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"Homemade" Screwed Rods

(Author: Paulo Kroeff de Souza)

Threaded bars are very practical and versatile parts that are, normally, available from fixings dealers. They are currently and easily available in sizes from some millimetres to a few centimetres in diameter and up to 3 metres in length. They have many uses as odd size fixings or small structural elements. Some special ones, with thread profiles more sophisticated than nut and bolt ones, produce linear movements in mechanisms. An example to observe in action, when you travel, are the airliner flap actuators (have a seat in a window behind the wing). As a professional engineer, I used threaded bars in a number of designs, and I did not learn about that in the engineering school, but way before, playing with Meccano screwed rods! Recently, in the internet I found the Screw Mechanisms S. M. 131 to 139 of section IX of the Standard Mechanisms Meccano Manual of 1925 and others appearing in subsequent versions of this publication.

Meccano screwed rods are parts that may originate from from threaded bars. However, it is very difficult, if not altogether impossible, to find 5/32” BSW threaded bars even in UK or in USA. Surprisingly, here in Brazil, I could readily obtain 5/32” UNC threaded bars. I even had the choice of buying them zinced or simply polished for a bit more than the equivalent of £1 for a 1m bar. Happily, the 5/32” UNC thread is 32 threads per inch, just as the 5/32” BSW. The UNC is just somewhat “fatter” than the BSW, that is: the BSW is deeper than the UNC. Therefore, if you have a 5/32”BSW die, you can convert a 5/32” UNC into a 5/32” BSW. Also surprisingly, it was very easy to obtain a pair of 5/32” BSW dies here in Brazil for something like £6 each. In “Mercado Livre”, a sort of Brazilian E-Bay, I found the suppliers for the rods and the dies. Of course you will also need a proper die handle. The figure below shows a piece of bar, the dies and a die handle with a die fitted.

 

For those not familiar with thread cutting with a die, I should mention that you normally rotate the die one to two turns into the rod and then backup a quarter to half a turn to brake the metal chips produced by the cut. Obviously, to cut a long part, this procedure will require hundreds of movements, which will take a very long time. For this reason, producing long threaded bars requires processes and tools quite different from the ones used for manual work. Happily, since we are not cutting the thread but just sort of trimming it, we can use the ordinary die with the part rotated in a conventional lathe. Of course, the lathe speed has to be quite low.

So, set the lathe to 500rpm or less and sparingly apply cutting fluid to the piece of threaded bar. Look for a cutting fluid, at your tool supplier, good for steel and, if you want to make brass plated rods, brass cutting. May be you will have to buy two different fluids. The cutting fluid is essential. Otherwise, the dies will wear very quickly and will damage the bar by breaking parts of the thread. I made two die passes and marked the die used for each pass because the die of the first pass will show more wear and will tend to produce a less accurate thread profile. This inaccuracy requires a second pass for correction.


After the two die passes, it is necessary to smooth the thread using steel wool. Contrary to what the photo shows it is advisable to use a protection for the fingers. I made some five passes with the wool and then the thread felt quite smooth when turned on bare fingers. I also inspected the parts with a magnifying glass to be sure they were smooth and clean, and then washed them in isopropyl alcohol for degreasing. I packed the parts, without hand touching, to take them to a galvanic shop for brass plating.

This way I produced four, part 81a, 1½” screwed rods, four 80d or X435, 2½” rods, both types quite difficult to obtain as Meccano originals, and two 4” rods, inexistent in the Meccano System. I also added two 17½” and two 23½” parts, those sizes relating to the 37-hole and 49-hole parts in a way similar to 11½” rods relating to the 25-hole strips and girders.

The plating service was quite good but, unfortunately, the coat applied was too thick and the thread became sticky. Then, I had to reprocess the parts with the dye and the steel wool to trim off the excess brass. I used the second pass die for the purpose, to obtain an accurate final thread. The coating was somewhat damaged, the parts looking like aged Meccano screwed rods.


If, your Meccano, has zinc screwed rods instead of brass screwed rods it will probably be easier to have your parts zinced and it will probably be easier to trim them after plating.

This procedure may result in rods of a somewhat lesser quality as compared to Meccano originals but they work well enough, are inexpensive and any lenght up to 1m is possible.

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