# Helical gear problems - Help



## Lagerbolzen (Sep 29, 2020)

Good afternoon guys
I’m based in the UK and have built the Chuck Fellows helical gear jig to try to machine some replacement gears for a smiths Chronometric Speedo gearbox using my Bridgeport without tilting its head.   
im trying to cut a 12 tooth 24DP 35degree helical gear with 0.625 blank diameter. 
So reading the cosine rule I’m using a no5 cutter 24DP ( theoretical 21.8 teeth)
I’ve used the GTBritnell centre setting method also. 
but the first cuts I take seem very wide. I’ve angled the fellows spindle 35 degrees up from horizontal.    
did I do something obviously wrong here With the angles maybe ??


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## Drawfiler (Oct 19, 2020)

I notice that you have had no responses to your post so here goes, I am not familiar with the method you are using but I think the helix at the tear of the mandrel is to turn the work as it advances.
i would look at 3 areas, 
1 are you using the right cutter Too low a tooth count will be too wide.
2 are you sure that you have 35 degree angle right
3 if above are OK have a look at the lead on the helical guide.


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## Misterg (Oct 19, 2020)

Lagerbolzen said:


> I’ve angled the fellows spindle 35 degrees up from horizontal.
> did I do something obviously wrong here With the angles maybe ??



I'm only familiar with the technique in theory - I haven't done it (yet).

If it's any help to cross-check your numbers, I've calculated the pitch diameter as 0.610" (so your blank diameter sounds about right). The angles look correct (to me). The only missing information is the lead on your helical guide - I calculate it should be 2.74" (pi x pitch diameter / tan[pitch angle] ).


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## Mechanicboy (Oct 20, 2020)

Learned out this is fault done of you.


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## gbritnell (Oct 20, 2020)

Sorry I didn't reply sooner but I just saw the thread. I had the same problem when I started making my first gear. If you cut on one side of the gear blank it will make an elliptical groove. If you cut on the other side it will make the proper cut to form the sides of the teeth. 
gbritnell


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## gbritnell (Oct 20, 2020)

Here's the video I made when I cut my first helical gears. You'll notice the side that the cutter is on to cut the teeth. If you move the cutter to the other side of the gear blank you get the elliptical shape I was talking about.

gbritnell


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## Steamchick (Oct 20, 2020)

Mechanic Boy, G.Britnell, and anyone else: Thanks for showing us the complexity of machining these gears. (New technology for me). I am looking for a pair of helical gears to translate the crankshaft rotation to camshaft rotation on the classic horizontal single engine as per Crossley et al. 
I haven't found anywhere on the web to get small enough gears. I am looking for the crankshaft gear to be about 10~12mm or 3/8"~ 1/2" OD with complimentary camshaft gear (same diameter?) - perpendicular to the crankshaft (6mm diameter shaft), but off-set by the gear centre distance - to run at half the rotational speed of the crankshaft. If anyone wants to make something, suggest a price and I'll have a think? I don't want to impose on anyone for a "freebie" - as I don't think it is fair. We all have costs of tools and materials, and time and heartache. But I don't want to buy a whole gear cutting set-up for 2 gears every decade... when there are others who enjoy making gears. 
I have made custom ceramic burners for those who prefer to buy, and charge a price to cover cost and a little more so I can buy materials for my next home job. It's a pleasure - for me -developing a new burner design. So anyone with a need for a "special" burner, please ask.
Ken2


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## gbritnell (Oct 20, 2020)

Hi Ken2,
Send me a PM and we'll talk about it. If I have a home-made cutter that will work for the needed pitch that will save a lot of time and money. 2 helical templates need to be made, 1.5 hours. Cutting the gears, 2 hours with tool setup etc. Shipping to you. I'm close to Cleveland, Ohio U.S.A. Maybe one of the fellow modelers in the U.K. could help you out. If not then I would be happy to make them for you. 
gbritnell


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## Mechanicboy (Oct 20, 2020)

With small gear cutter = no problem to mill the gear against the blank gear wheel in conventional milling when you are pushing the blank gear wheel against the gear cutter without the ball bearing are jumping out of cam disc. 

With large gear cutter, it can be more hard to mill against the gear wheel when you are pushing against the rotating gear cutter in conventional milling, the risk then the bearing ball are not on the cam disc in the whole time 
instead the gear cutter is in climp milling against the blank gear wheel and will hold the ball bearing on the cam disc in the whole time.


