How should I improve my drilling?

[pendragon1998]

I had a starter with a broken off non-hardened screw in about 1" of cast iron. The screw hole was open on te bottom, so I decides to drill it out from the bottom, then thread it for a larger screw. I selected a new #7 Chicago-Latrobe HSS black oxide 118 degree bit and an electric hand drill (don't own a drill press). I used a drilling and tapping lube.

The drill worked fairly easily through the work, with me taking my time, but I had it slightly angled, so the bit ended up starting to exit about half a diameter over towards the side of the starter. Basically part of the bit circumference was free and a crescent shaped portion was still going through the starter body. The bit jammed and the drill chuck spun around the shank. I pulled it out, tightened the chuck, lubed the hole, and started again. This time, it jammed and the bit shattered, cracking in half and shooting a piece into my stomach, cutting me a little.

It was my fault, but I would like to be educated on how to do better next time.

 

[csp]

Is your drill a variable speed? If so, were you running it at a slow speed or fast?

Any fluid used? Even WD40 makes a big improvement over drilling a hole dry.

 

[aircommuter]

On small bits you want to keep the speed up so they won't catch. They are drill guides available for holes, that way you would stay centered better.

 

[pendragon1998]

Is your drill a variable speed? If so, were you running it at a slow speed or fast?

Any fluid used? Even WD40 makes a big improvement over drilling a hole dry.

It's a DEWALT DWD110K 8.0 Amp 3/8-Inch VSR Pistol Grip Drill . It runs 0-2500 RPM. I was trying to keep the speed down fairly low, but not quite as low as the drill would run (maybe 3-500 RPM, if I had to guess), and I was applying moderate downward pressure. The chips were coming out wonderfully. When I drilled another section to enlarge a hole, it cut through like I was drilling wood - very nicely.

 

[larry_g]

One thing to understand is that when drilling in a situation like this where two different materials are involved the bit will migrate into the softer material. You should have known you were out of the bolt long before you broke through. Drilling out bolts is fiddly work and you should stop and inspect the process a few times as you progress.

lg
no neat sig line

 

[TLCObsession]

One other thing is that there are left handed drill bits that may back out the screw as you drill it.

 

[Kevin54]

One thing to understand is that when drilling in a situation like this where two different materials are involved the bit will migrate into the softer material. You should have known you were out of the bolt long before you broke through. Drilling out bolts is fiddly work and you should stop and inspect the process a few times as you progress.

lg
no neat sig line

^^^^This^^^^ I'm assuming that you use a #7 to drill out a 1/4" screw. You should have went smaller yet and tried to stay more on center, then step it up so you don't break out.

The bit jammed and the drill chuck spun around the shank. I pulled it out, tightened the chuck, lubed the hole, and started again. This time, it jammed and the bit shattered, cracking in half and shooting a piece into my stomach, cutting me a little.

This is where you should have went real slow when starting back up, but if you were already off center, you should have taken a smaller bit and tried to regain your center back by leaning the hand drill, going a little bit at a time.

Sometimes you can take a piece of steel, drill a hole through it, clamp it to your part, and use it as a drill guide to keep yourself centered on the piece you are drilling.

But as stated above, it is fiddly work. And for most people, if you haven't done it a lot, the chances are you getting in trouble with the drill and bit are pretty good.

Did you get the screw out and everything opened up to a larger size?

 

[pendragon1998]

I ended up enlarging the hole enough to get a bolt through for a temporary repair, but it wasn't as clean as I was hoping to accomplish. Thanks for the tips above. I will put them to use.

 

[bczygan]

I found that adjusting the torque down to a lower setting, so instead of the drill trying to rotate in my hand when it catches, or if I have a good grip, the bit shattering, the clutch just disengages.

Bill

 

[happymachinist]

I always start with a well centered center drill. Stopping often to check my progress.

Sent from my XT1254 using Tapatalk

 

[pepi]

Center punch it before you do any thing and use a cutting fluid as you go.

 

[2oolhound]

If you drilled it on an angle and/or off centre you can drill it out even bigger if the flange is big enough to allow that then tap it for a big bolt. Put the big bolt in with loctite and cut it off flush with the flange. Then redrill your little hole through the bolt. Or you can fill the hole with weld and redrill.