I've experienced more tap breakage with such taps. The strength of gun taps can't be overlooked.
A needle on an air hose does wonders towards removing chips from blind holes. I have a set at the ready, to accommodate tapped holes from 2-56 up to fractional hole sizes. Considering the majority of my work was tooling, the few moments of cleaning holes wasn't considered a big deal-------but I was known to use them for light production as well.
All I can say is that my experience leads me to suggest that if a slightly deeper hole is permissible, gun taps, on the whole, will out perform other taps, although I never used the thread forming variety, such as the Besly.
Harold and Susan Vordos wrote: .......those of us with experience tend to avoid
Thank you! I always did this. Once a coworker asked for some flat-bottom taps. I told him just to grind off the tip of a spiral point tap leaving two or three tapered threads and he looked at me like I was an idiot (in a polite way).
BTW, isn't that point just for holding the tap between centers for grinding?
IF the tap is not started perfectly straight to the hole, it will become increasingly hard to turn as it goes deeper into the hole. Unless the tap flexes. it will cut progressively deeper threads on one side of the hole until either the tap breaks or the tape self aligns in the hole.
A tapered tap is easier to start straight in the hole. If you start any tap straight it will be a lot easier to turn it and not break it off.
Gun taps are used when the hole is a thru hole, otherwise a sprial point tap is used so the chips wind out of the back of the tap and don't bunch up in the bottom of the hole when machine tapping. A gun tap in a blind hole is a sure way to get into trouble fast.
I can't see any other reason for it to exist, although it is helpful in getting the tap in the hole when production tapping with a tapping head. The blunted end is harder to center visually.
"Gun Tap" is a name one companies came up for a sprial point tap.
Taps are best classified on their chip control properties. Spiral point pust the chips ahead, hand taps (straight flute) keep the chips in the flutes (hence the need to keep cleaning them) and spiral flute taps pull the chips back out the flutes.
The 'H' number referes to the amount oversize the pitch diameter of the thread is that the tap cuts.
"G" infront of the "H' means Ground. A GH3 designation is quite standard for mild steel in normal usage, Where post tapping heat treat is needed, a GH5 will often be used. Where scaling of a blind hole is expected, a GH7 can be used. Of course, this should be matched to the mating threaded part.
Bottom, Plug and Taper refer to the length of the lead in on the end ot the tap.
On the parts we do there is little or no clearance at the bottom of the hole. A couple of chips at the bottom of the hole will break the tap on a blind hole by jamming the tap against the chips at the bottom of the hole. We have no problem if we use the right tap for the job. I guess you have the luxury of a lot of extra depth on the bottom of your tapped holes so you can get away with using any tap in the box.
I usually drill twice the depth of the required threads (in soft cast iron, screws require 2x their diameter, so I drill 4x deep). But then I make over-built large stamping dies.
Look up the term "DFM". Your products are designed in a non-ideal manner. Sometimes this cannot be helped, but the tone of your message suggests that Harold's workpieces/experience offends you. This attitude is ass-backwards, and an industry-breaker in the right circumstances. Tap-drill holes *should* be drilled excessively deep to prevent tap breakage in bad circumstances. Why make life difficult?
If you haven't already, you should look into roll-forming taps. They are chipless and excellent for applications like yours.
Allow me to explain my thinking on this, and perhaps you will still disabuse me of my ignorance, but I will graciously stand corrected if that is the case.
I was thinking that since the load on each cutting tooth on a taper tap would be lower then the tool life would be extended. Consider if you will a circumstance where multiple through holes are to be tapped. For the sake of the example let us assume that the plate to be tapped is about 1 diameter thick.
A tap would have to be cycled into the hole until the first full thread cleared the bottom of the hole. If a taper tap had to make 15 complete revolutions to a plug tap making only 7 complete revolutions then the time difference per hole would be a little over double. If the tap were running at 100 rpm, 14 revolutions would be 14/100 of a minute and 30 revolutions would 30/100 of a minute so the net effect on the cycle time would be 10 seconds, but if by having smaller size chips and longer tool life more cycles could be squeezed out of the tap before it needed to be replaced, then those factors might be greater than the cycle time loss.
