• Travis Gillin

Easy Gear Reduction for Stepper Based Mechanical Drives

If you have or are building your own CNC Plasma cutting or routing table i'm sure gear reduction has crossed your mind at some point in time. Stepper motors are available in many different sizes and holding torques but even if you're running a massive 1800oz Nema 34 at 230 Volts with 16x microstepping, odds are you will still need gear reduction to get steps smaller that 0.001" and be able to achieve smooth and useful acceleration and deceleration profiles. Have you thought about how your gonna implement this reduction? Belt drive's are great but timing belts, pillow blocks, and pulley's are not exactly cheap, you can easily have over $250 is just one unit (and you need one per motor...). Gears are nice, but tricky to work with and sometimes tough to decide exactly what you need because things like pitch diameters, gear modules, involute curves, and helical-cuts all make my head hurt when trying to order a matching set. Not to mention, the worst part of all is having a large gear or belt reduction assembly hanging off the side of each mechanical drive...

With all these things in mind, I decided that it was crazy that a small, inexpensive, bolt on gear reduction product for Nema Stepper Motors doesn't exist (At least I couldn't find it anywhere). So I decided to give a go at solving the problem. I'm a problem solver....

Design Goals are -

  1. Bolt directly to the end of a Nema stepper motors and provide gear reduction as if the unit wasn't there (output shaft and bolt holes are exactly the same as the motor)

  2. Slim profile, Add no more that a few inches to the height of the motor

  3. Cheap, less than the average cost of the motor its gonna be paired with

  4. Strong, Little to no backlash

  5. Low Parasitic loss

  6. Base platform that allows for multiple gear ratios without changing thickness or outside profile

Goal 2 pretty well rules out conventional tried and true planetary sets. Its probably the easy answer but they are fairly expensive to manufacture. Gears in general, usually hobbed means multiple setups and operations. So I thought about this for weeks until I hit me at an odd time, I was rebuilding an oil pump on an LS engine.

Once I started moving the cam around the outer ring, I realized I could use this as reduction. The inside and outside is easily machinable with common size cutting tools and a wide range of reduction is possible by changing the number of lobes. This type of reduction is referred as cycloidal for anyone interested in that.

So the next step was to draw something up, I started with a drawing of a 1600oz Nema 34 and worked up a bolt on 5:1 reducer assembly that meets my design goals. So 5 rotations of the motor shaft are divided equally and accurately into one revolution of the output shaft. I added some renderings of what the first iteration will look like. There's gonna be a few small changes down the road but this is the basic assembly

Now I have it all drawn up, so its time to make some chips and see if this thing is even practical. I decided to start with wood. I hate working with wood but I have a bunch of scrap pieces laying around from a building project so I wen't for it.... Its just a proof of concept anyways.

The proof of concept works and it's ready to be machine out of Aluminum and Steel, the final products will be grease-able and should handle any RPM and Load that the drive motor can throw at it.

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