A Question for the Machinists!

I want to thank everyone who participated in my question. Today after sharpening an old 3/8 inch drill bit on a Drill Doctor I bought about 3 years ago and never really used, and cutting the drill speed on my bench drill press to it's lowest, (about 650 rpm), I was able to drill 24 holes in a 1/4 inch angle iron with no problem. And the drill bit still seems sharp. I'll have to dig out the rest of my old bits and sharpen them all. Again thanks to all for your advice.

Glad to hear that after sharpening, and slowing it down, that you're having better success.

Guys have made notes here about speed, color, etc., and they're good rules-of-thumb to keep in mind.

One thing that's frequently overlooked-  there are many drill presses and hand drills out there.  You noted that your slowest speed is 650rpm...  your drill press obviously was intented for general-purpose drilling, mostly for smaller holes in materials like wood, plastics, and very soft metals.

There IS a difference between machine made for general purpose, and those made for doing real metalwork.  The Jancy mag-drill that Dream referred to, is specifically designed for drilling ferrous materials-  not just because of the magnetic base (and he's right, they're awesome!), but also because of the INTENDED SPEED RANGE.

Drilling holes falls into the exact same category of cutting as ANY OTHER type of cutting machinework... meaning... non-abrasive, and non-electrical machining.  Metal cutting tools, regardless of wether they're a twist-drill, a file, a lathe toolbit, or a milling cutter, all cut using a very basic technique-  sharp edge RIPPING AWAY metal in a methodical sweep.  The earliest chapters of ANY machinist's textbook, involves using basic cutting tools, and the tool bit geometry- rake, relief, speed, and feedrate.

You saw the reference to FPM... Feet Per Minute...  Consider a bandsaw... an average metal-cutting bandsaw... one of mine is set up for 120fpm.   That means a tooth crosses the workpiece at a speed of 120ft every minute... a 12' band blade makes 10 revolutions in one minute.  Compare that to a woodcutting bandsaw... it's flyin' along at 1000+ feet per minute.  This is because the material being cut (wood) is softer, and the blade teeth are much more coarse, than a metal-cutting bandsaw.

On a lathe, an example: the workpiece turns while the bit takes a cut along the outside diameter.  If the diameter of the workpiece is around 6", that means the circumference is a little over 18".  To make 120fpm at the CUTTING SURFACE, the workpiece must spin at  about 80rpm, in order to yield that 120fpm.   If the diameter of the workpiece is only 1", that means the necessary RPM to make 120fpm, is around 460rpm.

This is what they mean by 'surface feet per minute'.

Now, consider that instead of swinging a 6" workpiece in a lathe, you're spinning a 6" DRILL BIT...  yep, 80rpm.  Recalculate for 1"... now you're at 460.

Recalculate for a 1/4" drill bit... that's a circumference of 0.785"... which will have to turn  a bit over 1800 rotations in one minute, to yield a 120fpm speed.

Is it always the same case?  No.  Not all materials are the same.  Softer workpiece materials will cut at different speeds than harder, and harder-surfaced bits can cut faster than softer-surface bits. 

Not all drill bit metalurgy and coatings are equal, and quite frankly, the imported twist drill bits you buy nowdays are made of some of the lousiest excuses for tool steel on the face of the earth.  Some are so soft that they can be made incredibly sharp, but won't hold the edge if you put it in the front pocket of your Levi's.  Likewise, I've had some that were lousy metal, but hardened to the point that a brand-new bit fell out of a box, landed just two feet lower on the bed of my radial drill, and shattered like it was a wine glass.

As noted, the GEOMETRY of the drill bit... not just the point, but the rake and relief, determine how well a drill bit works in a given application.

Finally, the drill bit may, or may not have an additional cutting surface.  Most of my machine tools use high-speed-steel bits (HSS), but I also use carbide-tipped tools.  Generally speaking, high speeds will result in the metal hardening itself to an unworkable state (when the base metal's metallurgy will allow).  While it's a good general rule to go slow enough to avoid overheating, realize that carbide cutting tools MUST be run at higher speeds than HSS in order for the bit to 1) work properly and 2) last any length of time.  When I run carbide in my lathes, I find that the proper feedrate is typically TWICE (if not more) than running high-speed steel cutters... and when I'm cutting some types of stainless, the waste isn't chips or strings, it's swarf... long, blue swarf, that for the first inch or so coming off the cutter, is red-hot...  of course, I'm cutting lots of metal, moving fast, and getting mirror-finish cuts.  This doesn't happen with HSS... and unfortunately, once you START a cut like that, slowing down can be a serious thing, because that red-hot metal will basically freeze and jamb the tool bit IN the workpiece.

If you plan on doing lots of metal cutting, find any common machinist's textbook, and do some extended reading... it'll really help your understanding of the physics of cutting tools of ALL types.

Finally... if you don't weld, take the time to learn to use a stick and MIG welder-  neither har 'hard', but you'll never learn if you don't attack it.

I have a Drill Doctor that sharpens up to 1/2". I bought an additional chuck that will hold/sharpen to 3/4". It works well up to 23/32" but for some reason it doesn't do the 3/4" right.

Although most of the heat essential for efficient cutting exits with the chip, any increase in the temperature of the component will make accurate measuring difficult. Coolant can be used to reduce this effect.

Coolant can be used to drive away any swarf which might interfere with the cutting edge, particularly in boring operations. Coolant can help reduce vibration, especially where rigidity is limited.

Where interrupted cutting with CBN inserts occurs, coolant should not be used as this will thermally shock the CBN tool as it comes out of the cut, resulting in premature failure. Although interrupted cutting with coolant will never improve tool life, it is possible to use coolant machining hardened steel with DR-450 CBN inserts and coolant can be used when machining cast iron with CBN, that is not hardened, under all conditions.

http://carbideinserts.blogspot.com/2008/03/using-coolant-cbn-inserts-interrupted.html
 I was always told not to use coolants with carbide tools , this is something one might want to research more.

Edited by Coke-in-MN - 12 Nov 2010 at 12:29pm

dont drill too big of a pilot hole!