# Gears...



## precisionmetal

Hello all,

New to this forum. I'm in California, and my passion (as well as job part of the time) is wire edm work. Specifically, I enjoy developing and cutting gears via wire edm. Solving gear-related design problems or reverse engineering replacement gears is my forte. Needless to say, I also cut many things via wire edm that have nothing to do with gears at all.

Am always happy to help if I can provide anyone with geometry, CAD models, or anything else pertaining to gears.

A few photos of some fun stuff:

Photos 1 and 2 are of a 34 tooth 140 DP 20° PA gear, with a 9 tooth 80 DP 45° PA internal spline. This gear was cut from solid carbide.

Photo 3 is a 40 DP 20° PA pinion which was cut complete including the D-shaped bore from heat treated 440C.

Photo 4 is 2 Module 20° PA gear that was a replacement for a broken gear in an Italian-made wine bottle corking machine.

Photo 5 is 64 DP 20° PA ring gear cut from heat treated 15-5 stainless for a prototype planetary gear set in a small robotic vehicle.



























Pete


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## Powder keg

Nice work Pete!!! I've built a couple of gears. My 1935 LeBlond was missing a few teeth. The small one was fun. It had about a 5/8" 6 spline in the middle and the OD on the gear was about 1.125" if I remember right.

Thanks for sharing)


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## ChooChooMike

Those are some complex EDM calculations - very nice :bow:

A warm welcome to HMEM Pete !!

Mike


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## John S

Pete,
What gear software do you use ?

John S.


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## precisionmetal

Thanks for the welcome here!

John,

I use a few different programs when creating a gear profile -- most everything (other than a CAD and a CAM program) is custom written.


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## vlmarshall

Great stuff. I keep wishing for a wire EDM at work... no luck yet. ;D


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## doc1955

Welcome 
I'm pretty new here too. I have enjoyed setting back reading and taking note. Where I work we do very little with gear cutting I have done a few but not that small. I'm very impressed. We have a wire edm here we had a sinker edm but have gotten rid of it.
Do you work with cad now?
If so which cad system do you use? I use to work in the tooling shop but now work as a tool design engineer. I use NX6 and CATIA V5 and also FeatureCam or now known as DelCam.


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## 1hand

Real nice Pete. Kinda gives me a headache trying to imagine working with stuff that small. Welcome. th_wav


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## precisionmetal

Doc,

I've used a number of different CAD apps, but have to say that anything by Ashlar is probably my favorite. 

Of course there is always the old saying:  _"What's the best CAD program in the world? ..... the one you're most familiar with!"._  ;D


1Hand,

I enjoy making small parts, I admit. I have yet to really push the limits on size -- I _think_ I might be able to go to 200 DP on gears, but may have to have the moon in the right position before I can pull that off! The small gear above (first two photos) was cut using 70 micron wire (.0028"). That's already 30% smaller than my machine is rated for, but with a bit of tweaking I've had some pretty good success with that size wire. I've actually run 50 micron wire in my machine before, but it will take more work to get that to run reliably. The tension and power settings get so low with wire that small that everything has to be "babied" to pull it off.

Here's another small part I made (I think I made about 25 of these).  Looks simple... but this customer required every dimension to be ±.0001". I had the material centerless ground to ±.000050", then sliced off the "discs" in the wire machine. Then wire cut a fixture right in place and never moved anything -- inserted the blanks and cut them.

Fun stuff!

But I have to say that some of the model engines I've seen on this site make anything like this look trivial. There are some _phenomenally_ talented people here, and I'm really enjoying going through some of the posts and reading about what's been built.


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## doc1955

You are right with the saying I've heard that before. The reason I was asking is I usually attend except for this year the Seimens UG users groop meetings and though maybe by chance you used UG and attended also. Year before last it was in LongBeach CA matter fact 2 years in a row. Last year it was in Nashville and next year again in Nashville.
Anyway I'm totally amazed we run .005 wire dia. I don't believe we run any other dia.
I've never got to run the wire edm machine but have designed several holding fixtures for them.

Simply impressive!


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## precisionmetal

Aha,

No... not a UG user. 

You'll have to jump in there and play with the wire machine if you get a chance!


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## Deferr

WOW! I am amazed by this.


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## BillC.

140 dp? Where's my microscope? 