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## Lagerbolzen (Oct 20, 2020)

Thanks everyone,You we’re exactly correct I was cutting on the wrong side of the blank. 
now The technique works fine. 
thanks again to all
Mike


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## Drawfiler (Oct 21, 2020)

I can make them for you on my gear hobber, the long part of the job is doing the calculactions for the gear trains and I do have a lot on just now so you would have to be patient!


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## Misterg (Oct 21, 2020)

Lagerbolzen said:


> I’m using a no5 cutter 24DP ( theoretical 21.8 teeth)



How does one chose which # cutter to use for this technique? I had assumed that it would be the one for the actual tooth count (#8 for 12 T in this case).

The other possibility I had in mind was the cutter should match the tooth count of a spur gear the same PCD as the helical (24 x 0.610 = 14.6T which would be #7).

Neither of these give the result above... Would someone mind pointing me to the correct calculation? (I don't recall seeing anything on this in the original CF notes...)

Thanks.


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## gbritnell (Oct 21, 2020)

Don 1966 made a spreadsheet which gives the proper cutter for a given amount of teeth. Here's the simple explanation. Kind of. If you set a helical gear with the teeth down on a flat surface and looked at the center line of the gear. The exact shape of the tooth at that point would be the same as a spur gear. (Involute shape) Now as the point on the tooth moves away from that exact center it starts to curve away following the helical lead of that gear. When using an involute cutter the diameter of the cutter is large enough that it trims a little bit of material from the sides of the tooth away from center therefore not producing the proper involute shape. To create a closer involute shape a cutter having the shape for more teeth than what you are cutting has less of a curved side so as it's cutting an elliptical path it creates a closer shape to the side of the tooth. When I cut my helical gears I use home-made cutters no more than .50 O.D. so I can use a cutter with approximately the proper involute curve as the needed tooth count. 
A spur gear with the same diametral pitch should mesh with a helical gear of the same D.P. At the center point of the tooth. 
If one studies the math and construction of a helical gear it starts to make sense. 
gbritnell


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## Richard Carlstedt (Oct 21, 2020)

Just a general question George, is there a relationship between the cutter diameter
and the workpiece diameter ?  (Ratio?) 
 I can see that a large(r) cutter will wipe  more of the tooth walls and tooth form gets wider  and your cutters are quite small.


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## Misterg (Oct 21, 2020)

gbritnell said:


> Don 1966 made a spreadsheet which gives the proper cutter for a given amount of teeth. Here's the simple explanation....



Found it, thankyou! 

Here: Login

(Please let me know ASAP if it's not the done thing to link to other forums, and I'll edit the message while I still can.)

I can follow the logic of your explanation, thanks very much. Trying to calculate it is something else! I see from the spreadsheet, the equivalent number of spur gear teeth comes from  #Teeth on helical gear / ((cos(helix angle))^3) - it gives the 20.8 tooth equivalent for the original gear in this thread.

Yay! another piece of the jigsaw falls into place


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## SmithDoor (Oct 21, 2020)

You find helical gear can be pain on mill.
The large manufacturers use mostly gear shapers set at 30°. On mill everything must calculated out and miss just little you are SOL.
So machine shops make just little deeper cut and hope no one finds out.
There are tricks that makes replacement gear easier.
The other is use same pitch thread as well known mill with charts.

Dave



Lagerbolzen said:


> Good afternoon guys
> I’m based in the UK and have built the Chuck Fellows helical gear jig to try to machine some replacement gears for a smiths Chronometric Speedo gearbox using my Bridgeport without tilting its head.
> im trying to cut a 12 tooth 24DP 35degree helical gear with 0.625 blank diameter.
> So reading the cosine rule I’m using a no5 cutter 24DP ( theoretical 21.8 teeth)
> ...


