Hard Starting Taps - What gives?

[brucer]

I don't remember the numbers, but if you have H2 or H3 taps, that would explain your need for a bigger drill size because those are around .010 inch over per step (H1, H2, H3 etc...)

This allows the use of standard fasteners to accomplish different fits. There are also "L" fits that go small in similar steps.

Additionally, plug taps are a little harder starting, and restarting with a tapered tap doesn't always give the desired effect right away.

Bernie

I have never drilled a hole bigger than specified for a tapped hole.. I have tapped so many +.005 1"-8 holes (for heat treat) it isnt even funny, drilled them all .875 with an Allied drill.. Same goes for drilling and tapping 1/2-13 holes in H-13 cores and cavities pre-heat treat drill them all 27/64 and use a gh3 tap..

Each GH increment is the basic thread clearance, plus .0005

ie:
gh1 is +.0005
gh2 +.001
gh3 is +.0015

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Chamfer the hole.. The chamfer on the hole should be larger than the major diameter of the tap.. for instance a 1/2-13 hole I would chamfer it at minimum of .030 per side so the chamfer would measure .530 diameter I would shoot for .530 to .56 on the chamfer..


Tap magic isnt all that great of a tapping fluid, I do use it but I particularly dont care for it, I get it free, thats why I use it. I think transmission fluid and kerosene mix works just as well or better... I would suggest ezz-foamy, its about the best stuff i've come across..


I also like a starter tap.. Use high quality taps, I learned long ago purchasing for the shop I used to work for, I could buy 16 cheap import taps to 1 good osg, morse, greenfield.. They do cost more but they last longer and cut much better, I suggest osg taps.

Also the style of tap does help a bunch, try to buy cnc style taps, the have a shorter profile on them so they dont load up near as bad as standard cutting taps including the starter taps..

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WOW I guess I am in the minority about spiral flute taps. I use them for power tapping in a tapping head and hardly ever break one....with one of my tapping heads I can tap 10 holes drilled the correct size with a spiral flute 3/8-16 tap in 60 seconds !!! If the tap is a good brand, and sharp it wont break because it cuts and doesn't bind.......

only time I use spiral flute taps is in aluminum.. the rest of the time I use cnc taps.

 

[Breakit-Fixit]

When tapping something that may be difficult to cut the threads into due to the size of the hole, or the type of material being tapped, I usually consider how much engagement do I really need for the application.

If I need 75%, or close to it, then I use the standard Tap Drill Size charts, or I just subtract the pitch of the thread (1 divided by the threads per inch = the pitch) from the major diameter to find my Tap Drill size if I am in a hurry.

But if the application isn't critical or doesn't involve a safety issue, then I use the following formula to find a lower percentage of thread engagement to make the tapping go easier.

Tap Drill Diameter = Major Diameter minus ((0.1299 times the desired percentage of engagement) divided by the Threads Per Inch to be tapped) .

Sometimes you see this written as: TD = MD - ((0.01299 X %)/ TPI)

For example, if I decided that for my 1/4-20 thread application, whatever it is, and whatever the material to be tapped is, that a roughly 60% thread engagement would suffice, the math would look like this:

T.D. Ø = 0.250 - (( .01299 X 60)/20), which results in a T.D. Ø of 0.211 inches.

The closest existing drill size to this would be a 5.40mm (0.2126") , not one you are likely to have handy!

But if I did have one, it would be an easy hole to tap to 1/4-20 thread.

But hey, I do have a 7/32" drill bit here, which is a decimal 0.2188" , I wonder what percentage of thread engagement that would give me?

Well, easy enough to find out using this formula:

Percentage of Thread Engagement = Threads Per Inch Times (( Major Diameter minus Tap Drill Diameter) Divided by 0.01299)

Or shown this way: %= TPI X ((MD-TD) / 0.01299)

So if I plug in my values I get:

Percentage of Engagement is equal to: 20 X ((0.250-0.2188)/.01299) and the answer is , that's only going to be 48% thread engagement, not quite enough for my application, whatever it was.

Hmm, so I plug 65% into that first equation, and I get a T.D.Ø = 0.250-(0.01299 X 65)/20) , TD= 0.2077, which is close to a 5.30mm bit at 0.2087.

Hmm, don't have one of those either, but after a bit of scrounging around, rummaging through the cat's toy box, etc. I find that I do have a #6 bit at 0.2040" , lets see, that will give me......20 X ((0.250-0.2040)/0.01299 = 70% engagement.

Well, guess I will have to settle for that, but it will be easier to tap than using the normally called out #7 drill bit at 0.201 inch, or even the occasionally listed 13/64" bit at 0.203 inch.

I've used the above countless times over the last 50 years or so, saved me any number of times, and it also reduces the wear and breakage of taps a bit if you don't need a 75% or higher engagement.

Chamfering the first 1- 1.5 thread distance of the mouth of the hole, keeping the tap perfectly perpendicular to the hole, using the right cutting or tapping fluid for the material, and of course using a sharp tap of high quality , all as noted above, will also make it much easier to tap that hole, no matter what the diameter or the material being tapped.