Wire edm's are the greatest invention in machine work ever! The only method that rivals it is electron beam wire fed fabrication - but when finished the part still needs to be finish machined.

Good stuff!!

Bill C.


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## precisionmetal

More gears...

Cut these for a buddy who is working on a ¼-scale engine project. Crank gear (the small one), 2 cam gears, 2 idler gears and 1 magneto gear. The two 1/16" holes in the cam gears are for driving the cam (only one hole will be used). He wasn't sure yet whether he wanted the drive hole on a tooth, or on a gullet, so I put in one of each. Those gears have 40 teeth, so the drive holes are 175.5° apart. The crank gear has a .250" bore with a 1/16" key. Since it wasn't possible to put a _keyway_ in the gear, I just integrated the key into the part.

Gears are 64DP 20°, with the tooth thickness reduced about .0007" from standard in order to give each "pair" about .0015" total backlash. Material is A-2, heat treated to 55Rc.


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## Powder keg

Those sure are nice!


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## cfellows

How long does it take to make one of those gears with EDM?

Chuck


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## precisionmetal

Chuck,

The larger gears (40 teeth, .625" PD) took about 55 minutes each.

That includes wire cutting all 3 holes (.1250" in the center and the two .0625" holes), and then the gear.

I took 3 passes on the holes because I wanted plus/minus .0001" or less. The outside of the gear was done in one cut.

Pete


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## vlmarshall

What's the average amount of overburn with .0028" wire in a finish cut?


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## precisionmetal

Vernon,

On a _finish_ cut (i.e. the last of a few trim cuts), the overburn will be near zero... maybe 50 millionths at the most. Or to put it another way, on the final trim pass the "offset" will essentially be the radius of the wire, or maybe 50 millionths over.

On a _single pass_ cut, overburn will generally be around 25%-35% of the diameter of the wire. In other words, if one were using .010" wire and cut straight through a piece of 1" thick plate, the width of the cut would be around .013" or just over. 

Is that the question you were asking?


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## vlmarshall

precisionmetal  said:
			
		

> Vernon,
> 
> On a _finish_ cut (i.e. the last of a few trim cuts), the overburn will be near zero... maybe 50 millionths at the most. Or to put it another way, on the final trim pass the "offset" will essentially be the radius of the wire, or maybe 50 millionths over.
> 
> On a _single pass_ cut, overburn will generally be around 25%-35% of the diameter of the wire. In other words, if one were using .010" wire and cut straight through a piece of 1" thick plate, the width of the cut would be around .013" or just over.
> 
> Is that the question you were asking?



Yes, that's exactly what I was wondering, thank you!


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## Dan Rowe

Pete,
I have read a few of your posts on HSM and it is very clear that you know a considerable amount about gears and EDM. 

My question is simple can EDM be used to make a bevel gear? 

Dan


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## John S

Dan,
Take a look at this out of interest.

[ame]http://www.youtube.com/watch?v=RV8Dq6mslnE[/ame]

John S.


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## Dan Rowe

JohnS,
Very interesting. Was that done with a solid model and a ball end mill with a 3 axis cnc machine?

I have Mach3 and can draw what I need as a solid model. What did you use for CAM?

Dan


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## John S

Yes, solid model off the web, I thought it was the SPI site but can't find the model I used again.

Saved as an STL file, opened up in Vectric's Cut3D program, roughed out with 6mm ball nose and finished in two passes at 90 degrees to one another with 2mm ball nose.

The video is actually on the finish pass, that why it doesn't look to be cutting, never thought to video it on the initial path.

Just standard CNC bench mill on 3 axis, the chuck is just work holding bolted to the bed.

Took about an hour but the stepover was quite course, I just wanted to prove the concept instead of spending 4 or 5 hours with a small step over only to find it didn't work.

John S


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## John S

Found some gears on the Rush Gear site, [ no spiral bevels thought ]
Took a normal bevel and ran it thru Cut3D and got this.







John S.


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## shred

Cool stuff. I'd think there's a limit or relationship to to how small a CNC cutter has to be and the DP of the gear to be produced by it.  Say a 1/16" or 1mm cutter. What gears and/or DP's could that generate reasonably? It seems like the limit would be the space in the 'notch' of a gear (I've not got the proper name for the bit-the-tooth-goes-into handy)


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## Dan Rowe

shred  said:
			
		

> I'd think there's a limit or relationship to to how small a CNC cutter has to be and the DP of the gear to be produced by it.