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## gbritnell (Oct 22, 2020)

Hi Rich,
No relationship at all. I don't have many involute cutters so I make my own. To get the right tooth form for say 32 DP and have the cutter be strong enough I use .50 Dia. drill rod. For really small pitches I use .375 D.R. I will generally make the cutter one number up (profile) than what that particular gear (number of teeth) requires. I just made a 1/3 scale 1953 Ford 3 speed manual transmission with all helical gears (7) and I only had 1 gear that was a little tight. It turned but was tight so I remounted it on my fixture, aligned it, and took a couple of thousands off of it.
gbritnell


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## gbritnell (Oct 22, 2020)

Hi Dave,
The helix angle for a mating pair of gears is determined by the center to center distance. To maintain the proper CtoC distance the helical angle would need to be modified for a given pitch.
gbritnell


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## Misterg (Oct 22, 2020)

Richard Carlstedt said:


> I can see that a large(r) cutter will wipe more of the tooth walls and tooth form gets wider



I've found another discussion that goes into more detail on cutter choice with an exact(?) calculation which includes the pitch diameter of the cutter:





__





						Helical Gears  - Theory and Practical | Model Engineer
					






					www.model-engineer.co.uk
				




Seems the consensus is that the N/Cos^3(a) approximation is 'close enough' in most cases.


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## coulsea (Oct 22, 2020)

the more i read the more confused i get. when i first looked at spur gears i was just as confused until i finally worked out that using metric module involute cutters i only needed to know the outside diameter of the blank for a given number of teeth and how deep to cut, the rest of the formulas were irrelevant.
I now want to make a side shaft engine and helical gears would seem to be needed. i should be able to make the chuck fellows jig. i don't really care if the gears are not technically perfect as long as they work.
I think that i would be right in saying that if i made two gears with 45 degree helix one being 20 tooth and the other 40 it would work and i would have the 2:1 ratio needed for the exhaust valve. the problem is that the 40 tooth gear takes up too much space so having one closer to the size of the 20 tooth would be good.
So finally my questions 1) will two 20 tooth gears, one with a helix angle of 30 degrees and the other 60 give a 2:1 ratio and will they mesh with each other.
2) are the angles in Q1 too far apart to work and if so do you need to get some of the ratio from number of teeth and some from helix angle. 

Andrew


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## gbritnell (Oct 22, 2020)

Hi Andrew,
By going with the 2 different angles you can get the gears almost the same outside diameter but you will still need twice as many teeth on one as the other. 
Attached is my drawing for the gears I made for a tiny side shaft hit and miss engine. The pitch is tiny but if you look at the gears and information that will give you a better idea of what you are looking. at. Using the spreadsheet that Don1966 published all you have to do is plug in you diametral pitch, tooth count and helical angles and it will give you all the information you need.


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## teeleevs (Oct 23, 2020)

Oh Wow,  for our side shafter we used a 9 tooth Suzuki crankshaft gear which was more like a 9 start thread than a gear, it is much larger than the 18 tooth dizzy gear.  (Now you are more confused than ever ) we cut the crankshaft gear in half so now we have 2 narrow ones and put one away for future job.  We had measurements and angles of the dizzy gear and cut it in brass without trying to helix it, filed it a little and it runs ok. Don't know if I have photos. 
Ted from down under


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## gbritnell (Oct 23, 2020)

The reason the 9 tooth gear is larger than the 18 tooth gear is because as you increase the helix angle the gear gets larger. If you would take a cut through the 9 tooth gear at right angles to one tooth it would have the same profile as a spur gear of the same diametral pitch. So to evenly space this tooth you are calculating the hypotenuse of the triangle, therefore the steeper the helix angle the larger the gear gets to evenly space the teeth and spaces.


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## SmithDoor (Oct 23, 2020)

You may want a app for Helical gears

Dave 









						HelicalGear Calcula Engrenagem - Apps on Google Play
					

Calculator helical cylindrical gears and spur diameters




					play.google.com
				






Lagerbolzen said:


> Good afternoon guys
> I’m based in the UK and have built the Chuck Fellows helical gear jig to try to machine some replacement gears for a smiths Chronometric Speedo gearbox using my Bridgeport without tilting its head.
> im trying to cut a 12 tooth 24DP 35degree helical gear with 0.625 blank diameter.
> So reading the cosine rule I’m using a no5 cutter 24DP ( theoretical 21.8 teeth)
> ...


