4-40 Tap

[jbg230]

I set out to make a set of mini tools with carbide cutters since my larger carbide roughing tool worked out so well. I bought a 1/2" square bar and some mini cutters. Shaping the bar for the round cutter was easy. It's the tapping of the thread that has me depressed. (pardon the pun)
The screw for the cutter calls for a 4-40 tap. I drilled the hole with a 3/32" bit. No problem on the drill press. I used the cheap tap from my tap and die set and it broke about half way in the hole. The bar must be some sort of hardened steel. I purchased an Irwin HSS tap from ACE Hardware and the same thing happened. I am only using a "Super" household oil, but I'm doing the 1/4 - 1/2 clockwise and counter clockwise turns as I go.
S I cut off the bar end, re-shaped, and drilled again for the 3rd time. I then tried my handheld drill with the broken tap bit, forward and reverse, forward and reverse. Broke the tap down to the nub.
So I tried to get educated. It'd be nice, but $150 or more for a drill press tapping head isn't the answer.
I guess I need a cobalt 2 flute plug style tap. They're about $15.00. There are so many versions though, I'm kind of lost.
Can anyone recommend the correct 4-40 tap for stainless or hardened steel? What brand? Does Nitride play a role?
If it's recommended that I getting a tapping oil instead of my household oil, I'll get it.
Thanks for any help.
Jake

 

[Curly]

Get a spiral point tap if the hole goes right through the bar. The taper or starting tap. An example.

High Speed Spiral Point Taps - Gun Taps - for use in through holes and machine tapping.

It will also help to use proper cutting oil. These taps are use in machines and are not backed out like hand taps so run straight through. If your drill press goes slow, 30 to 60 rpm, then use it to power tap in with a quick on off. Also get a proper number size cobalt drill bit and drill the hole slowly with it and make sure it cuts as it drills. Otherwise the SS can work harden the metal which might be why you are having so much trouble.

 

[KenV]

Jake, if you purchased mystery metal bar stock from the big box store, your challenge is the steel is recycled from mixed stock sources with zip effort for consistency, and can be work hardened from cold rolling. For the price point, annealing by the supplier is not a requirement.

Added, 4-40 is a very small screw and fine thread.

Your processing may be adding a bit of work hardening also.

Added to the advise above, pull the tap out every few threads and clean the swarf off with a toothbrush. If there is build up of swarf in the hole, flush with WD-40 or mineral spirits. This is to avoid binding. Use a small tap wrench so you can feel the tap cutting. Small tap use is delicate work but being careful and using care is the alternative to repeating until luck strikes.

I like tap magic for tapping.

 

[jbg230]

Will do. Thanks for the advice and info.

 

[dogcatcher]

I would bump the drill bit size up to a #41 and use tapping oil. The fit will be a little sloppy, but not enough to matter. If you feel uncomfortable with that, drill with the 3/32 all the way through and then drill about 1/2 of the top of the hole with the #41. That will leave you with some tight threads in the bottom of the hole.
The 3/32 is .0938 The #41 is .0945

 

[More4dan]

Drill size for 4/40 is a 43, 3/32" is already 5 thou larger. Taps need to be held straight or they will break. I put this size tap in my drill press and turn by hand so I can feel resistance and I watch that the tap is turning instead of torquing up. It would help to drill all the way through if possible and be careful to drill slow and steady so you don't work harden the material. Use tap magic for both drilling and tapping. Chamfer the hole before tapping. Cutting fluid is really important for Stainless Steel to keep it from galling. Titanium coating does help prevent galling and sticking. When I tap Titanium and Stainless sheet with 1:72 taps I use what called a forming tap that uses a larger hole and the tap rolls the threads instead of cutting them similar to a self tapping screw. These taps don't have flutes so they are stronger.


Sent from my iPhone using Penturners.org mobile app

 

[monophoto]

Here's the rule: when you are buying a tap smaller than 1/8", you should always buy at least three because you are going to break some. DAMHIKT

Seriously, small taps are very fragile and break easily. Deal with it.

