Question about overheated drill bit...

[alanganes]

So I was drilling through a chunk of 316SS with a 3/8" bit. Got a bit distracted, not keeping the feed up, and ended up overheating my bit to the point where it discolored and the cutting edges look somewhat deformed. Yes, I'm an idiot.

Anyhow, this was a good quality US made bit (forget the brand) not a cheap hardware store or walmart thing. So if I were to regrind the point to like new geometry on the Darex, what would those in the know think the odds are that the bit is now too soft to ever work properly again? I'll likely try it just to see, but was wondering what the collective wisdom is on this.

I should know this. How hot can you get an HSS tool before it softens too much?

Thanks!

-Al

[Toolguy]

First, you would have to grind the bit back to where it wasn't overheated without overheating during grinding. That could be 1/4" or more. If it gets blue, it's too hot.
Second, you may need a carbide bit to finish the hole now.

[dp]

According to one youtube machinist you can get them to 1200؛ F. Sounds like you exceeded that, though, since it deformed.

[Rich Carlstedt]

Cut off a 1/2" with a cutoff blade while the drill is held with ice cubes, and then sharpen it.
OR, you could use it for wood.

Rich

[RussZHC]

Seem to recall reading drill materials varying...http://www.practicalmachinist.com/vb...l-type-169038/ is a thread where the last post (# 6?) talks a little bit about drills w cobalt as an additive to HSS having a different point at which high temperatures effect them...so, maybe like lathe tool bits, it can depend on...?

[jkilroy]

Where do you guys get this stuff? The annealing point of most HSS starts at around 1600 degrees F, somewhere between light cherry red and light orange, as in really damn hot. Purple is like 550, dark blue 560, blue 570, in other words blue is 1000 degrees from having any kind of negative effect on your HSS tools!

I seriously doubt most folks would stand at a drill press, crank the RPM up high enough, and pull hard enough on the handle to ever get anywhere near 1600! Help, most home shop drill presses could probably never get there if they tried! Even if you could get anywhere near the 1600 temp the soak times for HSS are long, very long, as in an HOUR per inch at least. Say that 3/8 drill would need at least 5 minutes at somewhere between 1500 and 1600 degrees for any serious damage to take place, and then a slow cool would be required or it would just harden back up.

[iMisspell]

As ToolGuy was heading... its possible you have "work hardened" the piece of 316 and if you try and drill the same piece with a freshly sharpened bit (or completely new bit), it will dull/burn-up the bit.

[macona]

HSS was intended to run into the red heat so dont worry about it, just resharpen it.

[bobw53]

and pull hard enough on the handle to ever get anywhere near 1600!

Don't discount the temperatures that you create when cutting metal, even if it is only in your garage.

Take a look at where different coatings and lubricants come into play.

The high pressure goodies in regular old coolant comes into play around 700F
Regular old sulfurized cutting oil doesn't do its thing until about 900F

TiCN coating, the light purple pink stuff, around 900F
TiAlN or AlTiN coatings, the dark purple stuff doesn't work its magic until around 1700F (which in my mind makes it pretty useless on a HSS tool)

We are talking very local temps here, high pressure and heat, just behind the cutting edge.


As for getting the whole drill or even the end to 1600F, that's basically a full blown melt down, which happens occasionally.
And then you have more problems than worrying about resharpening your drill.

I'd just sharpen her up and keep going.

[Jono]

What rpm? sounds a little high. I've only ever done that on 316 when I've had a brainfart and run it at 100 fpm. I just ground it a little further than just good shoulders and carried on. Low speed, high feed and a dead sharp tool should cope with any work hardening.

[SGW]

It's difficult to anneal HSS, even when you want to. I suspect the drill can just be resharpened.

[J Tiers]

Piling on......

HSS can certainly be run hot. it has the property of "red hardness", meaning that it retains significant hardness into the red heat range. Red heat does not mean "bright orange", though.....

Not a good thing to test in daily use, perhaps, but DO NOT mistake HSS for regular carbon steel or alloy carbon steels, which are easily annealed or softened by heating well below red heat. As mentioned, it's hard to anneal HSS, you almost certainly did not do that.

What you probably DID do was to workharden the part, and wear the drill point. It's also possible that the very end got hot and was cooled by the work in such a way as to "heat check" or develop micro-cracks.

Sharpen that sucker and move on, after grinding back a little bit to get past any possible heat checking. Don't overheat and then quench while grinding or you may propagate the heat checking.

[MrSleepy]

When re-sharpening, for SS the black book recommends a 135deg drill point angle with a 9deg lip clearance angle.

Rob

[ahidley]

toolguy is 100% correct

[Edwin Dirnbeck]

I have drilled many thousands of holes in diferent stainless and hard and semi hard mold steel. Your hardness of your drill is probably ok BUT you probably have worn the tip area to a reverse taper it happens all the time. What I do is measure the o.d., chop off the bad part then rgrind your drill with a whole lot of releif.Now turn your speed way down and approach the bottom of the hole very slow .Try to get past the work hardened area.If it squeels repeat this process until it drills without squeeling and yes some times the only thing that will break thru the workhardened area is a carbide drill.I allways keep a bunch of cheap masonary drills for this,usually you only need to go 1/8 inch or so .Now sharpen your drill normaly and finnish. I have found time and again when you are drilling nasty material ,once it squeels ,it is best to stop and touch up your drill. Edwin