The DP sets the size of the cutter for spur gears, and there is a reasonably priced cad/cam solution called Involute written by Graham Baxter.

It is a bit more complicated with a bevel gear because the DP describes the big end of the gear. The face length of the gear is what sets the small end which is what is needed for cutter diameter.

The reason I am making my own gears not using commercial ones is the face length of the bevel gears for a Shay locomotive are longer then current gear practice. 

John S, thanks for that screen shot that is one of the programs I have been considering.

Dan


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## woodchip85

Dan Rowe  said:
			
		

> Pete,
> I have read a few of your posts on HSM and it is very clear that you know a considerable amount about gears and EDM.
> 
> My question is simple can EDM be used to make a bevel gear?
> 
> Dan


As to your question yes, Wire EDMs can be used to create bevel gears, dependent on the size and distance of the taper, with abit ingenuity I've even cut helical gears on a wire EDM. The main things to consider when having any gear wire cut is the cost, a gear can take minutes or hours dependent on the thickness of the part,accuracy and surface finish.


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## precisionmetal

Dan Rowe  said:
			
		

> My question is simple can EDM be used to make a bevel gear?
> 
> Dan



Dan,

A straight bevel should be possible, yes. I have never cut one, however I have made a gear shaper cutter before -- a generating cutter like used in a Fellows gear shaper (actually just cut a section of one to prove the concept).

In a simple sense: if each "end" of a profile can be defined, then it can be wire cut. Needless to say, a bevel gear would require the blank to mounted at an inclined angle on a dividing head in the wire machine, but that's not an issue -- I have a 3R dividing head that I can mount at any angle in the machine.

PM


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## Schack

precisionmetal  said:
			
		

> More gears...
> 
> Cut these for a buddy who is working on a ¼-scale engine project. Crank gear (the small one), 2 cam gears, 2 idler gears and 1 magneto gear. The two 1/16" holes in the cam gears are for driving the cam (only one hole will be used). He wasn't sure yet whether he wanted the drive hole on a tooth, or on a gullet, so I put in one of each. Those gears have 40 teeth, so the drive holes are 175.5° apart. The crank gear has a .250" bore with a 1/16" key. Since it wasn't possible to put a _keyway_ in the gear, I just integrated the key into the part.
> 
> Gears are 64DP 20°, with the tooth thickness reduced about .0007" from standard in order to give each "pair" about .0015" total backlash. Material is A-2, heat treated to 55Rc.




Greetings All,
This is my first time posting on the board. Here are some shots of the model that I am working on. I have to say 
without Pete I would not be able to build this exactly to scale. He is a good friend and craftsman. I am starting with the running engine and gearbox. Then the rest of the bike. I have the basic short block finished. I am working on the heads this week. Hopefully I will have the models done tomorrow and in the machine this weekend. I need to make the cam followers (more work from Pete) and the valve gearing will be finished. Then carb and ignition. 
I hope you all enjoy. I will keep you posted. 
Thanks again Pete.






The real deal in Morro Bay Ca.






Without the fairing.






The model engine. Notice the gears.






More of the model on the full size bike.


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## Andrew Stewart

Help with DP (Diametral Pitch)?

Browsing catalogs on involute gears I see the sizes are quoted in dp and module, how does someone starting out decide what cutters to purchase?

I imagine many parameters have to be taken into acount such as diameter, material, torque just to name a few.

A higher dp I imagine allows for better accuracy but lower torque, these are only my thoughts.

Can someone enlighten me on this subject? Is there any articles, post dvd books that you can recommend?

Thanks In Advance
Andrew


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## Dan Rowe

Andrew,
Gears are math in metal. To understand things like DP, module and circular pitch you need the formulas. These can be found in most catalogs. The download section has a good gear book that was uploaded by Steamer which is a good place to start the study of gears.

Module is a metric gear and DP is imperial. The circular pitch can be expressed metric or inch.

Dan


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## mklotz

Gear formulae...

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

module = 25.4/DP


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## Andrew Stewart

Thanks For The Information Provided So Far.

I question I am asking is how does one decide what DP cutter to purchase.

For example say I want a 2:1 reduction gear, the larger blank being 4 inch OD.

