BLIND HOLE ANCHOR NUT FASTENER

Abstract: A blind hole anchor nut fastener comprises a tubular sleeve (10) and a screw-threaded nut (14) at the end of the sleeve, so that when a screw-threaded mandrel is screw-­ engaged with the nut, and is displaced axially in the appropriate direction, the nut can shear (at 16) from the sleeve and be drawn into the sleeve in wedging engagement. The basic nut shape (18) is modified by making it non-­ circular such as trochoidal (for example) at one end so that in the displacement into the sleeve the non-circular shape helps in anti-rotational hold relative to the sleeve and the reduced volume minimises actual extension of the sleeve under the wedging action so as to reduce the possibility of the sleeve splitting.

Description: BLIND HOLE ANCHOR NUT FASTENER This invention relates to blind hole anchor nut fasteners. British Patent No. 1093714 describes a blind hole anchor nut fastener which comprises a tubular sleeve and a screw-threaded nut disposed at the end of the sleeve, the arrangement being such that when a screw-threaded mandrel extending through the sleeve and engaged with the nut is displaced axially in the appropriate direction, the nut shears from the sleeve and is drawn into the sleeve in wedging engagement therewith. Effectively, the sleeve is expanded by the nut, and to enable this expansion to take place the nut is generally frusto-conical, with its smallest end at the shear point and of a suitable diameter to enter the sleeve. These anchor nuts are extremely difficult to manufacture satisfactorily. The tolerances are small, because if the nut is made too small relative to the sleeve it will not wedge satisfactorily, and if it is made too large the sleeve splits. If the shear point is too weak, nuts may shear as a result of handling in manufacture and transportation, and if the shear point is too strong, shearing will not take place but the sleeve will bell-out and be unsatisfactory for that reason. Satisfactory manufacture is possible, indeed it is believed that nuts of said Patent have been extremely successful commercially but this has only been so because of great care in manufacturing within the prescribed tolerances, and with great difficulties in selecting material. The object of the invention is to provide an improved nut of this kind which is capable of giving satisfactory results within wider tolerances of manufacture and/or material. In accordance with the invention, a blind hole anchor nut fastener device of the kind comprising a tubular sleeve and a screw-threaded nut disposed at the end of the sleeve, the arrangement being such that when a screw-threaded mandrel extending through the sleeve and engaged with the nut is displaced axially in the appropriate direction, the nut is drawn into the sleeve, is characterised in that the nut has at least one face which lies on the surface of a frusto-cone having its minimum size near the sleeve and which frusto-cone is co-axial with the fastener generally, and the nut also having at least one face which is arcuate about an eccentric axis. Preferably the face which is arcuate about the eccentric axis is concentric to that axis (but not necessarily so). According to a feature of the invention this further face (which may conveniently be called "the eccentric face"), is back tapered, that is to say it forms a portion of the surface of a frusto-cone having a maximum diameter adjacent the end of the sleeve. Whilst the invention comprehends the possibility of a single concentric face a preferred number is three of each. Usually the nut and sleeve will be integral with a shear zone at the junction of the two, as in said prior Patent, but it is within the bounds of possibility, and within the scope of the invention to make the two parts separate. According to another aspect of the invention, the improved fastener is manufactured on an automatic turning machine, firstly by producing to a form (which may be) generally similar to that as shown in Patent No. 1093714, and then by an additional operation removing metal from one or more parts of the periphery of the nut so as to form the eccentric faces. According to another aspect of the invention, a method of manufacturing the improved fastener comprises making the fastener in the form generally as shown in the said Patent No. 1093714 and then modifying the shape of the nut in accordance with the invention in a press operation or operations. The function or operation of the invention can be best understood by considering eh case where a fastener generally as in said prior Patent is manufactured and then portions of the nut are machined away. Suppose for example that the large end of the nut is left at full diameter at three equi-spaced points, and there are connected by arcs of larger radius then the original radius of the nut at this end. At the smaller end of the nut the original machined diameter is undisturbed. In other words a generally triangular patch of material is machined away to provide each of the three faces, each of these triangles being bounded by two points lying in the larger end of the nut and the third apex at the smaller end of the nut. When such a fastener is installed, and the nut is drawn into the sleeve, the required expansion of the sleeve is considerably reduced. Further the sleeve is drawn to a complementary shape to the nut and hence becomes non-circular which provides an additional security against rotation. One embodiment of the invention is now more particularly described with reference to the accompanying drawings, wherein: Figure 1 is a plan view of the improved nut; Figure 2 is an elevation of the same in the direction of the arrow A (Fig.l); and Figure 3 is an elevation of the same in the direction of the arrow B (Fig.l). Figure 4 is a perspective view illustrating the nut of Figures 1 to 3 installed on a piece of sheet material. Figure 5 is an elevation showing a possible turning tool set up. Figures 6 to 13 show eight successive stages in manufacture of the nut of the invention by one method. Figures 14 to 19 show six successive stages in the manufacture of a nut by another method. The anchor nut shown in Figures 1-4 of the drawings comprises a tubular sleeve 10 having a slight lip 12 at one end and provided with an integral nut 14 at the opposite end. The nut is connected to the sleeve via shear zone 16. The nut is initially made as a frusto-cone - see the broken line Figure 3 indicated by the reference numeral 18 showing the original full diameter of the frusto-cone. Subsequently this is machined away in three places (in this example) so as to provide three faces 20 each of which is tapered in the opposite direction to the original frusto-cone. A portion of the original frusto-cone is left as indicated by the reference numeral 22, extending in three peaks rising at the points 24 to the larger end of the frusto-cone and extending as a complete ring as indicated at 26 at the smaller end of the frusto-cone. The faces 20 may be stepped slightly inwards of the ring 26 to give rise to a shoulder 30. The amount of metal removed can be seen (at a maximum) by considering the portion bounded by the broken line 18 (Fig.3). Because of the angle selected for that view, this illustrates the maximum taken on any one face, and considered around the periphery of the nut, that amount reduces down to zero where the face 20 meets the untouched surface of the original frusto-cone at the peaks 24. It is the removal of metal which combined with the non-circular resulting nut which enables a satisfactory wedging action to be obtained but with a smaller displacement of metal than is required in the original design of the said prior Patent. It is this which enables a satisfactory hold to be obtained with less risk of splitting, and hence enabling a wider tolerance in the material specification and in manufacturing. One method of manufacturing is to produce the complete fasteners with the frusto-cone in an automatic machine with conventional techniques, and then to machine the three faces individually using cutting tools which rotate about the body of the work and about an axis eccentric to the body of the work. It will be appreciated that to produce the three faces, three separate cutting tools rotating about different and generally parallel axes are required successively, or the single cutting tool may be moved to rotate about the three different axles in turn. An alternative and preferred manufacturing method involves rotating the work about a fixed axis, and using a conventional laterally mounted cutter, which is mounted on a crank which is reciprocated by a rotating cam or eccentric. To arrange for sufficient reciprocations for the cutter to operate on all three faces in succession would involve problems with vibration, at cutting speeds which would otherwise be suitable for the material concerned in the sizes which are usual with fasteners of this kind. However, by suitably matching cam (or eccentric) shaft speed and design, the cutting tool can be moved in an arcuate path relative to the axis of the work so as it cuts one face of the nut and then misses the next x faces before its arcuate path again coincides and it cuts another face and so on. The parameters can be selected so that x has a high value, but for maximum speed of manufacture x may have a value of say 1 or 3. To explain this further, if x has a value of 1 and the three-sided nut shown in the drawing is to be made, the cutting ool will first encounter the nut and start to machine the first face and then when it next encounters the nut it will start to machine what will be the third face of the nut. It will then encounter the nut and machine what will be the second face, and at its next encounter will continue machining the first face and so on. The speed of feed of the cutting tool towards the nut will be adjusted appropriately. Referring now to Figure 5, this shows the nut blank 50 being turned to the required shape by a turning tool 52 which is moved in a complex arcuate path by carrier 54 journalled on eccentric 58 mounted on spindle 56. A swinging link 60 is pivoted to the carrier and on a fixed axis. The cutting tool will intersect the nut at intervals depending upon the relative speed of rotation of the shaft 56 and of the nut 50. A ratio between the speeds of 3:4 is at present considered to be the most satisfactory. Figures 6 to 13 show the sequence of stages in cold forming of the blank at least from the Figure 6 to the Figure 11 position, and possibly also the Figure 13 position apart from the formation of the screw thread internally of the nut, which may be done on a tapping machine. The actual number of stages between the blank of Fig.6 and the near or quite finished blank will depend upon the type of press used, amongst other factors. Figures 14 to 19 show the sequence of operations in the multi-spindle automatic turning machine manufacture from continuous rod, with the nut parted off from the rod in Figure 19 before or after screw-tapping.

