Measuring OD

Measuring calipers have been around for nearly 150 years. In that time they have evolved into full-featured digital measuring systems. But it is a testament to the simplicity and versatility of the original design that the evolution has been so slow and that so few changes have been required to produce the instruments we are familiar with today.

Here are some of the highpoints of the caliper’s evolution since it was introduced in the 1860s:

  • In 1899 a second pair of jaws was added, allowing inside as well as outside measurement.

  • In 1951 the Vernier scale was recessed for better protection.

  • Around the same time, a dial indicator was added to allow for fast, comparative readings and higher resolution.

  • The 1980s saw the first digital readout, an advance which eventually led to data collection.

  • The early 21st century saw the employment of shop hardened electronics for use in wet environments.

  • And just recently, improved scales incorporate a reference point so that they never lose their position.

    Today you are apt to see digital calipers as the tool of choice for the most common shop, automotive and hobby measuring tasks. But when most people pick up today’s digital caliper, they still see a tool for just measuring outside diameters (OD) and inside diameters (ID). Yet there is a lot of versatility built into this tool. Let’s take a closer look and see what a caliper can do.

  • Measuring ID Source: Mahr Federal Inc.

    Caliper Basics

    All calipers, whether vernier, dial or digital, include the same basic components. They have two jaws. The fixed one is the reference jaw and the sliding one makes the measurements. All calipers incorporate a little “hump” where your thumb goes to help move the measuring scale back and forth. Some incorporate a thumbwheel to help improve repeatability by trying to maintain constant gaging pressure (the wheel disengages when the sliding jaw comes into contact with the part).

    Usually, the sliding scale will have a round or flat rod associated with it. This extends out the back of the caliper and is used for making depth measurements. Electronic calipers usually also have controls to set the zero point, change units and turn the units on/off. There may even be a connector for sending measured values to a data collection device.

    As with any measuring tool, it is important to make sure a caliper is in proper operating condition and ready to do its work. Always check to make sure its calibration is current, the scale moves freely and there are no nicks or burrs on the jaws. Clean the jaws with a soft piece of cloth to remove any dirt. Close the jaws and make sure the instrument reads “0” or set it to zero, using the zero button on a digital caliper or set the dial to zero on a dial caliper.

    Fundamental Measurements

    When using a vernier, dial or digital caliper, the process of making fundamental OD, ID, depth and step measurements seems pretty obvious. But what may not be so obvious is how easy it can be to make them incorrectly. The caliper is a very operator-dependent tool. If the part is not placed in the middle of the jaws, if the caliper is not 90 degrees to the measurement, or if the operator applies too much force to the sliding scale, there is a good chance the measurement will not be very accurate. Here are some tips:

  • When making an outside measurement, place the jaws over the part so that it contacts the fixed jaw in the middle of the anvil.

  • Use the small jaws on the top of the caliper for inside measurements. Bring the sliding scale to “0,” then place the part over the jaws. Slide the jaws open until they make contact with the part and are squarely in place.

  • Measuring Depth Source: Mahr Federal Inc.

  • Depth measurements are made with the blade that extends out the back of the caliper. Place the end of the caliper squarely onto the top of the hole to be measured. Ensure the caliper reads zero, then open the caliper until the depth rod meets the bottom of the hole.

  • Step Measurements Source: Mahr Federal Inc.

  • Step measurements are an often overlooked way of making depth measurements. Zero the caliper and place the sliding jaw on the upper step of the part. Then open the jaws of the caliper until the reference jaw contacts the lower step.

  • When using a caliper for any measurement, always repeat the process to ensure the measurement is repeatable and reliable.

  • Measuring Center Distance Source: Mahr Federal Inc.

    Compound Measurements

    Digital electronics are great. The advent of the electronic calculator has saved countless hours. The electronics in the digital caliper can do the same by doing some of the math during the measurement process.

  • Center distance. One check that can be made by making two measurements is finding the center distance between two holes having the same diameter.

    The first step involves using the inside jaws to measure the diameter of one of the holes. After having the caliper in position and reading the diameter, use the zero button to set the caliper to zero.

    Then, while still using the inside jaws, measure the distance between the two furthest surfaces of the two holes. The caliper reading is the distance between the outside edges of the two holes, minus the diameter of one hole. Since the diameter is twice the radius, the reading is actually the distance between the centers of the two holes.

  • Comparing ID and OD Source: Mahr Federal Inc.

  • Comparing an ID and an OD for fit. Often one may be faced with boring a cylinder to fit a piston or making a shaft to fit a hole. The digital indicator can be used to calculate the fit so one can machine the part to the correct size. Let’s start with a hole and match a shaft for it.

    First, use the inside jaw set to find the diameter of the hole. While the caliper is in place, reset the zero.

    Now measure the shaft OD using the outside jaws. The caliper will show the difference between the shaft and the hole. A plus dimension will mean the shaft is oversize while a minus dimension will mean the shaft is smaller than the hole. The positive reading can be used to make the shaft to the proper size fit to the hole.

  • Measuring Thickness Left Source: Mahr Federal Inc.

  • How much thickness is left. When creating a blind hole in a part, it may be important to know how much material remains between the bottom of the hole and the other end of the workpiece. A digital caliper can help with this.

    First use the outside jaw set to measure the total width of the part. Again, while the caliper is in place, use the zero button on the digital caliper to set the tool to zero.

    Then use the depth rod/blade to measure the depth of the hole. The caliper reading is the remaining thickness left in the part. Using this process, the reading will be a negative number-but it represents how much material is left.