The tap drill size for a #4-40 hole is #43 or .0890".
If you are tapping in a machine (using rigid tapping or a floating tap holder), you can use a spiral point tap if the hole is a thru hole; it will push the chips ahead of the tap. If it is a blind hole you can use a spiral flute tap, which will bring the chips out through the entrance of the hole.
If you are doing this free hand, you can use a hand tap in a tap handle (the T shaped tool), but it will take some skill to line the tap up straight.
You can use the drill press to guide the tap. Most tap handles have a small dimple on the end (large taps have the dimple in their end). To use it, clamp your work to the drill press, drill the hole, put a pointed bit in the drill chuck (I have a special spring loaded point), put the tip of the tap into the hole, and bring the quill down so that the point locates in the dimple. Now the tap is lined up with the hole and you can tap the threads.
Once you get used to lining up the tap with the hole, you can use a tap mounted directly in the chuck of a hand drill. You'll be suprised at how quickly you learn to do the forward/reverse cycle with drill. I would only recommend this if you have lots of parts; if you break a tap, the part is scrap and is easily replaced. It's a little too risky on a single part, and you're not really going to save much time anyway.
I use Emuge and Yamawa taps, available from most local machine tool suppliers. I don't bother with cheap taps; saving money is great, until you break the tap on the last hole of a piece that you've been working on for several hours.
While you're at the tool shop, ask for a drill/tap chart; it's a handy chart with all the drill sizes from #80 up to 1" (usually) and tap drills for UN, metric, and pipe threads. I've got a large one on the wall and a small one at each machine and another at each computer. Though, after a while you will come to memorize the tap drills.
And make sure you use tapping oil. I like A-9 for hand tapping and blind holes in aluminum, and flood coolant (OakFlo somthing or other) from the machine for thru holes. There are plenty others that work; every one has their favorites.
Hope that helps,
Chris Kirchen  |