My guess is that you have a blunt tap.

Rik

If you've a grinding wheel with a good corner between side and face you can usually sharpen a tap by lightly touching the side of the wheel to the cutting edge, but be careful. Take a close look at it first to see if it's chipped.

You didn't say if it is a taper tap. If it is then a good 10mm of it is tapered so all of that length is cutting simultaneously as it gets deeper in. Also withdrawing it only once in that length is hardly enough. In a similar situation I have followed in with second and plug every 3-4 turns to ensure that the body of the hole is fully cut and the taper tap is only cutting a short bit of new thread at a time.

A quality taper tap should be able to thread the hole without needing to use second and plug taps. I see some issues. For a start a pilot hole of 8.6mm is nearer to 80% thread depth than 70%. Hand taps tend to extrude the thread form slightly effectively reducing the size of the pilot hole. That may well be your problem. I'd have drilled the pilot hole 8.8mm. When hand tapping I break the swarf by turning the tap back every turn or so. A 3/8" UNF thread isn't big, but some goo may help the threading process. I use Rocol RTD; makes a big difference to the torque needed to drive the tap.

Andrew

Hi, if you have drilled the hole square to the material but haven't tapped it square you will have the problem you have described. If you did tap it square to the material but the hole was not drilled square you will have the same problem. Regardless of what angle the hole is drilled, the tap should follow the hole to reduce the risk of breakage or binding up.

Regards Nick.

About 11mm in is where the taper on your taper tap(you did say so) will end and is where the tap starts to take off the most material, try going in with your second tap and then back in with the taper so that it will then have a bit less to cut.

It should not be a problem with a 3/8" tap but if it is a bit worn, gummy steel or you are not that strong the load may be a bit much.

I've never looked at those YouTubes, and I'm sure you're right about the difficulty of preserving the correct rake angle – but nevertheless a sharp tap is a helluva lot better than a blunt one, even if that angle ain't exactly what it should be.

I don’t think the OP has told us if this was simply tapped free-hand or if the tap was lined up properly. No real clue as to whether this is an isolated case or if the tap operates in the same way every time it is attempted to tap a similar diameter hole.

If the tap was started crooked, it will try to cut into the hole wall on one side more than the other. A dozen threads could be about the point when the cut becomes too much. Perhaps the Op should be looking down the hole, from the other end, to see if the tap is perfectly central when it jammed or off-centre.

Not straight will mean only a finite depth of thread is possible before the tap breaks.

Quote from AzoMaterials, my bold:

'Most steels with appreciable alloy content possess a complex crystal structure resulting in numerous potential slip planes and intersection points, consequently most engineering steels are highly susceptible to work hardening.' The link also describes the mechanism that causes work hardening.

We all know some stainless steels work harden if you look at them, but it may be news that mild-steel can do it too, albeit less aggressively.

Quite a common theme on the forum is the alleged inferiority of modern steel, particularly if it is made in the Far East. One complaint is the sudden discovery of a mysterious hard spot while working the metal that ruins the tool. It is often blamed on an unmelted ball-bearing or similar scrap nastiness surviving in the mix. Obviously orientals make steel by stirring old bicycles in a pot. Actually, given what really happens inside a furnace, unmelted bits are very unlikely even if the steelworks is completely naff. If it is a genuine inclusion, it's rather more likely to be eroded furnace lining than unmelted scrap. Another potential cause of hard spots in steel is what happens after it comes out of the furnace, in particular rolling, heat treatment and cutting, These are more likely to harden steel throughout or at the edges only rather than cause islands.

A curious feature of quality problems in steel is that they seem much more likely to be reported by small workshops than major users. (If modern steel really was of shoddy quality, it would soon bring mass production like car making to a dead stop, and that's rarely reported.)

I suspect that work hardening may be the root cause of Chris's problem and other similar difficulties. I suggest work-hardening is far more likely to happen to amateurs than professionals because of the type of work we do. In production heavy machines are used to remove metal at optimum rates, with suitable cooling, lubrication and swarf removal. Also tools are switched as soon as they stop cutting efficiently. In these conditions work hardening during a cut is unlikely. But, by it's nature, small-scale work is much more likely to cause work hardening. Our machines can't be optimised as they would be in industry, they tend not to be powerful, and we do a lot of hand work. As slow cutting is more likely to rub, lubrication indifferent, swarf removal slow, and peak overloading caused by jerky hand-tools, I think work hardening of metal in small workshops may be rather common, particularly so in amateur operations where blunt tools are tolerated for longer to keep costs down.

I suggest Chris' problem is:

1. Drilling the tapping hole went well, though the drill may have blunted as it went deeper causing the metal to progressively harden with depth.
2. Cutting with the tap got more difficult with depth because the tap encountered harder metal, does more work, and at the same time became more choked with swarf. These effects all tend to blunt the tap.
3. As a blunt tap is forced into the metal rather than cutting it, work hardening becomes worse, thus further blunting the tap until the job jams.

I think work hardening is less likely to occur on a lathe because the operator is warned by chatter and poor finish that the tool needs sharpening. Increasing bluntness is less obvious whilst sawing, drilling and milling; perhaps that's why 'hard spots' tend to be found during those operations. They're not inclusions, it's work-hardening.

Proving the hypothesis would require repeating the job several times with a new tap and a microscope. Comparing photographs of the state of the tap would show how quickly it was blunting (or not!) Bit stuck on the work-hardening part though – can anyone suggest a way of proving that work hardening has occurred as a result of cutting metal?

Dave

Edit: typos, typos…

Edited By SillyOldDuffer on 23/09/2018 11:56:36

You say that your set of taps and dies has UN and metric tools, are you sure you are not trying to put a 10 mm tap through, the thread is nearly 24 tpi, but the diameter is greater.

Ian S C

Dare I suggest a serial tap might be a solution to this situation

Neil

Serial taps are a good idea, but I would offer the observation that coated serial taps I have bought recently are definitely not as sharp as uncoated HSS taps. In fact, they cut as though they are slightly dull. One might argue that the coating does dull the cutting edges. So that might give rise to the same problem.

My coated serial taps have been consigned to the wish-I-hadn't-bothered bin. My uncoated serial taps continue to give sterling service.