More recently, drilling tools as well as milling tools have been developed, which, contrary to integral solid tools, are composed of two parts, viz. a basic body and a head detachably connected with the same and thereby being replaceable, in which head the requisite cutting edges are included. In such a way, the major part of the tool can be manufactured from a comparatively inexpensive material having a moderate modulus of elasticity, such as steel, while a smaller part, viz. the head, can be manufactured from a harder and more expensive material, such as cemented carbide, cermet, ceramics and the like, which gives the cutting edges a good chip-removing capacity, good machining precision and long service life. In other words, the head forms a wear part, which can be discarded after wear-out, while the basic body can be re-used several times. A now recognized denomination of such, cutting edge-carrying heads is “loose tops”, which henceforth will be used herein together with the concept “loose top tools”.
On rotatable tools of the loose top type, several requirements are made, one of which is that the loose top should be held centered in an exact and reliable way in relation to the basic body. Accordingly, each unintentional eccentricity between the center axis of the loose top and the center axis of the basic body should not be more than 0.01 mm. Another requirement or desire from the users' side is that the loose top should be mountable and dismountable in a rapid and convenient way without risk of incorrect mounting. Most preferably, mounting and dismounting should be possible to be carried out without the basic body necessarily having to be removed from the driving machine.
Drilling tools as well as milling tools (shank-end mills) of the loose top type are widely described in the patent literature and may be divided into a number of different categories depending on the ideas on which the designs are based. Thus, certain tools use loose tops having rear pins that entirely or partly (together with other coupling details) fulfill the task of centering the loose top in relation to the basic body. To this category belongs among others the tool disclosed in U.S. Pat. No. 6,012,881, which discloses a loose top drill in which a rear coupling part of the loose top is axially inserted in a jaw between two non-compliant drivers, the insides of which include axially extending, torque-transferring ridges that engage the corresponding chutes in the coupling part included in the loose top, besides which a centric pin protruding rearward from the coupling part is inserted in a center hole mouthing in the bottom of the jaw. With the pin, a screw mounted in a radial, threaded hole in the basic body co-operates, which has the purpose of locking the loose top in relation to the basic body. In that connection, the center pin is cylindrical and insertable at a close (form fitting) fit in a likewise cylindrical center hole, in order to, together with concave and convex contact surfaces of the drivers and the coupling part, respectively, fulfill the purpose of centering the loose top. However, the concurrent requirements of fit not only between the center pin and the hole, but also between the drivers and the coupling part, impose extreme, not to say unattainable, requirements of manufacturing precision. In addition, a conflicting problem will arise if good precision peradventure would be achieved, viz. that the mounting and dismounting of the loose top become difficult to carry out as a consequence of the centering pin requiring great force to be pushed into and pulled out of, respectively, the hole.
With the purpose of obviating the above-mentioned disadvantages of the tool according to U.S. Pat. No. 6,012,881, it has by EP 2266736 and EP 2266734 been proposed to form the centering pin of the loose top with, on one hand, an essentially semi-cylindrical contact surface concentric with the center axis of the loose top, and on the other hand a diametrically opposite clearance surface, the first-mentioned one of which can—by means of the screw of the tool—be pressed against the inside of a cylindrical centering hole in the basic body. In such a way, the centering pin can be given a cross-sectional area that is smaller than the cross-sectional area of the hole, the mounting and dismounting of the loose top being facilitated without the exact centering of the loose top being jeopardized. However, in this case, a 50% risk arises of incorrect mounting of the loose top in connection with the centering pin being inserted into the hole, in that the contact surface of the pin can be turned toward any half of the internal, cylindrical hole wall.
The present invention aims at obviating not only the serious disadvantages inherent in the loose top tool according to U.S. Pat. No. 6,012,881, but also the practical mounting inconveniences that have turned out to be associated with the tools and loose tops that are objects of EP 2266736 and EP 2266734.
An object of the invention to provide a loose top tool the loose top of which can, on one hand, be centered in an accurate way in relation to the basic body of the tool, and on the other hand be mounted in a simple way without risk of incorrect mounting. In other words, the operator should without mental effort be able to, in a foolproof way, mount the loose top in only one predetermined position.
Another object of the invention is to provide a loose top tool, the two main components of which, i.e., the basic body and the loose top, respectively, should be manufacturable in an effective and economical way while achieving the desired simplicity of the mounting.
Yet another object of the invention is to provide a loose top the centering pin of which is strong and robust.