V1 Rotor Testing

We previously described what a Dremelfuge is and why we need to build one. We also explained how we designed the stand for the Dremel and the rotor. Today we’re testing the rotor. 

First, we started to put the rotor we had printed on the stand. This involves taking the flywheel off the Dremel, and threading one part of the rotor onto the spindle of the dremel…. We soon found that the hole on the rotor was too small to fit the Dremel spindle.

 

The Dremel spindle doesn't fit through the hole in the rotor.

We used a drill to widen the too-small hole with (we used a 5/16ths bit). This is the point where the eye-protection came out-- it stayed out for the whole rest of the testing. 

 

After some modification, the spindle fits through the hole in the rotor.

After that, we tried to insert to flywheel--  dremel attachment that our rotor clamps onto, into the rotor.  As luck would have it, the flywheel on our actual dremel was bigger than the measurements suggested online.  We reduced the size of the flywheel using tin snips. This technique worked surprisingly well-- it took about 5-6 cuts to downsize the flywheel enough that fit in the slot. 

 

The resized fly wheel "fits" in the rotor.

 

As currently designed the entire rotor hangs off this flywheel.

This looked like it would be pretty scary, since the flywheel, which the rotor clamps onto, was much reduced in size. 

We used screws to clamp the flywheel into the assembly (our original design). 

 

The rotor with the top plate and screws. These parts didn't require modification.

The flywheel was secured by screwing in the top plate here (in the center.) We reapplied our eye protection and hid behind boards for the initial testing. 

 

Anticipating the rotor would break into many pieces, we put wood boards in between us and the dremelfuge for our initial test.

At about 15,000 rpm the rotor exploded into many many pieces. Like, very many pieces. We are still finding the pieces. 

 

Some pieces of the rotor we recovered, a good reminder that eye protection is essential.

Our friend Elliott, arrived home as I was glumly arranging the small pieces that had broken off into something resembling the rotor.  Not one for subtly, Elliott said “You guys want to put how many G’s on this thing?  You know a G is like….one gravity….right?” We all laughed, and then went back to looking for more pieces of the rotor. We also began to reflect on our disappointing test.

First, we thought about our safety and the whether the risk mitigation strategies we had used were appropriate.  Eye protection was a good idea.  However, our protection strategy of the boards, relied on the assumption that the rotor would fail in-plane (that the pieces would fly straight out).  We found pieces of the rotor around the living room that seemed like they had been ejected about 10-15 degrees off the spinning plane.  Hmmmm.

Our friend Jake, who arrived home after we had hidden evidence of our failure (or so we thought) went to grab some paper towels from the top of the fridge and found…..a piece of the rotor.   The top of the fridge is about 6’ off the floor and at least 30 degrees off the plane. 

Jake also laughed with us, and explained that in hiding behind the boards, we didn’t account for the flexural stress-- basically the rotor wants to flatten out into a pancake (and currently its curved) and so the force that it exerts as it tries to flatten is what caused the small pieces to be flung at such a large angle off the plane, like the one that we found on the fridge. We concluded that the next version of the assembly will need to have a very robust guard.  This guard will need to protect the user from rotor pieces ejected at a wide range of angles. Flexural stress. Huh.

If it worked the first time it wouldn't be called re-search. (Right?)

Stay tuned for a gif of Rotor V2!