Breaking through cast iron

[paul]

I decided to give my new giant bicep a rest from hours of hacksawing a lump of cast iron and resort to a bit of chain-drilling.

The part I'm working on is basically a 'cube' of cast iron with a 1" hole bored through it. I found that the drill would 'catch' as it began to break through on every single hole I drilled (drilling from the outside of the cube in to the bored hole).

Is this likely to be because the breaking through is happening on a curved surface or is this something that often happens with CI and can it be avoided? I'm concerned that the much smaller holes for the ports will present a significant problem/risk of breaking the drill bit.

Any suggestions/tips welcome ;D

[ron]

Paul, the only thing I can suggest is try a higher drill speed and a lighter touch, cast iron seems to be a very variable metal, of recent I have machined stuff that varies from the sublime to the bloody awful!
Ron

[GWRdriver]

of recent I have machined stuff that varies from the sublime to the bloody awful! - Ron

And so it is with me. At the moment I have a pair of cylinders which were cast as a "stick". When bandsawed apart the exposed faces looked absolutely magnificent but the metal has turned out to be brittle and crumbly. Very disappointing as C.I. is normally my favorite metal to machine.

[AndrewP]

Paul, try whacking a tight fitting wooden dowel in the bore before drilling. If you want to get really clever try a nice sliding fit, then wet the wood - expands and grips, drill holes then dry out wood and out it pops.
Cheers, Andy

[chris vine]

HI Paul,

Make sure the work is held securely in a heavy vice for example. Then the drill will have to continue to cut rather than grap.

Be very careful with the feed as the drill starts to break through and keep a firm arm so that you don't feed it through quicker as the metal resists it less.

Chris.

[SteveW]

Paul,

Have you tried reducing the cutting angle on the drill? This should reduce the grab/pull-though effect.

If you think about it you don't need much top rake for CI on lathe tooling so it follows for drills. Agreed you end up with a set of drills for CI (or brass) but this isn't the end of the world, just a bit expensive if you don't do drill sharpening.

[dickdastardly40]

I like the wooden plug idea from AndrewP.

My suggestion would be not to drill through so far the drill breaks into the bore, set your drill press depth stop to just as the tip causes a bulge.

I guess you'll have to drill from from both sides and will still have a little full thickness sawing to do but with less stress on you.

Let us know how you got on

Al