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 ]
To create the helix a template needs to be made. The calculations for this template are calculated like this. You have a shaft with a diameter that corresponds with the pitch diameter of the gear. You have a particular helical angle. From a point on the diameter if you created a line equal to the helical angle and rolled it around the diameter until it came back to a point in line with the starting point this would give you the advance of the helix in one revolution.
Attached are the PDF's that Chuck Fellows drew up laying out the creation of the fixture and the development of the helical templates.
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.
https://youtu.be/blaZ5tz0_6E
I can see this is going to require further study on the large screen home home computer
Way above my skill level, but very interesting to see how it's done
Impressive, way above my pay grade but I really enjoy threads like this.
That engine build is just WOW and the tutorial was fascinating, but I do wish that I could have watched the other end of the fixture operate for a couple of cuts, just to get the whole picture, as I'm unclear on how the template and the follower stay in contact throughout the forward and reverse motion. A close up of your cutter would not have gone amiss either.
Wonderful to have such talented and capable folks sharing their knowledge with us here. You do explain these technical matters in a very clear and matter of fact fashion!
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.
Hi Jim and Stoker,
The shouldered disc that the template is screwed to is free to turn on the shaft. It has 2 holes in it that align with the 2 rows of holes in the index plates. The index plate is secured to the shaft with a set screw. In operation the pin is placed in the first hole, the shaft is now secured to the helical template. The shaft is moved forward until the template contacts the bearing. Now by keeping forward pressure on the shaft, to keep the template against the roller wheel, the shaft is rolled against the template creating the helical motion. On the return stroke the same pressure is held against the roller wheel so the cutter tracks in the same cut.
Once returned to the start position the pin is pulled out and moved to the next index hole and the template disc is rotated until the pin drops into the corresponding hole then the procedure starts again.
As far as gear tooth cutters go most of my gears are no larger than 48 diametral pitch. That means the gear has 48 teeth per inch of diameter. The module formula is m=25.4\DP. Involute cutters come in sets of 8. This covers the proper gear tooth form from rack to infinity. A rack gear has a straight sided tooth with an angle corresponding to the pressure angle of the gear, 14.5 or 20 degrees. As the gears get smaller the involute curvature of the tooth gets more circular.
I only have a couple of commercially made involute cutters. These are 48 DP. I usually make up my own 'hob style' cutters for most of my gear cutting. I have made cutter all the way down to 80 DP. I draw the gear tooth form in my CAD program then take the dimensions and make a gear cutter from drill rod and harden it.
When cutting a particular spur gear, lets say it has 1.00" D.P. and has 48 teeth. You would use a #3- 48 DP cutter which will cut from 35-54 teeth. When cutting a helical gear with the same diameter and D.P. you would use a different numbered cutter due to the math in calculating the proper tooth spacing of the gear. There are charts that show how to calculate which cutter to use.
I hope this explains both questions.
gbritnell
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.
You really do have excellent command of technical language, so that your explanations are very understandable. I fully understood the indexing and diametral pitch from your earlier explanations, but was unclear about how your guide remained in contact with your follower through both forward and rearward movement, so was hoping to see the action on that side of the fixture. I now understand that it is only manual pressure that keeps the system properly engaged, so that a moment of inattention, slacking off on the contact pressure while rotating the shaft, would ruin your workpiece gear, as there is no "failsafe" pressure system, like a spring or captured groove, to otherwise assure proper guidance.
Thank you very kindly for taking the trouble to clarify this point and help me with my understanding of the fixtures function and process.
Interesting reading and I think i understand it all
But as for using this knowledge way above my pay grade and ability.
Cheers
Dennis