Machine Tapping

[Maplehead]

I'd like this post to answer the questions on machine tapping that I never come across.
I tap by hand and that has it's own inherent problems, like coming in at an angle, pressing too hard or too softly down into the hole,... One thing though that I always come across while manual tapping is that at some point the turning stops because the extra force needed to continue feels like it will break the tapping bit. When this happens I usually back off a little and then proceed forward. Sometimes I backout completely and I clean the hole and the tapping bit. In the beginning I used to break the tapping bits in the hole. So my first question has to do with that.
How do you control tapping on a mill when the spindle is turning the tapping bit, so that it doesn't break the bit?
More precisely, how does the mill know when to stop when the bit is getting into that dangerous breaking area?
The feeling in my hand signals me to stop when the torque is getting too close to the bit breaking point but how does a mill tell you that?
How does the mill tapping bit get drawn into the hole?
Obviously I don't lock the head of the mill and the piece being tapped is clamped down.
Does the tapping action alone pull the mill head down so the the tapping bit can go further into the hole?
Most of my tapping is done with #6x40 bits. Is that too small for a mill?
I have an LMS micro mill with forward and reverse and variable speed so I guess it's ready for tapping.
I think I'll start with just these few questions. Any and all replies are greatly appreciated.

 

[benmychree]

I use a drill chuck to hold taps for machine tapping and tighten the chuck so that the tap can slip in the chuck when there is resistance to turning that would break the tap. The type of tap is important when it is used for machine tapping, a chip driving 2 or 3 flute tap for through tapping and spiral fluted tap for blind holes; I think that #6 is pretty small for machine tapping. Best is a tapping head that has a friction clutch, and backs the tap out when feed pressure is reversed. Lubricants are quite important as well, I like TapMagic.

 

[Mitch Alsup]

I will Mill tap 1/4-20 and larger threads in aluminum. I do this by using a fairly slow speed the tap in the drill chuck and as the tap enters the hole the Mill is turned off so only the rotational inertia is used to perform the tapping. When the mill stops, I will reach up to the spindle belt pulleys and turn them by hand to complete the hole. Done just right, the 1/4-20 will just clear the 1/4" thick hole and your done.

I mainly hand tap because most of my tapped holes are in the 4-40 range. A 4-40 tap in steel is very delicate and requires good tapping fluid and a deft sense of torque in your tapping hand. But, here again, I mainly tap aluminum, and my last 4-40 tap got through about 400 holes before breaking.

 

[MrWhoopee]

Yes, the the quill is pulled down by the tap. How does the mill know when to stop? It doesn't. As John said, a tap can be driven in a drill chuck and will slip when it encounters sufficient resistance. Assuming you are using either a spiral point (gun) tap or spiral flute tap, the chips are either driven ahead of the tap or back up the flutes. A straight flute, hand tap does neither, necessitating backing out to clear the chips. I never (knowingly) buy hand taps. I generally prefer spiral point taps because they have shallower flutes and are therefore stronger than spiral flute taps. Care is required in blind holes to prevent packing the chips and possibly breaking the tap. Blind holes are where spiral flute taps come into play since they feed the chips back up the flutes like a drill. I will power tap down to about 10-32, below that it's hand tapping. A piloted tap wrench helps to keep the tap straight.


When I was an apprentice, I was drilling and tapping a large run of 1/4-20 holes in mild steel. I became impatient and stopped changing the speed between drilling and tapping. Fred nearly had a stroke when he saw me tapping at 1200 rpm, throwing the mill in reverse when I got within a few threads of the end. Made good time and never broke a tap.

 

[JimDawson]

Normally #8 and smaller I hand tap in the mill. I put the spindle drive in neutral so it freewheels, and turn the chuck by hand while maintaining a bit of down pressure with the quill. Above #8 I normally power tap. For really small taps, I use my sensitive drill chuck for the tap holder, I think the smallest I have tapped is 1mm x 6mm deep in 303 SS

 

[T Bredehoft]

I routinely tap 4-40 in aluminum in my PM25, but drill 1/4" deep, and tap just 1/8" and back out, finishing by hand. Yes, Tap Magic.

 

[projectnut]

I would agree with the advice offered so far. I would add to it be sure you use quality brand name HSS taps. I avoid carbon steel taps unless it's an odd size or a one time job. About the only carbon steel taps I have are 3/8-16 STI. They're oversize for thread inserts. I bought them for a one time job, and haven't used them more than half a dozen times in the last 10 years.

Personally I would purchase brand names like Triumph, Precision, OSG, YG1, Cleveland, Beloit Regal, Besly, Morse, Widia, and Greenfield. There are many more quality brands, but these are the ones I can think of off the top of my head. I probably have in the neighborhood of 500 taps of various sizes and thread configurations in the drawers. 90+% of them are either spiral point (gun) or spiral flute. I have an older set of Snap-On (Blue Point) hand taps and a few odd sizes that are needed for my 100+ year old lathe.

Another note of caution is Don't buy used taps. It's almost impossible to tell with the naked eye whether not they are sharp. All the brands I mentioned and many more are available on eBay for reasonable prices. I usually but them by the box rather as singles. I find I do use them often enough to justify the cost, and they are much less expensive per unit when buying multiples.

On Edit: I noticed earlier I referred to gun taps as "spiral flute" taps. I have corrected the error to the correct terminology spiral point

 

[Mini Cooper S]

I use a drill chuck to hold taps for machine tapping and tighten the chuck so that the tap can slip in the chuck when there is resistance to turning that would break the tap. The type of tap is important when it is used for machine tapping, a chip driving 2 or 3 flute tap for through tapping and spiral fluted tap for blind holes; I think that #6 is pretty small for machine tapping. Best is a tapping head that has a friction clutch, and backs the tap out when feed pressure is reversed. Lubricants are quite important as well, I like TapMagic.

What he said!
I have machine tapped as small as #4 taps, you develop a feel for it. Start with the chuck a little looser than you think you need, you can always tighten it up if the tap slips and won't tap. Another thing that I will do is drill the tap hole a little larger than the chart says, maybe .005"/.010" depending on tap size. This greatly reduces the force needed to drive the tap and in most general use cases won't weaken the thread enough to affect function. You might try some practice holes in a piece of scrap. Remember the only machinist who can honestly say that they never break taps, haven't tapped very many holes!