Broken Taps

Many years ago I broke an M4 tap in some ground flat stock. Until a couple of days ago that was the only tap I’d ever broken. I have in the past needed to put an M2.5 thread in steel and stainless steel and not had too much of a problem. I took things slowly and used a tap follower in the mill chuck. I always use tapping compound. I’ve done this operation maybe a dozen times in the past year. I did a while back switch from a 2.05 drill to a 2.1 to make things a bit easier. In the past two days I’ve snapped two taps and to add insult to injury I dropped my one remaining tap on the floor never to be found. I also noticed that one of my tapping drill bits was struggling a bit to get through the stainless rod. I’ve snapped taps in both 6mm and 8mm stainless rod from the same supplier. I’ve manage to tap the 6mm stock before without trouble.

I was thinking about another supplier for 316 stainless rod just in case it’s the material. After the episode with the drill bit yesterday though I’m wondering if my tooling is just worn? I’ve now ordered some more taps and also some 2.2mm drill bits.

I’m wondering if I need special taps for stainless or whether I should just replace my taps regularly. Any thoughts anyone?

Hi Vic,

I don’t know too much about machining SS, but most certainly some grades work harden more than others. Cutting with blunt drills may be hardening it and seeing off your taps?

I don’t often break taps and most of mine are really old (but not threading SS). The last one I broke recently was simply due to me being too blasé. Not starting straight is another recipe for breakage - the tap slowly tries to cut into the side of the drilled hole - and may be something that happens by not taking 100% care.

I don’t buy cheap taps - unless for a ‘use-once’ job. Same with dies - I bought one from the net and it was just not up to the job. Same steel, different threads on each end (imperial and metric), and the old imperial die coped easily but the metric one chipped - too brittle and not tough enough.

edited to add that if the twist drill is not cutting straight for some reason, the tap going in ‘straight’ might be giving it a hard time - similar to cutting at a crooked angle.

Edited By not done it yet on 22/12/2019 13:39:37

I do all my tapping of square stuff on the mill. It’s drilled then tapped without moving the table and I always use a tap follower and tapping compound. The drill hole for the tap I broke the other day went straight through without any excessive force. I’m hoping new tools and going up a drill size will sort the problem. If not I guess I’ll change material supplier.

Some types of SS can be problematic to drill and tap. The tooling needs to be sharp so that it gets a chance to cut. Drills and taps gradually loose their sharp edges and should then be relegated to less demanding metals. Ideally, a selection of drills and taps of a given size should be kept in stock, and swapped around to find the best performing ones for each job. I have tapped a tungsten alloy called Densimet which behaves very much like SS for 2.5mm holes. I bought new Europa taps for hard steels, and they didn't work well. I am in the fortunate position to have a large stock of donated taps to try out and an ancient one finished the holes easily. I think that taps designed for aluminium might work better in work hardening types of metal, but have yet to test the theory.

According to the second chart on this page a 2.2mm drill would give 50% thread and appears to be ok for steel, stainless and iron.

Perhaps a change to a spiral flute or spiral point tap could help? depending if these are 'through' or 'blind' holes.

Ed.

I'm impressed by Volkel brand serial taps for difficult materials. I get mine from Davethetools via E-Bay. 2.5 mm set would be £20. Not cheap but I think worth it. especially as having had the pleasure of shifting 3 broken 2.5 mm ones in sadly grabby aluminium!

Clive

They’re through holes but I’ve ordered both spiral flute and spiral point to try.

What type of cutting compound are you using? I find that the thick stuff, the type with the consistency of grease will hold the chips and clog small taps. Try using a very light spray of something like wd40.

Arc Euro recommend Spiral point taps for through holes (To push the swarf ahead of the Tap) and Spiral Flute taps for blind holes, to help remove swarf from the hole.

They offer Taps down to M2.

HTH

Howard

That’s who I’ve ordered some from Howard.

The first tap I broke was an M10 plug which bottomed, the remarkable thing was that it took very little effort and I was taking things steady. I think the sudden hard stop was the cause as it did not progressively get harder. I am now very wary if I have to use a plug tap and don’t have any possibility to drill deeper to avoid bottoming. That was 46 years ago.

Mike

Mystery solved I believe.

The tap I broke the other day was in a small part that had been turned and then a flat milled on it. The turning went ok and the milling presented no problems with the carbide mill I used.
Today I attempted to make a similar part using the same stainless steel bought specially for the job. The turning went ok but this time I needed to use a slightly bigger mill and the only one I had was HSS which started ok and then had the teeth stripped off it.

The material is supposed to be 316 but clearly isn’t. Whilst looking for something the other day I found some more stock of the same size material almost certainly from the same local supplier. I attempted the job again and completed it without any issues at all.

I’ve cleaned up, chamfered and marked the ends of the remaining problem stock with a red marker. I’ve made a mental note to myself to use it only for turning ...

I tap quite a lot of stainless but restrict myself to 303 and 304 grades which are much easier to tap than 316.

What I have found is that HQS taps from the Tap and Die Co cut stainless very well, just as they are advertised to do. However they seem to be some form of carbon steel and do not cut for long before they go blunt. I have now abandoned these and gone to HSS only.

For tapping I use Trefolex for general purpose tapping, in the possibly mistaken belief that this is what everyone else uses. For screw-cutting in the lathe I use an American product, Tap Magic which works just as well as Trefolex but has the added advantage that being transparent it makes it easier to see what is going on.

I am surprised that people find WD 40 suitable for tapping, I would have thought that its constituents of Paraffin and fish oil would not provide enough lubrication, except for Aluminium.

When I had to work for a living we had to make a small sample part from 316 SS. The shape of the part suggested that plate should be used, fortunately some was found. The material work hardened so fast that it was virtually impossible to machine. Some enquiries were made of the supplier and I was told that plate was made to be weldable rather than machineable. Bar was made to be machineable, and not weldable, and to make it machineable a small measure of (I think) sulphur was added. So some bar was obtained and found to machine beautifully.

This was up here in Australia, so it may only apply here, although apparently Australia hasn't made stainless steel for decades, or so I have been told.

Happy New Year and cheers!!

Bill

Or anything else. And now they are wondering why the economy is slowing down. Gee whiz, we can't base a nation on making cafe lattes for each other? Really?

I bought some stainless plate for a job many years ago and that work hardened very quickly. To finish the job I had to heat it up to red heat and let it cool slowly.

It’s strange about the stainless rod I bought locally. All the stuff I’ve bought from eBay labelled 316 recently has machined beautifully.

I tend to use either trefolex, rocol or ct90 depending on the job and material type. The suggestion to use wd40 is more about flushing away the chips than lubrication. A blast with an air compressor usually works too but you will flush away the cutting fluid at the same time. 

Edited By Hollowpoint on 29/12/2019 12:16:58