Jeweled vs. non-jeweled
The bearings in the movement are usually man-made rubies or sapphire. You won't get rich collecting these industrial grade gems, though. They're worth fractions of a penny but when they crack, your indicator suddenly becomes useless. It'll take quite a jolt to crack the jewel so don't bother worrying about it. That kind of jolt will also have damaged other parts of the indicator.
Although opinions vary, it is assumed that jeweled bearings need little or no lubrication (oil).
Non-jeweled movements have brass bushings which won't break but they will wear down. If the indicator is in constant use, the holes in these bearings get larger and larger and eventually the indicator's gears will wobble about, creating problems with repeatability and that elusive concept of hysteresis. In other words, measurements in the up direction may not be the same as measurements in the down direction.
If your indicator gets heavy use, you will profit from buying an indicator with a jeweled movement. Otherwise, a non-jeweled movement may actually be less subject to damage. Not every manufacturer will give you an option, however.
Stem diameters: 3/8 inch vs. 8 mm
If you are working outside of the US, then the choice is clear: you'll stay with metric dimensions. Within the US borders you'll be plagued with the non-ending duality of inch vs. metric.
The stem of the indicator is the preferred attachment point onto indicator stands and other fixtures. It is here that the indicator will be most stable and accuracy will not be compromised by a shaky setup. You will have to know which stem diameter your indicator stand will accept. Most stands are designed for 3/8" diameters and most indicators sold in the US will have 3/8" stems.
However, just because your indicator has a 3/8" diameter stem, doesn't necessarily mean the indicator has to be graduated in inches. The manufacturers have been forced to create hybrid versions for the US market. It will be possible to buy metric indicators with 3/8" stems, just as you can buy inch reading indicators with 8 mm stems.
Fortunately, 8 mm is a smaller diameter and a simple adapter sleeve can be used to make these stems fit into a 3/8" hole. It will not work in reverse. The 3/8" stem will never fit into an indicator stand that accepts only 8 mm stems.
Given these options, it would be wise to select one system for your shop and to stick with it.
Lug backs vs. flat backs
Most dial indicators come with lug backs. Compac (Swiss) dial indicators are the exception: they come with flat backs. Also, if the indicator is a component of a bore gage, thickness gage or a depth gage, then there is no need for a lug back.
The lug back screws onto the back of the indicator with 3 or 4 screws. These should be nice and tight because you will be mounting the indicator by its back. The lug has a (usually) 1/4" hole in it and with that you can bolt the indicator to a stand whether its a magnetic base, a granite base or a fixture of your own design. Normally, the lug is located smack in the middle of the back and that's termed "on center." Your particular setup may benefit from a back which has the lug "off center" and these backs are also available.
The backs from one manufacturer will not fit the indicators from another manufacturer. You can not use a Teclock back on a Starrett indicator, for example. The diameters will be wrong and the screws will probably be in slightly different locations.
Of course, you might be holding the indicator by its stem (see Stem Diameters - details above) in which case the back is immaterial. You'll probably find these indicators with either metal or plastic flat backs. It doesn't matter, as long as dirt and spray is kept out of the inner gears and movement parts.
When attaching an indicator, either by its back or its stem, make sure the stem is perpendicular to the surface being measured. If it's at a slant, then you'll have a cosine error which can be significant in a .0001" indicator. Try and see for yourself what a difference even a slight angle can make.
Right side up vs. upside down
We had a belligerent customer whose dial indicator wasn't returning to zero when he used it upside down. We told him that dial indicators are designed to be used right side up. He said "Bullshit," which put us on the defensive. We called one of the major US manufacturers, the one located in Athol, Mass., and asked them. They said, indeed, the dial indicator has to be used right side up, that "gravity is a factor in the equation." If the indicator has to be used inverted, then a much stronger return spring has to be installed.
The literature that comes with the brand new digital indicators from Japan's leading manufacturer states plainly and clearly that their state-of-the-art indicator will work anywhere between upright and horizontal. It says nothing about working between horizontal and upside down. Now we feel vindicated, and that complaining customer can eat his words.
Dial size: how big is too big?
Dial indicators come in 5 different dial sizes from "mini" Group 0 to Group 1, 2, 3, and 4. The average, most useful size is in Group 2 with a dial diameter of about 2-1/4 inches. These are easy to read and easy to handle. Don't buy any other size unless you have a good reason for it.
Group 1 indicators are about half the size and you may need them to fit into tighter spaces. Many grinding machines come with these smaller indicators because they take up less room. Compac makes the best Group 1 indicators and these would be a worthwhile investment. If you're designing a new set-up, you would be wise to incorporate these models.
