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I plan to build a hand cranked marble dispenser which uses larger 1 inch versus 5/8" diameter marbles.

The only problem is that each cycle of about 5 marbles only takes a few seconds to dispense them.

If I added a worm gear, I could slow down the cycle quite a bit.

The reduction ratio is determined by the number of teeth on the spur gear.  But the worm gear has a minimum diameter also in wood, maybe 1 inch diameter minimum, to mount on a 1/4" dowel??

about 4 seconds would be the right amount of time for each marble to be dispensed......versus maybe 5 seconds for all 6 marbles to be dispersed without any worm gear.

So 4 sec = 15 RPM desired crank speed

Kids can crank up to 100 RPM.............so desired speed reduction ratio would be 100/15 = 6.7.  The speed reduction ratio is determined by the # of teeth on the spur gear, so need an even number like 7 or 8. 

About as small as I want to go on spur gear is a 3.0 inch PD.............so with 8 teeth you get this.......

I want crank on RH side since most people right handed.  Might have to package worm and spur behind the sliding element crank, don't know yet.

On a previous project, I made a Youtube video on how to make the wood worm gear. You can use this link to watch that video.

Does not matter whether worm/spur are right or left handed on this model, the sliding element works either way ok.

So distance between teeth on spur gear with 3" PD and 8 teeth is circumference of pitch diameter divided by # of teeth...........Pi*D/8 = 3.1415*3/8 = 1.178 inches.

Now last question is, what diameter for worm gear?

First worm gear model I built, the diameter was 1.5 inches.

2nd worm gear brass model, diameter was 2.0 inches.

I have 1.25 inch diameter birch dowel, could try that on this project.

I will make the template in Sketchup, then print it out, and cut into 4 pieces to mark the 4 quadrants around the worm gear blank.

I was thinking of making the worm gear 3" long, but maybe I should go with 4 inches because the teeth are so relatively wide at 1.18 inches?

I don't have to design the worm gear itself, I can just simulate it with 1.25" diameter and 4 inches long in Sketchup, and design the gear later.

I will make the worm gear and spur gear, with their bearing mounts, and try them out before I start building the whole model.  I am also guessing on the teeth engagement depth, and will have to try that out as well.

I first cut a groove on each side of teeth using green carving or deburr tool on Dremel, then used new 1/2" drum sander on Dremel. Was not too bad making the teeth.

On blank, cut 4 inches from 1.25" dia dowel, then marked center of each end using dowel drilling fixture, then chucked in lathe and drilled from tailstock using 3/8" Forstner bit.

I made some bearings from plain pine and set up the test.  I got lucky, everything worked fine and smoothly !!!!!!!!

I have enough clearance for the tip of the spur gear teeth with the distance I arbitrarily picked, I did not have to make a gage to make sure each gullet was the same depth.

You can use this link to watch a video of the worm gear testing from my old Canon SureShot camera.  I made another one using my Samsung S8 cell phone camera and it is a little clearer. Use this link to see this one.

I used my earlier instructions from my Word file, still had a heck of a time, but I got it drawn in Sketchup.

On the first model I built with no worm gear, I used an 1/8" thick piece of clear plastic on the marble storage tower so you can see what is going on while you run the crank.  I got to thinking, why not eliminate the plastic piece and just use 2 pieces of maple to hold the sliding bar in place?  Because the plastic is prone to breaking when drilling the holes for the brass screws, it eliminates this potential problem.

You can download a copy of this model design using this link.

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

This was a fun project to make. The grandkids should have fun with this one!