Hole tapping Tips

Just thought I'd post this to break the silence.
Pete Stanaitis
--------------- Some basic tips on tapping holes. by Pete Stanaitis
This article is being written for people who have already done some hole tapping and who have had unexpected bad things happen to them during the process. I hope that some of this information will reduce tap breakage and frustration.
About Taps: Hand Taps come in 3 different basic styles, Taper, Plug and Bottoming. This relates to the number of chamfered threads at the business end of the tap. Taper taps have 8 to 10 chamfered threads, Plug taps have 3 to 5 chamfered threads and bottoming taps have only 1 to 1 1/2 chamfered threads. You should tap through holes with a plug or taper tap. If tapping a blind hole, start with a taper or plug tap, clean out the hole after going as far as you can then finish with a bottoming tap. Either taper or plug taps can be had in the "Spiral pointed" version. This spiral relates to the tap's ability to push the chips ahead of itself. Sometimes they are called "gun taps". This doesn't mean they are for use on guns; just that they "shoot" the chips ahead. This kind of tap doesn't need to be backed out after cutting a few threads. There are also many other innovations in tap design.
Tips on Tapping: 1. If I were going to risk OVERspending on any tool in my shop, it would be on taps. I would NEVER knowingly have a "carbon steel" tap in the house. HSS as a minimum, and made in USA, from a well known tool maker. 2. I almost always have at least the plug and bottoming tap of any size I use. The only exception is for the bigger (to me) sizes, like 5/8, 3/4 and 7/8". 3. I keep more than one tap of the sizes I use most often. I keep one Brand New tap to use to test to see if my other taps are getting dull. Just today I had to tap two 1/4-20 holes about 1" deep. I picked out a plug tap and started at it. The tap worked pretty hard, even though I used my normal sulpherized cutting oil liberally on the hole. I didn't want to break the tap, so I went for the new one. It tapped the other hole like a dream, without having to stop and back the tap out at all! What to do? ----Immediately, throw out the old tap!!!! 4. It seems that most hardware stores and even some tool catalogs sell the Plug tap as the "default" tap. It might pay to get a few taper taps in a few sizes to ease the task and the strain on the tap itself. 5. There are also taps that have short areas of reduced diameter shank. The idea is that if you break the tap, it will break well above the hole, making it easier to remove. 6. Always take the time to chamfer the hole before starting to tap it. This reduces stress on the lead threads on the tap and helps keep the tap straight. 7. Nothing is worse for a tapped hole than starting the tap crooked. And often as not, you wobble around for a few turns. If the hole is started crooked, the hole itself will eventually try to straighten the threads. This causes more friction and increases the likelihood that the tap will break. 8. As Dave Mariette said in a recent Bellows post, "use the drill press for tapping operations whenever possible". For me, instead of chucking the tap in the drill press chuck, I do the following: I chuck the tap in a "T" style tap wrench, just as you would do for hand tapping. Then I put a center drill into the drill chuck. The upper end of my tap wrenches all have a small hole bored in the end. Next, I stick the tap into the hole to be tapped, then run the drill press spindle down so the center drill enters the hole in the top of the tap wrench. This holds the tap exactly vertical so I can cut the first 2 or 3 threads. Then I tap the rest of the hole. 9. If you can't use the drill press, there are several alternatives: A. I often take a piece of square stock, let's say a 1" cube and drill a tap-size hole right through it. Then I set the block over the hole, stick the tap down through it and I'm off and running. B. Bob Walsh just told me that he uses the level that is built into his drill. C. I once saw a little tool that was just a short piece of angle iron with a vertical groove in one side. To use it, you push it up against the bit for one axis, then sight the vertical groove for the other.
My next-to-last comment is about drilling the correct size hole for the tap. There are tables you can use for this. I'm no expert, but I think those tables are usually designed to produce a 75% thread. If you are drilling in real tough material, and if there won't be a lot of stress, you can sneak the hole open a few extra thousandths. Did you know that you can calculate the tap drill size without the table? Here's how: -Subtract 1/Pitch (threads per inch) from the major diameter. That's the tap drill size! Example: 1/4-20: 1/4" = 0.250. 1/20= 0.050. 0.250-0.050=0.200. You need a drill bit that is about .200" diameter. (In this case, either a #7 or a 13/64", which is 0.203" will work fine.) Another example: 3/8-16: 3/8-1/16= 5/16.
Last comment: ALWAYS use plenty of appropriate lubricant. An old engineer that I worked with for many years told me that "anything, even water, is better than nothing". For all carbon steels, I use the sulpherized cutting oil that hardware stores sell for pipe threading. You need different lubes for other metals, except for cast iron, which needs none, usually.
I hope this helps some. If you have any additional tapping tips, please write them up and post them here.
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What, if anything else, would you recommend for someone who has never tapped a hole? Ken
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I guess I'd recommend that you do some practice tapping of mild steel holes BEFORE you need to do it on an actual project. I know this will probably sound terrible, but you might even buy a few CHEAP taps in sizes like 8-32, 1/4-20 and 3/8-16 for "testing". Drill some correct tap size holes (through holes for starters) and then try tapping them using the info in the previous article. The idea here is that if you break a tap, you are only out a buck or two and the broken off tap is not stuck in a project. You want to start to get a "feel" for cutting versus binding. You will notice, particularly with the smaller tap, that it can actually take SOME twisting before it breaks. If you tap two or three holes with each size and don't break any taps off, then drill some more tap holes, but make them several thousandths too small. Note the differences in the amount of force it takes to get the tap to cut. Try tapping some holes without lubrication (after you have done it properly a couple of times. After all that, try the same thing in aluminum, say T6061. Note the gummy-ness in the chips. Note that you can get the tap stuck more easily. Do the same thing with some scrap brass. Keep doing this until all the cheap taps are either stuck or borken off. Then you will be a lot wiser and the cheap taps won't be around to bite you in the behind some other day.
Pete Stanaitis
Ken Vale wrote:

