Harold Hall

Workshop Processes

Machining a Barrel Shaped Connecting Rod, Harold Hall

Originally, I included this within my article on building the horizontal steam engine “Tina” but chose to separate it for my web pages as it may have interest beyond just that engine.


The pages are therefore just about machining a barrel shaped con rod as is illustrated in Sk. 1, It is an interesting item to make and worth going into it in detail as to the method. It is made from 3/4" x 1/2" mild steel and is turned fully round over a length of 3-1/4". However, this is not parallel but barrel shaped, being 3/16" diameter at either end and 13/32" diameter in the centre.


Not having made a part like this previously I considered the possibilities. The first idea that came to mind was to produce a template that could be used with a follower to control the cross slide as it was moved along the length of the rod. The cross slide lead screw would have to be disconnected for this to work. Another method that would require the leadscrew to be disconnected was to use a pivoted arm whose length was equal to the radius to be produced. As the radius worked out at about 12 inches the rear fixed pivot would have to have a mounting well away from the bed of the lathe.


As both these methods would involve a lot of work making the required facilities I decided that I would use a less engineered approach that had been going through my mind. This involved roughly turning it to shape and finishing with a file and emery paper.


Two methods were considered, the simpler, to leave the centre portion parallel and turn a section tapered at both ends. I decided though to adopt a method that would more closely follow the curve required. This involved starting from the centre and traversing the saddle a set distance then reducing the diameter by a calculated amount again traversing by the same distance before reducing the diameter yet again. The process would be repeated until the rod reached its smallest diameter when it would be turned end on end in the lathe and the process repeated to complete the barrel shape in rough form.


My first idea regarding this method was to use the four jaw chuck with the outer end supported by the tailstock centre. Each end would have to be say 1/4" longer so that in the final machining the centre drilled impression would be removed. However, I was very concerned regarding the safety aspect of using a file to finish the barrel with a large four jaw chuck rotating at high speed very close by and would not recommend this unless you have a smaller three jaw, or better still, a collet chuck.


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