What You Need to Know About Depth Micrometers
While most machinists are familiar with and regularly use outside micrometers, there is a less common type of mic that is very helpful when measuring the depth of holes, slots, and recesses of keyways: the depth micrometer.
Anatomy of a depth micrometer
Image credit: chicagobrand.com
The two main parts that differentiate depth mics from outside mics are the flat base attached to the sleeve and the measurement rod. The flat base stabilizes the mic on the top surface of the recessed part. The measurement rod typically has a small diameter to facilitate the measurement of small holes or slots. Measurement rods come in different sizes and are interchangeable to allow the measurement of multiple depths.
Just like any other mic, depth micrometers feature a scale on the sleeve that shows the depth measurement. Unlike traditional mics, zero is located toward the end of the thimble. The measurement is read in reverse and increases as the thimble moves toward the base of the instrument.
Mechanical vs. digital depth micrometers
Depth micrometers come both in digital and mechanical styles. Mechanical depth micrometers apply the working principle of the screw and nut to display the reading on the sleeve. If properly maintained and calibrated, they can last forever but, as for any other type of micrometers, they require a bit of thinking when it comes to reading the measurement.
Digital depth micrometers, on the other hand, apply electronic principles to take measurements. They are extremely easy to read and can measure in both inches and millimeters but are also more delicate than their analog counterparts.
How to use a depth micrometer
First, you’ll want to select the right rod size (which depends on the depth of the recessed area you are trying to measure). Next, clean the mounting surfaces of the measurement rod and the rod-receiving shaft. Insert and rotate the measurement rod into the rod-receiving shaft to seat it completely. Then, turn the thimble counterclockwise so that when the base is flat on the top surface, the measurement rod does not touch the surface to be measured. Holding the base firmly on the reference surface, rotate the thimble ratchet clockwise until the rod stops on the measurement surface and the mic clicks three times. Once that is complete, simply, read the depth measurement.
How to calibrate a depth micrometer
Although depth micrometers tend to stay calibrated, it’s always a good idea to check your instrument before using it. The calibration process for this particular style of mic is quite simple.
All you have to do is roll the 0-1”rod all the way up into the base of the tool, place the base on a flat surface and turn the thimble ratchet clockwise until the measurement rod tip is stopped by the flat surface and the ratchet click” three times. The sleeve zero line should line up with the zero on the thimble scale. If they are not aligned, position the tip of the calibration wrench in the calibration wrench hole on the sleeve and rotate the sleeve to achieve alignment.
This video by Keith Rucker explains how to calibrate and use depth micrometers.
Recommended models
Starrett 440Z-6RL Vernier Depth Gauge
This complete set comes with 12 interchangeable rods that allow you to measure depths up to 12”. It comes with a ring-type knurled lock nut, ratchet as well as a speeder for stability and to apply uniform pressure and make quick adjustments for precise measurements.
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Mitutoyo 129-149 Vernier Depth Gauge
This set also comes with 12 interchangeable rods that allow for measurements up to 12” in depth. The diameter of the rods (0.157”) is slightly bigger than the Starrett model but will still allow you to measure almost any type of recess.
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Fowler 52-225-116 Economy Depth Micrometer
Less expensive than its more glamorous counterparts listed above, this set comes with 6 interchangeable rods and can measure holes up to 6” deep.
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Mitutoyo 329-350-30 DMC4-6″MX Depth Micrometer
The best option for machinists who prefer digital over manual tooling. This set can measure recesses up to 6” deep and can take measurements in both inches and millimeters.
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3 Comments
Before calibrating your depth micrometer, if it is not exactly on zero, make sure you are checking it on a clean extremely flat surface such as a granite surface plate and that the surface that the rod contacts the the micrometer is absolutely clean also.
For all this information in this article, I’m surprised they did not talk about “flat rod” depth mics and the advantage of them. The round rod spins as the thimble is turned. This, is some cases, can wreak havoc when the spindle touches off on the part being measured. A flat rod micrometer has an anti-rotating feature incorporated in the frame, allowing the flat rod to contact the part firmly without the rotation spinning on the part. It is easier to get a positive “feel” on what you are measuring.
They also come as optical. The problem with optical depth mics is the amount of flat surface around the area you are measuring has to be enough to support the legs of the mic, but they’re great for small areas where a standard type depth mic won’t work.