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2-flute gun taps are the easiest to keep square to the work, if you start it close to square it tends to right itself. and don't forget the spiral flute taps which pull the chips back out of a blind hole. i typically use TiN coated taps if for no other reason than to be able to have visual indication of the wear on the tap, the contrast between the gold coating and the silver of the hss gives an easy way to tell how bad it gets. and it does add to lubricity. tapping high alloy steels is a whole nother ball o' wax.(cpm 10v & 15v) i tap a dozen holes max and toss the tap, when your working on a plate that cost $500, you don't take chances. using dies to cut external threads are the hardest to keep square to the work, i chuck up on the stock in the lathe and use the tailstock to start it square.
lps edge creme is some good shit, i use it mostly on reamers and counterbores to extend their life and give me the right fits, but it works wonders on the cpm stuff too when i must drill 1/32" through 1" 15v(cause the boss won't spring 12grand for a small hole edm)

other.
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Good stuff Pete. Thanks, that's a keeper. I have been tapping holes for many years, and to date have never broken a tap in a hole, but I use great care. Your post has some great tips and info I wasn't knowledgeable about. Thanks.
Ron
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Ted,
tpi=threads per inch not turns per inch. To find the tap drill size subtract the distance from one thread to another from the major Dia.
1'-8 tpi pitch=1"-.125... .875 tap drill size _______________________

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gp wrote: To find the tap drill size subtract the distance from one thread to another from the major Dia. ^^^^^^^^^^^^ Another small nit: You formula will produce a thread with 100% engagement. Most threads are cut so there is a little clearance at the tops and bottoms of the threads, so the tap drill should be a tad bigger than your formula says.
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Check a few examples. You'll be amazed at how close it comes to the 75% tables. Did you forget the Cos 30 factor?
Ted
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Ted Edwards Ted snipped-for-privacy@telus.net
wrt tapping formula

Yes. That's what I thought, too. Had not seen the formula before, though. Frank Morrison
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