What happened? Broken bit...

It isn't really a pilot. The 1/4" hole is required, I was using the drill to hog out before final boring to ID. It is a cone baffle for a suppressor. I guess I could have drilled the large hole and gone back to a 1/4" but I was concerned with concentricity. I took out an 11/16" bit and finished the operation, no problems.

Attaching pics was failing so I put in links...I think I got it now. Drawings included for part reference...slightly modified now but you get the idea from the pics.

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IIRC I was turning at 418 RPM but moved the belt to the 266 RPM position when I went up to the 11/16" bit. I also didn't push it in as hard. I had just changed form the 1/4" to the 1/2" and might have been pushing too hard...with the 11/16" bit I just pecked at it as I was gunshy of breaking another bit!

I attributed the chatter to the long bit and the shallow hole @ start. Once the sides of the bit were inside the hole the chatter stopped. I would peck a few times, back out, and pour in more cutting oil. I was using high sulfur threading oil for my cutting lube. You can see how much was soaking everything.

I have aligned the heck out of my tailstock...it is as good as I can get it. I have no tools to measure actual alignment but a test bar is under .002 for 30" and some of that is bed wear at the headstock.
 
Its difficult to see in the photos but your bit appears to have the same black oxide surface where the flaw was. On the one I had fail the black surface ran down the web to where it fractured producing a clean metal surface.
The original flaw was there before the bit was heat treated hence the oxide finish inside the crack.

Greg
 
I'll look closer tonight when I get home, on first glance I though that was just wet with cutting oil making it dark.
 
I've had exactly the same thing happen with a new 1/2 HSS drill. Discoloration suggests there was a pre-existing crack in the web.
 
TS and HS are aligned by setting dead centers in each and dialing in point to point as perfectly as possible...shims under and offsetting screws built in. I'm not promising much beyond this but I've been using this setup for several years without similar issues. I don't have a MT2 test bar to test alignment further out nor have I heard of another way to guarantee the ram axis is perfectly aligned to the spindle axis.

On close inspection of the pics it may well be discoloration in the crack...much the same shade as the black oxide exterior in fact. I need to look closer at home tonight.
 
it is very hard on a sharp bit to keep from chattering on such a large pilot hole--an old trick on drilling a larger hole in thinner sheet plate to stop the chatter is to use a small scrap of cloth rag lightly held in your finger and thumb over the hole until the bit grabs it --and you let go---on a lathe since the part is spinning just place a small piece of cloth on the bit---it sounds dangerous to use cloth material around a spinning bit or spinning work piece , but it is just a small piece and you let go as soon as the bit grabs it---you almost have to see how well it works to understand how safe it is---I never tried it on a lathe but you wouldn't even need to hang on to the cloth piece--just lay it over the end of the bit---I learned this trick from watching old machinists drill larger holes in thin metal and not have the chatter and grabbing of the bit---Dave
 
The wobbling of the drill was caused by the 1/4" pilot hole; that causes a lot of strain on the drill itself; barring a flaw in the drill, that was most likely the cause of the breakage. Ideally, a spotting or small center drill should have been used to start the 1/2" hole, to make it easier to feed, a split point drill could have been used.
 
cdhknives,

When you drilled the 1/4" bore, did it go without a struggle or, did the bit heat-up and get dulled in the process of drilling?

On a fairly regular basis, I need to re-bore and re-thread small holes into bigger ones going upward at least 2 standard sizes. Have never busted a bit like that.

Anyhow, if the 1/4" hole was a struggle and the bit started to smoke, then there was localized heat treating taking place. The whole piece does not get heat treated but, the thin surface areas comprising the bore can get surprisingly hard. It's usually just a thin skin of hardened area. As the bit rotates and gets hot from cutting, it heats the metal it's in contact with. The outer layers of the piece are relatively cool and the heat transfers to the rest of the of the piece. This is called auto-quenching.

Ray
 
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