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Name: Mike
Status: Other
Grade: Other
Location: GA
Country: United States
Date: March 2007


Question:
Does drilling holes in metal make it stronger? Please explain how this works.



Replies:
Hi Mike,

Removing metal (by drilling or any other means) never adds strength to a part. This is an old wife's tale that is absolutely false!

This is something that probably started when someone noticed a high strength part that had parts of it machined away, and assumed that the high strength had something to do with the metal that had been machined away. One might well ask that if removal of SOME of the metal makes the part stronger, then it must be true that if you remove ALL the metal, a part will be even stronger! No, we both know that is nonsense!

It is certainly common for a part to have some of its metal removed that is not contributing anything to its strength. This is done generally to make the part lighter, and if done correctly does not significantly weaken the part. But under no circumstances does removing or drilling out metal actually increase its strength. At best, it just makes it no weaker.

Let me give you a simple example of this. The drive shaft of a car or truck is a long tube that connects the end of the transmission to the gears in the rear end that drive the rear wheels. All drive shafts are hollow tubes, as if someone took a long rod that was perhaps 4" in diameter and drilled out all the metal in the center. Now, a drive shaft is subjected to strong torque, because the transmission is twisting one end, causing the drive shaft to rotate and twist the gears in the rear end of the vehicle. If you analyze the stresses in a solid (not hollow) drive shaft, you will see that the stress is carried mainly by the outer periphery. The center has very little stress on it, and carries almost no load. It just adds useless weight. So drilling out the center to turn the drive shaft into a tube, has almost no effect on its ability to carry the torque load, and it has the benefit of cutting its weight dramatically. So what we have is much reduced weight, and the use of much less material, with no significant reduction in its strength.

Hope the above example is helpful.

Regards,

Bob Wilson.


No, drilling holes does not make a piece of metal stronger. But putting holes in metal in the right places can make the piece lighter without losing much strength. This is important on airplanes. Airplane frames often have many round holes.

When someone goes about designing or making a structure, such as the frame of an airplane, a bridge, or a car, that person usually starts with flat or round pieces metal. The pieces are bent and cut to make the parts. It turns out that it is possible to make holes in some locations (and lighten the part) and not lose much strength. This is because the part is not stressed uniformly, and places of little stress can be eliminated. Round holes are often cut into the parts. The edges of parts are usually carrying the most stress, so you put the holes away from the edges of the part.

In addition to cutting holes, it is possible to improve structures using parts of varying size. For example, bicycle frames need to be light and strong. They can be made strong by using thick material, but this makes them heavy. Instead, hollow tubing is chosen. No holes are drilled in bicycle frames for lightness, instead, the walls of the tubes are made thicker in places with a lot of stress, and thinner where there is less stress.

Robert Erck



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