Group 3 and 4 are really too large for comfort. You'll get a larger dial which means the graduations are spaced further apart. Just because the graduations are further apart doesn't mean the indicator is more accurate. Nor does it give you the right to subdivide the graduations. If the indicator is going to be situated at some distance from the operator, then a larger dial may be justified because it's easier to see. The larger mechanisms of oversized Group 4 are subject to more wear and tear and they can be more easily damaged as well.
The miniature Group 0 models are frail creatures. The .215" diameter stems of the Federal models are easily and quickly damaged, for example. Upgrade to Group 1 whenever possible.
For more specific information on dial indicator manufacturers take a look at Dial Indicator Brand Comparisons on page 14.
Why isn't the dial hand set to high noon?
Your dial indicator has its large hand set at about nine or ten o'clock. If you let the hand return to this resting position you might actually notice some deviation (poor repeatability) and you might think the indicator is malfunctioning. Actually, the manufacturers don't want you to check repeatability at this point and they want you to add an extra bit of pre-load on the hair spring so by the time the hand reaches the 12 o'clock position, everything will be ready for measuring. Check the gage now and you'll see that it repeats very nicely. There's a more practical reason though: if you need to set the indicator to the zero position it's much easier if the hand can travel in both directions. If the indicator hand were already at zero this would be tricky business.
What's to remember? If you take the indicator apart for any reason, put the hand back in the correct position; and, don't try to check repeatability while the indicator hand is in this resting position.
Dial configuration: continuous or balanced?
Balanced dials run from zero at the top, to a high number at the bottom, and back to zero again. They are “balanced.” These are the dials to use when your indicator is being used to check run-out. You’ll want the indicator’s hand to be set at zero and then to deflect one way or the other to show you whether the spindle is running true. You would reposition your work piece until the hand no longer deflects.
This same balanced dial is used to make comparison readings. With the indicator in an indicator stand, place the master piece (or the gage blocks) under the indicator and set the indicator hand to zero by turning the rotating bezel. Remove the blocks and insert your work piece. If it’s dead-on, then the hand will still read zero. If it’s off, then your indicator will show you by how much. Keep an eye on the telltale. That’s another—somewhat old-fashioned—word for the revolution counter hand. Why should you look at this? Because you might have over traveled by one or more complete revolutions. The large hand might still be on or near zero, but the revolution counter will tell you that you’re way off the mark.
Continuous dials start from zero at the top and then run completely around like a clock. Clockwise being the usual configuration. Counterclockwise is used with dial depth gages where the measurements are actually taken in a reverse direction. Use a continuous dial if you’ll be taking direct readings. Position the indicator on an indicator stand with a granite base so that it reads zero when just touching the base with the contact point. Now insert your piece to be measured and you can take a reading directly off the face of the indicator. Be sure to take the reading of the revolution counter hand into consideration. This setup can be modified many ways to take care of all sorts of measuring situations.
Both kinds of dials, balanced or continuous, can be used interchangeably. It’s just that one or the other dial may be a little easier to interpret under the different circumstances we have mentioned.
Digital vs. mechanical dial indicator
Digital indicators have undergone considerable transformation and improvement in recent years. SPC output, on some models, allows you to track reading results on a printer, on a spread-sheet, or in sophisticated programs.
Digital indicators have a few other great advantages: easy to read numbers (you don't have to interpret what the hands mean on the dial) and they are switchable from inch to metric. Some have high degree of liquid and dirt resistance which makes them ideal for messy work environments. Since no gears are involved, there's less likelihood of a spot error (an error at just one position, due to a bent gear tooth) nor are you likely to encounter hysteresis errors (the difference in readings when the plunger is going in and going out).
Disadvantage: when they break (and they will) there's little likelihood of having them repaired. If the electronic module is available then it can sometimes be switched out with little trouble although this can be an expensive replacement. Parts for older models may be impossible to find. Digital indicators may offer read-outs (.00005") which are sometimes beyond their limits of accuracy, giving the wrong impression.
Mechanical dial indicators have these advantages: can be cleaned and repaired when needed. Can be gotten in smaller, less bulky sizes. May outlast digital indicators by many years.
If you are doing production work with less-than-skilled workers, ironically, the digital will be a better choice. If you will be taking good care of the indicator and have a penchant for mechanical gages anyway, then the dial indicator is the better choice (particularly a good quality gage like Compac or Mercer). In final inspection, if extensive data collection is not required, we would only consider a mechanical indicator as suitable. This, however, is a personal preference.