Claims: CLAIMS 1. A blind hole anchor nut fastener device comprising a tubular sleeve 10 and a screw-threaded nut 14 disposed at the end of the sleeve, the arrangement being such that when a screw-threaded mandrel extending through the sleeve and engaged with the nut is displaced axially in the appropriate direction, the nut is drawn into the sleeve, characterised in that the nut has at least one face 22 which lies on the surface of a frusto-cone having its minimum size near the sleeve and which frusto-cone is co-axial with the fastener generally, and the nut also having at least one face 20 which is arcuate about an eccentric axis. 2. A nut as claimed in Claim 1 wherein the face which is arcuate about the eccentric axis is concentric with that axis. 3. A nut as claimed in Claim 2 wherein said face which is arcuate about the eccentric axis forms a portion of the surface of the frusto-cone having a maximum diameter near the sleeve. 4. A nut as claimed in any preceding claim provided with three portions of concentric face and three eccentric faces 5. A method of making the fastener according to Claim 1 on an automatic turning machine by producing to a formeincluding the sleeve and a single frusto-conical concentric face, and characterised by an additional operation of removing metal from one or more parts of the periphery of the said concentric face of the nut so as to for the eccentric faces. 6. A method of making the nut of any of Claims 1 to 4 comprising making the fastener with a sleeve and a nut having a concentric frusto-conical face, characterised in that the shape of the nut is modified in a press operation or operations. 7. A method of making a nut according to any of Claims 1 - 4 characterised by the performance of a series of cold forming operations on a blank including the step of forming said eccentric faces prior to a tapping operation. 8. Apparatus for making the nut as claimed in Claim 5 comprising a tool carrier which is cam or eccentrically urged to reciprocate and the path of reciprocation is modified by a fixed pivot whereby the cutting tip of the tool is displaced in a circular path.

Inventors: OLLIS, MARTIN GEORGE
Applicants: OLLIS, MARTIN GEORGE
Application year: 1982
Main classification: F16B19
IPC codes: B23B5, B21D53, F16B37, F16B13, B23Q27, B21H3