If you do guitar setup and repair, one of the more frustrating problems you run into is guitars that have weak necks that distort under string pressure, often a "sag" just in front of the neck heel. I have also seen the same problem on high quality guitars that have been strung with too heavy a gauge of strings. The results when leveling the fret board and re-fretting, or just leveling the frets produces mediocre results at best because the sag distortion rebounds with the strings off, but comes back once the guitar is re-strung. The neck jig is a tool that many luthiers use, and is supposed to simulate string tension, but in fact it exerts a force perpendicular to the neck that only counteracts the effect of the truss rod. It does not simulate the compression effect string tension places on the neck that produces the illustrated (exaggerated) sag.
The picture shows an adaptation to a neck jig to simulate string tension while leveling frets or fret boards. It reads out in lbs, and can easily simulate any reasonable string tension using compression created with the lead screw. Contact at the top of the head stock is cushioned with cork. Contact at the bottom is typically with the strap button.
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Step 1: Digital Readout Assembly
The digital readout is made from an inexpensive bathroom scale that has four strain gauges. Only 2 are needed for this tool. The strain gauges from the one I used were attached to a glass plate with very strong double sided tape. I was able gently warm the glass plate with a hair dryer and remove the strain gauges and display from the plate by carefully prying them away with a putty knife.
I opened the digital display and labeled the strain gauge wire colors on the pc board with a sharp tip marker. Each strain gauge has 3 wires. The wires are then desoldered so the extra plastic protrusions can be trimmed from the digital display housing and strain gauge housings. A jig saw and belt sander were my tools of choice for this step.
Cut two approximately 6 inch lengths of heat shrink tube to protect the wires on the two strain gauges. The digital display will not work if you don't simulate the presence of the two unused strain gauges. The strain gauges are a wheat stone bridge, so I was able to simulate each unused strain gauge with two 1K ohm resistors. For each unused strain gauge, I connected a 1K ohm resistor between where the black and red wire attached, and another 1K ohm resistor between where the red and white wire attached. Different weight scale manufacturers will use different wire color codes and gauges, so it is necessary to check the strain gauge you use with an ohm meter and adjust the resistor value and connections as needed.
The digital display is attached to the headstock stop with a small aluminum bracket. The two strain gauges are stuck onto the front of the headstock stop using the double sided tape that was used in the original scale. The reason for using the two strain gauges stacked on top of each other is to account for the height difference of the head stock in acoustic v.s. electric guitars. The small wood U shape protector device slides over the front of the headstock stop. Its purpose is to cushion the guitar headstock with the cork surface as well as distribute the force to both strain gauges.
Step 2: Lead Screw Assembly
The lead screw assembly is added onto the back end of the neck jig with 10 inch lengths of 1 x 4 wood. Sandwiched between the 1 x 4's is a 3 inch length of 2 x 4 that serves as the mount for the vertical angle iron. The angle iron is attached with lag bolts and washers on both the back and side of the 2 x 4 block for strength and stability. The lead screw itself is an inexpensive automotive brake caliper compression tool. The holes in the 9 inch length of angle iron had to be enlarged slightly to fit the lead screw. The end of the lead screw is cushioned with a circular wood block. One end of the bock is drilled to fit over the strap button, and the other side is countersunk so the metal lead screen end cup can be glued to the block. A small section of the center of the brake caliper tool is cut out and used to provide the threaded stop against the guitar side of the angle iron (acting like a large nut).
Step 3: Using the Guitar String Tension Simulator Tool
It should go without saying that you can damage a guitar using this tool incorrectly. Know what string tension is appropriate. This information is available from string manufacturers. D'Addario provides a tool for calculating your string tension here http://www.stringtensionpro.com/ . The bottom line is that you don't assume you know the correct string tension to simulate without checking first!
Start by strapping the guitar to the neck jig with the bottom of the guitar even with the back end of the neck jig. Set the head stock stop assembly with the strain gauge/head stock protector so it is lightly touching the top edge of the guitar headstock. I use two 2 inch drywall screws to attach the head stock stop assembly to the neck jig. Zero the guitar neck relief adjustment using a straight edge tool that measures the fret tops or fret board surface.
Loosen and move the strings to the side of the neck. Switch on the digital display using the switch under the back cover plate if it has one. Then turn the lead screw up to the desired number of lbs compression. If you increase the tension too slowly, the digital display may time out. In this happens, completely release the tension on the lead screw and start over because the digital readout will zero itself on the existing tension!
Once you have the guitar under tension, check that the body of the guitar is not arching itself off the surface of the neck jig. If so, loosen the lead screw tension and tighten the neck jig guitar hold down strap. If the guitar is correctly secured to the neck jig, and the appropriate amount of tension is applied with the lead screw, the neck should again have zero relief. If not, release the tension and adjust the way the guitar headstock contacts the headstock stop assembly, and re-tension.
Once the guitar is under tension and the neck is at zero relief, you are ready to level frets or the fretboard for a re-fret job.