Helical gear cutting

[gbritnell]

The other day I decided to build a small governor to go on the Maudslay Marine Engine. Basically cosmetic but I thought it would add some interesting movement to the engine. The design incorporates a helical gear set at 1.5:1 gear ratio. I opened up the helical gear chart that Don made and put in the numbers that I needed to get the specs. The crank gear is 50 degrees with 21 teeth and the governor gear is 40 degrees with 14 teeth. I only had a division plate for 14 holes so I had to make up another with 21 holes. I also needed to make the helical templates to create the helical teeth on the gear.

When Chuck Fellows designed the fixture he used 1/8 aluminum for the template. My first template was made from .062 brass and as I worked with the fixture I found that .031 would work just fine, and it was easier to form.
Ok I have the index plates and the templates made. Now I needed a cutter. The chart shows the required number involute cutter for each gear so being that the gears are tiny and they don't need extreme precision I decided to split the difference in tooth count. I made a layout of the tooth form in Autocad and came up with the number to cut the cutter from drill rod. I made the cutter, hardened and tempered it.
I set the helical fixture up in the mill, clamped in the vise at the proper angle for the larger gear, 50 degrees. I then mounted the gear blank onto the arbor and tightened the cutter in the spindle.

I have cut quite a few helical gears using this fixture over the past number of years and to be honest I don't remember exactly how I went about centering the cutter with the spindle. The gears were larger so maybe that aided in my eyeball calculations. Anyway I created a sketch to try and help explain what I need input for.

As most of you know I have been machining for 50 some odd years and have been able to solve all of my machining questions. It's not that I can't come up with a way of doing this job it's just that I would like to know if there is an 'ACCURATE' way of doing it. By accurate I mean within a couple of thousands.

I searched Youtube for helical gear cutting and there are a number of videos showing the process but none of them show a setup operation. My question applies to not only my fixture but on a horizontal/ universal mill using a universal dividing head.

The question is: how does one find the center of the shaft/gear blank at the point of contact with the cutter? Maybe it's right in front of me and I can't see it but try as I might I can't solve this.
Remember the key word is 'ACCURATELY'
gbritnell

 

[peterl95124]

George, maybe this, use an acorn nut to secure the gear blank, but turn a point on the end of the acorn nut (verify that when installed on the fixture it is "centered"), then using a magnifying loupe you can put the tip of the cutter right on that point, and then use trigonometry to calculate where to reposition the cutter to be centered on the gear blank (having previously measured how far the point is from the gear blank)

I find that a 5x loupe is good for a couple thou, and if you can get a 10x in there (not always possible) then you're good for a thou or two.

now I'm wondering what kind of favor I would owe you to cut some 15-tooth RH 45-degree helical gears, 48-[ab]normal-diametrical-pitch (IE pitch diameter is 1.4142 x normal PD for a 48 DP gear), 1/4" bore, 3/16" face width. These I need for a Hansen Diesel, the other sizes I have bought from SDP, but they don't carry 15-T, and haven't responded to a custom cut request-for-quote, perhaps I can share my design, or even a working fuel injector if/when I get to that point.

 

[gbritnell]

Peter , let me see if I have an indexing plate for 15 divisions and a cutter. If I do I'd be happy to make them for you. Cutting the gears is the easy part. Making the accessories to do the job is the time consuming part.

 

[peterl95124]

George, I've posted a couple questions on your YouTube video showing you operating your Chuck Fellows gear cutting jig, which I'll repeat here (this looks simple enough for me to make myself, already built my own cam grinder, so what the heck!)

George, you don't show indexing, I'm wondering if that's a thumb screw at the bottom for indexing, and if so which part is threaded, the index plate or the helical template holder ? TIA !

George, what is the practical maximum steepness angle of the template, I mean besides having enough space between the indexer and the roller follower, IE if the angle is too steep will there be too much tangental force on the shaft and not enough axial or vise-versa ? I'm intending to cut 45-deg helical gears (from just around 1/2" to 1-1/2"), and thinking I should make the template diameter about the same as the (actual) pitch diameter of the largest gear I intend to cut, for smaller gears the template will have a lower shallower slope, is that a reasonable approach ?

George, lastly (hopefully!) I see you're cutting a LH helix, but to mesh with the gears I've already bought I need a RH helix. Obviously I need a helical template that slopes the other way, but also. It seems I can either have the fixture on the left like you have but move the cutter to be in front rather than behind the blank (and pull the spindle down rather than push it up), or flip the fixture to the right side of the vise so it tilts the other way and continue cutting from behind the blank (and pushing the spindle up like you've been doing). So my question is can you see any pros vs cons between these two options ? Or to put it another way, what if you continued making LH helix but put your fixture on the other side of the vise so it slopes the other way and moved the cutter to in front rather than behind the gear blank, does this impact how you twist the spindle while holding the template against the roller ?

 

[Drawfiler]

My tip for entering a cutter when making a spur gear may help here, lets assume we are cutting on a horizontal mill with the blank on mandrel in a dividing head.
Align the cutter by eye over the centre of the blank and very gently bring it closer until it just starts to cut then wind the table back and forth with the Y axis making a tiny oval cut in the blank, now centre the cutter in the middle of the oval by eye, take a reading of the Z axis dial and drop the table a few thou, index to next tooth and wind up the table creep up on the Z axis reading, to just touch the blank with the cutter.
This is the best method I have found. In this application without trying it my guess would be to again touch the blank and move the Y axis to make the oval and set the cutter in the middle of it, the cutter would be diagonal across the oval but it should work.
Peter

 

[gbritnell]

Peter, contact me directly at: [email protected] That way I can send you pictures and more information.
Regards. George

 

[kvom]

Based on drawing, the critical settings are Y axis. Y0 is the centerline and can be found conventionally. Y-cutter is the cutter diameter minus the depth of cut of the teeth. Then position the cutter with X slightly forward (X+) of contact with the gear and Z slightly above the gear (Z-). Then as the gear ascends and turns it will be fed into the cutter.