Hello Everyone,
In this video I will discuss the hub design for this dome and the next video in the series
will detail the strut design. This is 3V 3/8th dome so it will require a total of 46 hubs
consisting of twenty-five 6-way, six 5-way and fifteen 4-way hubs that will be used along
the base. Because of the large span, this dome is going to need a hub that can withstand
a great deal of compression force. Plus, it must be able to withstand the downward force
from heavy snow loads during the winter.
The first hub design had hanger bolts anchored into the end of each strut. The end of each
bolt would go through a hole in a piece of schedule 40 3-1/2” steel pipe which was
used at the hub. The strut was tightened to the hub with a washer and nut. This is the
first prototype using the hanger bolt method and a PVC hub. The triangulation between each
section was extremely strong. When I created a full scale prototype, the leverage of a
full length strut and removal of the triangulated pieces cause the hanger bolts to easily pull
out of the end of the struts.
The second hub design was similar to the hanger bolt design, except a hole was drilled through
the strut and a washer and nut was inserted. This was to create enough surface area to
prevent the bolt from pulling through the wood. However, testing a full scale prototype
proved to be too much force against the strut and the bolt acted as a level and split the
wood.
The third hub design is completely different. The strut is sandwiched between two spokes.
Flat bar-stock is welded to the hub at the appropriate angles and a bolt is placed through
the top spoke, through the strut and fastened with a nut under the bottom spoke. This full-scale
prototype shows how 4 spokes of a 6 spoke hub would be assembled. A load test shows
it can support my body weight on only 4 spokes, plus none of the other struts are used to
strengthen the legs. There was some slight bending of the bolts from my bouncing but
there was no hardware failure. Regardless, the final design will use 3/8th inch bolts
instead of 5/16th.
The central hub is made from 3 ½” galvanized rigid conduit which is about ¼” thick.
The conduit is cut into 3 ½” lengths. The galvanized zinc is ground off wherever the
spokes are to be welded. Please note that working with galvanized material, especially
using abrasion cutting equipment or welding should be done in a well-ventilated area.
Also, you should always wear gloves and safety glasses.
I made up a jig that will safely hold the bar stock to the drill press. It allows me
to consistently drill a pilot hole in the same location near the end of the stock. Since
I have 480 holes to drill, creating jigs and templates is crucial for building an accurate
hub. Once the pilot holes are drilled, I switch over to a stepped bit that will finish the
hole at the proper size. I then flip the piece over and gently remove any tear-out from the
bottom of the hole. Once the holes are in the end of the stock, I then cut it to the
correct lengths. I built a stop for my abrasion saw so I could cut each piece to the same
length. After cutting off the ends, I can go back to the drill press and drill a new
set of holes for the next set of spokes.
Now it’s time to start assembling! This template has markings on it so I know where
the locations are for the 5 spoke or 6 spoke hubs. I just place the hub over the template
and draw a small mark on the hub.
This jig is used to accurately weld the spokes to the hub. The hub is placed over the post
and each spoke is held in place with a pin. By using this jig, I can weld a complete hub
in less than 15 minutes.
All the pieces are welded together with a standard MIG welder. Another safety note:
Please weld in a well-ventilated area and free from items that can catch on fire. Also
wear the appropriate gear to avoid burns from the hot metal and use a welding helmet to
prevent blindness. Keep fire suppression equipment nearby at all times. No one else should be
in the area while using a welder.
When each hub is complete, any dirt and rust is removed by sandblasting and is then painted
with cold-galvanizing paint to prevent future rusting. I hope you enjoyed this video on
making a hub for the geodesic dome. The next video will show how the struts are made. Thanks
for watching!