by Drew Koppelmann
Last month we introduced the concept of machine tool evaluation as a means of gaining control over production variables. To summarize very briefly: Evaluation (also known as calibration or characterization) assesses a machine tool's ability to produce good parts by directly measuring its geometry and positioning accuracy. Let's now look at one of the instruments used in evaluation: the electronic level.
Electronic levels measure the deviation in the right-angle relationship between a nominally horizontal surface and the earth's gravitational force, or the change in this relationship over time. Electronic level systems typically consist of an electronic gaging amplifier, one or two level sensing heads and, often, electronic dimensional gaging heads as well. The programmability of the amplifier makes the electronic level a flexible measurement system, and this is one of its advantages over precision spirit levels. The other advantage is accuracy: Spirit levels resolve to about 0.0001" per foot, compared with electronic levels that can resolve to 6 microinches per foot, or +0.1 arc seconds.
Electronic level sensors may be used singly or in pairs. When using a single sensing head, all measurements are relative to the earth's gravitational field; we'll call this "absolute" level sensing.
When used in pairs -- a practice known as differential level sensing -- electronic levels can compare the attitude of separate or adjacent horizontal surfaces: for example, the relationship between the spindle and the table of a machine tool, or the relationship between two nominally parallel ways upon which the table travels.
In machine tool evaluation, absolute levelness is usually not critical. If it were, machine tools would not work on ships. What is critical is the relationship of one surface to another. Machine tools must maintain strict parallel and/or right-angle relationships between the table and the spindle because this establishes the relationship between the work piece and the cutting tool.
To measure levelness on a differential basis, two sensing heads are connected to a single amplifier and arranged so as to respond opposite to each other. If both are tipped 1 arc-sec to the north, one senses +1 arc-sec and the other senses -1 arc-sec. The amplifier, adding the two values, cancels the two signals out, thus indicating a differential of zero (because the sensors are, in fact, tipped the same amount in the same direction). If, on the other hand, the two are tipped 1 arc-sec each, but in opposite directions, the amplifier reads either +2 or -2 arc-sec, that being the true difference in angular orientation.
When evaluating a machine tool, one of the sensing heads is designated as the reference. It is zeroed and is not moved during a trial. The other is designated the "active" sensor and, as it is repositioned, the amplifier registers its changes in attitude relative to the reference head.
Levels can be used to measure the pitch and roll of horizontal movements and the pitch and yaw of vertical movements. Take as an example a vertical-spindle machine: To measure table roll, the active sensor is placed on the table, with its direction of sensitive movement at 90ž to the axis of table movement being evaluated. The reference level is mounted on the spindle in the same orientation, using a simple right-angled bracket that fits in the tool holder. The table is then traversed along the axis, stopping at convenient intervals to record the measurement. Normally, the trial is run twice in both directions, and the average reading for each location along the axis is calculated. Roll can then be plotted on a graph.
If the trial reveals excessive roll (or other parameters out of tolerance), the level can be used as a diagnostic device. Machine tool travel typically occurs along pairs of nominally parallel ways, and these can be measured directly by levelness. Although the level cannot measure the relative height between the ways, it can show differences in attitude between the two surfaces.
Using very similar procedures, electronic levels can also be used to check the flatness of surface plates and to test the accuracy of coordinate measuring machines. Economical and very easy to operate, electronic levels are a good way to get started in machine tool evaluation, and an important part of the complete evaluation tool kit.
About the author
As applications manager, gaging products, at Federal Products Co. in Providence, Rhode Island, Drew Koppelmann provides dimensional gaging applications assistance to companies in a wide range of industries. He may be reached by fax at (401) 784-3246 or by e-mail at dkoppelmann@quality digest.com.