17.01.2021 Checking the accuracy of the arc ends.

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Sunday, after another night frost. I have been thinking about checking the marked ends of the arcs. I may be able to make a jig involving the laser level vertical beam. If the arcs are restrained by stops the marked end lines can be quickly checked for squareness. The alternative is to fix a straight edge on the centre line of the radius bar.

I don't have many candidates for a better radius bar due to the considerable length required. The straight edge would have to be screwed to the end of the rather thin, radius bar. That would mean drilling holes in a steel rule. Checking the true accuracy of the straight edge then becomes the next hurdle. I'd better think of something more sensible as I enjoy my morning walk! 

I used a cord from the radius bar pivot screw and all my lines looked fine. Sawed off the points and smoothed the cuts. Rebuilt the ring on the ground to 4m internal diameter and... I had to keep pushing the segments outwards. Until there were acute angles at the joints all the way around. I ended up using 10 full arcs to be able to reach 4m inside diameter.  The inside curves are cut to 2m radius. So should end up as a smooth ring regardless of the small [wedge shaped] gaps between the arcs. 

I now have to mark the ends of the arcs with accurate radial lines. Then saw to these lines to tidy up the joints. I don't want these gaps causing voids when laminated and glued to further layers. Worse, I cannot mark out the building's foundation blocks accurately. Not unless the ends of the arcs close neatly together without needing the ring to be measured across its diameter. Which is of course, impossible with the octagon building sitting in the middle.  

I found the centre of the circle with the 2m radius bar and then hammered a nail into the frozen ground. Laying a long straight edge against the nail provided the correct radial lines for the ends of one arc.  I can use this as a pattern for marking the other arcs. The arcs needed to be loaded down with bricks and other weights. To stay still enough during measurement and end marking. The ground isn't very flat where I had laid the circle out. With the hard frozen snow and ice further adding to the difficulty.

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