Technika pomiarowa
Technika pomiarowa
The Right Angle on Perpendicularity
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The Right Angle on Perpendicularity
George Schuetz, Mahr Federal Inc.
 

The right angle is one of those things that man has created in his mind. In nature it only happens by chance. But the importance of this concept—which results from the perpendicular intersection of lines or surfaces—applies to architecture, civil engineering, agriculture and manufacturing.

There are a number of different terms used to describe this relationship, including perpendicularity, 90 degrees, normalcy or squareness. In engineering products, which may not have perpendicular surfaces, the right angle is used in the dimensioning of rectangular coordinates. The implementation of perpendicularity in manufacturing and measuring is never perfect and is assessed as achieving a higher or lesser degree of perpendicularity.

The most common method to inspect for perpendicularity is to compare the part to a master square, which can be steel, granite, or cylindrical.

In the world of machine tools, perpendicularity is extremely important as it sets up the reference to which all parts are manufactured. For this application, manufactured reference masters are usually too small to provide a standard for the machine tool. For these applications, optics, in the form of autocollimators or lasers, are used as the reference standard. An optical square provides a very precise 90˚ reference for setting up the machine.

The steel square or cylindrical square are the most widely used tools for surface plate work. They function by determining whether two selected points on the part are located on a common plane at a right angle to the surface plate. For this check a special height stand is used, on which a spherical contact on the base and an indicator is held vertically on an adjustable bracket. The mastering process involves bringing the contact on the gaging stand to a reference square. The indicator is then moved to the selected point on the reference square, touched off and set to zero. This sets up a comparative square that can be used as a transferable master to measure parts. The reference square is replaced by the part and the two contacts on the stand are touched to the part. Any deviation, plus or minus, is read on the indicator as a deviation from perpendicularity. Based on having a right angle, and knowing the distance between the base reference contact and the indicator sensing tip, the deviation can be converted to an angular measurement.

One of the issues with this type of measurement is that it only measures two points on the perpendicular surface: the reference point, and the sensing point. A lot of things can be going on between two points. For example, the surface may have some out-of-straightness, which depending where the sensitive contact lands, can mean the difference between a good and bad part. A step up from the two point, fixed location method is to use a stand which is also a very precise square and allows for precision vertical movement while retaining its perpendicularity throughout the vertical travel. These tend to be very precise, but expensive, master squares that can provide measurement at numerous points and thus provide a true picture of the part's perpendicularity. 

A faster, more automated way of making this surface plate perpendicularity check is with an electronic height gage. While the electronic height gage may not have the level of mechanical perpendicularity that is built into a precise master square, it is a very repeatable gage. And since it is repeatable, any inherent inaccuracy can be measured and corrected for, thereby improving the performance of the gage.

Another advantage of the electronic height gage is that a digital indicator can be used to automatically record deviations as the slide moves up the axis. The result is a recording of multiple points along the part, and the simultaneous calculation of both straightness and perpendicularity in both angular and displacement readout.

It should also be pointed out that the perpendicularity measurement is only as good as the reference surface upon which it is based. Since these tend to be surface plate related measurements, the accuracy of the surface plate and its cleanliness are what determine the perpendicularity of the height gage. Therefore, to get the best measurements, the grade of the plate and its present condition must be known.

It is also possible to make the height gage perform better than the perpendicularity spec it has been pre-measured for at the time of manufacture. In some cases, the height gage can be fixed to a location on the surface plate and a reference square brought to it. The height gage is then used to measure the square and correct its perpendicularity error based on this particular spot on the plate: in other words it can be corrected for best performance at its location to the reference square. Once the correction factors are entered into the electronic height gage, the square is replaced by the part. The performance of the height gage has been improved for this particular measurement to a higher degree of perpendicularity. Note however, that the height gage cannot be moved, and the part must sit in the same location as the reference square was sitting. Any deviation from this negates the customization of the perpendicularity setup.

 

A faster, more automated way of making this surface plate perpendicularity check is with an electronic height gage, such as the Digimar CX2 Height Gage, which feature fully motorized measuring slides. A digital indicator can be used to automatically record deviations as the slide moves up the axis. The result is a recording of multiple points along the part, and the simultaneous calculation of both straightness and perpendicularity in both angular and displacement readout.

 

The angle  is calculated based on collecting samples along the trace and fitting a best fit line.