So how do you fix this?

[Lakc]

This is a nearly complete crankshaft with a special 1/16 chip clearing, deep hole drilling, drill bit with the tip snapped off as it started to break through.


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I didnt even realize it happened until I inspected the part, I dont think there is much depth to the broken tip. Crankshaft is 4140, drill is cobalt.

 

[fltenwheeler]

Make freinds with someone that has a tap burner.

Tim

 

[Tin Falcon]

if the tip broke off as the bit was just breaking through you may be able to take a prick punch or pin punch and pop it outwith a gentle tap from the back side.
Tin

 

[Maryak]

Lakc,

You could try annealing the drill by heating the whole thing to a cherry red and then bury it in lime and allow to cool slowly. The lime needs to be sufficient for a barrier of at least 1" all round, from the world at large. The other point is to heat it evenly as cranks are easily distorted.

Next day use a drill and drill out the old drill.

Hope this helps.

Best Regards
Bob

 

[rake60]

4140 and a 1/16 drill bit... WOW!

Is it a hole drilled the length of the shaft and the 1/16" hole intersects it?

I have broken many bits off under those circumstances over the past 20 years.

Sometimes a piece of drill rod the size of the larger hole can be tapped in to pop the
broken tip out. Just tap it gently. If you hammer it down it will probably bind in the
hole and compound the problem.

Rick

 

[Lakc]

4140 and a 1/16 drill bit... WOW!

Yes, and it almost worked :'(

Is it a hole drilled the length of the shaft and the 1/16" hole intersects it?

I have broken many bits off under those circumstances over the past 20 years.

Its 1/16 from rod journal #1 to the main at 42.4 degrees, hence the slightly fatter counterweights then I normally use. Hindsight tells me that I should have nibbled both the start and end of the holes with a 1/16 carbide endmill before drilling through, instead of just the start. Breaking through is going to be hard enough, doing so at an angle is just bit suicide. Its one of those learning moments.

Sometimes a piece of drill rod the size of the larger hole can be tapped in to pop the
broken tip out. Just tap it gently. If you hammer it down it will probably bind in the
hole and compound the problem.

Sounds like the best option so far. I will nudge the tip back into the hole slightly first, and break out the rest of the hole with an endmill before applying any force.

Thanks all for letting me pick your brains!

 

[Lakc]

Well I gently nibbled out the rest of the hole that the tip was stuck in, filled it with bear grease (rcbs case lube) to keep the punch friction down, and sucessfully punched the remaining bit (about 3/32 length) through.
Thanks for everyones help.

 

[mu38&Bg#]

Is a carbide drill applicable in this case? Is it just as likely to break? I use some of those little carbide PCB drills with the 1/8" shank, but they are pretty short. What about a straight flute die drill?

 

[Lakc]

I have been using a bunch of those little carbide endmills from the pcb industry. They shattered when involved any type of interrupted cut like the flutes of a drill. I did, once, unintentionally, cut through a 9/16 round toolbit with a carbide die drill, and that took vinegar as a cutting fluid, but it still shattered the bit when it exited.
Someday I will build that edm, but its always better to stay out of this kind of trouble.

 

[stevehuckss396]

and sucessfully punched the remaining bit (about 3/32 length) through.

That's good news! Sucks when you have to start over.