What happened? Broken bit...

cdhknives

cdhknives

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I was just starting this boring operation when the tip snapped in half. There was a fair amount of chatter but not an unusual amount. The pilot hole is 1/4". The drill is 1/2". It is a MT2 shank held directly in my tailstock. The bar is annealed (condition A per the manufacturer) 17-4. This was the first usage of this bit in the as-delivered condition (no sharpening by me).

Was I:

A) Cranking the feed in too fast?
B) The victim of poor material quality
C) Other

Links to pictures below.

Snapped tip
Just starting
 
There was probably a flaw in the drill bit. I've had the same thing happen first use too.
IMG_1520.jpg

Greg
 
Please post photos as it is easy and links not always work.

If something breaks fast it is being used beyond its design.

You state 1/4 hole increasing to 1/2, is said tool a boring tool (cutting one side or a drill like tool ( cutting both sides)?

Was the setup rigid?

It could have allowed the tool to bend slightly of too heavy of cut resulting in the break.

But not seeing anything we only guessing.

Sent from my SAMSUNG-SGH-I337Z using Tapatalk
 
BTW: This stuff will work harden in a heartbeat so, coolant is needed. It's possible you created a hard shell around the 1/4" hole when making the pilot hole. Since you had a pilot hole, a good starting point with a 1/2" bit would be about 350 RPM with a bump up if it feels OK. Once it's established, keep firm/steady pressure. Only peck if the chips get dangerous.

I'm guessing one of two things went wrong: The piece got slightly work hardened and when the edge broke through, it grabbed real hard. Or... the bit was defective.

Ray
 
Your pilot hole was larger than recommended; it should be no larger than the margin of the point, I think it allowed the drill to hog in, it being very sharp, as new, and I suspect the "heirloom" lathe may have a bunch of free play in its screw and nut, allowing the hogging in and overload to the point of shattering.
 
I would not have drilled a pilot for a 1/2" bit. When a MT drill grabs the drill is the only thing that can give.
 
kd4gij said:
I would not have drilled a pilot for a 1/2" bit. When a MT drill grabs the drill is the only thing that can give.[/QUI quite agree with you on this point, neither would I have drilled a pilot hole with that small of a hole to be drilled, and the proper size of pilot drill for a 1/2" drill could easily be jammed up with chips and itself break off in the hole.
Click to expand...
 
1/4" is way to big for a pilot hole for a 1/2" final. I would guess your "fair amount of chatter" is being caused by this. You should not be getting chatter, period. It means something is not right
Just drill the chisel point size as a pilot hole. I will admit it does look like odd breakage, (never seen that before) but I'm also of the opinion the 1/2 bit grabbed the oversize pilot hole, pulled in tight, and, well, SNAP!

Cheers Phil
 
another possibilty.....is your tailstock very closely aligned with the spindle axis? if not it puts a bunch of bending stress on a drill bit. the 1/4" pilot bit may have been flexible enough to deal with misalignment .
-brino
 
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