VOL. 4 May ISSUE YEAR 2003
in Vol. 4 - May Issue - Year 2003
Important Details for the Design of an Almen Gage
Indicator with an accuracy of one micron (0.001mm). SAE J442 only asks for a 0.0025mm graduations. Support Plate for Ceramic Balls made of Tungsten Carbide Ceramic Balls pressed into Aluminum Plate
Even though a gage looks like a simple device, there are a number of details, which are not found within the SAE J442 Dec-01, but certainly have to be considered to design a gage which keeps its accuracy over the years.
Positioning of the Support Balls
The right position of the support balls is not just a matter of keeping the requested dimensions and tolerances of the 4 support balls to each other. That is fairly easy. However, more demanding is to foresee how the system changes during the years of use. And with an indicator which has a resolution of only one micron, the smallest change will be recognized.
Possible Wear on the Almen Strip Support System
Since the placing of an Almen strip on and off a gage causes mechanical friction, wear is a main issue in designing gages. The use of ceramic balls instead of hardened ball bearings certainly is a large improvement in terms of ball wear. However, one important question was to find out how the mechanical pressure changes the position of the balls over time.
Tests have shown that balls supported by a hardened plate would slightly damage the support plate after using the gage many times. In other words, the pressure while measuring the strip would push the balls a few microns inside a hardened plate. Hard to imagine but fact. Therefore Peening Accessories GmbH decided to use tungsten carbide plates to keep the ceramic balls in position.
Display Accuracy of Indicator
The Swiss made water resistant digital indicator has a measurement accuracy of one micron (0.001mm). The SAE J442 only asks for 0.0025mm graduations. So the Swiss indicator is 2½ times more accurate than requested in the specification. However, this very high accuracy has the side effect that the last digit within the display (where micros are counted) may flicker a bit. Some may be bothered by that. Therefore the indicator can be switched to a 0.01mm display, but will still measure the microns and round up or down accordingly. For some users that might be the preferred setup of the gage.