2. Side rake (1) is ideally chosen to suit the material being machined, with 10 to
15° for mild steel and 20 to 25° for free cutting mild steel and aluminium being
normal in the case of a knife tool. However, in the case of a boring tool, increasing
the angle will reduce the strength of the head, (reference to Sk. 2 should make this
obvious) and a compromise has to be adopted. In this case 10° would be about right
but could be increased for larger boring tools which will have more substantial shanks.
Incidentally, the side rake for the commercially available solid carbide tool seen
in photograph 1 is zero degrees.
Whilst Sk. 2 quotes angles these are not critical, they are worth considering as
the minimum, a value of say 10° will still function OK but do attempt to get nearer
the 5° quoted as at this value the cutting edge/tip will be more robust and need
sharpening less often, though I doubt if you will notice the difference. In other
words, the effect in the home workshop is more theoretical than apparent, in industry
where every second counts it is different.
Having discussed the background details how then is the boring tool ground? Whilst
there is no essential order in which the facets are ground I will describe one that
works well and illustrate it using my controlled feed grinding rest. The operations
will be much the same even if ground manually
The Grinding Sequence.
First grind the side rake (1), Photograph 2 showing the operation and Photograph
3 the result for the three cutter sizes I am making (4, 5 and 6mm blanks). This is
followed by grinding the front clearance (4) and front relief (5) both being achieved
in the one operation, Photograph 4, again the result is seen for the three cutters
in Photograph 5. Secondary clearance (Z) can now be ground as seen in photograph
4 but at a greater angle.