Lightweight interchangeable magnetic sleeve and method of manufacture thereof

A method of manufacturing a sleeve assembly (2), the method comprising the following steps, performed on a cylindrical tube (4) formed from one of:

• an engineering plastics material, a metal or an alloy thereof, having a density less than 3.5 g/cm3 and having outer and inner diameter dimensions respectively greater than and less than an ultimately required outer diameter/inner diameter dimensions of the finished sleeve assembly,
• counterboring the tube to a precise inner diameter dimension,
• adhesively bonding a tubular compressible liner (6) having an outer diameter of the order of 0.2-1.5 mm less than the inner diameter of the tube, and being of comparable length, within said tube to create a combined sleeve assembly,
• cutting at least one length from the combined sleeve assembly, said length being of the order of 0.5-4 mm longer than an ultimately required axial length of the sleeve assembly,
• machining out rebates (8) at either end of the sleeve assembly, said rebates having an axial depth of at least 8 mm back from annular end surfaces (4A) of the tube (4), said machining comprising complete removal of liner over said axial depth as well as some amount of a tube interior, said amount being between 4-25% of an annular thickness of the tube such that annular shoulders (4D), lying substantially flush with adjacent annular end surfaces of the liner (6), are defined within the sleeve assembly at both ends thereof,
• machining threads on exposed internal cylindrical surfaces (4C) of said rebates (8);
• screwing end rings (10) into each sleeve assembly end and tightening said end rings against the annular shoulders (4D) such that the tube is axially compressed between said end rings to a predetermined degree, said end rings being of an axial depth which is marginally greater than the axial depth of the rebates,
• machining a plurality of magnet-receiving recesses (12) in the exterior cylindrical surface of the sleeve assembly (2), said recesses being of a depth greater than between 50%-90% annular thickness of the annular end surfaces of the tube and extending substantially axially from one end of the sleeve assembly to the other, and furthermore being generally evenly spaced circumferentially around said exterior surface, such that a remaining surface area of the tube is less than 50% of an original cylindrical exterior surface area, said recesses terminating, at both ends, in a location which is axially more distant from the most proximate annular end surface of the tube than that at which said annular shoulders are disposed,
• adhering within every one of said recesses a plurality of magnet and magnet keeper pairs (22, 24) so as to completely fill said recesses,
• machining the exterior cylindrical surface of the sleeve assembly, with magnet assemblies adhered in place therein, such that the outer diameter of the sleeve assembly is precisely determined relative to a datum axis of machining, and
• machining the annular end surfaces of the sleeve assembly at both ends, including said end rings, such that any portions of said end rings which stand axially proud of the adjacent annular end surfaces of the tube are removed, and such that the sleeve assembly is machined down to a precise, predetermined axial length.