Weird situation when tapping steel

Something I've never come across before, and am at a loss to explain.

Firstly, my materials stock is varied having been accumulated over many years from various sources so machining results have often been .' variable ' but I live with it, as nothing I do is that critical. I will buy the right stuff if necessary.

A few days ago I knocked up a simple arbor out of stock 20mm steel bar, this had an 11mm OD spigot to hold the gear I was planning to cut on my mill. Into the body I needed an M6 thread about 35mm deep. After turning the body and spigot I drilled the hole with an M5.0 bit (would have preferred a 5.2 but I had lost mine) but - I have used M5.0 before numerous times without issue. Started the thread while still in the chuck and was going ok until about 10 mm in started to feel tight and was getting difficult to turn in the lathe.

Took it all out of the lathe and into the bench vice, still difficult. at about 15 mm it was squeeking badly. Plenty of oil didn't improve things. Gave up at about 20mm. Difficult to remove the tap. Then followed down as far as I dared with a plug tap but again it was jamming so tight I was sure it was gong to break, so stopped.

A ' hard spot ' in the material? Had it work hardened when cutting the spigot (high carbon steel instead of mild)?

Got the blow torch on it in the hearth and annealed it giving it a long ' soak '. When done, put a drill down it again ( an odd imperial one I dug out that was about 5.1mm) and that went down full the 35mm no problem - problem solved?

NO. Tried tapping again and exactly the same, jams about 20mm in. Checked the taps, look fine. Did a spark test on the grinder of the bar it came from, looked pretty much what I would expect from MS.

Anyone any ideas what is going on? - this has really irritated me - it's not the end of the world, the work isn't that important in itself just gets up my nose when I cannot reason out a problem.

Thanks, Regards Mike

You need a bigger hole IMO

A possibility exists that the tap aint right, thread blunt/broke/bent, thickness etc

If it drills ok it should tap ok

Unless you need 30mm of thread drill out the top half with a 6mm

If the job "needs to be right" then you need to get the right kit first

Edited By Ady1 on 16/04/2023 10:51:59

Could be any number of things.

Worn/damaged tap

Out of spec drill bit size

Chips not clearing

Misalignment of tap in hole

I've tapped hundreds of holes over the years, and generally know what to look out for. The tapping drill is listed as 5.0 or 5.2 depending where you look! The hole I made was the right size; as noted originally the taps look perfectly ok (close inspection not just a quick look) and aligned / started in the lathe. Lube used and chips cleared regularly.

Yes, doesn't seem to make a lot of sense!

Maybe drill wander has given you a curved hole round a harder inclusion. Sometimes the scrap doesn't mix so well. I have some 3" u channel that has hard spots that won't drill.

As per Hollowpoint could be all sorts of things. But my money is on too small a drill size. For M6 in steel I drill 5.3mm. Hand taps tend to extrude the material slightly. That, plus too small a pilot hole, and a 'gummy' steel inevitably leads to a tap jamming.

Andrew

Question: Why would you need a 35mm thread depth at 6mm diameter? Seems pointless, unless there is an exceptionally good reason for it.

There is one other thing that I've experienced.

I presume that you drilled the hole in the lathe? Very occasionally when drilling in the lathe the drill bit will deflect or "wander" a small amount and be pulled ever so slightly out of true, leaving a hole that isn't square to the part.

This is most common when a part held in a 3 jaw chuck is centre drilled, then released from the chuck for whatever reason before being re tightened and then drilled.

The tap won't then follow the wonky hole because it is much more rigid.

Edited By Hollowpoint on 16/04/2023 11:12:16

5mm is the right drill for M6, that's what I use 99% of the time. I might use 5.1 on difficult materials.

The hole could be crocked, a dull or poorly centered drill can wander off significantly.

One thing that has just crossed my mind that may relevant - I did notice that when tuirning the main body & spigot was what a really nice finish I got! I had just earlier changed the inserts and put it down to that, but perhaps as Andrew says it was actually indicating a ' gummy ' material ? Who knows.

Perhaps I'll just add it to my ' experience ' box. Mark the remainder of the bar to indicate it's odd properties if I pick it up again. I'll also replace my tapping drills with 5.2 or 5.3 just to make life a bit easier, even though the vast majority of sources indicate 5.0 as the correct size.

Thanks for the comments guys. Mike

Errr, M6 X 35mm deep - you don't need anything like that amount of thread, but the clue lies a little further down.

