Peak displacement analysis: new metrology tool automates dimensional inspection of interrupted surfaces.
Typical digital dial indicators and electronic gage amplifiers can be set to capture peak readings as a part is moved or rotated while being measured. The highest point and lowest point are easily determined and the difference between them, the tir (total indicator reading value), easily calculated for flatness, concentricity, and other similar inspections. These peak holding capabilities are usually applicable in situations where the surfaces being scanned are continuous and uninterrupted.
A good example is measuring the runout of the cutting edges on a common six-flute endmill cutter. Setting a typical gage to display a tir value would be useless since a large, unwanted minimum peak value would be generated each time the probe falls off into the voids between cutting edges. Setting the gage to hold a maximum peak only would allow only the size of the highest edge to be determined without any regard for the others.
The only workable method would be to perform an individual maximum peak measurement for each edge. The operator would rotate the part past the first peak, record the maximum value, reset the peak hold function and repeat the process for all six edges. Then, the highest and lowest value of the six must be determined and the difference between them calculated for the actual runout amount.
Basically, that is what PDA does automatically and with great speed. In one continuous scan of the part the PDA gage identifies the individual values of all the peaks, disregards the voids between them, and calculates meaningful results.
A closer shave
One of the first applications of the PDA gage was by Schick, the Connecticut-based manufacturer of razor blade cartridges. Special gage amplifiers were designed and built to do the job of dimensions. The razor cartridge is mounted with the blade edges exposed upward on a motorized slide fixture that moves the blade under a lever-style LVDT gage probe. The probe tip exerts an extremely light downward force as it rides over the pair of blade edges and the plastic guards on either side of them. PDA logic in the gage calculated blade position results in seconds.
For another Illinois company, a producer of molded plastic components, a PDA gage solved a unique measurement problem. A plastic automotive alternator component has a hole through it into which a knurled metal shaft is later pressed. For a more secure press fit, the hole, rather than being round, is molded with a number of symmetrical flat facets. One style part has an octagonal, or eight-sided hole, while another has a sixteen-sided hole. The customer's specifications dictate that the dimension between each opposed pair of flats across the hole must be within certain limits.
By rotating the part on a typical indicating plug-type bore gage the minimum reading between opposed flat pairs is easily seen. With a normal digital or mechanical dial indicator on the bore gage, an operator would have to measure and record the four or eight flat pair dimensions, identify the maximum and minimum of the group, and compare each of these to limits to accept or reject the part. A Brunswick PDA system was supplied that replaces the normal bore gage indicator with a plunger-style LVDT gage probe. A part is placed on the bore gage and rotated just over half a revolution. The gage displays the size and provides go/no-go indications instantly by comparing the results to user-programmed tolerance limits.
How PDA works
A PDA-equipped gage amplifier incorporates high speed microprocessor circuitry that constantly analyzes the incoming displacement data to determine the positive and negative going trends that define the multiple peaks and valleys in a series of measurements. The gage applies certain intelligent logic while collecting the data so as not to be fooled into accepting small surface imperfections as valid peak or valley points. When a predetermined number of peaks have been observed the gage stops responding to incoming data and displays results until reset for another scan.
All individual peak values are stored in memory and are available for the gage to use in mathematically calculating results. Commonly calculated results include min peak, max peak, average of the peaks and tir value. The gage includes an RS-232 data output which can send data and results to a printer or data collector.
The gage is a portable bench top device, with measurement resolution of 0.000 010". It uses LVDT gage probes, either plunger-style or lever-style, and internally processes over one thousand discrete readings per second to accurately capture high resolution peak values.
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|Author:||Palmer, Kelvin M.|
|Publication:||Tooling & Production|
|Date:||Feb 1, 1996|
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