Test Indicators: What can go wrong?

A damaged Last Word test indicator
Test indicators are also called "dial test indicators" and "lever type" indicators and sometimes are given the abbreviation DTI. They're small and the gears, pinions and bearings are even smaller. That's why they're the most fragile of the mechanical instruments, the most easily damaged, and among the hardest to repair. Special tools and considerable practice are usually required.

Here are some of the things that are likely to go wrong, how you can prevent it, and what you can do when it happens.

crystal is the clear plastic lens through which you can see the dial and the hand. Several things can happen. The crystal will become cloudy or discolored with extreme age or solvents you're using in the shop. The solvents could be airborne mist. The crystal can fall out on its own and this happens when the plastic shrinks. This can be due to temperature fluctuations or age of the plastic. The crystal can also fall out when the bezel is damaged. The bezel is the metal or plastic rim around the face of the indicator. In most cases, the plastic crystal is inserted into the bezel with a press. It will snap into place in a groove and because the plastic is now dome shaped it will remain firmly seated. Some manufacturers now use one-piece plastic bezels with crystals. This makes the job of replacing a damaged crystal a snap. Literally. Just snap the new one in place. However, it also means that you will be able to break the plastic bezel with ease. More on that later.

The crystal can crack from mechanical damage but it can also show stress fractures, tiny cracks which may only show up from certain angles or under certain light conditions. These are most often associated with Compac indicators, even when brand new.

When the crystal is no longer properly seated in the bezel, several things can happen: fluids like oil or coolants will leak under the crystal, may discolor the dial or dissolve the numbers on the face. It will also leak underneath the dial and that's where the hair spring lies. If the hair spring becomes oily, it will no longer function and the indicator won't repeat.

dials are plastic or painted metal with the numbers and tick marks (graduations) printed on them. Some dials hold up better than others. The numbers can fade with time or because they're washed out by solvents or exposure to extreme light. Dirt and oil can make dials unreadable. If you can remove the bezel and crystal you can use a tissue to wipe the dial, but if you need to replace the dial, then you'll have to remove the hand (or hands) and that needs tools and experience.

hands (pointers) are preset at different locations on different models. The exact location is really arbitrary and, over the years, various manufacturers have changed their minds on the hand's starting position. Mitutoyo has made it easy—presumably for their own assembly line—by printing a tick mark along the outer edge of the inner dial. When assembling the indicator, the large hand should line up with this tick mark. More significantly, the large hand and the small hand, if there is one, should both coincide to reduce the possibility of reading errors.

Coolants and oils can also leak into the movement from the contact point end. There's no way around this. The body has to have an opening at this point so that the lever can move. Small amounts of fluid may not cause immediate harm, but they will eventually work their way towards the hair spring and then you'll have to have the indicator cleaned. Don't try to immerse the indicator in solvents. It won't work. The indicator has to be disassembled and the hair spring has to be cleaned and dried. Disassembly is complex. Just opening the side of the body won't give you access to the hair spring. You will have to remove the dial assembly and unless you know what you're doing, you'll probably do more harm than good.

In some indicators, the cover on the side of the body can be removed without it affecting the indicator one bit. The Bestest indicators are a prime example. Learning from the watch manufacturers, they've adopted the mono-bloc movement which is independent of the body. In theory, you can open the case, remove the old movement and drop in a new one. In theory. Most other indicators stop working the moment you open the case. And, you'll probably never get them to work again.
The bezel rotates with enough friction to hold it in place during measurements, but with enough ease so that it can be turned while in a set up. The correct balance is tricky and different models use different methods with different results. There is usually nothing you can do if the dial turns too hard or too easily. New plastic bezels of Mitutoyo indicators ride on an o-ring and this works fine while they're new. But, o-rings deteriorate and solvents will ruin their elasticity. The bezel becomes very hard to turn or it practically falls off on its own. Swiss made indicators use a metal spring which could be adjusted, but it requires disassembly. If you have special requirements, ask your repair shop to make the modifications for you.

