# Gear cutter question



## V 45 (Dec 27, 2015)

I'm having an issue trying to figure out what involute cutter to buy to make some gears.
What I'm trying to accomplish...one gear with 40 tooth with O.D. of .824", another 20 tooth with O.D. of .431", these mesh together. Then another set that mesh one with 15 tooth O.D. of .444" and the other is 27 tooth O.D. of .758".
 I have not cut any gears prior but I do have a rotary table with a index attachment that I can use. 
  Any help is greatly appreciated.
 Thanks again...  V 45


----------



## dnalot (Dec 27, 2015)

Hi

This might help

http://scriptasylum.com/rc_speed/two_speed.html


Mark T


----------



## V 45 (Dec 27, 2015)

That's a good for connecting gears but ... the ones I'm trying to make don't all mesh together. Just the 2 each in separate areas. I'm looking for the cutter size if I stated it wrong.
 Thanks again


----------



## gus (Dec 28, 2015)

Hi V 45.

The Involute Gear Cutters comes in a set of 8 per Module.  Each cutter will cut a range of tooth count and no more or less. For example No. 1  will cut 135 teeth to a rack. No. 2 will cut 55-----134. Please refer to Gears and Gear Cutting by Ivan Law. Chapter 9 page 65. With the correct cutter ,the cut gears will mesh.  By now there are about 10 HMEM Members cutting DIY Gears for our 4 stroke Gas Engines. See attached table foto. Gear cutting is not difficult but be prepared to scrap some gears as part of learning curve. Scrapped DIY Gears make very good paper weights or fishing sinkers . Book by Ivan Law is recommended for beginners.


----------



## Cogsy (Dec 28, 2015)

Rather than buying cutters you can easily make a single cutter per DP that closely approximates the correct involute shape. Lots of info on calculating, making and using them at this link. Lots of us have used them and they work just fine for timing gears, etc.


----------



## xpylonracer (Dec 28, 2015)

Hi V45

From the info given it seems your gears are not exact commonly used DP, closest to the 40T and 20T is 48DP, this DP would need diameters of .875" and .458" respectively. To cut these you will need 2 different cutters.
For the 15T and 27T gears the closest commonly used DP is 36. Diameters for these 36DP gears is .472" and .805" and again you will need 2 cutters.

FYI DP=(N+2)/OD  where N=number of teeth
so  OD=(N+2)/DP

Checked again and seems likely your 40 and 20T gears are MOD .50, the other pair don't come exactly MOD .6 but are close. If you don't make your own cutters you still need to buy 4 cutters, could make them expensive for just a one-off.

Emgee


----------



## steve-de24 (Dec 28, 2015)

V45, you don't say where in the world you are located but here are my thoughts on your gears,

I think your 40 and 20T gears could be metric Module=0.5
For the 40T gear :  OD = 42 x 0.5 = 21mm = 0.827"
For the 20T gear :  OD = 22 x 0.5 = 11mm = 0.433"

And the 15T and 27T gears could be 38DP
For the 15T gear : OD = 17/38 = 0.447"
For the 27T gear : OD = 29/38 = 0.763"
And before anyone says 38DP is non-standard, I agree, but it was the standard DP used by Meccano, a very popular machine modelling kit in the UK aimed at kids of all ages.

Regards,
Steve


----------



## Charles Lamont (Dec 28, 2015)

Rather than giving the ODs of the gears, Is the centre distance between the shafts fixed? If so, this would allow a more accurate suggestion to be made.


----------



## kvom (Dec 28, 2015)

Charles Lamont said:


> Rather than giving the ODs of the gears, Is the centre distance between the shafts fixed? If so, this would allow a more accurate suggestion to be made.



Agree with this.  If you're trying to replace some old gears with fixed shaft positions and required ratios, then the shaft distance is the critical factor.  There may be pitches and tooth counts that will do what you want with cheaper cutters.


----------



## canadianhorsepower (Dec 28, 2015)

xpylonracer said:


> Hi V45
> 
> From the info given it seems your gears are not exact commonly used DP, closest to the 40T and 20T is 48DP, this DP would need diameters of .875" and .458" respectively. To cut these you will need 2 different cutters.
> For the 15T and 27T gears the closest commonly used DP is 36. Diameters for these 36DP gears is .472" and .805" and again you will need 2 cutters.
> ...


