The easy was would seem to be to put 1/8 threasds on 1/8" rod.

To give an adjustable length, you either need Right and Left had threads on each end, or by putting 1/8 BSW 40 tpi on one end,and something different on the other end to get a differential thread effect, to allow adjustment.

Neither Zeus nor Tubal Cain quote 1/8 BSF, although Tracy Tools offer 1/8 BSF 44 tpi Taps and Dies.

Another approach, if you have the appropriate Taps and Dies available would be 5 BA giving 43.1 tpi with the thread a thou undersize, or 5 – 44 ANF, o r to turn down to 3 mm . The 0.5mm pitch gives 50.8 tpi.

HTH

Howard

Malcolm you might find this useful:
https://pages.cs.wisc.edu/~bolo/workshop/thread.html

Keith

BA isn't a good choice for this because it doesn't have fine and coarse thread ranges. 5BA is close enough to 1/8" and 43tpi.

Threading BSW on one end and BSF on the other would work except BSW doesn't go down to 1/8", and if it did I fear the thread would be too coarse. BSF in 1/8" diameter is exotic.

US Threads 5-40 (UNC) and 5-44 (UNF) might do: #5 is 0.125" with 40 coarse and 44 fine threads

Same idea with Metric M3 coarse (pitch 0.5) and M3 fine (p=0.35), but 3mm rod is thinner than 1/8" (3.2mm) if that matters. If fatter than 1/8" is acceptable, M3.5 coarse is p=0.6 and M3.5 fine is p=0.35.

Presumably the idea of using two different threads is to exploit the difference to get an extra fine adjustment? My tired brain remembers the method exists, but not what the rules are. For example would 1 tpi difference (5BA and #5-44) be more or less suitable than the 4 tpi difference between #5-40 and #5-44?

Dave

The idea is called the differential screw thread. If you have two pitches which differ by 0,05 mm ie 0.7 & 0.75 mm then one full turn of the threaded items will advance 0.05 mm. One thread is in a static member and one thread is in the sliding member.

I used this method successfully in a Micrometer Box Turning Tool recently described in Home Sop Machinist. This is used to run down long lengths of small diameter material in the lathe. (I have not listed a photo as I did not want to change the topic).

Regards

Gray,

S O D,

Sorry but 1/8 BSW does exist. I have a bag almost full, of such screws and nuts.

My suggestion of using different pitch threads on each end (Failing the availability of left hand threading tackle ) was to produce differential threads to provide, effectively, a vernier adjustment for length.

i.e Uscrewing one turn of 1/8 BSW moves the rod 1/40 of an inch Outwards, while the 5BA at the other end moves whatever is attached to it, 1/43 of an inch Inwards.

This will provide a fine adjustment of length. Which is what I imagined the OP wanted..

I don't think that the 5BA thread being 0.001" undersize will matter very much.

Howard

Mine go down to 1/8", but you can buy them down to 1/16".

Now that the post seems to be concluded I thought I would add another use of the differential thread principle that I used on my Squeeze riveter.

I wont bore you with why I chose the screw thread over the more conventional lever, but below is a close up of the working part.

The reader will clearly see the two different sizes of screw thread used. Below is the results on my Clayton Timber Wagon chassis.

If a torque wrench is used the tool gives extremely good and consistent results. There are a couple of more photographs in "My Album".

Regards

Gray,