Office of Steam Forum for Model & Toy Steam Gas & Hot Air Engines

Gentlemen,
 First off I want to say that this was not my idea. A very innovative and prolific builder, Mr. Chuck Fellows, who recently passed away, designed and developed this fixture for cutting helical gears.

 What is a helical gear? Most of us who have built some type of I.C. engine or possibly some other mechanical device have used spur gears. Simply a disc having a diameter which corresponds with the pitch diameter and having teeth laid out with uniform spacing around that diameter which relates to a specific diametral pitch (SAE) or module (metric). A gear with a given diametral pitch/module can be paired with any other gear with the same pitch. The shape of the teeth are created by what's known as an involute curve. This tooth curvature insures that when 2 gears are mated the mating of the teeth will produce a smooth rolling action as the teeth engage and disengage one another. These gears can only be paired up with parallel axes.
 Now helical gears. Helical gears have most of the same characteristics as spur gears, diametral pitch/module, involute tooth shape, pitch diameter etc. The one area that they differ is the teeth are not parallel to the axis.
They can be at any angle to the axis. These types of gears provide a smoother, quieter operation when mated with another gear. The reason being is that there is always tooth contact throughout the meshing action whereas spur gears, although small, have an intermittent contact area which produces a whine form the gear train. The drawback to helical gears is that the produce a thrust along the axis which increases with the angle of the helix. This needs to be addressed with bearings that will support this thrust. A unique thing about helical gears is that they can operate with parallel axes or at right angles to each other.
Another thing is that by changing the helical angle the pitch diameter changes therefore allowing pitch diameters to be incrementally altered. 
 
 The fixture that Mr. Fellows designed consists of a block with a reamed hole to accommodate a close fitting shaft. At one end of the shaft is a reamed hole to accept different sized arbors to hold the gear blanks being cut. The other end of the shaft has a shouldered arbor onto which is mounted a spiral template and an index plate. Mounted on the top of the block is a rectangular shaft with small bearing on the end which rides on the template. [ Guests cannot view attachments ] [ Guests cannot view attachments ] [ Guests cannot view attachments ] [ Guests cannot view attachments ] [ Guests cannot view attachments ] [ Guests cannot view attachments ]  

Here is the Youtube video that I created showing the use of the helical gear cutting fixture. The gear that I was cutting was for my miniature Ford 300 inline six cylinder engine.

I can see this is going to require further study on the large screen home home computer 

Impressive, way above my pay grade but I really enjoy threads like this.

That is really something!

I think I understand the theory here, and both the video and the drawings for the fixture look pretty clear to me; however, I'm not at all sure about the gear cutter you used.

I'd like to try cutting gears (spur, helical, or both) for a variety of small engines, but don't know what cutter pitch, OD, and bore to choose as an all-around set of 8. Can you help with this? I ask because the size of your inline six seems to fit the relative size of things that I have in mind....

Hope this makes sense to you.

Well, that certainly elucidates things for me.

In my own semi-intuitive by-guess-and-by-God way, I had arrived at 48DP as the magic number for a set of cutters (too lazy and unskilled to make my own, though I really should try it). Ain't it good to see one's own preconceptions confirmed?

Thank you.

Interesting reading and I think i understand it all
But as for using this knowledge way above my pay grade and ability.

Cheers
Dennis