Diametral Pitch = (Teeth + 2) ÷ Outside Diameter

6 = (22 + 2) / 4
6 = (11 + 2) / 2.166


11 = (42 + 2) / 4
11 = (21 + 2) / 2.090


16 = (62 + 2) / 4
16 = (31 + 2) / 2.0625


24 = (94 + 2) / 4
24 = (47 + 2) / 2.0416

How does one decide whether to cut your gears with a 6, 11, 16, 24 DP cutter for example?

I imagine there are many parameters that have to be taken into account?
i.e.
Material: Nylon, Aluminium, Mild Steel
Power: 0.1 KW, 100 KW, 1000 KW
RPM: 1 rpm, 100 rpm, 1000 rpm, 10,000 rpm
Application: swiss watch, chaff cutter

What are the advantages of using 6 DP over a 24 DP cutter, visa versa.

Thanks in advance


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## Dan Rowe

Andrew,
To calculate the strength of gear teeth the Lewis formulas are needed see:
http://www.roymech.co.uk/Useful_Tables/Drive/Gears.html
http://courses.washington.edu/mengr356/daly/Gear_stress.pdf

Dan


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## kuhncw

This is a very non-scientific answer and it is aimed at model engine construction since this is a model engine group. I pick DP and buy cutters by "what looks about" right with respect to engine scale. My engines are between 0.75 inch and 1.312 dia bore. I have 32 DP cutters and 48 DP cutters. Gear loading is very light. These small teeth look right and run fine. 

If you are designing gearing to transmitt significant power then my method is of little value. 

I don't mean to discourage you from studying the technical details of sizing gears. I just wanted to give you the approach that I suppose a lot of model builders use in our lightly loaded applications.

Regards,

Chuck Kuhn


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## precisionmetal

In the case of the 1/4-scale Indian engine mentioned earlier in this thread, the gears were simply "scaled" from the originals.

Andrew, unfortunately you ask some questions that can not be easily answered. There are so many variables in gear design that the subject could go on forever. Chuck's answer, along with a ... "use what's available to you" is probably a place to begin.

If you required a precise ratio of 2/1, you will undoubtedly find it easier to achieve with a higher DP, however if you need to transmit a lot of power, a numerically lower DP would be desirable. 

PM


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## djc

Andrew Stewart  said:
			
		

> The question I am asking is how does one decide what DP cutter to purchase.



To add to the other excellent replies; you need to consider the number of teeth in the smaller gear in the set.

Your example shows that for a 6DP gear, the small gear would have 11 teeth: this is too small to cut with a conventional cutter (12 is usually the smallest you'd go).

Big teeth on a small blank also limit the size of shaft that can go through the middle.

Cutters aren't generally available in odd-numbered DP, hence 11DP is unusual; in the module system, they generally move by increments of 0.25.

Finally, it's more conventional to work with the pitch diameter rather than the blank diameter as this makes determining the centre distance of the pair considerably more intuitive.


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## Andrew Stewart

Thankyou for your replys.

They have been most helpful

Andrew


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## Andrew Stewart

Just a question about buying "involute" or "hob" gear cutters.

I see specifications for:

1. Class "AA" "A" "B" Ground profile & "C"
2. IS, BS, DIN and ASA

Can anyone point me to or explain what these options are when buying a gear cutter and what are the most commonly used?

Thanks in advance.
Andrew


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## precisionmetal

Cut this gear over the weekend -- a sample for someone building a very tiny scale racecar model.

254 DP - 20° PA - 42 teeth.

This was done on an Agie wire edm machine using .0028" diameter wire.

Material was a piece of .025" Starrett shimstock.


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## dsquire

precisionmetal 

I am glad that you took the close up with the penny because at real size I would have thought that there was a speck of fly s4!t on it. That is amazing, great work. :bow: :bow:

Cheers


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## Niels Abildgaard

Hello PM

Can You use a wire edm machine as a measure thing?
I am asking because I want to make a better parting tool.
(Please se under specialist section)
It is obviously possible to glue ,brace or weld buisnes part of a 2 mm Tang-grip insert holder unto a suitable 3.4 mm prismatic steel bar but it lack elegance.
Will it be possible to use the wire to obtain the geometry for wire sparking a new seat from an existing blade?
We mustasume that the prismatic angles of the seat is 15 degree and rest is 0


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## precisionmetal

I'm sorry, but I don't understand what you mean by _"use a wire edm machine as a measure thing"_.

