Sigh, practicing a skill I would rather not need

I have just snapped another M5 tap on my current project.

<Sigh>

Now I get to practice tap removal again.

That is a skill I wish I didn’t get the opportunities to practice.

Dave

Noooooo.

Are you tapping a difficult material or just suffering from a wobble that snaps the tap? A difficult material can be made a bit easier by drilling for a less than full thread depth. To combat the wobbles it can help to use a drilling machine with a spring loaded centre to keep the tap true to the work, a mill or the lathe can also be used to provide the same service or a universal pillar tool if you have one.

Mike

The trouble is that good taps come in different guises, to optimise different materials. I had to tap a 3mm hole in Densimet which is a Tungsten Cobalt alloy and bought a green ring tap intended for hard material. It did not want to do the job, despite being new and industrial quality. I looked through the box of small metric taps and found one that tapped the hole easily and it looked very second hand. I might have been better off getting a tap intended for aluminium.

For hand tapping, you could make a tap guide that keeps the tap square to the hole, just a piece of metal with a hole drilled using a drill press or mill that the tap can just pass through.

I have found that you are more likely to break a tap if it is blunt. I use good quality British made HSS (Presto, Dormer etc) but even these get blunt eventually.

As for carbon steel jobbies - I would not give them house room even though some swear by them.

Rik

There are many reasons for tap breakage [here are just a few of them], but the most over-arching reason in hand-tapping is insensitivity on the part of the operator to what the tap is actually "going through" at all times during its use.

You have to develop a feel for the tap in the material and know whether the tap is liking or disliking what you are doing with it. Invaluable practice can be obtained without great expense by tapping many consecutive deep blind holes with small taps on a range of scrap pieces of material.

HSS from a reputable source. Nearly new, probably done under a dozen holes.

Just one of those things I guess, no difference in the procedure from how I’d normally work.

Some pics:

Carbide D bit:

Peck drill 1 thou at a time…

Then pick out the remains:

Rescued, thread ok and part usable.
It’s good that this is possible but a trick I would rather not need…

Dave

Hi

Where did your carbide D bit come from ? (asking for a friend !)

Cheers

Peter

I made it from a snapped PCB drill. I have a Deckel S0 grinder.

Dave

Is it just me or do the flutes look more than a little uneven?

My take on this caper -

1. I never buy 2 nd hand taps ( or drills) Only new, sharper.

2 Always use methods of guidance as outlined above by others.

3 Try to avoid tapping blind holes, only through holes.

4. If blind hole tapping unavoidable, don't use grease or any kind of tapping sludge, however good it's supposed to be. Only use thin oil. Much easier to wash out swarf from the bottom of the hole with say, paraffin.

5. If in any doubt, use next larger diameter tapping drill.

6. Consult a reputable drill and tap guide table and don't use the smallest drill sizes, look at the percentage clearances. Not normally necessary to go beyond 70-75%. Tight clearances put a lot of extra torque strain on the tap for very little gain in thread stripping resistance.

6. Frequently remove tap and clear out swarf then start again. Smaller the diameter, the more important it is, to avoid swarf compaction at the bottom of the hole. Compaction could break tap and certainly make it harder to remove said swarf.

Hope my notes prove useful to the OP and anyone else in difficulty.

Edited By DMB on 17/04/2022 22:17:36

Looks like an Acme tap to me. Good list of advise from my almost name-sake

Dennis

I was glad to have solid carbide drills when I snapped a 1/4" tap in steel, it was not very sharp and the hole was a bit small. The carbide stub drill made removing the tap look easy, although I was rather apprehensive of the outcome.

I agree with DMR, the remains of the tap do look strange.

Edited By old mart on 17/04/2022 23:00:52

The thread on the remaining bits of the tap looks wrecked.

When a tap is going to stick it, goes stiff, then springy, then bang. The trick is to let the pressure off when it goes stiff and try to back it out. Springy means it is a 50/50 chance of me having to reach for the plasma cutter to blow it out the busted bits.

Dave,

How small can you go with the plasma?

Without full details of what is ‘normal practice when tapping’, for the OP, it is difficult to give specific improvement suggestions.

I feel a lot of people only buy one tap of each size, when a set of three would be much less likely to end up with breakages. Indeed, a lot (most?) of cheap sets on the market only include single taps. If it is a through hole in relatively thin material, a taper tap may well be adequate, but certainly for deep tapping, starting with a second tap, at high thread engagement, is not such good practice.

Alignment, in deeper tappings is, very important as one side of the hole will be equivalent to tapping with increasingly less than the proper tapping size until the tap is cutting 100% of its depth on one side while trying to bend the tap as it cuts.

M6 (in that section ) would be my choice, if at all practicable.

Whilst I mostly just wanted to moan to a group of people who would understand there are a lot of useful

comments here.

The holes are thru, drilled with the tapping drill I always use. Whilst I only need a couple of diameters of engagement the back of the hole can poke through. I try and avoid blind holes if possible it just adds to the pucker factor…

The taps are serial ones - so the taper tap is actually smaller diameter, then the 2nd and then the thread is fully formed by the last tap. I have been using this style for metric holes for a long time, purchased from Arc.
That’s one of the reasons the thread look shot - they are not full depth.

The green goo is Trefolex. I find it helps loads.

I would have used M6, but the linear rails are designed for M5.

I managed to get the rest tapped using the older set of the same taps.

Dave

Hi Dave,

You are not the only one that has broken a M5 serial tap. I have never seen a carbide D-bit used to remove the broken tap, only a tool that was triangular in shape.

Thor

Thor cannot understand a word in the link you posted but they look like stellite drills, great for drilling anything hard but have to be run hot, no coolant. Unfortunately stellite is now difficult to get hold of.

Looks very like the job I did building my CNC router. Lots of M5 holes in steel tube. The tube was nominally 3mm but backed inside with 4mm strip for more thread depth. I used a spiral-point machine tap in a cordless drill, Trefolex, and straight through in one pass. Can't remember how many holes now but there were two rails 1800mm long so quite a few. Was I lucky? Have to say that in general I use machine taps these days even tapping by hand.

Sorry Baz, I used an example from a Swedish supplier I use, just to show the shape. They are used to drill out broken taps much the same way the D-bit Dave used. They are made from tungsten carbide and ned 1500 to 3500 RPM and a steady feed.

Thor