Wednesday, 23 May 2012

Special torque applications: dental abrasive disks

Abrasive discs are used by dental technicians to sculpt and polish dentures. The discs are spun on the technician’s motorised hand piece at speeds of up to 20000 rpm. The abrasive surface applies sculpting and polishing friction, so high torque loads are normal. In order to reduce the possibility of the disc’s two parts – centre ring and abrasive disc – separating during use, the manufacturer wanted to test the torque performance of the join between them.

The specially made adaptor/jig fits directly over the rotating drive shaft of the Vortex-i torque tester to hold the sample securely and centrally, so that drive-shaft and centre ring are exactly aligned. Once the disc is placed in the recessed holding plate (reversible to accommodate the two sizes that the disc comes in), the clamp plate is screwed down over it. A probe, which is connected to the torque-sensing transducer, is lowered till its point penetrates the centre ring and can travel no further.

This video introduces the adaptor and shows the test being set up.

The test is designed to test to destruction the join between the centre ring and the abrasive disc. Pressure from the clamp plate ensures the abrasive disc does not spin inside the jig but rotates in unison with the jig.

The jig is rotated to 90 degrees clockwise and then 180 degrees anti-clockwise. If the centre ring and abrasive disk remain aligned (the radial marking will indicate this), the join between them is good and the disc passes the test. If the quality of join between centre ring and disk is insufficient, the centre ring will rotate independently of the disc and the sample will fail.

In this picture, the post-test misalignment of the markings on the disc indicate that the outer abrasive disk has slipped around the centre ring, so this sample has failed. The torque data is exported via the Vortex-i’s RS232 port to the PC for monitoring, analysis, and storage.

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