Study of effects of temperature on steel drilling performance

[pitchblack00]

When drilling a thick peace of steel I noticed the drill bit would cut through much faster when it was cool. The difference was minutes versus seconds! Is this a known phenomena in machining or was my drill bit simply contaminated with something or maybe other factors played a role? I've heard of temperature having detrimental effect on tool life, but not on actual cutting time. Is there a detailed study someone could point me to that relates cutting (i.e. performance) and temperature of the tool? The way I understand it is that when tool heats up it becomes softer, so it's ability to cut is reduces.

 

[Bystander]

https://en.wikipedia.org/wiki/An_Ex...urce_of_the_Heat_which_is_Excited_by_Friction

 

[jrmichler]

Most of the heat is generated in the shear zone of the steel being drilled, so is independent of the drill bit temperature. If the drill bit gets too hot, it gets dull instantly. A dull drill cuts much slower than a sharp drill. The most common reason for drill bits getting dull fast is too high a cutting speed. A high speed steel drill bit (most twist drills are high speed steel) run at 50 ft/min peripheral speed will drill many holes in normal steel without needing sharpening. For a 1/2" diameter twist drill, this is 380 RPM. If you run that same bit at 100 ft/min, it will dull quickly.

Here's a good discussion of heat generated in metal cutting: http://www.nitc.ac.in/dept/me/jagadeesha/mev303/chapter6.pdf. Figure 2 is identical to a figure in Fundamentals of Machining and Machine Tools, Second Edition, by Boothroyd and Knight. Note the temperature of the chip. A cool bit will not stay cool very long when in intimate contact with the hot chip. And a dull bit will make hotter chips.