Carbon Steel vs HSS Taps & Dies

[HOWARDTParticipant @howardt]

I mainly use metric HSS taps but for a recent Stuart build decided to use the ba screws supplied. For this I bought sets of carbon taps and the correct diameter drills at the same time. For the ME threads I just bought the plug tap as the holes for these were all through holes. All worked out well, minimum size was 8 BA and most of the holes were in cast iron, so no real torque required to create the threads.

[SillyOldDufferModerator @sillyoldduffer]

I don't see much difference between my inexpensive HSS and Carbon Steel taps. But various factors are in play:

• I cut low tens of threads at hand speeds rather than hundreds as fast as possible.
• Unless strength matters I drill tap-holes deliberately oversize to reduce thread engagement. This reduces wear and stress on taps and dies significantly.
• Taps and dies are always held straight in lathe, mill or tap-stand so they can't go in at an angle: wonky hand threading breaks taps in a blink.
• Whenever possible I pre-form larger threads by lathe cutting them. This reduces the amount of work done by dies enormously.
• I always use cutting fluid
• I take great care to ensure taps and dies are never choked by swarf
• Different taps and dies are kept for brass and another for steel because steel soon blunts taps enough to cause trouble on Brass.
• Most tapped holes are made in Leaded Mild-Steel, Aluminium Alloy and Brass.
• Ordinary mild-steel and above are treated with extra care.
• Work-hardening stainless steel gets a guaranteed sharp tap, new if I'm in any doubt.
• Bronze tap holes are drilled slightly oversize because they tend to close up.

Though I can't find an authoritative reference I believe Tool Steel taps are sharper than HSS. Unfortunately, because tool steel is easier to sharpen than HSS because it's softer, so it's also much easier to blunt. HSS taps stay sharp longer.

Dave

[Clive Brown 1Participant @clivebrown1]

I've always preferred HSS taps, if I can afford them, as I understand that they are usually ground to shape. My belief is that this should give a better thread form and also allows back relief to give better cutting. My stock is about 50/50.

[Dave HalfordParticipant @davehalford22513]

Zombie thread

[DiogenesIIParticipant @diogenesii]

It's in the back of my mind that the relief on a lot of HSS taps is ground differently from that of CS taps as they are more likley to be used on Stainless steel and more 'difficult' materials – I think there's often a full-profile 'land' to give strength, but it can cause chips to jam – I only know this because I couldn't understand how I managed to bust two HSS ones in short order, both breaking whilst being being backed-out..

[Bill PudneyParticipant @billpudney37759]

If the OP is planning to tap threads in al. alloy, I would suggest that he examine a typical carbon steel tap under a loupe. The problem with a cut/gashed thread can then be seen, huge (relatively) burrs, which will cut a most peculiar thread, especially in the smaller sizes. No doubt cutting a thread in some of the harder materials would probably deburr the tap or die.

I ALWAYS tap by hand power, preferably in the mill or lathe to maintain alignment. As others have said, taps going in on the skew WILL BREAK. Over the years I have tapped certainly hundreds, possibly thousands of holes, always under M6, usually under M3. I cannot remember the last time I broke a tap, which obviously means that I should tap my next thread in a piece of scrap!!

Personally I never buy sets, only ever the taps/dies needed and I try and buy HSS, with a ground thread, and with a quality name on the thing.

best of luck

cheers

Bill

[Nigel Graham 2Participant @nigelgraham2]

Regarding drilling the hole above nominal tapping size, Tracy Tools' charts state having been calculated to give 60-7-% thread engagement but also suggest if this is not acceptable, "the user may wish to experiment" to obtain the required result.

Comparing a few M-series and inch-size threads with the BSI-based Zeus Book (mine is the 1979 Metric Revision) shows slight differences. Tracy Tools offers only one tapping-hole diameter, in mm, for all threads, and rounds up to 1 decimal place those BSI (hence Zeus) specify to 2 decimal places. That's probably for the practical reason that most drills commercially sold are in 1dp increments.

The BSI "Alternatives" to "Recommended" are rounded down, not up, giving more metal for the tap to remove.

In practice this would probably only matter if using some very exotic material or working to very tight tolerances; conditions unlikely to be met by most of us here, although some may have done so professionally.

Also of course, unless using a very high-quality new or accurately reground drill, in a good machine-tool, the hole won't be under but may well be slightly over, its drill's stated size. Also the normal commercial fastenings' crest diameters are usually slightly below nominal.

Put it this way: I have never encountered problems using either source of information, but I do use a size up on theirs for tapping something like stainless-steel. (0.1mm = 0.004" , as near as makes no odds.)

[Mike PooleParticipant @mikepoole82104]

HSS are ground thread as I think it would be difficult to make them any other way, this has the advantage of being able to produce a very accurate thread but that will depend on the maker. Carbon steel can make very good taps and dies but here you need to buy good quality and many carbon steel taps are poor quality as the makers save every penny they can on material and machining. If you are going to produce a HSS tap then you need to invest in the machines to make them and a higher cost material so it is probable that a budget HSS tap will be better than a budget carbon steel item. With the demise of most of the old brands that could be relied on to be a good quality item made in carbon steel and HSS being the material used by the well regarded brands today the idea that carbon steel is inferior has prevailed. The picture is becoming a bit cloudy now as some HSS taps are not as good as they should be and some carbon steel are excellent. Shopping for HSS or CS at the extreme economy end of the market is likely to be disappointing but some people will strike lucky and get a bargain.

Mike

[Neil LickfoldParticipant @neillickfold44316]

With Ba taps I go a slightly bigger drill size. Like 4ba, I drill 3.1mm instead of 3.0 in the drill charts. The thread is still very effective, but is a lot easier to tap, especially into steels. HC steel taps work in the softer materials. But harder materials that are over 20 Rc like 4340, or 4140, P20, etc , some stainless steels , those materials are very difficult with HC taps. Even with HSS taps I go over the normal chart sizes. Interestingly, the Mazak milling machine tapping drill sizes are quite a bit bigger than the normal charts as well.

Have fun in making the parts for the new project you are under taking.

Neil

[Laurie WParticipant @lauriew39626]

Personally I much prefer HSS in BA sizes, and have found that the best British makes can be found secondhand with a bit of effort. Perhaps I have been lucky but a lot of the ones I acquired in the 0BA – 16BA range seem to have had very little if any previous use. That said, my small pile of 2BA ones seem to have had the hardest previous lives. This approach has worked much better for me than when I did try buying the more economy end new ones in either HSS or carbon steel. Of course brand new Presto etc work very nicely too…..

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