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## L98fiero (Oct 24, 2020)

SmithDoor said:


> You may want a app for Helical gears


Don't speak Portuguese or Spanish☹


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## gbritnell (Oct 24, 2020)

An app is not necessary. Download the spreadsheet from Don1966 and it will do everything you need to calculate the gear information. 
gbritnell


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## SmithDoor (Oct 24, 2020)

May want to try Don1966 or get a app for Portuguese or Spanish



L98fiero said:


> Don't speak Portuguese or Spanish☹


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## SmithDoor (Oct 24, 2020)

L98fiero said:


> Don't speak Portuguese or Spanish☹


I did try app last night and could not find English 

Dave


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## coulsea (Oct 24, 2020)

This is another good spreadsheet


			http://www.hercus.com.au/uploads/29161/ufiles/Gear_Calculations_rev3.xls


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## coulsea (Oct 30, 2020)

I have been making the chuck fellows jig and am at the stage of cutting the main shaft to length and thought that i would run a few numbers through the spread sheet to check the shaft length and am not sure if i am doing it right.
I want two gears (about 20mm dia)to run at right angle with a 1:2 ratio 1 being the crank and 2 the cam and i am using a 0.5 mod metric cutter.
so in the spread sheet i have *Gear *40 teeth 30 helix angle dp 50.4 helix lead = 4.9 inches *Pinion* 20 teeth 60 helix angle dp 50.4 helix lead = 1.4 inches
My question is how does a template with a 4.9 inch helix lead fit on the attachment or did i make a stupid mistake in the spreadsheet.


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## teeleevs (Oct 31, 2020)

We began with a factory made 9 tooth gear from a Suzuki jeep engine, photo 1 , we only had to copy the 18 tooth distributor gear, zoom up photo 2. No calculations needed.
Ted from down under


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## Misterg (Oct 31, 2020)

coulsea said:


> My question is how does a template with a 4.9 inch helix lead fit on the attachment or did i make a stupid mistake in the spreadsheet.



I came up with the same numbers for lead as you (4.95 & 2.47"). 

I presume you're looking at CF's plans where the shaft is shown as 4.375" between shoulders and the support block is 2.5" long (giving a max of ~1.8" movement).  If you look at any pictures of the jig, the shaft is longer than that. I was going to do the same as you and make it to suit the longest template that I thought I might need.


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## TSutrina (Oct 31, 2020)

The KHK  reference document "calculation of gear dimension".  There are a few pages on helix gears.   many articles show cutting jigs for helix gears.  HMEM group web site has an article "helical gear cutting lathe attachment"   It is the original document I believe.  The basic setup will work for a mill also.  Jul 10, 2010 by Cfellow.  Mike shows one approach to making a gear cutting tool also the rack style will work.


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## Mechanicboy (Oct 31, 2020)

In 2009 I created the helical gear with own made milling machine, here is the movies..


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## coulsea (Oct 31, 2020)

Misterg said:


> I came up with the same numbers for lead as you (4.95 & 2.47").
> 
> I presume you're looking at CF's plans where the shaft is shown as 4.375" between shoulders and the support block is 2.5" long (giving a max of ~1.8" movement).  If you look at any pictures of the jig, the shaft is longer than that. I was going to do the same as you and make it to suit the longest template that I thought I might need.


one solution may be to make the template and then remove half of the height. my gears will be fairly narrow and the travel across will be quite short.


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## Misterg (Nov 1, 2020)

coulsea said:


> one solution may be to make the template and then remove half of the height. my gears will be fairly narrow and the travel across will be quite short.



Yes - it should be possible to truncate the template - I'm trying to get my head around just how much of the template would be needed. Some travel is lost at the start because of interference between the rolling follower and the edge of the template, and some over-travel would be needed for the cutter to clear the work at either end. I think I need to sketch out a few examples, but it's making my head hurt at the moment!


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## gbritnell (Nov 1, 2020)

I don't remember the dimensions of the shaft from the original drawings but I made mine .50 diameter. Helical gears don't need to be made too long because they're not like spur gears in the sense that the whole width of gear has tooth contact. Helicals only have contact for a short distance because it's like laying to pieces for round stock on top of each other. I have made high helix templates and then just cut them off to give ample travel for cutting the gear. I don't use commercial involute cutters for cutting (generally). I make my own from .50-.63 diameter drill rod so I don't need as much travel of the fixture to clear the gear blank at both ends. 
gbritnell


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## TSutrina (Nov 1, 2020)

gbritnell said:


> I don't remember the dimensions of the shaft from the original drawings but I made mine .50 diameter. Helical gears don't need to be made too long because they're not like spur gears in the sense that the whole width of gear has tooth contact. Helicals only have contact for a short distance because it's like laying to pieces for round stock on top of each other. I have made high helix templates and then just cut them off to give ample travel for cutting the gear. I don't use commercial involute cutters for cutting (generally). I make my own from .50-.63 diameter drill rod so I don't need as much travel of the fixture to clear the gear blank at both ends.
> gbritnell


