Next make fixture 3 as follows. Cut a length of 1-1/2” diameter steel (or larger)
, place it in the four jaw chuck and face both ends. Then, with the part still in
the chuck set it 3/8” off centre. That is, 3/4” total indicator reading, or use a
3/4” thick test piece using the method suggested for the eccentric sheave. With that
done drill through 19/64” diameter.
When setting a part off centre in the four jaw it would seem logical to use a pair
of opposing jaws to achieve the offset required and the other pair positioned equally
just to secure the part. However, if you are using a small lathe with a nominal 3”
chuck you will find that one jaw is too far out of the body. The amount of eccentricity
can though be increased by making the point of maximum throw between two adjacent
jaws. If you have a large chuck then either method can be adopted.
Drill and tap for the grub screws and drill and fit the pins using a suitable adhesive.
File the inside faces of the pins to achieve a close fit on the Crankshaft webs.
Cut out the waste, assisted by the hole already drilled in the crankshaft, leaving
just a little to machine away. Fit the Crankshaft into the fixture and secure firmly
using the grub screws. Place the assembly into the four jaw and set the fixture to
run true and machine the Crank pin, Photograph 65. If you have a large enough three
jaw then this can be used but in either case use the standard and not the reverse
jaws as you need to achieve the best grip and accuracy.
Using the rear tool post parting tool, very carefully plunge it in leaving the left
crankshaft flange just a little over on its width and stopping when you arrive at
a complete circle being turned. Leave in the fixture and repeat for the right hand
flange. There will still be a portion in the middle to be removed but do not do this
by plunging the parting tool but by traversing it left and right gradually reducing
the diameter until you are down to the diameters already produced. Now continue to
reduce the diameter by this method to achieve the 9/32” diameter required followed
by skimming the two flanges to open up the space to 5/16”, at the same time ensuring
the the flanges are the same width, say within a few thou.
If you do not have a rear mounted parting tool, do attempt to remove as much as is
possible by other means,such as sawing, filing, drilling small holes, etc. before
finalising the process using a top slide mounted parting tool and traversing it left
and right as mentioned above.
Having opened up the space between the two shafts it is possible that internal stresses
in the material, now being relieved, will have resulted in the shafts being very
slightly out of line. Therefore, return the part to the lathe, mounting it between
centres, and reduce the first shaft to 9/32”. Remove, rotate, refit and turn the
other shaft similarly.
If though you have made the part longer and now wish to remove the drilled centres
use the three jaw to hold one shaft whilst the other is machined to remove the drilled
Do wind a piece of soft copper strip around the shaft being gripped to avoid it being
damaged. Photograph 66 shows the two completed crankshafts together with fixture