Turnbuckle style control rods

Hi Chaps,

I want to make some "turnbuckle" style control rods for my RC toy aeroplanes but I am stuck for a technique / tool for making them. For those who don't know, the items I'm referring to are ally rods threaded at both ends, one end with a rh thread, t'other with a lh thread. This means that when the rod is turned the fittings screwed to either end move together or move apart. This saves having to detach one of the fittings to adjust the linkage. Making these threads is not the issue at all, I know how to do that.

The problem is I want them to be thicker in the middle than at the ends. But they are pretty thin anyway (say 5mm in the centre, tapering to 3mm at the ends) and they are around 200 mm long. This means that any attempt to taper turn them is doomed because you can't use the normal travelling steady on a taper, and they are so thin they'll just bend.

So my question is : Is there a taper turning mechanism that allows the tool to move back while at the same time the steady moves in the opposite direction ? (ie the tool and steady either move closer, or further apart).

I have vague ideas in my head about maybe having the steady and toolpost on the same slide but each having oppisite handed nuts and the feedscrew being comprised of two sections of lh and rh thread. This would give the motion I require, provided this feedscrew could be driven at an adjustable proportion of the lathes' main leadscrew rotation rate (so I can set the taper).

Anyway, I just wondered if there is a mechanism like this (maybe in an old ME ?), or if anyone has any ideas about it ?



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There is/was a turret tool made to do that sort of thing, though whether it would be viable on thayt sort of length/diameter, I have no idea. I've got one somewhere, if I can lay my hands on it I'll post a pic. It's incomplete in that anything like that will need linking to a fixed point.


Dutton Dry-Dock Traditional & Modern canal craft repairs Vintage diesel engine service

Reply to
Tim Leech

Use a Vastly Expensive and Complicated Machine to make them.

Or else find some cheap commercial product that already makes use of similarly tapered stock, probably by using the aforementioned VEACM. Butted bike spokes any use to you?

I imagine the commercial solution to this is a rolling mill.

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Boo, Save yourself the bother and use a carbon fibre tube instead. Stiffer and lighter. Haven't bought any lately but kite shops were a good source.

Cheers Henry

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Have you really, really, really got a requirement for this loss of weight?

Assuming by "ally" you meant aluminium then assuming a density of

2.7g/cm3 and ignoring the weight loss due to the thread form, a plain 5mm diameter 200mm long rod weighs around 10.6 grams

Having a 10mm threaded section at each end and tapering the central

180mm from 5mm at the centre to 3mm results in a weight of around 6.6 grams, a difference of 4 grams per rod in exchange for what could be tens of hours of machining for a full set of rods.

So, have you really, really, really got a requirement for this loss of weight?

If so then take a 1mm dia aluminium rod former, glue on the 3mm threaded ends and then wind a couple of mm thick of carbon tow soaked in epoxy along the whole length of the 1mm section, overlapping the threaded rod bond line. The overall volume will be less, stiffness will be much higher and you will also get a significant weight reduction, down to about 3.5 grams if I got my calculations right.

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8< stuff about turning thin tapered rod >8

Don't be put off by those who say you are wasting your time. Could you rig up your travelling steady on a slide (rigid along rotating axis, movable perpendicular to this) and use a pulley and weights to provide the pressure to resist the cutting force?

Your idea of opposing screws has merit; you can achieve the same thing many other ways. Inter alia, a gear with a fixed axle, with a rack either side of it; two levers with the fulcra in line and a rigid diagonal connection.


Reply to
John Montrose

If you really must have rods that are thicker in the centre than the ends, how about:- Start with 5mm (3/16") 22-20SWG wall tube. Make a pair of slightly domed, smooth rollers for a scissors type knurling tool. Swage the tube down with the rollers on the lathe. Adjust the rollers by hand as you traverse along the spinning tube.

Might not be worth the hassle, and might not work, but you don't know until you try :-)

Alternatively, run solid bar in the lathe, use a garden sprayer full of white spirit for swarf removal and just and hold a bit of 120 grit wet and dry paper around the bar until it is of the shape you want. It probably wont take very long at all.

Mark Rand RTFM

Reply to
Mark Rand

Why?, just do what I and every other modeller does and rotate the turn buckle to change the pushrod length.


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Well, thanlks all for the replies. I've pretty much decided against it but istm that something based on a parallellogram with a fixed point to the bed might do the trick. I think I'll either have straight section rods or else only taper the ends (just for the look), that way I can use a travelling steady on the fixed portion and the top slide angled over to make the tapered section. A bit of cut and try required I think...

Thanks again,

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