeth in a helix like a drill.
This is evidenced in Blogs picture above. If my eyes aren't lying to me.

Can you use this type of hob to do a straight cut gear? or are they for helical gears.

I've shied away from buying any yet, not knowing the answer. I'd do the Google, but hearing from someone who has done it would be better,

Cheers
Phil


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## Jasonb

The teeth on bogs hobs are indeed cut like a helix, in fact it is possible to use a tap to do the same thing.

The action of the helix rotates the gear blank as it is cut so you do not need a means of indexing the teeth, just the correct OD to suit the PCD of the gears. This rotating action also cuts the correct profile on the teeth which needs to be more curved on smaller tooth count gears, thats why with involute cutters you get about 7 in a set to cover the range of tooth numbers.

This is one in use

[ame]http://www.youtube.com/watch?v=-7Kzy8OySFw&feature=related[/ame]


Jason


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## kcmillin

It seems like the gear hob in that video needs a special maching that turns the gear at the same rate as the cutter is cutting. All I have is a manual mill. Can I use hobs without spinning the blank? 1 toothe at a time I mean. This seems like a much more cost effective way to make gears.


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## hobby

I read so much on the innerent about diametral pitch, and always still came away with not an explanation of what it really is, but only equations. I love algebra, and solving equations, but I wanted answers to WHAT diametral pitch is all about in producing gears.

Denofid:

THANKYOU for taking the time to explain all that you did in reply #5.

That was the best explanation, of *what* diametral pitch is and how to work with it.


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## ksouers

Hobby,
I'm no expert but the best I've been able to deduce is the DP is analogous to the TPI of a thread.


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## hobby

kcmillin  said:
			
		

> It seems like the gear hob in that video needs a special maching that turns the gear at the same rate as the cutter is cutting.



According to the video put out by Jose rodrigous, the gear blank is loose on it's own arbor, and the cutter just traverses the blank as it is cutting.

He did his on a lathe set up.

He also shjowed how to use a straight hob (non helical), and his mill with an indexer, to make a gear one tooth at a time.


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## hobby

ksouers  said:
			
		

> Hobby,
> I'm no expert but the best I've been able to deduce is the DP is analogous to the TPI of a thread.



Thankyou, that is the best way to remember it's use in making gears.


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## Majorstrain

Jasonb said:
			
		

> The teeth on bogs hobs are indeed cut like a helix, in fact it is possible to use a tap to do the same thing.
> The action of the helix rotates the gear blank as it is cut so you do not need a means of indexing the teeth, just the correct OD to suit the PCD of the gears.
> Jason



Thanks Jason, :bow:
Cheers, that cleared it up for me. 
Phil


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## 1hand

And a box of matches to hold my eyelids open while watching it. He has a very 'put you to sleep' drawl. I would suggest watching it in stages.


Blogs
[/quote]

I agree! It would be nice if Swarf Rat would come out with a gear cutting how to video. Seems their videos are far more exciting. I have both threading videos, one from Swarf Rat, and Jose. Swarf Rats far more interesting, and you get the cool software with their videos. I dropped Swarf Rat an email to included a Gear cutting video to their collection, with software to figuring gears.

Bit off topic here, but not too far........LOL

Matt


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## Blogwitch

I just put my bit as an insight into another method, not to hijack this thread.

But just to clear it up so that it is understood, the partially pre slashed gear blank (done with a slitting saw and dividing head) will freewheel, and the driven gear hob (sitting at an angle but with the helix teeth perfectly horizontal) will drive the gear blank, as previously mentioned, while it is also cutting the gear teeth horizontally across the blank's edge.

I don't want to start a debate on it, as it would spoil this already informative post, and also the doubters will come out of the woodwork, just as they did with the swing up screwcutting tool (and they were proved to be emphatically wrong). But I would prefer the idea to be tried and tested a little before being put into open discussion in a post of it's own.

Blogs


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## kcmillin

wow. hobbing is easier than i tbought. I was certain I needed specilized, and hard to obtain, equiptment.
thanks guys.

kel


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## mklotz

> To figure how deep to run your cutting tool into the gear blank when cutting teeth, use the constant of 2.157. Divide the gear pitch into that number and it will give you the depth of cut for gears of that particular pitch.



Dean,

I twitched when I read that since my GEARSPUR program uses 2.2/DP to calculate the working depth. I assumed that anyone as clever as you hadn't made a mistake but, OTOH, I didn't make up the number I used.