My experience is that you need to be very careful to not advance the tap more than about 1/4 turn at a time - better yet, 1/8 turn - and then back it out to clear the swarf. If the tap feels like it's starting to bind - STOP and back it out.

 

[frank123]

As mentioned, #43 is the specified bit. A 3/32 may work in something soft like aluminum or maybe eve brass but you're unlikely to get away with it in any grade of steel.

4-40 is difficult to tap because of the small size of the tap and how easy it is to put way too much torque on it without realizing you are.

Do it by hand, do not use a smaller than specified bit if you can avoid it, and use a high grade tapping fluid (don't be afraid to use too much, you can't). Turn it in no more than a half turn at a time and then back it out half of whatever you turn it in (i.e. a quarter back out for a half turn in or an eighth for a quarter turn) to clear chips and prevent binding. It doesn't hurt to remove it occasionally and clean the tap and hole before continuing to tap it if the hole is very deep.

Be sure your tap is exactly aligned with the hole axis. If you drilled the hole on a drill press remove the bit and start the tap by chucking it and gently HAND turning the chuck with just a little down pressure to get it started (without moving the piece from its initial drilling position on the drill press). This assures perfect alignment.

 

[Woodster Will]

4-40 sounds like a pretty small fastener, what inserts have you got?

 

[hanau]

I tried and tried drilling and tapping 4-40 holes and broken numerous taps. Go to 8-32 if you are using the 12mm round insert or 10-24 if you are using the 16mm round insert.


What size of inserts are you using?

 

[dogcatcher]

Buy a "T handle tap wrench". One of these things, T-Handle Tap Wrenches - Tools - IRWIN TOOLS Make sure it is the correct size for your tap. A real hardware store employee should be able to steer you to the correct one. Buy a can of tapping fluid, I prefer Tap Magic, but any brand should do. As stated above by Frank 123, follow his directions.

 

[randyrls]

Jake; I would go a little larger on the drill bit. #41 will give you 50% thread engagement in steel. I would be very careful tapping. You may be able put the tap into the drill press chuck. Don't tighten it, you just want it to keep the tap straight. Remove the tap about every two turns and take the swarf out of the tap cutting channels.

 

[SteveG]

Off topic, but on topic :wink:

Many of us are only occasional "Metal-Workers". I know as a woodworker who on occasion does a little metal work, there is a bad habit that is much worse when associated with metal-working. I refer to using the air hose/gun for cleanup purposes. I believe (not sure) that machine shops tend to NOT put air hoses at work stations because of the risk of eye injury perpetuated by flying swarf. Cleanup is done with a paint brush dedicated to that purpose, vs. a blast of 100 psi sir. So this is a safety reminder for when we cut metal. Avoid the air hose and retain the eyesight.

 

[campzeke]

I had the exact same problem a few weeks ago. I was working with 316 stainless hex stock and broke 3 taps off in three holes. I asked a machinist friend to help me drill and tap the stainless. He mounted the stock in a vise attached to his vertical mill and drilled it like it was warm butter. He then put the tap in the mill ran it through with no problem at all. He told me the reason I broke my taps was because I was doing it by hand and was not starting the tap straight. If you think about it, it makes sense. If the tap does not start straight, it is trying to cut more material the deeper it travels into the hole. Someone above mentioned drilling and tapping using a drill press. That accomplishes the same thing the mill did in my case. My friend also did not tighten the tap in the mill real tight. He tightened it just enough to allow it to cut the thread and pull itself through. If it bound up, it would slip in the chuck. I learned a lot in the hour I spent with my friend drilling and tapping a few holes. Hope this helps.

 

[Curly]

Steve you can add that you save your hearing too by not using air.

That said I worked in a machine shop making aerospace parts and there were no brushes in the entire shop. There was only air and on some machines they were 1/2" hoses to clear the chips away from the parts being machined. Because it takes longer to clean with brushes and rags, air was king.