Building / Retrofitting a NeuFinder 4 with a

MyWeigh UltraShip Digital Scale

1. Excellent quality and accuracy, based on reviews and some personal testing.
2. Higher maximum load (16Kg, as opposed to 10Kg for the old model).
3. Slightly better precision (5g, as opposed to 10g for the old model.).
4. Bigger and brighter display.
5. The control buttons are not in the measurement chain, so pressing the zero/tare button does not have to be done carefully any more.
6. Option for AC power as well as batteries.
7. Option for the digital display and buttons to be remote from the rest of the NF4. (Well, not very remote. The display/control unit is on a coiled-wire tether.)

• Many of the pictures are thumbnails which, when clicked, open a full-size version of the picture in another window.
• There is one step explicitly labeled "Retrofit Only" which does not apply if you are building a new NF4.

Materials

Tools

• Set of open-end wrenches.
• 3/16" Allen wrench.
• Set of drills that work in wood and aluminum, up to 1/4".
• 3/4" bit for plywood. I used a Forstner bit for an extra-clean hole.
• Circular saw (such as radial arm or table saw) with a carbide-tip blade.

Preparing the scale

Here is the MyWeigh UltraShip 35 scale, as it comes out of the box. It has buttons for "on/off", "tare/zero", and "hold reading". The black button on the side selects the units; for the NF4, we use kilograms. The scale comes in silver or black. This one is silver. Pick the color you like; it doesn't matter to the operation of the NF4. I recommend testing the scale before you continue. Here is a link for testing digital scales.

Begin the disassembly of the scale by removing the plastic top tray. It is press-fit on, and can be pried off by hand.

Remove the metal plate. This requires removing two hex socket machine screws. While the screws are a metric size (6mm-1mm), they can be removed using a 3/16" Allen wrench.

This is what the innards of the scale looks like. The metal beam with the gray plastic pad resting on one end of it is the "load cell" for the scale. The beam has a pair of strain gauges, one on the top of the beam and the other on the bottom. Each strain gauge is encapsulated under a white resin for protection. When the beam is flexed, the strain gauges record the tension on one side and compression on the other side. This difference is converted by the electronics into a weight. All the electronics is in the display unit at the front of the scale. That unit is removable, remaining attached to the rest of the scale by a coiled wire.

Unscrew the bottom of the scale. The four screws are hidden under four adhesive "feet" that you must peel off. Save all the screws, bolts, and washers you remove from the scale. You will use several of them before the installation is complete.

Remove the electronics (attached to the bottom of the scale) from the case. No need to do any fancy prying; start it by just putting your fingers underneath and pushing up. Then detach the display/control unit from the case. It was designed to slide off, per the instructions on the bottom of the unit. In case you have trouble with the instructions (they are printed very small), with the scale still upside down, just push the latch tab forward and push the display/control unit down until it slides off the main unit.

You now have the main unit (sans cover) and the display/control unit. Measure the load cell beam. It should be 100mm long (about 4"), with threaded holes at 6mm and 24mm from each end. If it is significantly different from this, do not continue. Instead, notify us. It would indicate that MyWeigh has changed the design enough that we need to evaluate whether our design is still functional. If all is still "go", remove the screws indicated in the picture.

The key pieces in this picture are the load cell and a tiny connector board for all the wires in the main unit. Look at the load cell beam. It should have an arrow at the unsecured end, as shown in the photos. (By "unsecured", I mean the end not screwed to the steel plate.) The arrow should point toward the steel plate. If your load cell does not have an arrow, then take a waterproof marker and draw one. It should be at the unsecured end, and point toward the steel plate, as the picture shows. Look at the wires coming off the connector board. Slide the wires and cable tie off the plastic post.

Remove the load cell beam from its steel plate mounting. Unscrew the screws with the 3/16" Allen wrench. Unscrew the connector board from the gray plastic base.

