Repairing a DRO scale using a Harbor Freight Digital Caliper


By R.G. Sparber

February 4, 2007

Copyleft protects this article.

Here we have a great collection of spare parts. I buy these digital calipers at Harbor Freight. One of them has the inside jaws cut off and it is now my height gage. One has both jaws cut off and is my Z axis scale for my Shumatech DRO. I even use one as a caliper. It replaced a digital caliper I bought for $150 about 15 years ago. The rest of my uses involve taking the slider apart and using the enclosed circuit board to repair scales used by my DRO. Lets just say that I forgot to wear my wrist strap and I live in Phoenix, AZ where the humidity is very low. Even if your outer case looks a little different, more often than not, the circuit boards are identical. How else can they make money selling these calipers for under $17?

So if you want to take one of these calipers apart, the first step is to wear a wrist strap. If you do not own one, they can be bought at Radio Shack for not a lot of money.

Once you are wearing your wrist strap and have it connected to some grounded object, work can safely begin. Remove the caliper's battery. Turn the caliper over and pull off the sticker. Then take a sharp object and clear any bits of adhesive away from the screw heads shown here. Remove the 4 screws from the back plus the bolt from the bracket that holds the thumb wheel.



Here you see the 4 screws and 1 bolt removed.

You can now lift off the metal shell. Under it you will see the two strips of phosphorus bronze. Note that each strip has a unique hole pattern to prevent it from going on the wrong side.


Inside the plastic shell you just removed is the circuit board. Note the 4 tiny screws. Time to take them out and put them in a safe place.

With the screws out, you can lift out the circuit board. Do it carefully as the buttons are just molded strips of conductive plastic that can easily fall off.


If you plan to connect wires to the data port, now is a good time to tin the clock and data pads as shown here.

I have put the circuit board back into its plastic shell. It will be mounted on my Y axis scale, which has holes drilled and tapped into the back of the metal body. Note that the phosphor bronze strips are positioned over the holes of the circuit board.

Now flip the metal plate over onto the circuit board and carefully tighten down the 4 screws. If you like put the bracket with bolt back on the body too.

As you can see here, the scale does at least display a number. It does, in fact, work fine in its new home.

Here you see the low frequency and high frequency capacitors soldered into the battery compartment. The other option is to solder them onto the tiny fingers of the data port. You see these fingers just above the cylindrical capacitor. Note the solder blobs on the two center fingers. I did reinstall the thumb wheel bracket because it will be used as a cable relief point.

Here is my "old" Y-axis scale with its new circuit board installed. The power wires feed into the battery compartment and the clock and data leads go into the data port.

Comments and questions are welcome. All of us are smarting than any one of us.

Rick Sparber