As far as cutting -- if the shape can be defined in a computer, I can cut it (within limits of course -- too large it won't fit in the machine, and if the part is so small it can't be seen, that might be a problem as well). 

PM


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## Niels Abildgaard

Excuse me for being unclear.Not first time and with luck not last either.

What I wanted was to use the EDM as a copy machine.
First using the wire to measure a master form and the duplicate that on a virgin piece of metal.

It is for making a special parting blade holder explaned in a thread close to this.I have asked a friend if this is possible and we will try it tomorrow.

I will post pictures if succesfull.


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## precisionmetal

I suppose that's theoretically possible, however I'd think it would be far more efficient to optically inspect your current part and then create a CAD model from that inspection data.


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## Ken I

Wire Cut EDM is also my weapon of choice for small gears.






This is a 10T x 64 DP gear (used for Slotcar racing).

When wire cutting the external gear the wire cut MUST start in the middle of a tooth crest as the end leaves an unavoidable "spike" that must be removed - if this is on a tooth flank the gear is useless.

Using EDM you can also cut from pre-hardened and tempered stock.

Also for small numbers of teeth you can moderate the tooth profiles by generating them on Autocad - to eliminate the interference problems.

You can also do non-standard gears such as this 7T 64 DP below.







The profile to the left is a theoretical "Rack-Cut" involute profile - note the severe undercutting in the dedendum and the root diameter is too small in any case (had to run on a 2 dia shaft) - so I generated the profile for an oversize PCD using the 38T gear profile as the "generator".






The illustration above shows the generation steps in a CAD drawing (the hard way).

Once you have developed your profile you go directly via dwg, dxf Solid Edge etc direct to your wire cut machine.

I generally don't approve of non-standard gearing but with small numbers of teeth or through dint of circumstance its sometimes the only thing you can do.

Regards,
      Ken 

View attachment Dump.dwg


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## precisionmetal

If the situation allows it, I will often enter/exit the cut dead center in the bottom of the gullet.

Then just stop the machine a tiny bit before the profile is finished and apply a clamp to hold the gear in position (and give conductivity to the drop) and finish the cut. More often than not, it's near impossible to see where the cut started and stopped.

Many ways to skin the cat.


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## Ken I

I'll try that next time - thanks.

Regards,
      Ken


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## DTANNER

WoW I have only been using a EDM machine for a few months and you pictures have told me, I have a long way to go. I use it for a varity of things but know I must try a minature gear. If you have time I would like to see some more receant things you have managed to cut.  :bow:


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## precisionmetal

Here you go -- toward the other end of the spectrum:

A rear end spool out of a Trophy Truck. Customer was stepping up in axle size, but wanted to continue using an existing spool. Part was fully heat treated.

The spool, the axle and the drop in the first photo, and just a photo of the drop in the second photo as a size reference.


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## Ken I

That's one long cut - I presume one or two finishing passes ? - what was the total cutting time ?

Ken


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## precisionmetal

As I recall it took half a day or so. One skim pass after the drop was out.


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## popnrattle

Hello, you may be interested in a video I uploaded to youtube of a procedure to draw the components of a planetary.(Both external and internal TRUE INVOLUTE with backlash gears using Autocad '04). A 9-tooth Sun, an 18 tooth planet(3), and a 45-tooth ring(internal). I would make lots of gears if I had one of those wire EDM's. ;D Later, Rick.

[ame]http://www.youtube.com/watch?v=Hvfytr7DuGk[/ame]


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## Ken I

Thanks for the video - very illustrative. KP for effort.

How does your CAD develop the involute ? I see how you generate a series of points on the involute path - what happens then - do you use the cubic spine interpolation ?

My very old CAD (steam driven Ver 10) turns everything into an approximation via a number of circular segments - but that's still very close.

Also I see you didn't bother with the corner root radii in the clearance - on very small gears the wire cut can be bigger than this and you need to allow for it (or use a smaller wire - a PITB) on bigger gears you would want the more generous rad for fatigue reasons.

