How To Make Skew Gears
If you are wondering what a "skew gear" is, or why anyone would ever need to make one, this page is a fairly good place to start. If you already know the what and the why, but need a 'how', then this page is a good place to be. And if you've done it before and know all about it, this page should at least be good for a laugh.
Turns out (groan) that there are many ways of making skew, or helical gears. Making perfect skew gears requires some rather specialized machinery. It can be done by hobbing, where all teeth are cut at the same time, or one tooth at a time using a standard involute cutter while simultaneously rotating the blank and moving the cutter. I'm not going to describe either of those any further here as an amateur machinist is unlikely to have the required equipment. What I will describe is how acceptable helical gears can be machined with a mill, a rotary table, and a shop-made cutter.
This subject got attention as a result of needing a single skew gear to compliment the one received with a casting kit for an old four-stroker called the Feeney. To build this engine, I had to find a way to make the missing gear. This led to research into the gear type and ways that others had made them in the past, followed by experiments and eventually, making more than twenty of the things. The links below should provide all you will need to replicate the approach I took.
But there are several other ways to make skew gears without expensive hardware. One very unique and innovative approach was described by Alan Suttie (Canada) in Model Engine Builder. Another rather elegant approach (because it is similar to the one I came up with ) was written up by Rex Swensen in Model Engineer dated March 17, 2006. It uses a hobb-like cutter mounted on a vertical slide on the lathe saddle with the headstock acting as the dividing head.
Then there's various universal gear cutting machines also able to cut spiral gears, such as the one pictured here which appeared as a construction series in the Model Engineer way back as 1947. A more recent and more complex but capable machine appeared in companion magazine, Model Engineers' Workshop. More recent yet are devices using home made CNC control to perform the complex movements once done by the specialized gear train driven dividing heads. These are major projects in themselves, but I know many model engineers take great enjoyment out of building complex devices that receive only sporadic use.
Hopefully, you now have a better appreciation of how skew gears can be made to a standard suitable for use in a model engine for no more outlay than a little time, and maybe a few conversation pieces for the scrap bin along the way.
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Copyright (c) Ronald A Chernich, 2009. All rights reserved worldwide.