gifFace Strut Tetragon Butt Mitre

This strut type is butt mitred and staggered round the face. Because the miters are simple to cut, this is the most popular strut for panelled domes. The picture shows the staggered joins for a dual face. The white lines mark the face boundaries.

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Once again, you may use the ‘Hub Normal Cylinder’ intersect option with this strut type. When turned ON, an extra vertex is added to the bottom of the strut that terminates in the face corner. This gives an extra triangular face at the hub normal. The following screenshot shows the result from the inside of the sphere.

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If your strut depth is small, your envelope spherical, and your frequency low, it is possible to leave the ‘Hub Normal Cylinder’ intersect option OFF. But I recommend you leave this option ON. I have seen videos of some domes on YouTube that use this strut type; I don't know what software was used to generate the struts, (if any), but the builders obviously did not account for strut intersection problems on the interior of the dome. Consequently they had to jostle the panels about to get a rough fit. This is no good! If you leave this option off, it is up to you to ensure there are no strut intersections. This tedious process entails going round the inside of the dome with the navigator and examinng the struts in detail by selecting and hiding struts in turn. Obviously such a manual examination method is error prone, and intersections are hard to spot unless struts are coloured individually. So why not just leave the ‘Hub Normal Cylinder’ intersect option ON? Then all you need do to resolve any problems is enlarge the radius of the hub normal cylinder. The following screenshots show a typical intersection problem with struts that have a large depth. As is evident, it only takes a small sliver on one strut to spoil what would otherwise be a perfect join

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A intesects with B

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Hiding B shows the problem area.

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Problem resolved with ‘Hub Normal Cylinder’ intersect.

A diagram of the Hub Normal Cylinder intersection plane is shown below. Fig. 1 shows the strut and hub normal cylinder at the Dest end. Fig. 2 shows the results after intersection. Fig. 3 shows the compound angles. The bevel angle is shaded is blue and the mitre angle yellow. Note that both angles are measured from the middle position of the saw. So the bevel angle pivots up and to the right, whilst the mitre angle swings horizontally to the left.

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Note that HNC intersection is only used on the Dest end of a butt mitred strut. Typical log output when using the HNC intersect option is:

Hub Normal Cylinder Intersect Option

  HNC Compound angles at ORG:
    Org HNC Bevel = 0.00000000
                  = 000° 00' 0.0000000"

    Org HNC Mitre = 0.00000000
                  = 000° 00' 0.0000000"

  HNC Compound angles at DEST:
   Dest HNC Bevel = 41.08703571
                  = 041° 05' 13.3285564"

   Dest HNC Mitre is OBTUSE.
   Dest HNC Mitre = 25.26632803
                  = 025° 15' 58.7809077"

Do not be confused by the OBTUSE warning for the mitre angle. This simply informs you that the saw swings horizontally to the LEFT at the Dest end of the strut. Thus the actual angle measured from the main fence is 90° + 025° 15' 58.7809077".