Accuracy Analysis of PLA Bearing Part

    Each driver wheel has a bearing that rides up and down in a slot of the chassis frame.  The bearing surrounds the axle near each wheel.  On top of the bearing is mounted a spring connecting plate that couples the load from the driver to the chassis through a set of equalization levers.
    Four copies where made during the Small Parts Kit build.  The picture at left shows the parts which have been numbered for the accuracy analysis.

    The picture at left shows the bearing parts next to a ruler for size reference.  The bearings are 0.81" X 0.95" width and length.  Each has an axle hole of 0.42" target dimension.

    The table above provides a comparison of design dimensions for key features vs. measured values averaged over the four parts.  Statistics were also computed that measure the dispersion (sigma) and likely range of values (Error +/- 3.3*sigma) that may occur for other copies of the bearing that may be built in future.  As can be seen the greatest error arose building the axle holes.  Those are noticeably undersized.  The next largest error arose with the slot that interfaces with the chassis.  The slot span side to side is oversize by a bit over 5%.  Other key dimensions are also in error as shown.  In order to compensate for the errors either post processing (drilling, sanding, etc.) would need be performed, or alternatively, modify the design to adjust those dimensions in the opposite direction in an attempt to obtain better accuracy.  Perhaps both will be needed for some parts that are expected to move in respect to one another as will be the case for the bearing in which  the driver axle rotates and the bearing itself moves up and down in the chassis as the equalization scheme adjusts for small irregularities in the track, as is done on the prototype.

The chart above summarizes the nominal and predicted range of possible errors based on the statistics of measured values for key features of the four bearing assemblies.  The green squares are the average errors while the red bars depict the range of possible errors based on the application of sigma variance to the averages.  Corrections to the design dimensions should reduce and center the error bars around zero.  However, several of the dimensions have rather large error bars which might prove difficult to use without considering post machining of the parts.  On the other hand, it is not always necessary to obtain tight fit tolerances.  The primary objective will be to ensure parts stay together and those that move do so without slipping off or out of position.  In the end it will be necessary to build and fit check actual parts, not just designs.

    The preceding chart shows the trend of bearing model errors vs. size of feature.  It appears that feature sizes of approximately 1/2" are most accurate, those having lower dimensions tend to be undersized and those greater tend to be oversize.   Charts such as these will aid in developing methods to obtain suitable accuracy for critical features that must interface with appropriate precision, such as moving parts.