So, I did a bit of research (ain't the web fantastic?) and discovered, via...

http://www.engineersedge.com/gear_formula.htm

that we're both right and also that we're both a bit wrong.

It would seem that the right calculation is to compute the working depth, W, as:

W = 2.2/DP   if DP <= 20
W = 2.157/DP  if DP > 20

then:

A = addendum = 1/DP
D = dedendum = W - A

I'll be changing my program to include the calculations above.

This may sound like nit-picking to a lot of the readers but, if you're going to do something as intricate as gear making, you might as well have the correct formulae. It's particularly important with my program since I have no idea who might use it and for what purpose.


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## hobby

mklotz  said:
			
		

> This may sound like nit-picking to a lot of the readers but, if you're going to do something as intricate as gear making, you might as well have the correct formulae. It's particularly important with my program since I have no idea who might use it and for what purpose.



NOT to worry,
That is NOT nit-picking, but showing good responsibility,
it shows that you care about the other guy...


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## bob ward

There is one thing about cutting gears with hobs that I've never quite got my head around.

I assume that as well as setting the helix hob at the proper angle to the blank, there needs to be a side feed so the hob traverses the face of the gear.

Have I got that right?


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## cfellows

bob ward  said:
			
		

> There is one thing about cutting gears with hobs that I've never quite got my head around.
> 
> I assume that as well as setting the helix hob at the proper angle to the blank, there needs to be a side feed so the hob traverses the face of the gear.
> 
> Have I got that right?



Yes, and a way to spin the gear so it rotates in sync with the screw action of the hob. If the gear blank is gashed well enough with an indexing head, the screw action of the hob can turn it, like a worm gear.

Chuck


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## Deanofid

mklotz  said:
			
		

> Dean,
> 
> I twitched when I read that since my GEARSPUR program uses 2.2/DP to calculate the working depth. I assumed that anyone as clever as you hadn't made a mistake but, OTOH, I didn't make up the number I used.
> 
> So, I did a bit of research (ain't the web fantastic?) and discovered, via...
> 
> http://www.engineersedge.com/gear_formula.htm
> 
> that we're both right and also that we're both a bit wrong.
> 
> It would seem that the right calculation is to compute the working depth, W, as:
> 
> W = 2.2/DP  if DP <= 20
> W = 2.157/DP if DP > 20
> 
> then:
> 
> A = addendum = 1/DP
> D = dedendum = W - A
> 
> I'll be changing my program to include the calculations above.
> 
> This may sound like nit-picking to a lot of the readers but, if you're going to do something as intricate as gear making, you might as well have the correct formulae. It's particularly important with my program since I have no idea who might use it and for what purpose.



Marv, good catch! Digging through a few books before coming to the computer to address this post, I found nothing other than the 2.157 constant I had used as an example. Then, shuffling through my folder of printed off pages, I found the same page from your link.

That link you provided must have been there for quite some time, as the copy I printed off was from about 10 years ago. Strange that other printed matter from known sources skipped the part about using different constants for certain pitches. 
Admittedly, I have only ever used the 2.157 constant for all my gears over the years, but might as well do it right! 

What that one says is what you have here, (and duly noted). It puts it slightly different, and I think bears repeating as they have it. (In other words, it's easier to understand for people with brains like mine.) The pitch system on involute gears is inverted in respect to tooth size. Big tooth size = smaller pitch number, and vise-versa, so;

Whole cut depth, (doc) for 20 pitch and finer, use the constant of 2.2/P +.002.
Whole cut depth, (doc) for coarser than 20 pitch, use the constant of 2.157/P

The difference isn't much, but as Marv says, there is a difference, and we're talking gears here. Getting it right is the goal. If we are machinists, or if we want to be, we should do our best to follow prescribed formulas, especially when cutting things like gears.

Dean


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## mklotz

The program has been updated and is available for downloading.


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## Blogwitch

> Yes, and a way to spin the gear so it rotates in sync with the screw action of the hob. If the gear blank is gashed well enough with an indexing head, the screw action of the hob can turn it, like a worm gear.
> 
> Chuck



Like Chuck says, and a very good explanation, but he missed a few words off the end. "as the cutter traverses along the face of the blank".

I am looking to make blanks 3" to 4" long, maybe even longer. Only the first intitial cuts will require the slashing. As the cutter starts to 'bite', I am hoping that the slashing will not then be required. I am contemplating long series of gear blanks, and then just parting them off to whatever thickness I require.

