By John Kleespies
One of the most commonly used tools in a hand loader’s arsenal is a measuring caliper. However, that caliper cannot be trusted to be accurate: it must be known to be accurate. When building high-pressure ammunition, a caliper that provides incorrect measurements is worse than worthless – it is dangerous.
For about $40, rectangular gage blocks can be purchased to inexpensively and accurately check the calibration of 75% of a caliper’s measuring functions. These rectangular gage blocks typically feature three varying widths amongst the different surfaces, providing verifying sizes during the check.
To check your caliper, follow these steps:
- Begin with a general exam of the caliper by running the slide up and down with the thumb wheel, pressing buttons, working tension screws, and eyeballing for any obvious physical damage.
- Clean the jaws by pinching a damp cloth between them and pulling it out. Do this multiple times, even after debris is no longer visible on the cloth. Now is a good time to clean the gage block, as well.
- Check for chips or misalignment of the lower jaws (the big ones) by closing them and holding them up to a light to spot for damage: the jaws should form a perfect seal with no light shining through. Note that if you do see light shining through, give the jaws another thorough cleaning and check again.
- Perform a similar check of the upper jaws (the smaller ones).
- Next, securely close the jaws and zero them; this will be the only time the jaws should be zeroed for the test.
- Returning to the lower jaws, place the thinnest edges of the gage block deeply into the jaws and close them with a secure – though not harsh – and consistent pressure. Slide the block back and forth across the length of the jaws. The measurement on the scale should stay consistent, verifying that the jaws are parallel.
- With the lower jaws, measure the outside diameter of the gage block, always using the thinnest sides of the block. Be sure to slowly rock the caliper and note the lowest value, as any misalignment will result in a larger value being registered: the lowest value will be the actual measured size of your object. Repeat this on all three sides of the gage block, writing down whether or not each measurement is dead on, or +/- the amount off.
- Set the gage block on a clean and flat surface, such as the face of a mirror, and measure depth off the side the block with the caliper’s depth probe. Again, this can be verified off all three sides of the block, writing down the results with each test.
- Similarly, measure step height, writing down the results.
- Finally, return to the lower jaws and verify that it is showing as zeroed: the zero should not have changed through the duration of the test.
The above steps, performed with a rectangular gage block, will have verified 75% of a caliper’s functions: outside diameter, depth, and step.
To calibrate all four of a caliper’s measuring aspects – including inside diameter – it is possible to purchase a gage block with a precision machined hole bored through the center, allowing for an inside diameter check. These are sometimes referred to as “square gage blocks” versus the less expensive “rectangular gage blocks” used above. Obviously, a square block will be more expensive than the basic rectangular version, but a quality square block can be purchased in the $60 range.
Finally, analyze your measurement notes with a discerning eye. If the caliper consistently checked out perfectly on two sides of the block, but the third side is consistently off for the same amount of measurement… then it’s possible that the gage block itself is out of spec, or perhaps it needed to be cleaned better.
With all of the results recorded, refer to the caliper’s rated “precision” range. Precision range indicates the acceptable level of measurement error that can be expected from the caliper. Take note that the precision range is different from the “resolution” range, which indicates the amount of measurement that the caliper will display.
In many cases, for a caliper measuring in SAE, the precision range will fall within +/- .001”. To be clear, this means that after all of the calibration measurements are performed on the gage block, the recorded measurements should fall somewhere between +.001” and -.001”. In this case, the caliper is checking out within its acceptable window of operation. If, however, it is measuring beyond a +/-.001” range, then the caliper measured out of specifications and all of the above steps should be repeated to verify that your recordings were correct.
If a caliper verifies as out of spec, then it should no longer be used for operations that are critical for safety, such as hand loading ammunition. If it is a less expensive caliper, then repairing it will likely cost more than simply replacing it. If it is an expensive caliper, then contact the manufacturer for repair information.
Engineers and machinists perform calibration checks on their measuring equipment quite often because they are constantly using these tools. For the weekend warrior who is only employing his calipers every now and then, the calibration checks can be far less frequent without sacrificing safety… as long as the calibration checks aren’t ignored altogether.
