Wednesday, July 06, 2011

Nose cone-from-tube experiment

I didn't completely follow the method outlined in the video attached to this post.  Here's what I tried, what worked, and what didn't. 

The attached photo is as of the last step.
  1. Start with a free  2.1875" dia. tube (thanks Stephen!)
  2. Mark 6 equally spaced spots around the tube near what will become the base.
  3. Draw lines to the end as you would to mark fins.
  4. Make marks half way between the lines, also on the base end.
  5. Draw lines from each line on the tip end to the intermediate marks on the base end.
  6. Cut out the resulting triangles using a Dremel with a diamond cut-off wheel (this worked great).
  7. Glue the tip ends together.  I went a pair at a time and use tacky trim and molding glue.  A small clamp was used until the glue held (this worked out well).
  8. The resulting shape was OK without the intermediate rings referenced in the video.  YMMV depending on the L/D ratio, the exact shape you want, etc.
  9. If you wanted to move up to, say, a 3" tube, I'd use 9 or maybe even 12 marks vs. 6.  The fewer the cuts, the chunkier the resulting cone
  10. I lightly sanded all cuts using a Dremel sander.
  11. I wanted to use 2-part foam vs. 5-min. epoxy as shown in the video.
  12. I basically covered the gaps in the cone using a paper mache mix. I slid a temporary foam plug into the cone during this step (not to shape it, but to keep its shape).
  13. Let it dry fully.
  14. One part of the 2-part foam had hardened during the 1+ yr. since I had last used it.  I cracked the shell and managed to get some out.
  15. Issue #1: I had 0.85 oz of one part and 0.7 of the other.  I should had separately poured them to equalize the mix.  The foam at the end of the cone was rigid and filled the gaps.  However, whether due to the mix or the fact that the foam was old, the foam at the tip end was cured but was mushy, for lack of a better term.  It also didn't fill the gaps very well.  I thought the gaps would bulge and I'd trim them.
  16. I removed the paper to expose the gaps.
  17. I compacted the squishy foam with a thin dowel and then back filled all the gaps with Fill'n'Finish.
  18. Sand, repeat.
  19. Issue #2:  The overall look of the cone at this step was pretty good.  However, after several filling/sanding cycles, the gap areas appear to be flatter than the cardboard area.  Close enough for this try.
  20. Cut the cone off the tube and make a shoulder using the 'section and rejoin method'.
  21. I laminated the cone with casting epoxy.  The project I got it for fell through so I might as well try to use it.  This stuff is slow cure to the max.  The instructions say the thicker the part, the faster the cure and estimates 24 hrs. for a 1" part.  I periodically heated the lamination with a heat gun and it was set the next morning (~18 hrs.).
  22. There are some flaws that I was hoping the fill with several coats of the epoxy.  However, the cure is too slow for my level of patience.  I'll prime it, paint it and post a final photo at that point.