Fairing Model: Hot Wire
Dateline: 05/11/99Moving the Tool
Now that most of the foam is on the tool, it's time to move it to the winder and finish it up. The center bay has been left open for access to the lifting point.
As we lifted the tool, some of the foam panels started coming loose. Just putting the panels in place and filling the gaps with Liquid Nails hadn't formed a very good bond. To tighten up the foam, we injected some more Liquid Nails in the gaps and cinched the panels down with ratchet straps.
Once we knew the tool would survive the short trip to the winder, we put the foam on the bottom half of the central bay and lifted the entire thing with the overhead crane.
Excessive Sag
Once the tool was on the winder, the sag seemed to be much greater than what we had measured when it was supported on the ladders. The sag should have been smaller, because one end was clamped in the winder's chuck. But the sag at the tail end, which was simply supported, was noticeable.
At this point, we started brainstorming for solutions. We had already passed the point where it would be feasible to replace the shaft (as we should have done long ago).
The simplest option would be to do nothing. The design called for us to lay up a ply or two of glass fabric, let it cure, and then wind over it. The purpose of the fabric plies was to protect the foam from the winding tension. However, we thought it would help to stiffen up the tool.
Unfortunately, after getting the tool on the winder, we realized there wasn't enough clearance for the winding head. We could use the winder to support the tool and to rotate it, but we couldn't actually wind. That was an easy problem to fix--we switched to an all-fabric layup--but we still had the immediate problem of the sag.
Another option, which was my preferred solution, was to place a roller under the center of the tool. This would eliminate the sag, but would damage the layup as we rotated the tool. I thought we could solve that problem by making local repairs after the layup was complete. Thus, we had the machine shop start building the rollers.
While that was going on, one of our technicians came up with a simple idea: he hung a strap from the overhead crane and lifted up on the shaft near the tail end. This eliminated just about all of the sag.
Hot Wire Cutting
With the tool finally in place and the sag eliminated, we installed the rest of the foam and were ready to shape the tool.
The bulkheads had been sized to the nominal inside diameter of the part. To cut the foam down, we simply built a hot wire bow which spanned between the bulkheads, and used the bulkheads as guide templates.
As we rotated the tool, we found there was still a bit of flex. The foam started separating in a few spots.
Patching the Tool
To tighten up the tool, we decided to strengthen the foam joints and the foam-bulkhead joints. In most cases, the joints were still pressed together. Without gaps, a thick adhesive like Liquid Nails was not necessary. Instead, we simply drilled some holes along the joints and injected an epoxy.
The remaining gaps were fairly narrow. Inserting blue foam would be difficult, and we'd still have to inject epoxy to get a good bond. It was time for another trip to Home Depot, this time for Great Stuff expanding foam (normally used for insulating cracks in homes). The Great Stuff filled in the gaps quite nicely, formed a good bond, and could even be cut with the hot wire.
With all of the foam in place and roughly cut to shape, all that remained before starting the layup was to finish the surface.