Actually the contact of a helix gear is the same as a spur gear.   The difference is that a set of spur gears contact on a line at a particular radius for each gear.  contact at best moves from the tip to the bottom near the root and then back out.  Helix gears due to the helix could have a line of contact that covers the full radius form the tip to the the bottom near the root and for another pair from the bottom near the root to the tip.  Helix gears run quieter and with less vibration, but create a thrust on both gears.  Spur gears push, do not slide.  Helix gears also just push even if the axis are not parallel.  Worm gear slide as they push thus are with a shallow lead angle can not be back driven due to sliding friction.  Helix gears can be back driven.   If you move away from involute gears you can create gears sets that can not be back driven. and generally they are helix gears that have sliding occurring.


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## coulsea (Nov 1, 2020)

making progress. I was worried about how well i would be able to bend the template and what i had to make it out of, thought it would be a good project for the plastic printer. this is a 4.9 inch template printed as 2.45 for half a turn, I use Fusion 360 to draw a cylinder and then cut out a square section coil the same dimension as the wall thickness then split the body vertically to remove some of the unwanted areas.


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## Misterg (Nov 1, 2020)

coulsea said:


> thought it would be a good project for the plastic printer.



What an excellent idea.

I've been working on a parametric template model in F360 using exactly the same method to generate it. I reckon your 40T gear only needs ~35°of useful rotation to cut an 8mm thick blank with 6mm over-travel, so some of the templates can be truncated a lot.


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## gbritnell (Nov 1, 2020)

Tsutrina,
You are correct in what you stated. I guess what I tried to explain was 2 spur gears .50 wide have an instant contact of .50 whereas 2, 45 degree helical gears .50 wide will have an instant contact of much less. Yes as the 2 helical gears rotate each pair of teeth will mesh throughout the whole helical lead but the contact patch is much less.
gbritnell


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## coulsea (Nov 2, 2020)

My first helical gear. of course i tried it on the wrong side of the spindle first hence the big chunk missing.


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## TSutrina (Nov 21, 2020)

gbritnell said:


> Tsutrina,
> You are correct in what you stated. I guess what I tried to explain was 2 spur gears .50 wide have an instant contact of .50 whereas 2, 45 degree helical gears .50 wide will have an instant contact of much less. Yes as the 2 helical gears rotate each pair of teeth will mesh throughout the whole helical lead but the contact patch is much less.
> gbritnell


Here is something to visualize.  Think of the gear made up of thin pieces of metal foil like a motor lamination.  The mating gear is also made this way and every piece of foil in one gear has a corresponding foil sheet in the other.   Thus every pair is a thin spur gear.  So what is true for a spur gear is true for each pair in the stack even if they foil pieces are skewed, off set by some tiny angle to create a helix gear.   Just something to think about.


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## nel2lar (Nov 22, 2020)

gbritnell said:


> Tsutrina,
> You are correct in what you stated. I guess what I tried to explain was 2 spur gears .50 wide have an instant contact of .50 whereas 2, 45 degree helical gears .50 wide will have an instant contact of much less. Yes as the 2 helical gears rotate each pair of teeth will mesh throughout the whole helical lead but the contact patch is much less.
> gbritnell


*gbritnell*
I think you are looking at it wrongly. If you look at the tooth and measure the tooth, it will be more than the .5 of your material. The 45 degree will make it grow.
I'll be watching and most ot the time silent.
Nelson


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## TSutrina (Nov 23, 2020)

nel2lar said:


> *gbritnell*
> I think you are looking at it wrongly. If you look at the tooth and measure the tooth, it will be more than the .5 of your material. The 45 degree will make it grow.
> I'll be watching and most ot the time silent.
> Nelson


Trigonometry will change the tooth cut when the two axis are not parallel.  Image cutting a helix gear with a  hobbing tool.   It's proportions do not change, just altered by trigonometry.  The pitch dimension is also altered by trig, everything changes by the sine and cosine multipliers.  Thus the proportions stay the same.  Tooth will not get thicker.    Measuring between parallel faces the hob dimension is what you will get.  To cut a spur gear the helix hobbing tool axis is place at an angle to the spur gear axis.


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