IF, and I emphasise the word IF, it works, then I should be able to make gears with whatever number of teeth that are required and unlimited quantities for my use in a fraction of the time it will take to cut just one DP cutter gear. All that will need to be done is the blanks to be turned to the correct size for the number of teeth required, the hob and depth of cut then takes care of everything else. Of course the feed along the blank will come into it, but once I have the power feed set, things should become fairly well automated.

Isn't R & D exciting? A bit like a box of chocolates, and some chap called Gump.

Blogs


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## cfellows

Blogs, 

I'm no expert by any means, but in cutting a 3 or 4 in blank, I would worry about about the hobb creeping slightly as you traversed the face of the blank. In other words, seems to me like you might wind up with a gear that is very slightly helical. Of course, the accuracy might still be plenty good for your needs. Just thinking out loud.

Chuck


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## Blogwitch

Chuck,

That is where getting cutting angle of the hob spot on is the main concern, as I mentioned before. That will also control the width of tooth as well. I am hoping to make a jig that will ensure the hob teeth run perfectly parallel to the table

I am personally not too worried about the gears being slightly skew, in fact they can be really badly out and still run together fairly well. In the sort of work we are doing, they will be a lot more than adequate.

These gears I will be making aren't intended for life threatening applications, just a way to save money, rather than going out and buying from a gear vendor. I have already calculated out that I only need to make four pairs of gears for the hobs to pay for themselves. Also, using these hobs, it should also be able to easily make internal cutting hobs as well, which then opens up a whole new world on saving money. Have you seen the price of internal gears recently? 

I already have things worked out and plans afoot to cut internal gears on the mill, and if things work out as expected, I might even use the mill to power the cutting action, rather than by hand.

But getting the external gears cut is the priority at this time.

Lots to do, but no time to do it in.

Blogs


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## cfellows

Blogs,

I will be very interested in seeing your method for cutting internal gears. The only way I've ever figured out to do it is either by shaping or broach.

Chuc


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## Powder keg

This has been posted here before. Have a look.


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## Blogwitch

Chuck,

It will be a shaping exercise, using cutters made in the form of an outside gear, where teeth either side of the main cutting tooth preform part of the next tooth to be cut.

As I said, still in the design, planning and thinking stage yet, so I won't get too involved just yet. 

Why waste thoughts and talk until the first stages are tried out.

When a little further into making it, then will be the time for discussion, and maybe a change of ideas.

All this was discussed a while back, when I first started to enquire about ways to cut internal gears, so rather than repeat everything, you can read all about it until I got to the stop point, which is where I am still at.

http://madmodder.net/index.php?topic=1913.0


Blogs


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## cfellows

No problem, Blogs, I can wait. Unfortunately, not much info on internal gear tooth shape.

Chuck


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## cjparsons

Thank you for your comprehensive and helpful information - I am experimenting with gear cutting and have bought a Mod 0.5 20 degree #6 34-54 tooth cutter? (I tend to work in metric, I am in the UK)

I also found some information on this site http://khkgears.net/gear-knowledge/gear-technical-reference/involute-gear-profile/

My calculations are the diametrical pitch for this cutter is apparently 50.8 (inches?) - so to cut a 40 tooth gear the diameter of the blank would have to be 42/50.8 = 0.82677 inches or 21mm and the depth to cut the teeth would be 2.157 / 50.8 = 0.04246 inches or 1.0785mm (which doesn't seem very deep??)

Does this look correct?

Thank you

Chris


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## RM-MN

cjparsons said:


> Thank you for your comprehensive and helpful information - I am experimenting with gear cutting and have bought a Mod 0.5 20 degree #6 34-54 tooth cutter? (I tend to work in metric, I am in the UK)
> 
> I also found some information on this site http://khkgears.net/gear-knowledge/gear-technical-reference/involute-gear-profile/
> 
> My calculations are the diametrical pitch for this cutter is apparently 50.8 (inches?) - so to cut a 40 tooth gear the diameter of the blank would have to be 42/50.8 = 0.82677 inches or 21mm and the depth to cut the teeth would be 2.157 / 50.8 = 0.04246 inches or 1.0785mm (which doesn't seem very deep??)
> 
> Does this look correct?
> 
> Thank you
> 
> Chris



This calculator works well with metric gears.  http://www.technobotsonline.com/gear-size-calculator.html


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## cjparsons

Excellent - thank you very much

...and even better my calculations were correct 

Regards

Chris


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