Re: Machining

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: Terry Fritz <teslalist-at-twfpowerelectronics-dot-com>

Hi Gerry,

High speed steel is a relatively cheap hard tool steel used for most 
ordinary purposes.  It cuts wood plastic and other easy materials 
well.  The bits can bend so they are very durable for say hand drills.  The 
steel is tempered so if they overheat they are softened and ruined.

Tungsten Carbide is a very hard material and can cut hard materials like 
glass, rock, stainless steel...  It is pretty heat resistant and will not 
de-temper.  Difficult to sharpen and it is usually braised to a steel 
backing.  It weakness is that it is very brittle and can crack easily.  It 
is often used in wood working tools where cracking is not a big problem 
with the soft wood.  Very expensive compared to steel.  In our case, 
carbide tools stand up to FR-4 phenolics very well where tool steel dulls 
fast.  Almost all printed circuit board drilling and routing is done with 
very high speed tungsten carbide tools.  Carbide is often used in metal 
working lathes where a small tool edge can last a long time in a well 
controlled situation without being subject to impact damage.

Cobalt alloys are super hard like carbide but less likely to shatter or 
break and not terribly expensive.  But when they do, the are almost 
explosive and send shrapnel all over the place.  Cobalt actually gets 
harder as it heats to about 500F.  Cobalt cutters and drills are used where 
cutting very hard materials is needed and the cutter has to take some abuse 
without breaking.  It can be used in hand drills and is popular with 
machine tools.  It has high wear resistance and is difficult for a normal 
person to dull.  Sharpening really isn't an issue since the tool is 
probably worn out buy the time it dulls.  They do fatigue over time so best 
to replace them on time.  When I cut 6-4 titanium with cobalt tipped 
blades, the blades do not dull but the teeth eventually fall off.

Titanium-nitride is that gold coating you often see on tools these 
days.  It structure is supposed to be harder than diamond.  It is vacuum 
deposited in a thin coating rather than a tool material.  It mainly helps 
chips slid off the flutes.  Nice for say soft aluminum, titanium, copper, 
and other materials that like to stick to and gum up tools.  It also adds 
corrosion resistance and holds cutting oil.  It can be thought of much like 
the coating on Teflon cookware in its function.  Since it is so hard, the 
coating is super durable.  Even my most used cutters just show small wear 
of the coating at the very cutting edges.

I think the best bang for the buck is Ti-N coated Cobalt.  It is strong and 
the tools will last a very long time.  You do have to be a bit gentle with 
them but they will not dull and they can cut anything from wood and 
plastics to titanium and stainless steel alloys.  6% aluminum - 4 % 
vanadium - 92% titanium alloy will easily over heat tools do to the very 
poor thermal conductivity, likes to grab and gum tools, and is super hard 
(designed to defend against high-velocity bullets...).  Ti-N Co tools are 
just about the only thing that can touch it.  Otherwise it is ECM, 
waterjet, and plasma torch...

With all the hard alloy tools, safety glasses are NOT optional!!  They can 
all shatter very violently!!

Cheers,

         Terry


At 09:43 PM 4/20/2004, you wrote:

>Hi All,
>
>So far in this thread, I've heard of four types of drill bits:
>
>1. High speed steel
>2. Carbide
>3. Titanium Nitride
>4. Colbalt
>
>Could someone rank these as to hardness and cost and comment on application.
>Also, are there other bits to consider (not counting wood applications)?
>
>Gerry R