·        Cut the threaded female couplings in half on a band saw (below left).  These are the dynamometer collars that are stuck into intertidal rocks using marine epoxy. 

·        Ream out the collar threads using a ½” – 14npt tapered tapping bit (above right).  These couplers have a tapered thread, by re-threading them past the taper so they have a straight thread makes it easier to place meters in and remove meters from the collar.

·        Cut the female half off the threaded male end coupling/female coupling (see below).  Leave the hex nut shoulder on the male threaded end.

· Cut the Molybdenum Filled Nylon Rod (black) guides into 3/4” lengths.

· Cut the CPVC tubing into 6 ½” lengths (below left).

·        Drill down the center of the Molybdenum string guide, using a 0.199 #8 bit (slightly bigger than nose of router bit).  Use a lathe so that the holes are accurately centered (below left).

·        Hollow out a cavity in the center of the Delrin end plugs.  Use a bit that is a slightly larger diameter than the spring (u 0.368"), such as a 0.377" bit.  Leave approximately 1/16" of material at the bottom end of the plug to seal out sand in the field.  This operation is best carried out on a lathe so that the holes are accurately centered (above right).

·        Bevel the top end of the string guide using a quarter-round router bit (below left).  The router bit has a 1/4" shank, 5/8" diameter and rounding-over bit with no ball bearing on the head.  Remove sharp edges from the string guide.

·        On the lathe, turn out the inner diameter of the CPVC shafts just enough for a press fit with the end plugs and string guides (above right).  This can be done either with a standard boring bar on the lathe, or using a reamer of the correct size. A suitable reamer would be a 0.5010" oversize reamer available from McMaster-Carr (part number 3087A37).

·        On the lathe, turn out the inside edge of the male threaded coupling that forms the top of the dynamometer just enough to accomodate the CPVC shaft.  (Or turn down the outside of the CPVC tube, which may be easier to do while the tube is already in the lathe, although it may make the structure less sturdy). We recommend using the same reamer (0.5010") as above to open up the inside diameter of the CPVC.

·        Insert the Delrin rod end plug into the dynamometer shaft, with the hollowed out cavity facing upward (above right).  Drill a hole (using a size 29 = 0.136” diameter bit) 3/8" from the bottom end of the dynamometer through the shaft and the end plug (below left).  

·        Countersink one side of the hole, so that the screw will sit flush with the shaft (below right). 

·        Tap the hole for the 8-32 5/8” screws.  This is the screw that holds the spring in place. 

·        Use all-purpose PVC cement and purple primer to glue the threaded male coupling head onto the CPVC shaft, with the threaded portion facing down towards the bottom end of the shaft (above right).  (Put glue on each surface being glued together).  Make sure the flat top of the threaded male coupling head is flush with the end of the shaft.  Note:  CPVC glue does not glue PVC and vice versa.  Make sure to use all-purpose glue that glues both.

·        Mill out a 3 1/8” by ¼” window (above left) in the side of the dynamometer (long enough to see the top loop of the spring and the bottom edge of the black string guide). You'll need to cut into the threaded portion of the dynamometer head for this.  Therefore, it is important to do this step before gluing the black string guides in place (since they are used for a measurement reference point). 

·        Glue (with epoxy) the string guides into the top of the dynamometers so they are flush with the top of the CPVC and male threaded coupling (above right).

·        To roughen the polypropylene balls, use the lathe to cut 5 parallel grooves ~1mm deep into their surface (above left).  Drill a hole through the ball to thread the string.

·        Cut rubber stopper pieces (above right).  Make them big enough so they do not slip through string guide hole, yet small enough so they don't rub against the inside walls of the shaft (approximately 7mm square).