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Why did the tap stick?

Not a water tap, that is

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Chris Gill22/09/2018 20:05:42
40 forum posts
15 photos

Hi all

I was trying to tap a through hole with a 3/8" UNF tap but it got stuck half way.

The material was a bit uncertain but probably EN8 or similar (based on spark tests). The tap was an HSS taper tap, the pilot hole was 8.6mm (about 70%) and the hole was supposed to be about 20mm right through. But after around 11 or 12 mm of reasonably easy progress it stuck and was difficult to withdraw.

I'd already backed it out and cleaned it once and no end of cleaning made any difference. I tried opening the pilot hole to 8.7mm but it didn't help much.

My thread gauges agree the tap is 24 TPI (though they aren't as long as the tap) and a 3/8" bolt went into the hole easily to a point. I wondered if I'd managed to work harden the steel but it turned, drilled, knurled and parted quite nicely.

When I set the tap against a bit of threaded bar I discovered that there was a tiny discrepancy that looks like the tap is fractionally less than 24 TPI.

To my mind, if the tap isn't quite right it should go through the hole and the bolt should get stuck.

The tap is part of a set I bought years ago (UNC, UNF, MC, a few MF and others). This might be the first time out for this one but the others have been brilliant.

All I can think is that the pitch changes slightly along the length of the tap but is that feasible? Or have I missed something blindingly obvious?

Thanks in advance ...

Rik Shaw22/09/2018 20:13:36
1313 forum posts
352 photos

My guess is that you have a blunt tap.


not done it yet22/09/2018 20:37:07
4123 forum posts
15 photos

That, or the hole is not straight.

Mick B122/09/2018 20:59:05
1422 forum posts
76 photos

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.

Chris Gill22/09/2018 21:17:20
40 forum posts
15 photos

Aha! Didn't think of the blunt tap option - just assumed it was good. Sadly my grinder is just the off-hand type with wheels that tend to pummel things into submission.

I'm really hoping the 20mm-deep hole is straight, otherwise I've got serious problems. Though I can see the logic of that with much deeper holes.

Thanks all

Bazyle22/09/2018 22:10:20
4977 forum posts
198 photos

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.

Marcus Bowman22/09/2018 22:36:43
162 forum posts

Did you use a tap lubricant? I find Trefolex paste is best, but Dormer tapping fluid is fairly good. Or just cutting oil. I agree with Bazyle re: withdrawing more frequently to clear the flutes, as well as the obvious slight reverse turn every 1 to 1.5 turns.

If the tap is blunt, you would need to take some care to preserve the hook angle when resharpening. I see lots of youTube videos showing how to sharpen taps, but I have yet to see one that does it properly.

In my experience, EN8 should tap without the difficulty you describe.

Andrew Johnston22/09/2018 22:42:12
5172 forum posts
599 photos

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.


Muzzer22/09/2018 23:15:23
2904 forum posts
448 photos

Backing off every turn or so requires the "other" side of the flutes to be sharp. It may be that the main cutting edges are sharp - but not the ones used when reversing the tap.


Nicholas Farr23/09/2018 08:04:40
2107 forum posts
1017 photos

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.

XD 35123/09/2018 08:15:22
1409 forum posts
1 photos

Also measure the drill bit you used - would't be the first time i have seen undersize or incorrectly marked drill bits!

JasonB23/09/2018 08:19:06
17259 forum posts
1858 photos
1 articles

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.

Kettrinboy23/09/2018 08:51:29
77 forum posts
39 photos

I,d go with what others have said and always use a quality lubricant ,it really does make all the difference , especially on the small BA taps, you say the steel is maybe EN8 but if its actually something else its quite feasible for a less than sharp tap to jam up in a deeper hole, if you keep having problems try a different tap , ive got several taps of each size and one usually cuts better than the rest .

regards Geoff

Mick B123/09/2018 09:34:18
1422 forum posts
76 photos
Posted by Marcus Bowman on 22/09/2018 22:36:43:


If the tap is blunt, you would need to take some care to preserve the hook angle when resharpening. I see lots of youTube videos showing how to sharpen taps, but I have yet to see one that does it properly.


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.


Vic23/09/2018 09:53:25
2427 forum posts
12 photos
Posted by Bazyle on 22/09/2018 22:10:20:

You didn't say if it is a taper tap.

Yes he did.

Posted by Chris Gill on 22/09/2018 20:05:42:

Hi all

I was trying to tap a through hole with a 3/8" UNF tap but it got stuck half way.

The material was a bit uncertain but probably EN8 or similar (based on spark tests). The tap was an HSS taper tap, the pilot hole was 8.6mm (about 70%) and the hole was supposed to be about 20mm right through. But after around 11 or 12 mm of reasonably easy progress it stuck and was difficult to withdraw.

not done it yet23/09/2018 11:31:16
4123 forum posts
15 photos

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.

Hopper23/09/2018 11:48:25
4110 forum posts
89 photos

Lube up your plug tap, or second tap if you have one, and run it down the hole as far as it will go, with the usual backing off every turn etc. Then try the taper tap again. Sometimes with deep holes you have to work your way down like this.

SillyOldDuffer23/09/2018 11:54:10
5294 forum posts
1086 photos

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?



Edit: typos, typos...


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

pgk pgk23/09/2018 12:00:01
1611 forum posts
286 photos

..or a four foot scaflold pole over the end of each tap handle....


Ian S C23/09/2018 12:45:19
7468 forum posts
230 photos

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

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