Civil Engineering Reference
In-Depth Information
acy may be suspect unless you move from co-ordinate to co-ordinate in such a way
that avoids backlash becoming a problem.
The illustration shows all the required co-ordinates for drilling the hole positions
for a small steam engine bedplate casting. The machining datum is the top left inter-
section of the two datum lines. This is where all the co-ordinate dimensions are taken
from. I have also added letters to the hole centres to show the drilling sequence I
used.
Begin with the wobbler
First you have to find the edge of the back face and then the left-hand end of the cast-
ing in turn. Wobble off the back face first, moving the table very slowly until the wob-
bler rolls along the edge of the casting. Raise the spindle and move the table half the
diameter of the edge finder. Zero the dial (or make a note of the reading of the dial if
you cannot zero it). The centre of the edge finder should be over the edge of the job.
Move the table another turn so that the edge finder is over the job; the distance is
unimportant as long as the edge finder is on the end of the casting and not the corner
radius. Now you need to use the edge finder to find the left-hand edge, then move the
table half the wobbler diameter towards the middle of the casting. Zero the dial for the
axis for the edge you have just found. Wind back in the first axis you set so the edge
finder is a turn or two away from the job. Wind back to zero so the centre of the spindle
is over the datum point. You are now at the theoretical intersection point of the top left
corner of the job.
Fig. 11.19 Make a drawing before pitching out holes. It does not need to be high
quality, just an accurate sketch will do.
Centre drilling holes A-F
Now we can start to pitch out the holes. Move the table in the longitudinal axis to
0.225in so you are in line with the row of four holes at the left of the job. Centre drill
the first hole. This hole may wander slightly as it has already got a dimple from the
