Broken tap in cast iron... anyway to drill around to get it out?

[Dave Smith]

one thing I have learned in my past experience on critical tapping small holes, is to drill the hole with the largest size drill you can so tapping is easier and chances of breaking a tap is minimized. you DON'T always need to have deep threads on all tapping, and if you do twist and break a tap the broken peice is easier to remove. thinking ahead counts for less chances of problems.
Dave

 

[skcncx]

Why were you tapping so deep, Scales are not that heavy 1 to 1.5 diameters of thread is all that is really needed. That is enough thread that the screw will fail before the threads fail.

It's not the scales, it's a large bracket that will be holding the DRO screen and other items... but yes, I probably don't need to go so deep.

With a spiral point tap into a blind hole I never go more that 50% of the hole depth. All the chips driven ahead of the tap will fill up the hole and cause to to bottom out way before the hole depth is reached.

It was cutting like butter, and wasn't paying attention and felt it get tight as I was getting to the bottom, then (afterwards realized) I too quickly tried to reverse it to back out, jerked and broke the tap right at the very end (as in, where it's at full depth). Being a spiral flute on such a small diameter, theirs not much body to it so I just went to fast, going too deep, a lot of things the wrong way for sure. All other three holes I used a taper tap, straight flute and it was much better. Lesson learned for sure.

 

[Dave Smith]

don't feel so bad----we've all broken taps and wishin we hadn't.
Dave

 

[gjmontll]

I've been amazed at how well my inexpensive set of "Drill-Master" left-hand drill bits have worked to remove broken bolts and taps.

 

[woodchucker]

I've been amazed at how well my inexpensive set of "Drill-Master" left-hand drill bits have worked to remove broken bolts and taps.

that cheap set of soft bits, can remove taps? I get that it can remove bolts, and screws, but taps??

 

[gjmontll]

Yes, maybe I've just be lucky, but it doesn't really dig into the tap as much as just catch the broken edges and backs them out. (and pre-treating with PB-Blaster doesn't hurt.)

 

[skcncx]

I ended up using a 5/16 ceramic hole saw/bit (no center) to drill around my tap, which was the right size for a 3/8-16 tap. Every time I exposed about 1/4" of an inch of my tap, I still could not get it to twist out. The dust/paste/sludge from the cast iron and a cheap twist style tap basically just glued that thing in there. Never could get a good grip on it anyway.

Then drilled and tapped my M5 bolt into the center of a 3/8 bolt which had JUST enough clearance. Used some medium strength (blue) thread locker to lock the 3/8-16 threaded insert/plug. If I was thinking ahead, I should have not tapped the 3/8-16 all the way through the casting so it would have had a natural place to stop, then no need for Loctite. Oh well, a lot of good things come to mind after it's too late.

Now I'm down the rabbit hole of getting a better set of drill bits (M2, M7, M35, M42... rabbit hole) and proper style and quality taps for my next adventure.

Not a bad outcome, hole doesn't need moved which was my primary (hopeful) goal.

I did learn that the cheap $7 for a 5 pack of diamond hole saw bits is basically worthless. The diamond'like particles stripped off within seconds.... purchased a single bosch brand bit for $15 and still looks new after through 3/4" of cast iron.

 

[Dave Smith]

I did learn that the cheap $7 for a 5 pack of diamond hole saw bits is basically worthless. The diamond'like particles stripped off within seconds....

diamond grit coated drills and blades and grinding wheels are NOT for using on steel or cast iron, or aluminum, they are for carbides, glass, fiberglass, cement, stones or bricks which they work great. *** drills for steel or cast iron or aluminum should be carbide grit or solid carbide or HSS.
Dave

 

[skcncx]

diamond grit coated drills and blades and grinding wheels are NOT for using on steel or cast iron, or aluminum, they are for carbides, glass, fiberglass, cement, stones or bricks which they work great. *** drills for steel or cast iron or aluminum should be carbide grit or solid carbide or HSS.
Dave

Yes, I know. But, I could not find a carbide grit hole saw at 5/16" or any other hole saw, at least not easily available in that size. All other hole saws were much larger. I knew it was a once and done bit.. just commenting on the fact that the cheap ones lasted 30 seconds or less, the better quality one lasted much longer and still useable. Even after using it on a material it was not mean for.

 

[savarin]

I'm a bit late to this post but maybe for the future.
Consider building one of these

A spark erosion apparatus

Ever since I broke a tap whilst making the Improved chuck for taps I have been thinking of making a simple spark erosion apparatus. When making the hexagonal hole in the mill quill lock lever I again...

mikesworkshop.weebly.com

I made mine but simplified the construction to what I had laying around.
It has saved my bacon many times over.
One advantage is the threads from the tap can be picked out once the middle is burnt through and the hole re tapped to the same dimensions as the original.
Also odd shaped electrodes can be used to burn odd shaped holes.
One caveat is its a bit slow but it does do the job.