Contact point (tip) types

Contact point (tip) types


Plunger (drop) indicator tips

On drop indicators, the tip of the probe usually may be interchanged with a range of shapes and sizes depending on the application. The tips typically are attached with either a #4-48 or an M2.5 screw thread. Spherical tips are often used to give point contact. Cylindrical and flat tips are also used as the need arises. Needle-shaped tips allow the tip to enter a small hole or slot. Accessory sets of tips are sold separately and inexpensively, so that even indicators that have no set of tips may be augmented with a new set.

Dial test indicator tips

Dial test indicators, whose tips swing in an arc rather than plunging linearly, usually have spherical tips. This shape gives point contact, allowing for consistent measurements as the tip moves through its arc (via consistent offset distance from ball surface to center point, regardless of ball contact angle with the measured surface). Several spherical diameters are commercially offered; 1mm, 2mm, and 3mm are the standard sizes.

Despite the advantage just mentioned (regarding contact angle irrelevance) of the ball (sphere) itself, the contact angle of the lever overall does matter. On most DTIs it must be parallel (0°, 180°) to the surface being measured in order for the measurement to be truly accurate, that is, for the magnitude of the dial reading to reflect the true tip movement distance without cosine error. In other words, the path of the tip's movement must coincide with the vector that is being measured; otherwise, only the cosine of the vector is being measured (yielding the error called cosine error). In such cases the indicator may still be useful, but an offset (multiplier or correction factor) must be applied to achieve a correct measurement (where the measurement is absolute rather than merely comparative). The same principle is also employed with CMM touch trigger probes (TTPs), where the machine (when used correctly) adjusts its ball-offset compensation to account for any difference between the approach vector and the surface vector.

Some DTIs (such as the Interapid line and its competitors) are made with a built-in allowance such that a 12° tip angle (between the lever and the surface being measured) is the angle that corresponds to zero cosine error. This is a great convenience to the user because of the practicality of having the ball being clear of the indicator body such that the unit may pass over a surface.

Changing the tip of a DTI is not as simple an affair as changing the tip of a drop indicator, because the tip, being a lever, has its length precisely matched to the clockwork inside the indicator, so that the length of the arc of its extremity's movement has a known ratio to the gears that drive the dial's needle. Thus to add a longer or shorter tip requires a correction factor to be multiplied with the dial reading in order to yield a true distance reading. DTI tips are often threaded for interchange (like drop indicator tips), with small flats to accept a spanner; but the intent regarding user-serviceable tip change is limited only to the tips that originally came with the indicator, because of the above-mentioned importance of the length. Typically a DTI comes with only a few tips, such as a small-ball tip and a large-ball tip.

Neither of the above considerations (cosine error or lever length error) matters if the dial reading is being used only comparatively (rather than absolutely). But the avoidance of mistakes of the comparative-versus-absolute-confounding type rests with the knowledge and attention of the user, rather than with the instrument itself, and thus repairers of DTIs usually will not certify the accuracy of a DTI that cannot offer an accurate absolute measurement—even if it is perfectly good for comparative use alone. Such a DTI could still be certified (and labeled) for comparative use only, but because risk of user error is involved, gauge calibration rules in machine shops either demand a "comparative use only" label (if the users can be trusted to understand and follow it) or demand that the indicator be removed from service (if not).