PSN-L Email List Message

Subject: RE: A "new" seismo base plate adjustment screw/s approach
From: "Meredith Lamb" meredithlamb@.............
Date: Mon, 12 Jan 2004 20:06:04 -0700


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 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
 
 
 
 
 
 

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