I'm a little confuse here:wall:

you are saying OD=(N+2)/DP 
and on the site that  Cogsy sujest they are not.


----------



## V 45 (Dec 28, 2015)

Hello all,
  Thanks for the help !!
Here's some more dimensions that might help.
Center of shaft to shaft for the 40T (Teeth cut .047") and 20T is *.589"* (Teeth cut .048") 
Center of shaft to shaft for the 27T (Teeth cut .068")  and 15T is* .550"* (Teeth cut .065")

These are fixed dimensions of the shafts the gears would be on. This is on a print and not making replacement gears but new for a project. The original print was scaled down to fit my small machines. 

Also added my location.


----------



## kvom (Dec 28, 2015)

You are getting into the realm of watch gears.  My math might be a bit suspect, but here goes:

1) The pitch circle radii of the two meshing gears together equals the shaft separation, or .589
2) The tooth ratio is 2:1, so the larger gear's radius is twice the smaller.  Thus 3*B = .589, B=.196
3) The resulting PCD for the large gear is .784 and the smaller is .392
4) This results in a diametrical pitch of 51.

As stated earlier, a .5 module cutter = 50.4DP would be close enough.  Note that the tooth depth is 2/DP = .04".

You might also choose a smaller number of teeth.  30 and 15 would be 38DP.  The teeth would be larger.


----------



## xpylonracer (Dec 28, 2015)

Hi LUC

I quote OD (outside diameter) the highlighted part in the article posted is describing circular pitch (PCD).
OD is more useful because that's the calculated dimension of the blank gear with a known DP and number of teeth.

Emgee


----------



## V 45 (Dec 28, 2015)

The highlighted figures are the O.D (Outside diameter).


----------



## xpylonracer (Dec 28, 2015)

Afraid not, highlighted figures refer to Circular Pitch not OD, check the drawing again and you will see the arrow pointing to CP which is almost centre of addendum and dedendum.

Emgee


----------



## V 45 (Dec 28, 2015)

I highlighted the number. On the drawing the arrow is pointing to the very tip of the tooth. I attached the page I'm working on. 

View attachment SHEET 12 Gears.pdf


----------



## xpylonracer (Dec 28, 2015)

The highlighted text is in an earlier post by Canadianhorsepower (another forum member), there is confusion because I think you mean I am referring to your *BOLD* text for the OD of your gears in your post.

Emgee


----------



## V 45 (Dec 28, 2015)

Sorry for the confusion.

Still looking for help with a cutter !
 Thanks again


----------



## steve-de24 (Dec 28, 2015)

V45, the drawing explains a lot.  Because this design is a scaling of a previous design the gears have ended up being defined by 
(a) number of teeth
(b) outer diameter
(c) depth of tooth.
This is very non-standard way of doing things but ok if you are making your own gears ( and your own gear cutters if you want to be exact!).
Close enough cutters would be metric Module = 0.5 for the timing gears and DP = 38 for the kicker gears (if you can find them). 

Alternatively you could play around with the design a bit (by changing tooth numbers and minor changes to the distance between centres) and see if it is possible to use something like DP=40 cutters for both sets of gears. I'm assuming DP=40 will be easier and cheaper to obtain in USA.

Steve


----------



## V 45 (Dec 28, 2015)

I have a cutter drawing,  it's been almost impossible for me to make.  That's why I am looking for a commercial made cutter. I've made several but it's still not right.


----------



## Cogsy (Dec 28, 2015)

Have a look at the site I posted, the cutter/'hob' is just a straight rod with several v's cut into it, then some flats are milled to form the teeth. The most difficult bit (once the math is complete) is grinding the lathe tool the correct angle. Here's the link again.


----------



## V 45 (Dec 30, 2015)

Here's the type of cutter I've been trying to make. I've made several but none the right shape.How Do I cut a R.100 (100 degree radius) on a lathe tool bit?