Ken


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## popnrattle

Thanks for the kind comments. I connect the "dots" with the "SPLINE" command. It is a curve with no center point. Yeah, depending on the material you can put fillets. At times I have used a full arc to connect teeth at the root as the following link shows:

http://s489.photobucket.com/albums/rr258/popnrattle/?action=view&current=SHEETMETALGEARS.jpg

These laminated gears show the root as being the diameter of a 1/8"dia. drill. I drill all of the roots before I machine the teeth. You may notice the ring gear segments(internal teeth) whose roots had to be drilled to accomodate external tooth tip clearance. I made these laminated gears to insure alignment. Thanks for your input and interest. Later, Rick.


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## cfellows

This is probably not news to anyone else, but a couple of days I came to the realization that an internal gear can be made by taking a regular gear and using it as a template for cutting out the waste in a blank to form the internal gear. In other words, the part that is taken out of a disk to form an internal gear looks exactly like a positive gear of the same pitch, number of teeth, etc. Wonder if it would be possible to use a small brass gear, for example, as the profile on the end of an EDM tool to cut an internal gear? 

Chuck


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## Ken I

Chuck,
     Yes and no - a pinnion shaped EDM electrode would need to be "rolled around" A'La a Fellows gear shaper to generate the shape.

Besides we are talking wire cut EDM so its moot.

You could spark erode (plunge) use a properly profiled electrode - but this would not be the same profile as a pinnion.

However if I want to generate the wire cut path on CAD I would indeed generate it from the pinnion.

Think of all gears as a "rack" - then bend the rack to radius of your gear.

My explanations oft degenerate to the clarity of mud on rereading them.

Ken


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## Maryak

Ken I  said:
			
		

> My explanations oft degenerate to the clarity of mud on rereading them.
> 
> Ken



Nah, I don't believe that for a moment 

Best Regards
Bob


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## Charles Lamont

Also for small numbers of teeth you can moderate the tooth profiles by generating them on Autocad - to eliminate the interference problems.

You can also do non-standard gears such as this 7T 64 DP below.

The profile to the left is a theoretical "Rack-Cut" involute profile - note the severe undercutting in the dedendum and the root diameter is too small in any case (had to run on a 2 dia shaft) - so I generated the profile for an oversize PCD using the 38T gear profile as the "generator".

I generally don't approve of non-standard gearing but with small numbers of teeth or through dint of circumstance its sometimes the only thing you can do.

[/quote]

As I understand it, the normal way of reducing undercut is 'addendum modification', which is subtly different from what you are doing. The pitch circle remains the same but you increase the addendum of the smaller gear and reduce the dedendum, and vice-versa on the mating gear. This will give a slightly different profile.


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## Ken I

Charles,
      Two different problems

1) Non-Standard pitch circle - in the case of the 7T 64 DP gear it had to go over a 2mm shaft so an oversize pitch circle was used.

The modified PC approach is used when circumstances dictate you cannot use the correct theoretical value (shaft size, axle pitching etc.).

2) Interference when using small tooth count pinnions - Can be solved by addendum modification or as you pointed out by my slightly different regeneration method - there are probably a number of tooth profile modification methods.

Either way, the moment you deviate from theoretical you introduce an element of "sliding" - relative motion of one tooth face to the other at point of contact - as opposed to the correct "rolling" motion of one tooth against the other with no relative motion at the point of contact.

Which of course leads to greater friction losses and wear problems.

I was not advocating pitch circle enlargement as a means of getting away from interference.

If you have a pair of ruined gears - all you need is the number of teeth and the axle pitches and everything else follows - some people are amazed by this resurection of manufacturing data from apparently little to go on - however when a non-standard approach has been used, all bets are off - hence my dislike of non-standard gearing.

Ken


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## precisionmetal

As an FYI -- the point where undercut happens on an external gear is...

2 / (sine of PA)^2 &#8804; = number of teeth where undercut happens

(Two, divided by the sine of the pressure angle squared)

17 teeth on 20°
32 teeth on 14.5°

PM


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## Todd...

this might be a  dumb question, but i just got a dividing head to try and make a new gear for my lathe.  when counting the teeth of the gear, what is being counted the tooth itself or the void in between the teeth?

also i see the angles of the teeth mentioned a lot in gear making, i am going to be using my shaper and grinding a form cutter by using the old gear.  are the angles as important when reproducing a gear like this? or just when making one from scratch

are there any online articles or literature explaining some of these things more? i have the machinerys handbook 28, and get the ideas and theory of it but am looking for something more applied rather then theory