You now have a large plastic piece wired to the connector board, the load cell, and the display/control unit. The only useful part of the large plastic piece is on the right side (originally the rear), where we find the battery compartment and the jack for the AC adapter. Everything to the left of the green line is superfluous for the NF4, so we are going to cut it off. Cut the plastic base at the green line, making sure as you cut that everything to the right remains intact. I used a radial arm saw with a 10-inch 40-tooth carbide tip blade. I'm sure a table saw would work as well. A finer tooth blade might give a smoother cut, but there is nothing wrong with the cut I got from the 40-tooth blade.

The disassembly of the digital scale is complete. You now have four pieces: The power unit will be mounted behind the backer board. It contains the batteries and the connector for the AC adapter. The connector board will also be mounted behind the backer board, near the power unit. The load cell will be mounted on the front of the backer board, so that rotation of the backer board will flex the beam and create a reading. The display/control unit is at the end of a coiled cable, and can be placed wherever you find it convenient.

Installing the scale

Retrofit only: Remove the old scale. Remove the locking nut from the upper scale bolt. Slide the top ring of the scale off 5/16" bolt. Remove the eye bolt and all the hardware securing it.

Drill four holes in the aluminum angle, as shown. Be sure to start the holes with a center punch or nail, so the drill doesn't wander. You might even use a smaller drill for a guide hole. If holes "A" and "B" are not in exactly the right place the load cell bolts will not line up. If that happens, either you will not be able to bolt the load cell to the aluminum angle or the load cell will not point in the proper direction. In the event that this happens, fix the problem by enlarging hole "B" (not hole "A") with a file or reamer.

Turn the rotator board out of the way, so you can drill the holes shown in the picture. Locate and drill the holes. A few notes about placement: The most critical dimension is the spacing between the pilot holes. It is better if you measure this spacing with the aluminum angle you just drilled, rather than a ruler. The spacing must match the holes; if they match the holes but they are not exactly 2½ inches, that's OK. The next most critical dimension is the distance from the left edge of the backer board to the leftmost pilot hole.

Temporarily fasten the load cell to the aluminum angle, as shown. Use the screws that originally held the beam to the steel mounting plate -- the longer pair of screws. See the arrow for orientation of the load cell beam. Temporarily attach the eye bolt to the outer hole of the load cell beam, as shown. Don't worry too much about the height; it isn't important at this time. Use this assembly to test the placement of the components, as follows: Position the upper scale bolt at the mid-point of its slot in the rotator board. Turn the rotator board into its proper position, resting on the stop. Hold the angle/load cell assembly against the backer board. Be sure its holes are lined up with the pilot holes in the backer board. The eyebolt's position should be such that: It is neither too far from the backer board nor too close for the eye to latch onto the scale bolt. If this is a problem, enlarge hole "B" in the aluminum angle, so that the load cell beam can rotate closer to or farther from the backer board. It is neither too far left nor right for the scale bolt to be adjusted in the slot to latch the eye. If this is a problem, re-drill the pilot holes left or right (but at the same height), so the eye bolt comes to about the middle of the slot in the rotator board. When everything passes this "fit test", turn the rotator board out of the way again. remove the eyebolt and the aluminum angle from the load cell beam.

Mount the power unit (the large gray plastic piece) to the back of the backer board, using the #8 x 2" screws. They take a 1/8" pilot hole. Placement is not critical. The primary placement considerations are: The 3/4" hole must not be blocked. There must be room for the battery compartment door to open and close. Now mount the connector board, using the tiny screws that originally secured it to the plastic base. The screws take a 3/32" pilot hole. The only placement consideration is that it be close enough to the 3/4" hole in the backer board.

In real-world use, there may be an occasional rude yank on the coiled cable to the display/control unit. You don't want the strain to be felt where the individual wires are soldered to the connector board. So we provide "strain relief", in the form of a 1/4" plastic cable clamp. Place a couple of loops through the clamp. Use enough so the wire does not slip freely in the clamp. The picture shows two loops, giving three thicknesses of cable in the clamp. Also, allow a loop's worth of slack between the clamp and the connector board, as shown. Now screw the cable clamp to the backer board.  Remember all those screws we removed that held the scale together? Use one of them here. It takes a 1/8" pilot hole.

