Old post from 7/1/11

Motor mounts have always been a problem. SPIRIT uses two 3″ Magmotors, which have no mounting screws or protrusions. They are just plain cylinders. We attempted to make a two piece wooden mount with rubber inserts that clamp down on the face of the motor. However this approach risks the rubber being sheared off the wood by the torque of the motor.

Commercial motor mounts for this motor are generally expensive. They are just pieces of aluminum that clamp down with bolt tension to apply friction to the motors. After shipping, these mounts would total hundreds of dollars. I thought of directly casting the motor mounts as a possible method of making adequately strong motor mounts. Since the dimensions were listed online for the commercial mounts, I could just copy and mold that instead.

For our first attempt, we tried to make a sand casting rig with bentonite, sand and water. Green sand for sand casting can be made that way. Too bad the source of bentonite we used, kitty litter, had so many insoluble impurities that the mold was difficult to make even. With our lack of prime moulding materials and experience, we decided to make a muffin-tin mold and just pour from the top without a casting spout or a two part mold.

The dirt-n-brick furnace I have is pretty badly designed. It’s just a pile of cinder blocks shielded by a dirt mound. The heat efficiency is terrible, and it took a long time to melt the aluminum. However, once a puddle of liquid aluminum formed on the bottom of the crucible, melting happened very quickly. And did I mention that the crucible we used was just a tin soup can. This wasn’t an emergency casting attempt #1 for nothing. Two blow dryers were directed at the furnace opening as bellows, and paraffin tea lights were thrown in the crucible for bursts of heat (at the expense of adding impurities).

Since this was such a brash procedure, we didn’t bother to scoop off the surface slag. That was later very apparent in the quality of the final product. W also didn’t wait for the mold to dry most of its water, and we had used way too much water during the green sand mixing process. As a result the pour went as expected. At parts water spot boiled leaving large vacuums where the aluminum would not flow. The slag accumulated near the top and weakened one side of the cast. The lack of a two part mold encouraged heavy oxidation on the top side. Here’s what happened on video:

The end product was pretty disfigured, but the concept works. Had it not been for the steam bubbles, the cast would have been high enough quality to be functional as a motor mount. The resulting product was around half a pound and was heavily oxidized on one side. Regardless, the attempt was just an experiment and shows promise, so we will try again with a little more effort on the mold and get things right the second time around.


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