Helical gear problems - Help

[Lagerbolzen]

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 ??

 

[Drawfiler]

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.

 

[Misterg]

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] ).

 

[Mechanicboy]

Learned out this is fault done of you.

 

[gbritnell]

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

 

[gbritnell]

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

 

[Steamchick]

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

 

[gbritnell]

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

 

[Mechanicboy]

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.

 

[Lagerbolzen]

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

 

[Drawfiler]

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!

 

[Misterg]

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.

 

[gbritnell]

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

 

[Richard Carlstedt]

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.

 

[Misterg]

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

 

[SmithDoor]

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

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 ??

 

[gbritnell]

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

 

[gbritnell]

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

 

[Misterg]

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:

https://www.model-engineer.co.uk/forums/postings.asp?th=109176
Seems the consensus is that the N/Cos^3(a) approximation is 'close enough' in most cases.

 

[coulsea]

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