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
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
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
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 approach if you have a smaller three jaw, or better still a collet chuck.