Automated Checking FixturesChecking Fixtures Design — By Gladys on 11 August 2007 at 12:33 pm
When checking fixtures are used intensively, some automation must be carried out to reduce the cost for the checking of parts. Thanks to a minimal investment, the duration of the measurement process can be decreased by 90% by avoiding the introduction of the data obtained for the statistical study.
To get a sample of that type of checking fixtures, we created a copy of one of our customers´ conventional checking fixture – with his agreement – to use it as a sample and test bench.
The initial checking fixture had 6 dial indicator positions and only one dial indicator. Its use consisted in setting the dial indicator on the various positions and manually writing down the results. This process can be carried out in 2:10 minutes for each part and implies some risk of transcription mistakes. As a result, the statistical processing of a 50-part batch could imply 30 extra minutes, that is, a total of 2´5 hours dedicated to the capability study of this batch.
On this automated sample, we have set manual movement sensors as well as a Merlin data recollection terminal from the Marposs firm.
This automation enabled to reduce the measuring time down to 16 seconds, since, contrary to the first system, the data did not have to be manually transcribed and the Cp and Cpk data were instantaneously obtained during the measurement process of each part, allowing to follow up their evolution. The data can be directly exported under Excel format. Consequently, with this automation, the results for 50 parts can be obtained in 13 minutes instead of dedicating them 2 hours.
This decreasing in time can be bigger: for the same cost, the process can last half the duration if air pressure load sensors are used instead of manual load sensors.
Economically, there was only 30% more invested in the checking fixture, due to the sensors cost, but the return on investment goes far beyond this total. The Merlin unit can be reused with a great number of checking fixtures, and one terminal unit can be paid off with all the checking fixtures available at the plant. Sensors detection is automatic since they carry a recognition system when being connected to the terminal, and each checking fixture own measurement programme can be stored in the terminal.
What is more, using that type of sensors brings the following advantages:
- A known part can be used as a master to reset all the zeros of the sensors without having to modify the checking fixture.
- Easily undone corrections can be applied on each position to compensate a possible deviation from the nominal without having to modify the checking fixture. Such corrections can be used for painted or non-painted parts which thickness can vary, chromed parts, parts with an admitted deviation, etc.
- The Merlin terminal enables to make all kinds of arithmetic operations with the results: calcultation of the excentricity, values sum…and display it as another result.