----------



## kvom (Dec 30, 2015)

What is the purpose of the mechanism?  If constant velocity or a perfect mesh isn't needed you could cut triangular teeth much easier with a single cutter.


----------



## V 45 (Dec 30, 2015)

It's for the Hoglet engine gears. .. reduced

Edit: Here's what I tried today. It doesn't seem to be working out. The cutter has a 3/32" radius, it's the closest I could do with tools available. Which equals .093 radius. I used index circle 18 on my rotary table plate. Using 3 turns plus 6 spaces. It's a 90-1 table. I'm using a 5/32" lathe mandrel and used a .155" reamer for the center hole. I turned a aluminum blank to try it. The blank actually started to slide down the mandrel. I have the thicker end towards the tail stock. I was thinking about putting a metal tube over the mandrel to stop this. The project gears will be brass.  Thoughts?
It posted the pics sideways !!


----------



## V 45 (Dec 30, 2015)

Any help with this?
 Thanks again


----------



## steve-de24 (Dec 30, 2015)

The mandrel holding the gear blank is too flexible - it needs to be as rigid as possible.  I would make one similar to the one you have holding the cutter.  Diameter sticking out of the three jaw should be a piece of bar just smaller than the diam across the roots of the gear teeth, then a shoulder down to the gear shaft diameter - a good fit on that diam - then a thread for a nut. Between the gear blank and the nut you need to make a big washer - outer diam same as the bar. Make sure everything nuts up hard with no interferences in the corners, undercut the corners if necessary. A centre at the end of the bar for the tailstock as well.   
Make sure the direction of the cut the gear cutter is making is towards the shoulder on the bar.

Take a look at gear cutting set ups on the web.  Make it as rigid as possible.

Good luck
Steve


----------



## AgfaClack (Dec 30, 2015)

I used this author's setup; he describes the "rig" in detail after a few pages. Shows how to make a single-tooth cutter pretty quickly, and some other things.  I had a good time doing it.  

http://users.picknowl.com.au/~gloaming_agnet/cq9325rev7.html

I like the way you made your cutter; that's another way.

I used the Ivan Law book too.  There's a good computer program to download, referred to on Steve Bedair's site.  I used it to make sure I had the gear-blank's diameter correct, and it worked for me.

Also see myfordboy's vids on YouTube, he uses the Ivan Law book.  Which I downloaded on Scribd.

Pete


----------



## gus (Dec 30, 2015)

AgfaClack said:


> I used this author's setup; he describes the "rig" in detail after a few pages. Shows how to make a single-tooth cutter pretty quickly, and some other things.  I had a good time doing it.
> 
> http://users.picknowl.com.au/~gloaming_agnet/cq9325rev7.html
> 
> ...



Hi Pete,

Ivan Law's book is a must have and must read for first timers/newbies going into gear cutting. I am in the very last prepping stage to start Howell V-2 Engine which had so many gears to cut and mesh with pin-point center drilling. The gear vendors usually run out stock of the particular gears we want and as such we end up with DIY gears. Been fun cutting gears. Happy New Year from faraway Singapore.


----------



## Jasonb (Jan 6, 2016)

Just looked this thread up after your query elsewhere.

As Kvom says us 0.5MOD cutters.

Your 0.589" ctr to ctr distance is 14.96mm or near as dam 15mm

20T PCD = 10mm

40T PCD  = 20mm

Distance between ctrs  = (10 + 20 ) / 2 = 15

You also give a gear blank dia for one of the gears of  0 .431" this again is basically 11mm 

To get the OD of a gear the calculation is (Number of teeth +2) x MOD

So we get (20 + 2) x  0.5 = 11


----------



## petertha (Jan 6, 2016)

V 45 said:


> I highlighted the number. On the drawing the arrow is pointing to the very tip of the tooth. I attached the page I'm working on.


 
I'm not sure if an additional complication might be 'pointing' to a tooth edge vs. defining the diameter. I'm referring to the 0.444" and 0.431", but I also see a corner fillet radius of 0.010". Depending on the cad program & how the dimension are picked/defined, it could be picking up center to fillet edge (which is shorter than) center to nominal tooth circle OD?


----------

