Prototyping: Part 3 – Design & Fabrication

2011 May 23

Parts Arrive and Machining Begins

With the basic design and mechanical drawings of the prototype completed, we ordered parts and began some of the machining. The parts we are not making were ordered from McMaster-Carr. In this post, I wanted to discuss some of the parts we have received prior to assembling them into the complete prototype.

The bowl shaft was ordered as a basic precision shaft from McMaster-Carr. It is 1 1/4″ in diameter. Acme  Metal Works than did three machining operations. They threaded the outside of the shaft so that we can thread the bowl onto the shaft and a lock nut tightened down to hold it in place. A keyway was machined into the side to match up with the keyway in the drive gear to keep the shaft and gear locked together (but the key can shear, if there is a serious problem). Finally, an internal threaded hole was machined to allow us to bolt the diverter cone onto the top of the shaft.

There are two mounted bearings that hold the shaft. One is located at the top of the table and one at the bottom for stability. They are ball bearings designed to handle the load and speed of the roaster bowl.

A variable speed brushless DC motor will provide power to drive the bowl. This motor is 1/3 hp and operates in the range of 0-1750 RPM.

Our calculations show that it is likely that the speed we will need for the bowl is about 100 RPM. Therefore, we are using a speed reducing chain drive. The gear on the motor is small (10 teeth) and the gear on the bowl is large (96 teeth). That results in a 1:10 gear ratio and maximum bowl speed of about 180 RPM. If our calculations are wrong and we need to go faster, we will increase the size of the small gear.

This week we will continue fabrication of the table and bowl.


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