Broken Center Drill In Work Piece

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HBilly1022

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I'm in the process of making a bushing using 4340 steel. The OD is 1.570", ID is 1.250" and it is 2.050" long. After turning the outside to diameter I started to center drill the second end and the bit broke, leaving the whole tip, below the taper in the work piece. I put the work in the rotary table, thinking I could mill around the broken bit deep enough that I could drill from the opposite end and then bore through to the milling cut byt eh broken center drill bit. Bad idea. I got everything set up in the rotary table and using a 1/4" 4 flute mill end I started taking a 0.025" DOC and made it around one time and noticed it was getting harder to cut. Dropped another 0.020" and started the second pass and the mill end was done. I tried again with a 2 flute and a 0.010" DOC and it didn't make it 1/4 of the way around before it was too dull to continue. These were both new mill ends.

I thought about just drilling through from the good end until I hit the center drill but figure that would lead to a new set of problems when the drill bit made contact with the center drill.

I'm at a loss as to how to work around this broken center drill. Help please.
 
Holding the piece in your hand, with the broken ctr drill part up, rap it with a hammer, try to jostle the piece out. When using a high speed end mill in 4140, run it fairly slow, 4140 is kinda nasty, unless it is annealed way down. Normal state is RC 25-30.
 
Welcome to lathe work, I haven't met a single person that has not broken off a center drill pilot in a part in 30 years in this business. This is more problematic if the part is long and has no through hole, I do keep handy some straight flute carbide die sinking drills for broken taps and such.
I stopped using center drills for spotting holes years ago as this is what spotting drills are for, center drills are for producing centers.

As to your problem drill from the other end and it will likely push the broken pilot out, worst case is that you will bugger up the point of a cheap twist drill or two.

Then buy some spotting drills.

Good Luck
 
Wreck™Wreck said:
Welcome to lathe work, I haven't met a single person that has not broken off a center drill pilot in a part in 30 years in this business. This is more problematic if the part is long and has no through hole, I do keep handy some straight flute carbide die sinking drills for broken taps and such.
I stopped using center drills for spotting holes years ago as this is what spotting drills are for, center drills are for producing centers.

As to your problem drill from the other end and it will likely push the broken pilot out, worst case is that you will bugger up the point of a cheap twist drill or two.

Then buy some spotting drills.

Good Luck
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Geez now I have to get some spotting drills and a replacement center drill. :D This is one very expensive hobby but I'm still enjoying it.

Right after I posted this I had a brain storm (might have been a brain f*rt though). I chain drilled small holes around the broken bit then drilled through from the other end with a larger bit. Now I have created a new problem. All the rough areas between the chain drill holes are causing me grief. I tried using a boring bar and taking light cuts but the carbide doesn't like those intermittent cuts. Already chipped one. Now I have to come up with a different approach. I've got a carbide burring bit for my air die grinder and might try using that in the mill, with the rotary table, to see if I can remove the high points enough to leave something suitable for the boring bar.
 
Its a pisser breaking off a tap or center drill in the work. Unless the part is expensive or irreplaceable its often easier to scrap the part and start over. Like WW said.... center drills are for making centers, even then they can be problematic if they hit an inclusion in cast iron or a hard spot. Nice spotting drills can be had from Enco, take care of em and dont abuse em and they last for years. You can also buy screw machine drills with wide angle and forgo the spotting drill in a lot of instances. After you break your second center drill you usually avoid them for all jobs except cutting centers for turning stock.

Stay far away from Chinese center drills unless you are cutting peanut butter or margarine.
 
I usually can get a center drill tip out by using a small center punch to tap it backwards then turn the part over and bang it on the bench ant it falls out. On a part like yours If it didn't come out just drill from the back and when you hit the broken bit use a center punch to knock it out.
 
kd4gij said:
I usually can get a center drill tip out by using a small center punch to tap it backwards then turn the part over and bang it on the bench ant it falls out. On a part like yours If it didn't come out just drill from the back and when you hit the broken bit use a center punch to knock it out.
Click to expand...

I will definitely remember this for the next time. The way I ended up doing it created more problems.
 
A center drill tip is tool steel. It can be cut through with a small carbide end mill.
 
As someone above said it's easier to make a new one. The other way is to regirind that broken center drill to make it a one flute hollow cutter. Make it look like a plug cutter for wood. Chuck it on the lathe and drill slowly deep enough to the depth of the broken piece. Then break off the small piece that is left after drilling. You are good to go.


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Diamond coated dremmel bits and lots of them. I used them to remove a broken drill bit and small tap. Princess auto sells inexpensive packs.
My last venture of broken in a hole was a drill bit trying the remove broken head bolt in my cub cadet 122. Lucky there was hole all way through. I used a punch same size as the drill and punched it through.
Jack

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