How to Evaluate a Micrometer

You are looking for better, more accurate tools. You have reached the point where your inexpensive, multipurpose caliper doesn't always do it anymore, and the extra accuracy given by a micrometer is needed.

As you wander around the local flea market, you stumble across just what you are looking for: "Made in USA". "Clean". "Classic". "Vintage". It was made back when things made in the USA were the best. It is shiny. It has fraction equivalents in the frame. It looks barely used. Only $20. You have your wallet out and the "Crack" as air rushes in to fill the void left by the bill startles everyone in the area, lest someone else jump on this golden opportunity before you.

When you get it home, you find that it doesn't seem to agree with your caliper, and measurements don't seem consistent. What gives?

Unfortunately, you didn't do a good examination of the tool before spending your hard-earned money, and you are now the proud owner of a shiny holiday ornament shaped like a tool.

How do you avoid the same mistake in the future?

There is an old saying about motorcycles: Chrome don't make it go fast.

The same concept holds with quality tools. Good tools generally look fairly attractive, but are not usually intended to dazzle. Good tools are attractive because their appearance compliments function.

Lets examine a few cases to see what makes a bad micrometer, and one case of a decent tool in the rough. We'll start with the nice, shiny GL No. 85.

Evaluation doesn't take a lot of work, just a close look and a good feel. Maybe a minute to make a decision. Sometimes only a few seconds.

With this micrometer, the first thing that is apparent on a close look is that, though the frame is polished chrome, it is light in weight. It doesn't feel like steel or cast iron. When we look at the numerals and marks on the scale, they are raised. This looks like die cast aluminum or pot metal. Steel and good cast iron tend to be stable, strong, and at least reasonably tough. Pot metal tends to be less stable and may be soft, and, depending on the composition, may be quite brittle. Not a desirable quality, but not damning.

Removing the spindle assembly, we see where the chrome stops around the bore. The threads are tapped directly into the frame casting, and have no provision for taking up wear. No way to take up wear is bad.

When we look at the bottom and inside the frame, we see that the chrome is rough and flaky. The zero adjustment is done by threading the anvil in and out. The measuring face of the spindle is clean, but dull looking and a little rough, not flat and smooth. The anvil surface is the same. Not good at all.

The sum total of all of this is cheap. There are very good micrometers that might have one of these factors, but all together is a red flag.

A basic test, without getting out gage blocks, is to feel how it operates.

Start with how the spindle moves. Is it pretty much free-moving, but with a solid feel, not a loose feel? Does the spindle assemble wiggle, either axially or side to side? Are there tight or loose spots?

If the answer to any of these questions is not satisfactory, walk away. The pictures show the axial and side to side play in the GL micrometer, and it isn't a particularly bad example. The play is quite easy to feel. The axial play is a bit less than 1/1000", and the side to side play is about 2/1000, though these numbers vary with spindle position and where the assembly is wiggled.

Even though the tool makes measurements in the axial direction, the side to side is a bigger issue, since, in use, the threads always register to the same side axially, but the side to side play allows the measuring faces to come out of parallel and changes where on the thread face the spindle meets the threads in the frame.

The seller of this really did want $20. I actually got it for $5 at the end of the flea market since it hadn't sold.I overpaid since I got it for use as an example.

Lets look at a few other examples

Next up on the hit list is an Omega. These turn up fairly often. The were (maybe still are?) made to be sold under several badges by Central Tool in Rhode Island, so you will see this with assorted names. This was reputedly an apprentice-level micrometer.

This example doesn't look bad on first glance. Sharp engraved marks on the thimble and the barrel, the brand name is clear, made in the USA, steel frame, with a satin chrome finish. These things all say Quality.

A little rust on the sides of the anvil, but the faces are clean and fairly smooth, though not mirror polished. The spindle moves freely all the way closed. Zero adjustment is by a screw in the bottom to move the anvil, which isn't great, but if the tool is otherwise good, that is acceptable for a 1/1000" micrometer.

But then comes the first problem, and it is a deal breaker. The measuring faces don't align. When the spindle comes out, there is no adjustment for wear, we look down the bore and see that the bore really doesn't align with the anvil. It is easier to see if the anvil is properly flat and smooth, so the surface is like a mirror, but it is pretty visible here.

How does this happen? It can be from a number of things, among them dropping the tool, trying to use it as a clamp, or even poor manufacture. I have no idea how it happened with this one.

