Part 2: Fixed and Adjustable Boring Tools,
Special Lathe Tool Holders, Milling Cutter, Speeds and Feeds
by Paul Grodinsk
This article first appeared in the Volume 4, Number 42 (April, 1942) issue of Aircraft Production magazine, and is presented here through the kind permission of Flight International. Thanks also to Bruce Vander Mark for furnishing volumes of Aircraft Production for scanning.
The rapid advance in the use o f diamond tools for final machining operations is well illustrated in this concluding installment that describes some o f the most recent types of holding fixtures. In one example all the ring grooves on a piston may be machined simultaneously by multiple tools, three for each groove, held in a single-piece holder. In this article mention is made o f the employment o f diamond tools for surface milling, a subject of one of the many patented devices included in this review.
A much more elaborate design is required for boring tools than for those used for turning, due to the fact that the boring bar is required to perform duties usually carried out by the lathe compound slide. It is only comparatively recently, however, that the problem has been tackled in a satisfactory manner and practical designs evolved to meet the need for finely adjustable, but rigid tools. Care must be taken to distinguish between boring bars fitted with fixed tools—adjustment being made by moving the bar off-centre, and those which revolve on a fixed centre but are equipped with tools which may be adjusted in a vertical position in relation to the axis of the bar. There are other types of tools which have the shank permanently fixed in the holder, but with these there is no provision for adjustment. Another type has a rotatable head (similar to those in Part 1, Fig. 16), which permits a certain amount of adjustment both for size and the angle of the cutting edge.
Earlier types had a cylindrical tool shank inserted in the boring bar and clamped by means of a screw, the point of which pressed on the shank in a vertical direction. This primitive design had the disadvantage that when actuating the screw the setting of the shank was often disturbed. Consequently, direct-acting screw points, which also are liable to damage the tool shank, should be avoided and, if this is not , possible, a pad of soft metal should be inserted between the shank and screw point.
The principles under (a) and (b) can only be applied to relatively large boring bars with a minimum diameter not less than approximately 3/4 ”, and using tool shanks of about 3/8” diameter. Those under (c), however, are suitable for shanks as small as 1/8” diameter, and may be incorporated in boring bars of 3/8” to 1/2” diameter. A special advantage of types (b) and (c) is that the tool may be clamped before the final setting is made, so eliminating chances of movement which may produce inaccurate results. In the following descriptions main representatives of each type will be discussed.
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