Question about overheated drill bit...

[alanganes]

OK, post mortem:
so I got a few minutes to sneak back out to the shop last night. Took my discolored bit with the slightly malformed edges and had a closer look. It was not smoking hot anymore, so I could do so without burning my fingers.

Turns out the end was not as bad as it first looked, most of what appeared to be mashed edges was actually chips that had sort of fused to the end of the bit. Most of it chipped off, it was sort of melded/stuck to the bit. The cutting edges were clearly ruined by the episode.

So I made enough passes on the Darex to make the drill point look like a drill again. I did not remove all that much of the end, maybe 1/16" I'd guess. I put another piece of SS in the lathe, slowed WAY down and made sure the tailstock was locked in place(!) and proceeded to drill a nice 3.8 hole in it. So it looks like, aside from being a bit shorter with a bit of discoloration still near the end, the bit is not really any worse for wear. So I did get the thing stinking hot, but apparently not hot enough to unduly louse up the heat treat.

I used Mr. Dirnbeck's idea of opening the original work piece up with an old masonry bit or two (thanks, good idea!) to get past any work hardened stuff and then finished the piece off.

Thanks for all of the comments. There is quite a lot to digest just reading through these replies.
Much appreciated, guys!
-Al

[Mark Rand]

If it's an HSS drill then stick it in the Darex and clean it up.

It may help to break through the work hardened stainless by re-starting with a 1/4" drill for 1/8" depth and then going back to the newly sharpened 3/8" drill.

Lock the tailstock up this time

[alanganes]

Thanks for all the replies guys. Loads of good advice and ideas here.

I have not gotten back to this yet, but the story is this:

Part is just a 2" diameter disk, 1/2" thick with a 0.375" hole in it. I had the disk in the lathe, faced it off flat and spotted the center with a spotting drill. Slid the tailstock back and swapped the drill. I turned the speed down, but was surely spinning at least somewhat too fast. The real killer was that when I slid the tailstock back up to start drilling, I locked it down but apparently did not do so securely enough. I started drilling, keeping the pressure up to keep it cutting, but did not realize that in addition to sinking the drill into the part, I was also pushing the tailstock back. As the bit was not cutting as it should, it must have dragged enough to work harden the piece and you know the rest.

I'm going to try regrinding the bit an see what happens. I can always cut it back and re-point and thus have a nice new stub bit.
we'll see how this goes...

Thanks again!

[J Tiers]

... 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

And that goes for ANY cutting tool, in the mill or lathe, or drill press, etc.

Normally from going too fast, and/or from allowing the tool to "rub", which nearly always causes trouble with dulling the tool. After it's dull, it will rub even more, and squeal.

[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

[ahidley]

toolguy is 100% correct

[MrSleepy]

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

Rob

[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.

[SGW]

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

[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.

[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.

[macona]

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

[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.

[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.

[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...?