Remove the standard from the fixture and the fixture from the faceplate and reposition
the fixture on the standard's feet again securing it with a central screw. Adjust
the feet such that the four holes will be accurately placed, using the scribed lines
on the fixture as an aid in positioning the standard lengthwise and place it central
across its width checking with a rule.
With a 2.1mm drill spot through to position the holes in the feet, remove and reposition
the fixture on the standard's flange and drill through 7BA clearance, Photograph
Return the fixture to the faceplate after first having tapped the four holes 7BA
and mount the standard onto it setting its flange to run reasonably true, using the
fixture's fixings to provide the means of adjustment. The flange can now be machined
on its three surfaces with Photograph 14 showing typically the process for the inner
face. With the standard still on the faceplate bore the through hole, Photograph
15, using the 0.625” hole gauge made earlier to enable a precise diameter to be achieved.
Drilling the holes in the flange will be detailed later.
When attempting to turn accurate diameters, inside or outside, set the top slide
to approximately 6° which gives nominally 1 unit of radial movement for 10 units
of axial movement of the top slide, typically 0.1 thou radial for 1 thou axial making
it much easier to set on very small depths of cut.
Boring holes can sometimes be a problem, especially if near the minimum diameter
for the tool being used, as unlike turning outer diameters where the curvature of
the workpiece increases the clearance to the tool it is decreased in the case of
an internal diameter. If the tool is not cutting as it should try raising the tool
above centre which will then increase the clearance, often eliminating the problem.
At this stage make fixture 2 but for this, and some later operations, it is necessary
to know the lathe's centre height above the top slide's surface. To determine this,
place a short piece of steel into the three jaw and turn a short length to a known
diameter. Move the top slide under this and check the distance between it and the
top slide using some packing pieces and feeler gauges. The distance required will
be the packing thickness plus half the diameter of the turned part, make a note of
this as it is also required for later items. In the case of fixture 2 the dimension
is not overly critical and a rule dimension will suit, greater accuracy is required
for latter operations.
With that done, mount the casting in the four jaw chuck, face the outer end and bore
as shown in Photograph 16, using the 0.750” hole gauge already made to check the
diameter. This process ensures that the bore and the end are accurately at 90°, also
necessary for the second end. To achieve this, reverse in the chuck paying particular
attention to the machined end being firmly against the face of the chuck and face
the second end.