Making of an Accurate Hole with a Boring Bars

Cylinder boring 
  Cylinder bores of cast iron cylinder block were machined earlier with multi-blade cutters-firstly with grind type and then with insert type. Heat and distortion of the cylinder bores were minimised through positive cutting geometry of the blades that reduce the cutting force imparted to bore. Presently, single point ceramic tools with various means of size compensation are used for finishing the cylinder bores. Sometimes, the single spindle high speed boring takes a semi-finish cut in forward downfeed and then, switches over to finish cut in upward feed. The machine may be with double spindle with adjustable centre distance, if production rate is high enough. Bore pitches are adjustable through numerically controlled worktable slide. With the advent of aluminium cylinder block, tooling techniques have further improved productivity. In one case, a tool head featuring three MAPAL finishing cutting edges, three diamond guide pads and a taper hollow shank with face contact (HSK- DIN 69893) has been used for precision boring of cylinder bores that enables at least triple feed at the same cutting speed. Compared with single point machining, the number of parts per tool change increased from 10,000 to 25,000 cylinder bores. The surface finish improved from Ra 1.0 micron to 0.2 micron, the straightness changed from 8 microns to 4 microns and the roundness improved from 13 microns to 3 microns. Further, liners could be pressed without cooling (earlier nitrogen cooled to -150 degree centigrade) in aluminium block. The contact area of liners gets increased from 60% to over 90% that resulted into substantially better thermal conditions. A very exact face of the counter bores at the top of the bore to locate the cast iron liner is machined through two slides – one working slide and one balancing slide because of the high rpm used. Some semi flexible special machines have been used to machine the joint face and the cylinder bores in one setup, that results in an especially high accuracy of the cylinder block.