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Updated Hand Cranked Worm Gear Model

I found a neat pattern for a wood worm gear model in this book. It suggested carving the worm gear with a pocket knive........

Finished Model in the Book

In the book, the finished model looks like this..........

Google Sketchup

I made a Google Sketchup model of this design, but I don't know how to draw a worm gear in Sketchup, so no worm gear is shown........

Making the Worm Gear

The first step was to turn a blank to have a 1.5" OD and 2.5 inches long. I decided to drill the center 1/2" hole in the worm gear after I made the threads.

Then I used Sketchup to determine where to mark the top of the teeth every 90 degrees around the gear. I printed out the tooth layout from Sketchup, then pencil marked where the teeth go at every 90 degrees around the circumference of the gear.  I then hand connected the points by eye using a pencil.

Teeth market out in pencil

More on Worm Gear teeth

Then I used a plastic flexible straight-edge to connect up the lines. I made the tooth thickness 1/8" at top with 9/16" centerline distance between teeth. Then I used the Dremel with the small 5/16" OD sanding drum to make the grooves between the teeth. I rotated the blank about every 30 degrees to work the groove around the blank.

 Once I got the teeth done, I put on the 9/16" OD sanding drum and used it to reduce the top width of the teeth down to 1/8" wide.  Later when I tried the fit between the 2 gears, I used the smaller 9/16" to thin up the thickness of the teeth about 1/2 way along the height of the tooth.

 Here is the blank for the spur gear:

Misc Parts needed to build model

Base and BackBoard being glued up

Small Washer

I needed a small washer 3/8" thick, so I cut some yellow heartwood to a 3/8" thickness using this fixture..........

Drilling gear holes

 I made a jig for drilling all the holes on the spur gear. It has a 1/4" dowel pin glued......which becomes the center to rotate the spur gear on the drill press.......

Cutting the Teeth on the Scroll Saw

Final Assembly

Next is checking the gear fits, and positioning the worm gear with respect to the spur gear......

 I used blue masking tape to mark where to place the 2 red bearings when I glued and clamped them. Then I don't have to erase pencil marks.  The height of the 2 bearings turned out to be 1/8" different in height after gluing. If I make this model again, I need to measure from the base to the top of each bearing, and try to get them the same height.

The gear reduction ratio is 32:1. It is determined by the number of teeth on the spur gear.

Finished Model

YouTube Video

I made a YouTube video of the model in action. You can use this link to watch the video.

Sketchup Warehouse

I uploaded the model to the warehouse where anyone can access it. You can use this link to see or download the Sketchup model.

Closing Thoughts on this Project

Making the worm gear turned out to be a lot easier than I expected. The book called for whittling the gear with a sharp knife.  Safety wise, I prefer using the Dremel with 2 different sizes of sanding drums. The model works well.

If I build this model again, remember to check the distance from the base to the top of both bearings, and make sure they are the same before gluing. I was about 1/8" off in height, and did not realize it until after I glued the bearings onto the back board. This is easy to forget to check because you are busy trying to get just the right clearance between the two gears.  Too tight and they might not work if they swell due to temp and humidity changes, and too loose they look bad and might slip.

March 2021 Comments

Since I did this model, I found 2 different gear design programs that I used to print a paper pattern of the gear, glue the pattern onto the blank, and cut the teeth of a scroll saw. Neither program will do worm gear design.

On March 13, 2021, I figured out how to draw a worm gear in Sketchup :)    I found a Youtube video which explained 1 process, and I got it to work.  I went back and added the Worm Gear to the Sketchup model.  You can use this link to see how I drew the worm gear.