Originally Posted by
Scott T Smith
As George stated (and ask any machinist), cast iron is typically drilled dry, as it is self lubricating. I used to own an automotive machine shop, and have drilled many holes in CI over the years.
You do not need to step the hole in small sizes either. Simply drill a pilot hole, and then use the correct bit for the tap that you will be using. Using a countersink on the top of the hold before tapping is a good idea.
A cordless drill will work just fine. Use a sharp bit and keep good pressure on it when drilling. If you have access to a Hougan mag drill, then fine, but it's overkill (and not usually used to drill the small holes that you'll probably use to mount the feder).
The exception to the rule is if the cast iron that you're drilling is of extremely poor quality, and has hunks of unmelted recycled material in it. Then, it's anybody's guess.
Be sure to check the bottom side of the casting for ridges before you start to drill. If you catch the edge of one with your hole, it may cause your tap to break when you tap it.
I've tapped CI both with, and w/o a tapping fluid. It seems to work fine either way, but usually I'll use a small amount of tap-magic as a precaution.
+1 on "everything" you said. One thing NOT mentioned here is that when bit starts out the back side, you need to ease off on the pressure but keep a good grip on the drill, because the bit will try to grab the remaining bit of metal and run on through (like a bolt in a threaded nut) and this will many times cause a chip/chunk to break out on the backside. You can also prevent this if you have room to clamp a piece of hardwood on the backside the same as you would when drilling through wood to prevent tearout on the backside of the hole. (and yes, drill it dry)
"Some Mistakes provide Too many Learning Opportunities to Make only Once".