Civil Engineering Reference
In-Depth Information
Enlarging a hole by offsetting the cutter
Sometimes you will not have a drill large enough to drill a hole of the size re-
quired. In this case, it is possible to open the hole out larger with a slot drill
or end mill. Say the hole required is 22mm in diameter and the largest cutter
you have is 20mm; all you have to do after drilling the hole is to put the cutter
through the hole at the centre point then put it through the hole at 0, 90, 180
and 270 degrees, all offset by 1mm. Then you can finish the hole by boring.
This is a lot quicker than trying to remove 2mm from the hole with a boring
bar.
Fig. 11.18. Exaggerated view of using an offset cutter to enlarge a hole. The blue is
the original hole and the red shows the four offset holes. If you look carefully, you can
see the lobbing at the top and bottom and left and right.
PITCHING OUT HOLES USING CO-ORDINATE DRILLING
Set up your component on the milling machine's table and select a datum for the ma-
chining. If you have a digital readout, all you have to do is carefully pitch out the holes
and they should be accurate. However, if you do not have a digital readout the accur-
