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Adjustable segmented arm

I recently tried making a segmented arm, (similar to the type used to hold dial indicators), with 20 or so 1" diameter spherical segments. Each segment is cupped on one end and ball shaped on the opposite. The segments fit together -male/female, and a 7/16" hole is bored thru the ID through which a cable is fitted. When the cable is pulled tight the friction between segments make the arm rigid enough to hold a light load. The segments I made are about half as long as their diameter. While the arm works after a fashion, the segments tend to have an "over center" type feel as they are flexed under slight cable tension. The net effect is that the arm does not flex as smoothly as I would like. Is there a "magic" length to diameter ratio? Or perhaps the center holes I made for the cables are too large in relation to the segment outside diameter?

Reply to
oldjag
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I don't know, but thinking about the problem suggests that you need the 7/16" hole to go from the male hemispherical end as far as the centre of the sphere, then a small diameter from there to the female end. This would encourage the wire to follow a path that is always a radius of the spherical bits, that way the length should stay the same when the joints are moved.

Did any of that make sense?

Regards Mark Rand RTFM

Reply to
Mark Rand

The one I have has steel cylindrical segments with hemispherical cavities at both ends combined with spherical segments. I suppose they're in the 1/2" dia. area. Centered holes from end to end with a cable threaded through. I could never get the cable tight enough to withstand the minor spring pressure from the dial indicator, the thing always crept out of position under that load, even after totally degreasing everything. Kind of useless for an indicator holder. Neat idea, but the implementation in reality sucked. Going back to rigid arms.

Stan

Reply to
stans4

What's the cable diameter and material type?

Hul

oldjag wrote:

Reply to
dr

Well the cable is 3/16" dia 304 SS and the segments are aluminum. Today I put a nylon tube over the cable and this seemed to help a lot. When locked, it's definitly seems stiff enough to hold a dial indicator, but I need to measure the deflection vs. force. Mark's tapered hole idea sounds interesting, maybe I'll order some sort of tapered reamer to make another set of segments.

Reply to
oldjag

Doesn't have to be tapered, just big at one end and little in the male hemispherical end. I would do pilot hole/counterbore.

Mark Rand RTFM

Reply to
Mark Rand

Possibilities...

With a 3/16 cable the hole diameter at the center of the ball should be

3/16 also. In concept, this would require a cable that would make a perfect bend, ie bend with a 3/16 radius and fold on itself. If the cable can't bend so sharply, then the hole at the center of the ball would need a little greater diameter to compensate. Differently stated, the stiffness of the cable could compensate for a little extra diameter at the center of the ball.

3 more segments with a hole 3/16 plus a few thousands extra would be a starting point of a test. Try it once then drill a little more clearence untill there is no over center effect and no greater resistance when curving.

Hul

oldjag wrote:

Reply to
dr

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