The drill " went down full the 35mm no problem ".

I suspect you were fighting the drill-point cone if you measured the drilling from tip not full land, and the tap itself. Was the tap significantly shorter than the hole depth? If so were you coming up too close to the top of its own thread?

I take it you started with a taper-tap and it is the second tap that is binding. If so, the latter may be running into the un-tapped region left by the first tap's taper.

.

Your say you tried annealing the work so that meant removing it from the lathe and replacing it. Hollowpoint has addressed the problem that can cause. My way round that would be to hold the part-made component in the bench-vice and finish tapping it by hand... very carefully!

.

Regarding the steel, using "pre-loved" material rather than known specifications is always fraught (I use enough of it!) but I don't think a "hard spot" very likely even if using some really tatty stuff intended only for rough work. However if mild-steel, annealing it probably won't do anything anyway

.

If you can run an M6 screw down the thread to more than 12 or 15mm you've done more than you need. It won't grip the gear blank any more tightly.

Perfectly valid points Nigel. The amount of thread I achieved anyway was perfectly adequate for the task, as you indicted. More just irritating to determine what was going on!

I always tend to overkill things and try belt & braces approaches, the wife often says that in a hundred years or whenever they decide to knock my house down, all my fittings and fittings will still remain upright as I used so many bolts ,screws glue etc

regards

Surely oil is not the thing to use for tapping, use a cutting compound. This may of course have not been the reason for jamming.

Nigel is spot on I think, but it begs the question "how deep should a tap be expected to go?" .

Not far apparently! Between one diameter and 3 diameters, and most ordinary taps are probably designed for about 2x diameter. Tapping M6 deeper than 12mm is starting to push one's luck. 18mm is the conventional upper limit, so making a 35mm deep M6 thread is definitely a walk on the wild side! Not impossible, but likely to run into trouble.

I guess tap cutting depth is limited by friction. When a tap starts cutting at the top of a hole, almost all the turning energy is available for cutting. As the tap goes deeper the flanks engage and require ever more force to turn as well as cut. Friction will be much higher if the hole is too small, and in this case 5mm and 5.1mm are both undersize. My book suggests 5.5mm for M6 x 0.5 and 5.25mm for M6 x 0.75

So a combination of narrow hole, tapping much too deeply, and perhaps work-hardening because the tap is grinding rather than cutting at the bottom of the hole, and forcing the sides.

Extra deep threads add nothing to strength. After about 4 or 5 turns the female is stronger than the bolt.

Dave

Edited By SillyOldDuffer on 16/04/2023 12:34:45

Are you sure it was not a ferritic stainless steel. They can look like mild steel and be magnetic but tough to work compared to the real thing.

Martin C

I had that very same problem with a bit of hot-rolled, maybe this was from the same batch.

I assumed medical-grade radioactive cobalt from a scrap radiotherapy machine that had gotten into the mix.

The Americans usually add those to re-bar so entire buildings have to be torn down when they find out.

@Duffer: Your data for M6x0,5 and M6x0,75 is right. If you do the math it's the major diameter minus the pitch. A metric thread with no pitch designation means "standard coarse thread" which for M6 is 1mm pitch. So the correct drill diameter for M6 is 5.0mm.

The depth alone is unlikely to be the main issue, I've tapped to the full length of the tap many a time without any issues. But if the hole is crooked it's going to bind up as you go deeper.

I think as SOD mentioned friction can be a problem when the back half of the tap has no cutting to do it might be a thou less worn so just wedges itself in, with the V shape providing more surface area to grab.
I think I recall at work where a deep screw hole was genuinely needed (yes after the foreman had had his rant at the DO) they ground off a tap to leave only half a dozen turns of cutter. In fact can't you buy that sort now for CNC to avoid the risk of jamming?

My largest M6 (M6x1) hand tap is 21 mm. The machine tap is 23 mm and the dril/tap bit is 19 mm. I would drill M6x1 5 mm for steel and 5.1 mm for stainless. Always use cutting oil.

The machine tap requires the lowest tapping force because is only has 3 starter threads.

When the tapping force increases, the tap will twist and that will increase the tapping force even more.

As I'm sure Bazyle knows, but for the sake of others, the thread mill isn't a tap and is programmed to move around the hole synchronised with its movement downwards forming the pitch.

But he is of course correct about short machine taps .

Bill