Also if the quality of the thread is better by taking a smaller bite per revolution, then that may be a factor also.
The castings we machine are supplied with a set of drawings that we must adhere to. On our own fixtures the tapped holes are drilled with more clearance. Almost all the tapped holes are done with a spiral flute helix tap which we find works very well in a cnc machine. Pulling long stringy chips out of a hole can be a pain and also takes time.
We do not have a problem to machine the parts as per the drawings. Tapping , like shoveling snow, you try not to throw the stuff in front of you to reshovel or recut it. I look at machining from a production standpoint. If I can do the tapped hole and clear the chips in one op why would I want to generate a second op and drill deeper than necessary for the tapped hole. It all takes time.
I have only an eighth of an inch clearance in the bottom of the hole. A spiral flute tap produce a thread that will go almost to the bottom of the hole and will produce a deep enough thread to meet the spec.
Come to think of it, neither can I, but deep in my memory I have visions of those spiral fluted taps shattering under less than ideal conditions. That's what they do when they fail, they don't snap off clean, and you can't access them to extract them. My small operation had no Elox (edm of sorts) to burn out broken taps, so it was important that I not break them in costly components. It's the chief reason I started using gun taps that were modified for blind hole tapping. They work surprisingly well, assuming you don't mind the little time it takes to clear the holes of chips. Considering the vast majority of the work I did was tooling (often allowing for a slightly deeper hole, which takes almost no time to achieve), that was a small issue. I can see where high production would demand a different solution, at which time I'd likely change my approach.
As you may understand, I've used those taps, and don't like them. You can see them wind up as they load if you watch carefully----and that always spooks me. Those that are in my shop pretty much remain unused except for special circumstances.
Yep, that's exactly the case, John. The majority of work I did was tooling, where hole depth often was not an issue. As I just commented in another post, I have used those spiral fluted taps when required----I just don't like them, especially in stainless or chrome moly. They break far too easily to suit me.
As I recall there are a family of taps of the same size - both bottom and normal, but then in each there are % of depth in thread - e.g. one cuts a certain width of major diameter and then the next cuts more and the last cuts the rest.
As I recall they are suffix H1, H2 and H3. Look these up on a good supplier site - e.g. MSC that has thousands of taps - Perhaps easing into the work and doing a little more each time is the way to go. Not a high volume production concept, but then maybe 65% threads is all you want...
Chip load plays little to no role in power tap life, particularly with modern cutter geometry. Taps such as the spiral pointed variety cut beautiful threads and will do so for a much longer time than you might expect, particularly when they're run at proper speed with proper lubrication, in a tapping head. Material plays a role, but it always does, regardless of the nature of the tap. If you've ever tapped mild steel with a spiral pointed tap, using the old Tap Magic (1,1,1, trichlor.), you may have noticed the shiny surface of the thread flanks. That material is difficult to machine with a good finish, but those taps accomplish it well. There is nothing to be gained by using a taper tap under normal circumstances, and plenty to lose.
A taper tap would see more miles per thread, so, if anything, it would wear prematurely, especially considering you have to back the tap every quarter turn, and remove it from the hole to clear chips on a regular basis. Because the chip accumulates in the flute, where they ball up if you don't clear them often enough, occasionally resulting in a chipped tooth when you back the tap, and usually a degraded thread. Unless a thread is very shallow in depth (1/4 diameter or less), if you attempt power tapping with hand taps, the chip load usually fills the flute and results in torn threads, or a broken tap. It gets worse. Assuming you're using a taper tap, when you go back in the hole, even by hand, you run the risk of screwing up the threads. One shot tapping with the proper tap, run quickly and withdrawn even faster, has the potential to produce threads of exceedingly high quality.
If you've never tapped with power, none of this may make much sense, Roger. Hand tapping is a whole different world. Taper taps are very desirable in the hand craft world, but, like buggy whips, they've outlived their usefulness in the modern shop.
OK, I got your point. The gun taps are the most rigid. No doubt. I prefer the spiral fluted (bottoming type). On risky operations, I use the set of 2 (or three) hand tapping type.
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