MICROMETERS ACCURACY – Drunken Threads and Slip-sticks
Micrometers Accuracy- With the number of micrometers on the shop floor and in inspection areas these days, it is important to understand the degree of confidence that one can expect from these instruments. Whether it is a screw thread or digital micrometers, the level of precision depends on two factors: the inherent accuracy of the reference (the screw thread or the digital scale) and process errors.
With screw micrometers, accuracy relies on the lead of the screw built into the micrometers barrel. As with any screw-based movement, error in this type of micrometers tends to be cumulative and increases with the length of the spindle travel. This is one reason micrometers come in 1 in. (25 mm) measuring ranges. Apart from the difficulty of making long, fine threads, the error generated over the longer lengths may not be acceptable enough to meet performance requirements.
One approach that sometimes improves the overall performance of the measurement is to tune the micrometers to the range where is likely to be used. For example, if a (zero – 1 in (0 – 25 mm) micrometers) are to be used on parts towards the largest size, the micrometers could be calibrated and set up so that the optimum accuracy is at some other point in its travel than at its starting point. You could choose the middle to balance any errors at the end points, or elsewhere to maximize performance at any particular point of travel.
Aside from the calibration error of the thread, which reflects the accuracy of its movement per rotation, there are also two other thread related errors you should be aware of; one is error within the rotation known as drunken thread. Because of slight, thread waver over the course of a rotation. The other is slip-stick (or backlash), which is caused by unwanted slop between the mesh of the threads. This is a common cause of reversal errors. As a point of reference, the drunken thread is like profile error on a machined surface, while slip-stick is similar to backlash errors seen in gears on dial indicators.
With electronic micrometers, the thread usually drives a sensing head over a scale, or uses a rotary encoder as the displacement indicator. Both can induce slight errors, but the thread of the barrel remains the single largest source of error. However, electronic micrometers can remember and correct for such errors and, in the end, provides better performance than the interpreted mechanical micrometers.
The process for checking the performance of micrometers is similar to that of other comparative or scale based instruments. Gauge blocks of known sizes are measured and the deviations from the expected values plotted. Usually the gauge blocks are chosen so that the spindle travels for a full or half turn of the screw. Taking this one-step further, a single rotation of the screw can be analysed by taking very small increments of measurements around the peaks discovered on the first pass. These small increments—maybe ten steps in one revolution—may reveal even larger errors, or show patterns machined into the screw threads.
Besides thread errors, the other significant cause for errors found in the parallelism of the anvils. The precision method for inspecting the condition of the anvils is with an optical flat. It is generally acceptable to allow two visible bands when assessing individual anvil flatness by using a monolithic light source. For inspecting parallelism, six bands in total are observation, the combined total of both sides.
The applied measuring force of the sensing anvil on the part and the reference anvil is the other source of process measuring error. The friction of ratchet drive thimbles does reduce the deflection of the micrometers frame, but condition still exists as a source of error. With about two pounds of measuring force, typical frame deflection is roughly 50 micro inches, although this is apt to increase on larger micrometers where the rigidity of the frame increases.
As with any hand tool measurement, other sources of errors will also sneak in. Temperature, dirt, and the means by which the operator aligns the gauge to the part play a large role in the overall performance of any micrometers.
George Schuetz, Mahr Federal Inc.