Drill Bit Breaking Through

[Tozguy]

Drilling a 3/8'' hole through 5/16'' cast iron, top side of work is flat but backside is slanted. A new cobalt split point drill goes through very well but catches real bad when breaking through the backside. Don't want to force the issue and ruin the drill. Could try to slow the feed towards the end of the cut but that seems iffy. Drill press is already set at slowest speed. What about using a 4 flute end mill to finish the hole? Don't want to ruin an end mill either. Have three holes to do. What would you do? Thanks for reading.

 

[Ed ke6bnl]

can you tighten the quill and put drag on it so it will not drop when it starts to exit the hole? could help. no sure you are using a drill press of a mill maybe slow feed drive on the mill also with some drag on the quill lock.

 

[Tozguy]

No mill, just have a drill press and lathe. The few end mills I have are for milling on the lathe.

 

[RJSakowski]

Depending upon material being drilled, it is sometimes hard to control the flutes "screwing" the drill through the work. As you feel the drill start to break through, ease the pressure. A sacrificial backing plate will also help to control this. Finally, on materials that are prone to this problem, securely clamp them so they can't lift up as the drill breaks though.

 

[Paul in OKC]

You can use the endmill to finish off the hole. The drill bit kind of starts acting like a screw when breaking through material and wants to drive itself into and thru the rest of the part. Your end mill being flat wont have the same effect. If there is a way to put some drag on the quill as you feed that will help.
And as RJ said above, if you can put a sacrificial piece underneath where the hole is being drilled, that helps a lot.

 

[ch2co]

I think that first I would drill a pilot hole, then a larger pilot hole, then the 3/8 hole easing off on the drill press handle before final break through.
I've only drilled a couple dozen or so holes in cast iron, and this always worked for me and the cast iron I was drilling. Your cast iron might
be a totally different animal.

CHuck the grumpy old guy

 

[Tozguy]

Thanks folks, all good ideas.

Backside of the cast iron is too uneven for a sacrificial backing. The 1/8th and 1/4 drills did fine. But when the 3/8ths drill poked through, the work launched like RJ warns about in his post (No.3). My finger got gashed but the drill bit was unscathed.
With a bandaid on my finger and two clamps tight on the work this time, the drill bit was used again but stopped short before breaking through. A 4 flute end mill in the drill press was used to finish the hole. Much less drama than the first time around. Leaves me dreaming about getting a mill.
It is probably a good idea also for me to do some research on how to grind drill tips for cast iron

 

[echesak]

I've done similar, but with drills on the opposite end of the size spectrum. These are 0.020" and finish through a tilted surface. I have to be honest and say that I've broken more than a few. But I peck drill a little at a time, until it gets through. This keeps the bit from grabbing the edge and then when it finally passes through, the material on the angled surface is so thin it doesn't harm the bit. Something like this might be adaptable to your situation.

 

[Silverbullet]

Could you leave a small amount in the bottom and turn the piece over and use a countersink to finish the hole ?? That's what I'd do!

 

[gaston]

Just as you start to break through, stop , speed up the drill to a much higher speed and increase drag on the quill down feed. Forces the drill to cut rather than try to screw its self in, finish at the higher speed