Hard Starting Taps - What gives?

Pmedic828

Pmedic828

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I am attempting to tap some small holes in medium steel. I am using a HSS hand tap turning the tap with a tap wrench. The problem seems to be too small a hole when I look at the charts for drill size vs tap size for specific size. I think that the last hole I attempted was a 1/2 - 13. I drilled the 27 / 64 inch hole in the steel plate and attempted to start the tap. I was using a plug tap. I started the tap and after 1 revolution, it wouldn't turn any more. I used tap magic on the tap and in the hole. After seizing the tap, I carefully backed it out and changed to a taper tap of the same size. This also would not turn after 2-3 revs. I bored out the hole to a larger size and after increasing the size of the hole I was finally able to turn the tap and cut threads. The two mating surfaces seemed to mesh well and were tight. The taps were also new and purchased from a quality machine shop supplier. These were not Chinese taps.
One other question - What does H1, H2, H3 on the taps and how do they differ? I think that it is the tightness of the threads or something like that. Can someone explain what is meant by the different grades to a novice. Thanks.
 
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'm too inexperienced to be a wise guy on this stuff, but my wise word is "spiral" as in "spiral point tap." Once I discovered gun taps, I literally gave away all my carbon steel Hanson and other hardware store taps. I believe spiral point taps are called "gun taps" because they "shoot" the chips ahead as they cut. Stellar performers on through hole tapping, I use them for everything. For blind holes, I just drill a bit extra depth and pick out the chips if they appear to get in the way.
 
Make sure you are aligned with the bore of the hole that you drilled. A mis-aligned tap will bind very quickly. Easiest way to avoid this is to make a guide block, or buy a commercial one. Just find a piece of flat sided stock, usually at least a half inch thick or so, enough to support the tap, and drill a clearance hole for the tap. Then clamp the guide over the hole, and the point on the tap will center it with the drilled hole. Clamp the piece down, and tap away!

Also make sure that you break the chip as you go. Generally you can get about a half turn or so on the tap, then it binds. When this happens, give the tap about a quarter turn in the opposite direction, and you should feel it turn easily, until it hits a small chip. Continue in the reverse direction until you feel it break off. Now you can give it about a three-quarter turn forward, or until it gets too hard to turn. Repeat until you are through the part.

Unless you oversize the hole, reducing the thread depth in the part, you will be hard pressed to drive a large tap, by hand, straight through a piece of steel without snapping it.

-Cody

P.S.

"Gun" or Spiral point taps were originally developed for power tapping machines. The drill like point allows for easier starting, and keeps the machine from tearing out the first couple of threads. This of course makes it easier to drive by hand as well, though I find they are harder to get aligned initially.
 
spiral flute tap

I use to use standard taps and then discovered spiral point taps, and they did tap better than the standard taps......now I only use spiral flute taps, its like night and day they are so much better than the spiral point taps its unreal, and it was a real treat to see the chips come out of the work from the top, you can tap right down to a blind hole, no stopping/backing up stuff........they are not cheap but how much is your time worth.......I wont use anything else now....
 
Simply put, a 1/2-13 is a bear. big holes, steel = BEAR. 2 or 3, maybe 4 at most, back out, clean re-lube. repeat. takes a while. And oh yea, after it was well started and straight (done in the mill with a spring center),out came a cheater bar in the form of a box end wrench slipped over the tap handle.

- - - Updated - - -

good lord, why doesn't the return key work here...!! what is this MS Word ? ;)
 
The "H" number refer to how much the tap will cut over basic pitch diameter. H1 = basic to .0005" over, H2 .0005"-.001", H3 .001"-.0015".

A plug tap is hard to start and turn by hand. That's the reason hand taps come as taper, plug and bottom taps. You start the thread with the taper tap and then run the plug tap in the hole.

I'm with the other posters and use spiral point taps for 99 percent of the time.
 
broken return key

timbertoes said:
good lord, why doesn't the return key work here...!! what is this MS Word ? ;)
Click to expand...

Change your settings to use the "Standard Editor" rather than the "WYSIWYG" editor and it will work much better!
Back to our regularly scheduled program...............

change_editor.png
 
Pmedic828 said:
I am attempting to tap some small holes in medium steel. I am using a HSS hand tap turning the tap with a tap wrench. The problem seems to be too small a hole when I look at the charts for drill size vs tap size for specific size. I think that the last hole I attempted was a 1/2 - 13. I drilled the 27 / 64 inch hole in the steel plate and attempted to start the tap. I was using a plug tap. I started the tap and after 1 revolution, it wouldn't turn any more. I used tap magic on the tap and in the hole. After seizing the tap, I carefully backed it out and changed to a taper tap of the same size. This also would not turn after 2-3 revs. I bored out the hole to a larger size and after increasing the size of the hole I was finally able to turn the tap and cut threads. The two mating surfaces seemed to mesh well and were tight. The taps were also new and purchased from a quality machine shop supplier. These were not Chinese taps.
One other question - What does H1, H2, H3 on the taps and how do they differ? I think that it is the tightness of the threads or something like that. Can someone explain what is meant by the different grades to a novice. Thanks.
Click to expand...

That's good advice from PMedic, check those H numbers. Additionally (depending on the alloy), I frequently go oversize on the hole. Also, try using "Moly D" instead of the tap magic. And finally, throw some magnification on those starting edges and make sure they are sharp.
 
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