Helical gear cutting

Discussion in 'The Break Room' started by gbritnell, Mar 17, 2019.

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  1. Mar 17, 2019 #1

    gbritnell

    gbritnell

    gbritnell

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    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
     

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