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The Case of the Peaked Planetor
Kenneth Russell, Professor Emeritus, MIT -- Design News, July 13, 2008
Type the word “planetor” into a computer search and you will get back a bunch of references to a blocky type face. The planetor in this article is a carpentry tool for rapidly drilling large holes in heavy wood beams. The tool is strictly for professional use, mostly by plumbers and electricians.
The planetor rather resembles an airplane propeller with sharpened steel wings welded to a central shaft. Planetors may drill holes several inches in diameter in heavy timber, which is far beyond the capabilities of ordinary drill bits.
Such heavy-duty drilling calls for large torques and a powerful electric motor. I was warned that use by amateurs, such as myself, was likely to result in a broken arm. The tool is strictly for the pros who need to get through heavy timber in a hurry.
The Scene of the Crime
In this case the sharpened wings were snapping off near where they were welded to the central shaft. Such failure, of course, resulted in replacement costs and lost time. In addition, the wings were the size of my little finger. Fracture under heavy load could turn the wing into a projectile capable of causing serious injury, particularly to eyes. The wings were massive enough to go right through most eye protection.
The Investigation
I was contacted by the manufacturer in this case. There was no immediate threat of legal action and the company wanted to make sure none occurred in the future.
My main tools in this case were the scanning electron microscope (SEM) and Rockwell hardness tester. I have discussed the SEM in earlier articles and just mention here that it is capable of magnifications of many thousand times while still keeping all of a rough fracture surface in focus. The hardness tester presses a hard indenter into the surface of a metal under a prescribed load. The depth of penetration is converted to a hardness number. The test is simple, fast, reproducible and widely accepted.
I recently discussed three of my Calamities cases in a keynote address at a major quality assurance conference. I did a bit of head scratching and realized the most common cause for failures I investigated was poor quality control. Such was the case here.
Drill bits must be hard to work. One very popular way of gaining this needed hardness is to heat the steel to a bright red and quench it into water or oil. The result is a very hard and brittle steel. Tempering at a few hundred degrees Celsius gives significant ductility at the cost of only a little hardness.
Specifications called for a particular medium carbon steel to be heat-treated to a certain hardness. The planetor failed on both counts.
The Smoking Gun
The steel had significantly more carbon than specified and the hardness was significantly higher. The two points are related as higher carbon gives higher hardness.
However, the high hardness made for a brittle steel. The figure, above, is a SEM picture of the fracture surface on a planetor wing. The facets show that the fracture was brittle with the metal cracking along the grain boundaries. A fracture through the grains that produced ductile dimples was desired.
My fix was fairly simple. I recommended the lower carbon steel be used and that it be tempered to the specified hardness. I also recommended the planetor be annealed at a low temperature both before and after welding. These extra heat treatments relieve internal stresses and reduce cracking in the hard steel. The added processing steps add to the cost of the part but are part of the price to pay for using high-strength steel.
| Author Information |
| Ken Russell (kenruss@mit.edu) is professor emeritus of Metallurgy and Nuclear Engineering at MIT. He specializes in physical metallurgy, forensic metallurgy and failure analysis. Cases presented here are drawn from his actual forensic files. |
