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Drive Assembly - Prepare Flywheel
#1
Ok, so you've gone through almost a whole roll of plastic printing the flywheel for the drive - now what do you do with it?

The purpose of the flywheel is basically weight - the weight is used to help balance the droid and to provide kinetic energy to spin the robot in place. I've seen a range of weights used, from 7 lbs to 17 lbs - I figured 10 lbs would be a good target weight to start.

The best material to use to create the weight would be lead. To use lead, you would either have to melt it directly into the pockets in the flywheel (not going to happen with plastic) or make a mold of the flywheel to melt the lead into and then transfer the solid lead to the flywheel - a lot of work just to create 12 lead weights, not to mention the health hazards.

The safest, and easiest material to use is steel (or iron) blasting material. The steel itself is rather inexpensive, but shipping can be expensive - so it helps to find some place local to get it. It can be found at some firearm stores and you can try checking with companies in your area that do sand blasting - they go through lots of it and they may be able to sell you a 10 - 15 lb. bag for a good price. I couldn't find anybody local that would sell me that small of an amount, so I purchased a 10 lb bag of 170 grit steel shot on eBay for $18 + $12 to ship from NY to CA.

https://www.ebay.com/itm/122924133749

To get the steel to stay inside the flywheel pockets, use an epoxy to seal it in. I used 12 tubes of JB Weld ClearWeld that I picked up at the big box store - the one I used had a self mixing tip, so the two parts were mixed as they came out of the syringe.

[Image: attachment.php?aid=1142]

Because the flywheel will be spinning, you want it to be as close to balanced as possible, or it can cause the droid to wiggle and it causes large amounts of stress on the internal parts. So this means trying to put an equal amount of weight in each pocket of the flywheel. To do this, I purchased a cheap digital scale that had a 'grain' setting and could weigh up to 2 lbs of material.

[Image: attachment.php?aid=1143]

I wanted to use V3 of the flywheel, but was not able to find a split version. After looking at the files, I figured it would be easier to modify the (already split) V2 flywheel, than figure out how to split V3. I modified V2 by slicing it, on the Z axis, at 48mm. Doing this removes a little over 20mm of depth from the flywheel, removing the bottoms from 3rd and 4th rings. Another advantage of doing this is that it saves a bunch of print time, and leaves a flat bottom on the flywheel, which will make assembly easier later down the line. I probably could have removed the inner rings completely, but I wanted to keep the second set of screw holes and I liked the bullseye shape.

[Image: attachment.php?aid=1148]

This creates a flywheel that has 2 rings, each with 6 individual sections for holding material. Since I wanted a 10 lb weight, I would start with putting 5 lbs in the outer ring. This would require roughly 5800 grains of steel shot per section.

Before adding the steel shot to each section, I used about 1/3 of a container of the epoxy to cover the bottom, and then poured in the steel shot (while the epoxy was still wet) - my goal was to thicken/harden the bottom of the flywheel to give it a bit more strength.

[Image: attachment.php?aid=1144]

I shook the flywheel gently to level out the steel shot, then used the epoxy to coat the top of the steel shot - effectively sealing it into each section of the flywheel. 10 lbs is starting to look like a good target. 5 lbs of shot, with epoxy, filled each section of the outer ring to the upper rim.

[Image: attachment.php?aid=1145]
[Image: attachment.php?aid=1146]

After filling the outer ring, I weighed the flywheel to see where I was at. It was about 6.75 lbs - which made sense, 5 lbs of shot, about 1.5 lbs of plastic and .25 lb epoxy and hardware (holding the flywheel pieces together). Since I was aiming for 10 lbs total, I would need to add another 3.25 lbs of shot. This would require about 3800 grains in each of the 6 remaining sections.

[Image: attachment.php?aid=1147]

Once finished, I weighed the flywheel again and it was 164 ounces - not perfect, but pretty close to my target weight of 10 lbs - with room for a bit more, if needed.
Note: I should have used 3 lbs of shot (instead of 3.25), when filling the second ring - I didn't figure in the extra weight of the epoxy.

After letting the epoxy cure for 24 hours, the flywheel is ready to be installed on the drive.


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Updated with pics
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