Why are two electronic levels required for accurate flatness measurement?

The industry sees itself confronted with a permanently increasing demand regarding accuracy. To manufacture precise parts require accurate machines and accurate measuring tools to measure and control the parts during and after the manufacturing process.Typically, these parts are checked on a reference plate made of granite. If the parts have to be measured accurately then the reference plate has to be very accurate itself. For example, in today's automotive industry these reference plates have to be re-tested and re calibrated regularly.

Earlier, spirit levels were used to calibrate these plates. In line with the increasing demand in accuracy these spirit levels have been replaced over the years by high precision electronic levels with an accuracy of up to 1?m/m. These high precision levels are 20 times more sensitive and accurate than spirit levels.

Why was it possible earlier to measure flatness with one single spirit level and now you need two electronic levels for the same task? There are two main reasons, which both go in line with the increased requirement regarding accuracy:

1. The Weight of the Level Itself

A typical high precision level has a weight of slightly over one kg. When measuring flatness you are supposed to put the plate on 3 supports according to Bessel:

Even the most stable granite plate will adapt its form slightly to the support: (exaggerated graphics!)

What happens now if you measure flatness with one electronic level only? The following graph should illustrate this: the weight of the instrument itself has a small but still measurable influence on the shape of the granite.

For a plate with the size of 1000 mm x 630 mm the impact is in the range of several ?m/m and therewith in the same magnitude as the total error respectively the flatness of that plate!

Placing the second instrument, called "reference instrument", on the same plate, this second instrument will see the movement of the plate as a consequence of moving the measuring instrument on the plate. Measuring A-B (whereby A is the inclination of the measuring instrument and B is the inclination of the reference instrument) the error of the weight of the measuring instrument can be eliminated to a large extend.

Watch a video demo on youtube http://www.youtube.com/watch?v=MHFDQRaiU-k

2. Elimination of External Influences

Using the same differential measurement, it is possible to eliminate other external influences like

a. Vibrations from e.g. trucks: such trucks are very often used in production and they do not have to be in the same hall; even trucks running in a nearby hall can influence the measurement.
b. Vibrations from close by roads or trains
c. Vibrations from engines running
d. Vibrations from emergency power supplies.
e. Unstable ground: in some locations the ground is influenced by the tide or by underground water
f. For high precision measurements even people moving around on the floor can influence the measuring results

Summary

Using a differential measurement A-B with two instruments allows therewith measuring straightness of a straight edge or flatness of a surface and - very important - flatness and straightness on a machine much more accurately than with one instrument only. (Article courtesy: WYLER AG, Switzerland. Product information: Bombay Tools Supplying Agency, Mumbai. Tel: 022-23426495. Fax: 91-22-23401933. Email: info@bombaytools.com)