How hard is it to drill and tap cast iron?

I’m building some extension wings for my Jet contractor’s saw and it’d be simpler if some attachment points on the edge of the cast-iron tabletop were in different locations.

Given that I’d have to drill and then tap a couple of 3/8" holes through about 1/4" of cast iron, is this something that I can do with a good quality bit in my Milwaukee hand drill and a regular hand-driven tap set? Or is this drill press work?

I offer second-hand knowledge. An old friend of mine is a retired toolmaker, working part-time in a pump building shop. He says cast iron is not always homogenous. Sometimes you can drill and tap as smooth as silk. Often, though, you’ll find a hard spot, or a void, and your bit will chatter and go astray. Yes, you’ll need a drill press.

IMO big mistake to use a hand drill, particularly a pistol handle type. With a D handle drill you can do an okay job if you’re really skilled but you can still end up with egg shaped holes. Not a good thing if you intend to tap threads. You’ll need to work your way up, starting with a pilot hole and drilling the hole slight larger in steps instead of starting with the final size for your tap. The chips won’t come out as a nice continuous string but break off short causing the bit to chatter badly.

Disassembling it and clamping the top to a drill press is no small task and getting it reassembled in proper alignment will be harder but if you want it done right that’s the way to do it. I’m sure that contractor saw wasn’t free so there’s no sense in screwing it up. Do it that way and you can drill the hole, tap it (using the drill press quill to hold the tap handle) and countersink the hole slightly to chamfer the edge without any misalignment. Personally I’d try to find a way to use the existing holes in the saw top.

Yeah, I’m probably going to work with the existing holes, it just means either fastening a hunk of angle iron to the side and bolting extensions to that or making the whole thing out of wood and the underside of the extension will have some big grooves cut in it to allow for inserting and tightening the bolts that hold the extension to the iron tabletop.

Now that $300 Biesemeyer fence (which has stronger rails that can be used to mount the extension tables directly, rather than fastening them to the tabletop) is starting to look kinda good…

…going to disagree with the majority opinion here.
I do this type of thing nearly every day and it’s really not that big a deal. As long as you are careful and patient, you’ll be fine with the hand-held drill.

Just figure out where you want your holes. Then make a template out of heavy paper or cardboard. Lay it all out, centerpunch the holes, and double check their locations.
Start drilling (slow speed, with cutting oil) with about 3/16" or so bit and work up to your thread minor diameter. Check frequently that your holes are still centered where you want them.
Tap slowly using plenty of lubrication. Stop every half turn or so and back the tap off a little before proceeding (IME this makes cleaner threads in cast material).

I will agree with AskNott about cast iron being “grainy” though. So, I’d also recommend giong a bit larger and installing Heli-coils if there’s room.

Instead of using a hand held drill, see if you can rent a portable drill press. That’s what I call it anyway.

It’s a heavy duty drill mounted on a column. This column has a magnetic base. Place the base on a magnetic surface, and cast iron is ideal, and the drill is anchored. Then you use the drill just as you would a drill press. Feed is controlled with a wheel or lever.

Of course you need to be sure that the surface you’re anchored to is at the correct angle in relation to the top surface because you probably want your holes parellel to that surface.

Here’s what I’m talking about

But a Google search for “portable drill press” turned up this tool which looks kind of neat too.