Homebrew PCBs

I just tried to grind a simple cutter out of a broken carbide drill.
The 1/8th inch shank is just the right diameter for M3 mounting holes.
I've seen pictures of cutters on the web where half of the drill is simply  
ground away flat, so i tried that.
I used a white grinding wheel in a plain (dry) bench grinder, and it was  
slow going, but the result works well enough to drill mounting holes.  
There is some burr at the exit, but compared to the commercial drills this  
tapered drill does not stall the small spindle i use for PCBs, which is a  
plus.

That drill material is very tough stuff, and i don't have the right wheel,  
but it worked OK for a first try. Does carbide lose hardness if it gets  
too hot like hardened steel? I didn't bother to cool it much and it got  
orange hot quite quickly. It was only a trial so i do not mind if it is  
weak.


ST

Hi Stefan,

You need a green wheel (silicon carbide) or diamond to grind tungsten 
carbide. Diamond is great but rather expensive. Use a good quality wheel 
as the cheap ones don't seem to cut as well. Wear a mask as you really 
don't want to be breathing that dust. Don't use green grit or diamond on 
steel as they will wear very quickly.

Carbide can take very high temperatures but it is sensitive to thermal 
shock so let it cool in air. Quenching in water tends to cause fractures 
in the material.

Les

Stefan Trethan wrote:

> I just tried to grind a simple cutter out of a broken carbide drill.
> The 1/8th inch shank is just the right diameter for M3 mounting holes.
> I've seen pictures of cutters on the web where half of the drill is simply  
> ground away flat, so i tried that.
> I used a white grinding wheel in a plain (dry) bench grinder, and it was  
> slow going, but the result works well enough to drill mounting holes.  
> There is some burr at the exit, but compared to the commercial drills this  
> tapered drill does not stall the small spindle i use for PCBs, which is a  
> plus.
>
> That drill material is very tough stuff, and i don't have the right wheel,  
> but it worked OK for a first try. Does carbide lose hardness if it gets  
> too hot like hardened steel? I didn't bother to cool it much and it got  
> orange hot quite quickly. It was only a trial so i do not mind if it is  
> weak.
>
>
> ST
>

On Tue, 30 Jan 2007 18:55:30 +0100, Leslie Newell <lesnewell@...>  
wrote:

> Hi Stefan,
> You need a green wheel (silicon carbide) or diamond to grind tungsten
> carbide. Diamond is great but rather expensive. Use a good quality wheel
> as the cheap ones don't seem to cut as well. Wear a mask as you really
> don't want to be breathing that dust. Don't use green grit or diamond on
> steel as they will wear very quickly.

Ok, i think i even have green wheels somehere. I hope the colors are the  
same here (is it inherent to the material or a code)?

> Carbide can take very high temperatures but it is sensitive to thermal
> shock so let it cool in air. Quenching in water tends to cause fractures
> in the material.

Uh, oh, .. good to know.. first one was only a trial, as i said... ;-)

I remembered that the tips are brazed onto drills and blades, so carbide  
must be able to take some heat. But probably i should take it more slowly  
so that it does not glow quite as bright....


Thanks

ST

"Stefan Trethan" <stefan_trethan@...> writes:
> (is it inherent to the material or a code)?

Sometimes.  Check the code on the wheel, it will have some letters
(material), numbers (grit), and letters (hardness).

http://www.woodcentral.com/cgi-bin/readarticle.pl?dir=handtools&file=articles_557.shtml

http://www.peterchild.co.uk/info1/wheelinfo.htm

Stefan,

Carbide can take a LOT of heat, but glowing red or white hot isn't 
such a good idea.  As mentioned, don't quench it if you don't have 
to but it shouldn't hurt in reality.  A green grinding wheel is very 
soft by comparison to the white one you are using now.  It will 
abrade away very quickly - the nature of the beast when grinding 
carbide.  Don't inhale any of the carbide dust - it's nasty stuff.

The type of tool you ground is referred to commonly as a half round 
drill.  They work great in many materials including brass, copper, 
plastics and even glass.

Recently I tried to grind one for isolation milling - what a joke 
that was.  I just don't have the eyes for it anymore.  I tried all 
my optical aids and nothing would bring the grind into focus to see 
what I was doing. 

The burr you are getting is probably caused by not having clearance 
on the backside of the cutting edge.  It is very hard to visualize 
where it (the clearance) needs to be but if you look at a normal 
drill, you will see how the backside of the cutting edge rises up.  
The half round drill needs this as well, but it is very minimal when 
done correctly.

With all that being said, you must have done pretty darn good if you 
ground the drill with a white wheel and got it to cut a hole in the 
end!!!

Chris

Stefan, to forgot to explain the "heat" issue.

Carbide, with and without various coatings is used extensively in 
metal machining.  Carbide can take a tremendous amount of abuse and 
keep on cutting just fine.  As an example, when milling with a 
carbide indexable insert endmill (or face mill etc.), the chips come 
off the part Blue-Hot, hot enough to cause an instant burn on the 
skin that will blister.  These cutters can have anywhere from 2 
teeth to dozens or even more teeth.  More often than not, people 
will use coolant (a water with synthetic lubricants) on these 
cutters when cutting all sorts of hard and soft steels.  In those 
cases, that insert is getting very hot while in the cut, then as the 
tip exits the cut with each rotation it is blasted with cold water.  
The inserts survive that very nicely.  What most machinists don't 
realize is that you can cut just as effectively and get longer tool 
life without using coolant on these types of cutters.

However, as I have done during demonstrations with big machines, you 
have to watch where all those chips pile up. On one particular 
machine I was deonstrating to Snap-On tools, we were cutting a large 
die base of tool steel.  We were hogging LARGE amounts of material 
off the die base without coolant and everything was going along fine 
till things got blury.  What happened was the chips piled up against 
a plexiglass door, built up enough heat to ignite the plexiglass.  
Even with the small fire, the customer was still happy as a pig in 
mud to buy the machine :-)

Chris

In a message dated 1/30/2007 10:15:24 A.M. Central Standard Time,  
stefan_trethan@... writes:

Does  carbide lose hardness if it gets too hot like hardened steel? I didn't  
bother to cool it much and it got  
orange hot quite  quickly.<<
ST!  No, WC does not anneal like steel does.   Getting it "red" does no harm. 
 It is back to just as hard when new, once  cooled, again.  The ONLY way to 
properly sharpen WC is with a DIAMOND  GRIT.  Some will tell you a "green 
wheel" will work (silicone grit), but  this is only SLIGHTLY better than 
aluminum-oxide (cheap grit).  The edge  will be "rounded" if you attempt to grind with 
ANYthing other than diamond  grit!  Too, you must not try to grind-off any more 
than 0.02-0.03 mm per  pass.  Else, you WASTE the grit on the diamond wheel.  
Takes some  experience; perhaps best if an old, skilled machinist (Schlosser) 
demonstrates,  once or twice!  You MUST have a fixture to hold the bit being 
sharpened, to  adjust each pass! Jan Rowland

 


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