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Date:  1-5-2008
Number of Hours:  4.00
Manual Reference:  
Brief Description:  Measure beam and work table flatness

1. I know that the work table is not perfectly level or planar.

2. I purchased two 2"x2"x12' steel beams to give me a flatter work surface for building.

3. The ailerons require a slight twist which I can get by raising one end of one beam. However, I have never quantatively evaluated the flatness of the beams or the table, so I decided this would be a good time.

4. I used a 2' carpenter's level and a feeler gauge with leaves ranging from 0.006" to 0.020" by 0.001" as follows:

5. Starting at the NW end of the table, I placed the level on the long edge of the table and shimmed up the lower end until I achieved level. Then I summed the thickness of the shims and recorded the total thickness. See Fig. 1

6. To complete the longitudinal data, I repeated this at 2' intervals along the edge of the table, then along the center of the table, and finally along the opposite edge.

7. Next I did the same thing along the N edge of the table and repeated this every 2' to gather a latitudinal data set.

8. I entered the data into an Excel workbook, and calculated the height variation longitudinally along the table. The results are plotted in Fig. 2. It can be seen that the SW corner is ~10-mm higher than the NE corner. Much of the variation is near the ends of the table.

9. Next I took the same data on each side of the steel beams. There was a 1/4" plywood shim under the N end of each beam where the curvature of the table leaves a gap between the table and the beam.

10. The data were very similar for each side. The data for the side I've been using as "top" are plotted in Fig. 3. The beams are essentially linear within the error of measurment. The S-end is about 7-mm above the N-end. The S-ends of the beam are within about 1-mm -- close enough for government work.
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Measuring beam flatness

Measuring beam flatness

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Table height variation

Table height variation

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Beam flatness data

Beam flatness data

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