LEG RELEASING LEVER FOR A BOW COMPASS

Abstract: This lever of the first class with accessories for releasing and allowing quick movement of each leg (5A, 5B) of a bow compass has an intermediate bearing fulcrum (3) which is kept in contact with the surface of the leg (5A, 5B) by the thrust of a reaction spring (11) and is assisted in this by the cooperation of two transversely-retaining side plates (4A, 4B) engaged against their respective leg (5A, 5B) and comprising auxiliary inward fixing projections (4C, 4D) at their rear end. Said reaction spring (11) directly acts on a bush (12) carrying a half lead nut (16), so reducing the amount by which the operating end (1) of the lever (L) projects beyond the plane of the leg (5A, 5B) on which it is mounted. The bush (12) performs a fixing function by its insertion though a hole (6) provided at the reaction end (2) of the lever (L) followed by its rotation through 90° to cause undercut elements of the head (9) provided on the bush (12) to engage with said reaction end (2) of the lever.

Description: This invention concerns the field of bow and like compasses of the kind recited in the preamble of the main claim. To allow precise adjustment of the degree of opening of compasses and to prevent them opening further during use it is known to use a transverse screw engaged with their legs. This method distinguishes the bow compass class. The incorporation of a screw does not however allow the legs to be moved outwards or inwards with speed, this problem being overcome by using one of two possible expedients, namely making the screw thread of elongated pitch (to the point where motion is reversible), or using a disengageable lead nut. This latter method has found widespread use, especially in larger bow compasses of normal compass size. GB-A-720.853 discloses (fig. 5) a bow compass comprising legs movable by a screw engaging nut halves of bushes slidably mounted in said legs, each bush cooperating with a first class lever to be disengaged from the screw against the reaction of spring means for releasing and allowing quick movement of each leg, each lever being fulcrumed on the external surface of the corresponding leg and being biased by the spring means. The lever represents also the spring means inasmuch as the lever constitutes a resilient tail-like extension of a split collar. An annular recess must be provided in the leg to accomodate the spring collar. The nut half is irremovably fixed to the lever and in order for the lever to operate as such a fulcrum-like shoulder must be provided on an enlargement of the leg. The enlargement and the annular recess complicate the form of the leg and the fixing of nut half involves a mechanical operation. Two nut halves and two levers are provided in each leg. This double half nut system the purpose of which is to allow the adjustment screw to engage with a large (double) lead nut area, has the drawback of inevitable play between the two coaxial bushes which have the half lead nut portions at their ends. When this play between the two bushes is added to that between the outer bush and the hole in the metal leg into which it is inserted, a certain mobility arises in the connection between the two compass legs, the compass therefore being inaccurate. In addition to this basic drawback said double half lead nut method has the disadvantage of greater complexity, resulting in possible accidental jamming during the release operation and in particular increased cost. The inventive features of the novel bow compass are recited in the characterizing clause of the appended main claim. The invention is illustrated by way of non-limiting example on the accompanying drawings, in which: Figures 1, 2 and 3 are conventional orthogonal views of a lever; Figure 4 is a section through the lever on the line A-A; Figures 5, 6, 7, 8 are conventional orthogonal views of a bush comprising a half lead nut; Figure 9 shows a spring for preloading a half lead nut against a usual transverse adjustment screw; Figures 10 and 11 represent a bow compass showing, in an assembled state to illustrate their operation, the levers and their accessories for releasing and allowing quick movement of the legs. With reference to the aforesaid figures, each lever L is provided with an operating end 1, a reaction end 2 and a fulcrum 3. This configuration shows clearly that the lever is of the first class. At the sides of the fulcrum 3 there are two plates 4A and 4B provided at their ends with a retention tooth 4C and 4D which hook behind the preferably quadrangular cross-section 5S of the compass legs 5A and 5B. The outer surface of the operating end 1 is provided with anti-slip projections 1A. The reaction end 2 is provided with a rectangular hole 6 within a circular seat 7 having a flat base 8. The rectangular hole is provided to allow a head 9 of conjugate rectangular profile to pass through. The head 9 comprises undercuts 10 which bear against the flat base 8 at the reaction end of the lever. This bearing action is obtained by inserting the head through the hole 9 in the direction of the arrow F and then rotating it through 90° as indicated by the arrow G. This is done during assembly for each of the two legs 5A and 5B of the compass after inserting the bush shown in Figures 5, 6, 7, 8 into a spring 11. By this means, said spring 11 is preloaded between its support edge 13 on the bush 12 and the flat surface 14 of the respective leg 5A, 5B. The leg is provided with a hole 15 through which the bush 12 penetrates so that its head 9 can engage the reaction end 2 of the lever L. The bush is provided with a half lead nut 16 for engagement with a usual transverse screw with a central wheel 17G. This screw has a right hand thread 17A on one end and a left hand thread 17B on the other end. For this reason the two bushes 8 are given conventional markings to enable them to be distinguished during assembly and thus correctly associated with the right hand thread 17A or left hand thread 17B of the transverse screw 17. After the lever L and its accessories 11, 12 have been mounted on the leg 5A, 5B, the thrust of the spring 11 causes the reaction end 2 to rest securely against the upper flat surface 18, so that the system is independent of the presence of the screw 17. When assembly is complete, the insertion of the ends of this screw into their respective slot 19 is facilitated by operating the lever L. When the lever L is released, the half lead nut 16 bears against its respective screw portion 17A, 17B, and the end 2 bears against the upper flat surface 18. On operating the end 1 of the lever the relative half lead nuts are disengaged from the screw, to allow fast inward or outward movement of the legs.

Claims: A bow compass comprising legs (5A,B) movable by a screw (17A, B) engaging nut halves (16) of bushes (12) slidably mounted in said legs, each bush (12) cooperating with a first class lever (L) to be disengaged from the screw (17A, B) against the reaction of spring means (11) for releasing and allowing quick movement of each leg (5A, B), each lever (L) being fulcrumed on the external surface (18) of the corresponding leg (5A, 6B) and being biased by the spring means (11), characterized in that the spring means reacts between the internal surface (14) of the leg (5A, B) and an abutment (13) of the bush (12). A bow compass according to claim 1, characterized in that the fulcrums are represented by projections (3) of the levers (L) and the fulcrums (3) are assisted in keeping contact with the external surfaces (18) of the legs (5A, B) by the cooperation of two transversely-retaining side plates (4A, B) engaged against the legs (5A, B) and comprising auxiliary inward projections at their free ends for engagement with the internal surfaces (14) of the legs (5A, B). A bow compass according to claim 1 or 1 and 2, characterized in that the bushes (12) are engaged with the reaction end of the levers (L) through a bayonet joint (6,8,9,10).

Inventors: ARICCI, LUCIO
Applicants: ARICCI COMPASSI S.N.C. DI ARICCI GILBERTO & C.
Application year: 1988
Main classification: B43L9
IPC codes: B43L9