How will this tool perform in the real world? A test of the axial and the side to side play tell us that it isn't a good tool to begin with, as there is 2.5/1000" axial play, and 5/1000" side to side, at the position tested. This is enough to be felt easily, and can even be felt wearing gloves. I doubt it was this bad when it left the factory, as Central Tool produced (and still produces) good quality under their own label and for other labels, but this one isn't even pretty enough to hang up for the holidays.

Next up, a real sloppy tool that often goes for a high price.

It says right on the thimble that it is handy. This is a Hanson Tools Handy micrometer (not to be confused with the Central Die Casting Handi-Mike, though they are very, very similar). Though these were sold in huge numbers through an assortment of suppliers, branded Fulton (for Sears) as well as Hanson Tools. The appearance is similar to a better make, and the feel isn't horrible, but the frame is pot metal and obviously so, Was it a toy? Amazingly, no. Just made to a price point, though you wouldn't know it by the prices these sometimes list at on a certain auction site.

The spindle threads are accurate, and the movement, in this example, is fairly smooth. The problems show when we look at how the anvil and spindle meet. On The reading is zero, but there is light visible between the faces. Neither face is flat. This tool can not ever be accurate.

There are special purpose micrometers with faces that are pointed, spherical, and vee shaped, to name a few, but these are supposed to be flat. They are not.

The final proof of the poor quality is in looking at the play: 2.5/1000" axial and about 9/1000 side to side.

Though this was not sold as a toy, that is all that it is.

This is a micrometer that cost a dollar. It looked rough. There was rust. It didn't move. For all practical purpose, it belonged in a skip on the way to be turned into a Toyota.

But, I took a shot (full story in my bestselling saga "Evaluate, Repair and Adjust a Second-Hand Micrometer", also available from this fine publisher).

The things that differentiate this tool from the ones we have looked at include: The adjustment ring at the end of the spindle bore to take up wear by slightly squeezing the end of the threads; the tool can be fully disassembled in a straightforward manner (removing screws); the zero adjustment is well designed (it can be adjusted while the tool reads zero and not done by moving a working part); the frame is made of a high quality, stable metal (a high grade cast iron); the faces, though not perfect and with a small ding, are still mirrorlike.

There are few top quality that miss more than one of these points.

Despite the initial condition, how did it fare when tested? When adjusted, it passed basic calibration tests. When checked for play (which is the easiest thing to feel when buying), none can be felt. The measurements are 3/10000" (0.3/1000") axial, and less than that side to side at the point tested. This is just enough for the light oil film that lubricates the spindle threads and helps protect from rust. In fact, to show the axial play took more force than any of the other micrometers, and several seconds for the position to settle as oil moved through the clearance in the threads.

This is a quality tool and quite usable as it is to 1/1000", though it will get the faces lapped back to perfection at some point.

Now, for a final example....

This resembles a classic design from the late 19th and early 20th century (the Brown and Sharpe #15, pat 1884), as long as you don't look too closely.

I looked.

The design was outdated before WW-II. The zero adjustment is, again, by turning the anvil on a screw thread.

The chrome is shiny, and the surface of the spindle is polished, but the machining is very, very sloppy. The marks are chromed, mostly, and not very deep, so they are difficult to read. They should be clear. Glare from the chrome around them would pretty much make them difficult to read anyway, even if they were sharper and not chromed.

The end of the bore is rough, the threading isn't straight, and the spindle doesn't come close to meeting the anvil correctly. It is offset, and the anvil was ground at an angle to the axis. Unfortunately, not a right angle. The end of the spindle was also not ground at the correct angle. That this was still in the unopened package leads me to believe that the manufacturer has no idea what the intent of the tool is. Or, maybe, no care. This micrometer is still made, I have no idea who makes it or where, as all it says is 0-25mm and is sold by a number of mass market sources.

Beware the "appearance may differ" in the ad. That isn't a warning, it is a threat. You may get this instead of the real tool in the picture.

The axial slop is pretty bad at 3.5/1000" (0.08mm), and the side to side is 6.5/1000" (0.17mm). It feels horrible.

We have seen many of the things that make a micrometer bad, a few that make them good, and you now know that you want to examine and feel the tool before buying, unless it is a new tool from a reputable manufacturer, or a used high quality tool from a reputable seller.

There are plenty of other ways for a poor manufacturer to make a bad micrometer, but looking at the tool and manipulating it will generally find them.

Long term care of a good tool is straightforward for the most part, and can pretty much be summarized as: Keep it clean, keep it dry, and light lubrication for protection. Too much oil and heavy lubricants like grease hold grime and harden over time. Avoid abrasives and grit.

Oh, and don't drop it.