The bezel can be bent out of round in any number of ways. I'm sure you can think of some. This will cause the crystal to pop out. Sometimes the bezel can be bent back in shape, but often you'll have to replace it. Bezels can be bought with crystals already installed which is a good thing if you don't have access to a crystal press. The bezels of Bestest indicators can not simply be replaced. They require instrument disassembly. Bestest bezels are quite thin walled and will easily be bent out of shape. Compac and Interapid have more substantial bezels which can take a bit of abuse. Forget about the plastic bezels of Mitutoyo. They'll crack for any number of mysterious reasons.

contact points on test indicators are model specific. Each model, each manufacturer, has a specific length of point and this is important since the measurement depends on a leverage system. The length of the points from the center of the ball to the center of the pivot enters into the calculation. Information on contact points can be found on page 21.

The contact point swivels on all indicators. They’re held in place by a friction clutch in most cases. The Last Word indicator and Gem indicator have a ratchet instead. You can move the point almost 180 degrees in some cases. Do this with your thumb and finger even if the friction seems very stiff. Compac and Interapid indicators are notoriously stiff. It’s designed that way so that the point won’t move on its own when you are measuring. Do not use pliers. It’s not necessary and it will probably cause harm.

To obtain maximum repeatability it will be necessary to mount your indicator firmly. Nothing must move except the contact point. Mounting is done using the dovetails or mounting stems which most indicators have. A few (Starrett Last Word) don't have dovetails and you'll have to use special holders. Interapid indicators have mounting stems attached to the body as well as dovetails but even here the manufacturer suggests mounting by the dovetails for maximum stability. The amount of friction on the Interapid stem can be adjusted by tightening the oversized mounting screw. If these attachments are not tight, or if the fixture you're using (perhaps a magnetic base) is flimsy, then you'll have problems with repeatability.
Other repeatability problems can be traced to the contact point. It may not be screwed in tight enough. Or, you may feel sideways play in the pivot. In quality indicators, the pivot rests in two ball bearings which may be adjustable. If there is sideways play, and the bearings are not damaged, then they can be carefully tightened. Special tools are used to accomplish this task. Of course, it's possible that the ball bearings are damaged, and you may not see this from the outside. Most ball bearings are small and hitting the contact point (or dropping it on the floor) will damage the bearings. Nothing can be done but to replace them. It's not an easy thing to do. It actually requires complete instrument disassembly which you know if you've ever tried it. Compac indicators have oversized ball bearings which almost never need replacing. Bestest bearings are very susceptible to damage.

The indicator hand may not repeat if the
hair spring is bent out of shape or has gotten oily (as mentioned before). This requires professional help. In some cases, an indicator can become magnetized and this will affect repeatability. Run it through a demagnetizer to see if it helps. Starrett Last Word indicators can suffer from this malady. Mitutoyo indicators are "antimagnetic."

The indicator will be sluggish if the
ball bearings have become gummed up, or if they're too tight, or if the hair spring is oily. Amateur repairmen will usually over-tighten the bearings, thus ruining them. In other cases, cleaning or bearing replacement is necessary.

If the indicator
hand hangs up at one spot, or if it appears to jump, then there may be a damaged gear tooth. Several small gears are used in translating the movement of the contact point into the movement of the hand. These gears are small and the teeth are easily bent out of shape due to shock. It could also be the case that a bit of dirt, a metal chip, some dried grease has gotten into the teeth. Or, take a close look, the large hand may be touching the dial or the crystal or, if it has two hands, one might be touching the other. You may be able to bend the hand but you'll probably break the center pinion in the process. Be warned.
Dovetails are (or should be) of uniform dimensions from one manufacturer to the next, making it possible to mount any indicator on any fixture. We've noticed some Chinese indicators and mag bases with tolerances so lax that they don't easily fit. Here we give the dimensions (in metric) of an ideal, standard, dovetail using Compac indicators as an example.

In severe cases, the
pinions on the gears break off or become corroded. Corrosion will happen in all indicators exposed long enough to water or humidity. It's called rust. Rust usually makes an indicator not work. It will probably be necessary to replace many of the internal parts when this happens. If your indicators continually become rusty, and you can't change the environment, then consider switching to another brand of indicator. Although we don't know this for sure, some brands such as Mitutoyo, which contains more plastic parts, may be less susceptible to rust than others.

  • This website contains many other pages with information on test indicators. Please refer to our index page for subjects. Additionally, you will find repair manuals and other useful reference books listed below.
More About Test Indicators

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