Hi all, Forgive this mistake; but I left out portions of the latest draft on the subject by Chris Chapman. The subject is quite relivant to amateur seismology. Below is the complete draft...... Hi Meredith, Ordinary grease is made from oil and soap. The soap decays with time, but some soaps are better than others. The high temperature Molydenum disulphide grease used on cars is fairly good and the moving surfaces get coated with the solid MoS2 lubricant. Vaseline also lasts a very long time. Don't use graphite grease; it is inclined to promote corrosion. The critical point is that the expansion coefficients of the adjustment bolt and the nut or threaded baseplate need to be identical. This greatly reduces any tendenccy for the adjustment to creep as the temperature changes. You can buy large mulit flute countersink cutters quite cheaply, but I am not sure that this angle is quite correct for the nuts. Countersinks may have angles of 60, 82, or 90 degrees, with 90 being the most common. You can get drills with a cone of 118 degrees, but 135 degrees is more common. The ordinary drills with two flutes do not centre very well over a smaller hole and are inclined to chatter. You can get smoother cutting by using a drill press and opening up the hole first with a standard countersink. The bevel angle cut on nuts may not be very well defined. It seems to vary from 100 to 110 degrees in the ones that I have measured and the depth of the cut may be asymmetrical - check both ends of the nut. The 5/16" UNF bolts seem to have asymmetrical chamfers, but these are closer to 90 degree. The outside tips have a ridge on them, so the outside diameter of the countersunk hole should be the same as that measured across the flats - 0.500" in this case. Drilling an accurate clearance hole in the baseplate with a drill press and then sticking a nut onto the flat surface is another option and this does give a high strength glue joint. You can buy special nuts which have a large 45 degree chamfer on one end. These should be excellent when used with holes shaped with a 90 degree multi fluted countersink cutter. You can also buy special nuts designed for insertion into sheet metal. They are longer than ordinary nuts and are turned down to a small tube at one end. When used in sheet metal, the turned down end is inserted through a hole in the sheet and the end is splayed with a press or a ball hammer. However, they can also be pushed into a plain hole drilled in say aluminum plate and secured with epoxy or loctite. You can use one at both ends of a hole drilled through thick plate. This can provide a very precisely aligned rigid mounting. Another method of providing a thread in solft aluminum plate is to drill out a central clearance hole for the bolt and then use a "special" counterbore cutter to drill a flat bottomed hole slightly smaller than the outer diameter of the nut, to the length of the nut. You then press in a nut into the hole, maybe adding epoxy or Loctite, using a vice or a press. The six corners of the nut bed into the softer aluminum quite easily. For epoxy to give a high strength joint, the surfaces need to conform closely and there needs to ba an appreciable glue area. Epoxy will not stick a sharp edge to a conical hole very well. One thing that I have been doing is to drill the end of the vertical adjusting screws with a centre drill. I then stick a stainless steel ball bearing into the V cup, with epoxy. The diameter of the bearing is slightly less than the tap drill size for the thread used, i.e., use a 5 mm ball for a 6 mm OD thread. This seems to give a contact which does not change with time. The wide angle points which are sometimes used seem to "bed into" the mounting plates and may wander a bit. The seismometer is mounted on 2" squares of 1/4" thick stainless steel bonded to a concrete floor. I drill and tap the hole in the steel seismometer baseplate. I put a nut and a wavy washer on the bolt, screw it into position and tension the nut. You can get wavy / crinkle washers in phosphor bonze and stainless steel. They look like a very thin ordinary washer but are bent to have three S shaped curves which act as a spring. A dab of rubber contact adhesive will keep the nut from rotating as the suspension is leveled and the wavy washer keeps the screw thread under tension. Regards, Chris Chapman
Hi all, Forgive this mistake; but I left out portions of the latest draft on the subject by ChrisChapman. The subject is quite relivant to amateur seismology. Below is the complete draft......Hi Meredith,Ordinary grease is made from oil and soap. The soap decays with time, but some soapsare better than others. The high temperature Molydenum disulphide grease used on carsis fairly good and the moving surfaces get coated with the solid MoS2 lubricant. Vaselinealso lasts a very long time. Don't use graphite grease; it is inclined to promote corrosion.The critical point is that the expansion coefficients of the adjustment bolt and the nut orthreaded baseplate need to be identical. This greatly reduces any tendenccy for theadjustment to creep as the temperature changes.You can buy large mulit flute countersink cutters quite cheaply, but I am not sure thatthis angle is quite correct for the nuts. Countersinks may have angles of 60, 82, or 90degrees, with 90 being the most common. You can get drills with a cone of 118 degrees,but 135 degrees is more common. The ordinary drills with two flutes do not centre verywell over a smaller hole and are inclined to chatter. You can get smoother cutting byusing a drill press and opening up the hole first with a standard countersink. The bevelangle cut on nuts may not be very well defined. It seems to vary from 100 to 110 degreesin the ones that I have measured and the depth of the cut may be asymmetrical - checkboth ends of the nut. The 5/16" UNF bolts seem to have asymmetrical chamfers, butthese are closer to 90 degree. The outside tips have a ridge on them, so the outsidediameter of the countersunk hole should be the same as that measured across the flats- 0.500" in this case.Drilling an accurate clearance hole in the baseplate with a drill press and then stickinga nut onto the flat surface is another option and this does give a high strength glue joint.You can buy special nuts which have a large 45 degree chamfer on one end. Theseshould be excellent when used with holes shaped with a 90 degree multi flutedcountersink cutter.You can also buy special nuts designed for insertion into sheet metal. They are longerthan ordinary nuts and are turned down to a small tube at one end. When used in sheetmetal, the turned down end is inserted through a hole in the sheet and the end is splayedwith a press or a ball hammer. However, they can also be pushed into a plain holedrilled in say aluminum plate and secured with epoxy or loctite. You can use one atboth ends of a hole drilled through thick plate. This can provide a very preciselyaligned rigid mounting.Another method of providing a thread in solft aluminum plate is to drill out a centralclearance hole for the bolt and then use a "special" counterbore cutter to drill a flatbottomed hole slightly smaller than the outer diameter of the nut, to the length of thenut. You then press in a nut into the hole, maybe adding epoxy or Loctite, using a viceor a press. The six corners of the nut bed into the softer aluminum quite easily.For epoxy to give a high strength joint, the surfaces need to conform closely and thereneeds to ba an appreciable glue area. Epoxy will not stick a sharp edge to a conicalhole very well.One thing that I have been doing is to drill the end of the vertical adjusting screws witha centre drill. I then stick a stainless steel ball bearing into the V cup, with epoxy. Thediameter of the bearing is slightly less than the tap drill size for the thread used, i.e., usea 5 mm ball for a 6 mm OD thread. This seems to give a contact which does not changewith time. The wide angle points which are sometimes used seem to "bed into" themounting plates and may wander a bit. The seismometer is mounted on 2" squares of1/4" thick stainless steel bonded to a concrete floor.I drill and tap the hole in the steel seismometer baseplate. I put a nut and a wavywasher on the bolt, screw it into position and tension the nut. You can get wavy /crinkle washers in phosphor bonze and stainless steel. They look like a very thinordinary washer but are bent to have three S shaped curves which act as a spring. Adab of rubber contact adhesive will keep the nut from rotating as the suspension isleveled and the wavy washer keeps the screw thread under tension.Regards, Chris Chapman