Thanks
Todd


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## Tin Falcon

> this might be a dumb question, but i just got a dividing head to try and make a new gear for my lathe. when counting the teeth of the gear, what is being counted the tooth itself or the void in between the teeth?


well I always count teeth but do not think it matter unless I am miss remembering the # of teeth and spaces are equal. 

if you are trying to copy an existing gear just coy it as close as you can and do not worry about numbers . be careful not to copy wear. I have a couple of acquaintances  that were handed parts and told make like this. and they reproduced them to every detail. This included the wear that made the original part useless and in need of a new replacement. 
so an understanding of the original specification can be valuable info.
Tin


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## precisionmetal

Todd,

Hopefully every gear you check has the same number of teeth as it has spaces!  

Are you referring to "pressure angle" when you ask about angles?

PM


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## Todd...

pressure angle i guess, as well as DP, how can i measure an existing gear to find out what DP it is? i have read through machinery handbook and understand the theory of the different measurements of the gear, but how do i obtain them?


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## Tin Falcon

The proper method of measuring gear teeth is a gear tooth caliper. 
again if you are making a copy of the existing gear just make it match the old one.   
Tin


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## Ken I

Measure the OD of the gear and the root diameter (arkward if an odd number) subtract one from the other and divide by 2 to get total tooth height (including clearance) divide that by 2.139 (the .139 being the clearance) and that will give you the Module of the gear - if that turns up a metric value like 1mm then you have a metric gear.

If not divide it (in inches) into 1 to get the DP. If neither gell try CP - same as module but in inches like 0.1" etc.

Once you have established the basics of the gear - ie the number of teeth and its module / DP and this tallies with the diameters - you've almost got it.

Next check out the pressure angle - that is the angle that would exist for a rack or a gear with an infinite number of teeth - most modern gears are 20° and older being 14½° - generally the a 14½° looks a lot "skinnier".

However a wide range of angles is possible (but unlikely).

There are ways to determine this using rods as measuring wires (use drill shanks etc.) - consult your handbooks.

Regards,
            Ken


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## precisionmetal

A quick and dirty way to get very close is to simply measure the base pitch.

Pick 2 "numbers of teeth" where it's clear you can get the jaws of a vernier onto the *profiles* of the teeth (don't touch the tip or the root).   e.g.  3 teeth and 4 teeth, or 6 teeth and 7 teeth, etc.

Span one "set" of teeth, and zero the calipers, then check the other "set" (whether one tooth up or down).  Read the caliper.  That's the base pitch.

While the base pitch will not give you the DP *or* the Pressure Angle, it *will* point you to the combination.   In other words, there is a given DP/PA combo that will yield a certain base pitch.  If you have a good idea that a gear is 20°PA by looking at it, then the base pitch will show you what DP combines with 20° PA.

Base pitch is the magic number around which all gear design is based... even if it's a term that's not often used.

The great thing about using base pitch as a measurement is that it doesn't matter if the gear was cut on an oversize blank, if the OD was reduced for some reason, if some non-standard cutter was used, etc.  Base pitch still shows the DP/PA combo that did (or could) produce that gear.

... and more interesting yet is, you can use a different combo DP/PA to arrive at the same gear.  e.g.  Say you have a gear that is 16DP 30°PA.  That has a base pitch of .1700".  An 18DP 14.5° gear has a base pitch of .1691"  (.0009" off).  Pretty darn close.  So if you happened to need an 18DP 14.5° gear, and you only had a 16DP 30° cutter (I'm speaking about hobs or shaper cutters now, that "generate" the tooth form), you could probably get by.



PM


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## MachineTom

Measure the OD of the gear, the more teeth the better, even teeth is also better. count the # of teeth, add 2 to the number of teeth, then divide the number of teeth +2 by the diameter, the result will be the DP of the gear. This for 20° PA gears, which is a large % of all gears

example OD of gear 1.042, number of teeth 48.  t+2=50, 50/1.042=47.98~48dp


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## popnrattle

The video demonstrates how I draw the gears I need to make from metal or import sketches into 3-D software to prototype them with an ABS plastic modeler.

[ame]http://www.youtube.com/watch?v=xjQ9R7-vEvs[/ame]


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## viumau

Wonderful work

https://www.youtube.com/watch?v=1rckGCN1kwY


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