by Jay Freeman
Recurring looseness in my NexStar 8 tripod prompted a fix. The tripod is the early model, with telescoping legs made of lengths of satin-finish aluminum extrusion. I believe similar tripods are used for other telescopes, not only Celestron's but also other brands.
Each telescoping leg uses three lengths of aluminum extrusion. Two lengths, spaced a little apart, form the upper part of the leg, which is attached to the tripod head. The third length slides between the first two. The extrusions are shaped to guide the sliding piece.
Each upper extrusion has an end fitting which fits into its top end, that is pierced for a long bolt. The bolt goes through one end fitting, the tripod head, and the other end fitting, to a washer and acorn nut.
The problem is that the end fittings are plastic, and are fastened in place with little sheet-metal screws that grip poorly. The screws loosen up easily, and after being retightened a few times, the inside surface of the plastic holes becomes too scuffed for them to take hold.
Briefly, the fix was to replace the sheet metal screws with machine screws and T-nuts. A T-nut is a thin-walled metal cylinder, threaded on the inside, with a flange at one end. Seen from the side, it resembles a letter "T" with a very thick vertical stem -- the cylinder. T-nuts are available in hardware stores in many sizes.
Size 6-32 T-nuts had outer diameters slightly larger than the holes in the plastic end fittings. There was enough extra plastic so that I felt comfortable enlarging the holes, but I worked carefully, in small increments, inserting drill bits in a hand drill and twisting the chuck by hand. The plastic was so soft that using a power drill, or even cranking the handle of a hand-powered unit, might have risked cracking or melting the material, or having the bit not follow the existing hole. To fit the T-nuts I used, I bored out the holes from their original diameter of about 1/8 inch to 13/64 inch, and then counterbored the inside end of the hole about 1/8 inch deep, using a 7/32 inch drill. The counterbore was because the T-nut barrels tapered slightly.
The T-nuts I used were 1/4 inch long. A length of 3/8 inch might have worked, or might have been a hair too long. The T-nuts had four barbs sticking down from the flange, parallel to the barrel. They were intended for use in wood or similar material to keep the nut from rotating in place. By coincidence, the barbs were placed just right to fit the inside of the plastic end fittings. I did not have to modify the barbs or flatten them out.
I inserted the T-nuts from the inside of the end fittings, then reattached the end fittings to the extrusions using 6-32 x 1/2-inch round-headed machine screws, with a star washer and a flat washer between the screw head and the extrusion. With the screws tight, and the bolts that attach the end fittings to the tripod head snug, the tripod was a lot more rigid.
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