Tapping holes for 4-40 machine screws

[fivefishcnc]

Newbie alert! Sorry if this is such a basic question but I'm not a machinist.

So I have a 1/8" thick aluminum, I need to make threads in it so I can screw/use a 4-40 machine screw.

1. What size of drill bit do I need to initially bore the holes? 1/16" ?

2. I know I need to buy a 4-40 tap set, but what tool do I use for this tap?

I'm thinking surely I can't use a drill press since that will just strip the holes of it's thread, once going down the material, and when I go up.

Or do I need one of those hand-cranked T-shape tool and manually tap the hole.

Is there an easier and faster way to do this using some power tool?


3. And lastly, any tap set brand you can recommend? I just need holes from the 1/16 to 1/4" range. Are the ones on eBay good enough? (Material to be tapped will be 6061-T6 aluminum.)


Thanks for any advice you can give me.

[John Walker]

The hole size for drilling would be 2.35mm which is a letter 43, the closest imperial is 5/64".

1/16" is a bit small. Sounds like it is manual tapping for you. Hope that helps a bit.

[ckirchen]

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

[fivefishcnc]

Chris., THANK YOU for that excellent reply. The procedure is a little clearer to me now.

Also saw a video on Youtube using a drill press to hold the tap and rotating the chuck by hand.

I'll be buying some taps and do some practice runs.

John... thanks for the info! Much appreciated!

[Geof]

When tappiong with a Tap Handle one way to hold the tap perpendicular to the material you are tapping is to make a guide; drill a clearance hole through a small block of metal, wood or plastic that has a thickness of about 3 times your tap diameter. Put the tap through the guidehole into the part and just hold the guide firmly against the part while tapping.

[fivefishcnc]

Another question... I'm looking to get a tap and die set. While I'm not rich, I do value good tools. They make the difference between an easy job and a hard job.

I'm looking at this IRWIN set. It seems an American company and doesn't seem like a China made import. (I hope!)

I'm deciding between the 76pc set and their 117pc set.

From my comparison, the only difference is the 117pc. set comes with drill bit sizes (some I've never even heard of..... no.3, no.7, Letter B, Letter F... huh? )

I'm thinking of pulling the trigger on the 117-set IRWIN at $240.

Good deal? Go? No go?

Any other "better" deal for the same price?

[ckirchen]

Personally, I don't like tap sets; I prefer to buy the taps and tap drills one by one. Yes, it is more expensive in the long run, but I found that there are a lot of taps sizes in the set that I don't use.

For each tap size, there are many different types taps, each used for different scenarios and different materials. I also found that the tap set only had the right tap for the job once in a blue moon.

When I need a certain tap that I don't yet have, I will go buy 2 or three of them (or more, depending on the job) and usually half a dozen tap drills to go with them. Then, they go into one of those storage centers with lots of small drawers; each drawer is labeled for the tap or tap drill.

#3, #7, B, F are all drill sizes (.213", .201", .238", .257" respectively). They will also be listed on the drill/tap chart.

Chris Kirchen

[Geof]

The letter and number drills are to fill in size requirements between common fractional sizes. Commonly the drill size used for tapping is intended to give a thread that only has 75% of its theoretical height; the hole is intentionally oversize by a small amount. This causes a negligible weakening of the thread but trying to tap a full height thread by using a smaller drill dramatically increases the load on the tap and in steel may be impossible with small taps. With some thread sizes using a fractional drill will be too loose or too tight.

Actually when starting out it is sometimes a good idea to use a slightly larger drill and some tappin charts give an alternate for a 65% thread. This makes it much easier for a novice to learn the feel of tapping without breaking too many taps. It is simple to compensate for the reduced strength in the tapped hole by using a slightly deeper hole and a longer bolt engagement.