Slip the load cell beam through the 3/4" hole as shown. It should go arrow-end first. Take this opportunity to clean up the wiring a bit. I wrapped the excessive wire with a plastic twist-tie to keep it from flying loose.

Mount the load cell to the aluminum angle, as you did before. But this time, it's permanent. If you had to enlarge hole "B" earlier, be sure you orient the load cell beam properly in the hole.

Mount the aluminum angle to the backer board, using the #12 x 1" screws. While applying the final tightening to the screws, hold the aluminum angle in a clockwise position. (That is the direction of the strain on it in use. It is better if it is already "against the stops".) Pull the excess wire through the hole to the back, and wrap it with a twist tie if there is a lot of extra in back.

Loosen the upper scale bolt enough so it can slide in the rotator board slot. Hang the eye bolt on the scale bolt as shown, then turn the rotator board back into position. As the tip of the eye bolt nears the load cell beam, guide it into the outer hole of the load cell. The eye bolt diameter is a lot smaller than the hole, so it should slide in (with a little bouncing of threads). If it does not go straight in, slide the upper scale bolt in the slot until it fits as near to centered in the hole as possible. Tighten the nut on the upper scale bolt to secure it in the slot. Double-check to make sure the eye bolt is still aligned with the outer hole of the load cell beam.

Spin the plastic-insert locking nut onto the eye bolt. It does not need to be tight. That nut will be the adjustment nut when we set the rotator board angle during calibration. Note: The eye bolt should hang loosely in the load cell. Resist your temptation to add another bolt above the load cell, so the eye bolt is secured rigidly. It may please your sense of "right", but it will give wrong or flaky readings.

The display/control unit can sit on the workbench, or it can be mounted to the backer board. The mounting is accomplished with two screws, which are not tightened down flush. Leave the heads about 1/8" away from the surface of the backer board. The slots on the back of the display/control unit act as keyways to hang the unit on the screws. Remember all those screws we removed that held the scale together? Turns out they are just the right size for mounting screws for the display/control unit. Use two of the screws that you saved. They take a 1/8" pilot hole. The screws should be mounted exactly 5 1/2" apart, because that is the spacing of the keyway slots. Placement is for your convenience. I like it low and toward the right of the NF4 because it is out of the way, but still close to the toggle. That way, I do not have to move my hand very far to go between the toggle and the tare/zero button. Even if you prefer to use the display/control unit on the bench, you should add a pair of mounting screws. That way, the display/control unit is not flapping around when the NF4 is stored or transported.

Add power, and you are ready to calibrate and use the NF4. Power comes in the form of an AC adapter that plugs into the power unit on the rear of the NF4, or 4 "C" cells that go into a compartment in the power unit. The calibration procedure is as before, except that: The angle of the rotator board is adjusted with the locking nut on the eye bolt, as noted above. Leave the upper scale bolt where it is in the slot, and do all the calibration using the toggle stop adjustments.

Here the NF4 is in use. Calibration has been completed, and I am taking the last measurement on my calibration shaft. A few things worth noting: The shaft (as you can see from the previous picture) should read 4.54Kg. In fact, it reads 4.545Kg. That reflects the increased precision of the new scale over the old. The old scale would have read 4.54Kg, because it doesn't have the fourth digit. The increased precision is a bit of a double-edged sword. Tare/zero is more sensitive now, because the scale reads to 2 grams rather than 10 grams. Don't worry about a 2g reading after hitting the tare/zero button. When a non-zero reading appears on the display, it is backlit in blue for a while. I don't know how to turn this off, but it may cut down on battery life if you don't use AC power. If the display is trying to zero but is jumping around a very low-gram reading, try pressing the tare/zero button. That usually settles it down.