Tubal Cain broke my taps...

Looking for an 8BA tap, I noticed that the only two I have are both broken with the end touched up. That's odd I thought the hole looks rather small. Checked and, yes number 50 is the drill Tubal Cain recommends in his Model Engineer's Handbook.

It seemed to be rather tight, even in CI, even allowing for the re-worked end of the tap. looked closely at the table - why is there a much smaller gap between tapping drills for 8 and 9 BA than 7 and 8?

Just looked in the Handbook and the size he recommends for 8BA is Number 50 or 1.85mm.

But 1.85mm is number 49, number 50 is 1.78mm. Only 4% smaller, but presumably enough to explain why I break more 8BA taps than any other size

So, never take anything for granted, even when it comes from the top!

Hopefully the last seven 8BA holes will tap a bit easier.

Neil

I came across this years ago when making a fishing reel. I followed the Zeus book, which says 1.8mm, which is just 0.02mm off No.50. I used No.50. After breaking two 8BA taps, I switched to No.49 (1.85mm) with total success. Surprising how a few hundredths of a mil makes so much difference. I was working with a rather "clingy" sample of aluminium - didn't know the spec as it was a scrapyard find. Interesting that Zeus gives 1.75mm as an alternative. That seems impossibly tight.

Hope I've got all that right - these days I need a magnifying glass to read the Zeus book!

1.8mm seems to work for me

In Tubal Cain's book Drills, Taps and Dies (Workshop Practice Series No 12) he states clearly tapping drill No 49 for 8BA.

I know he was a remarkable man but did not realise he still operated from beyond the grave.....

I would suggest measuring the drill especially on smaller sizes. These days they tend to be under where ever they come from.

As to tap drill size the suggestion I received is still the one I stick too - look at the tables and think just in case.

Taps seem to be over size too but checking some metric serials in smaller sizes I bought recently they can be a touch under too. The aim seems to meet some mid tolerance spec. I'm not sure that Zeus and other sources were based on what seems to be modern practice. Actually I feel that the clearance on a screw they give is way more than it need be.

John

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I've put my own tapping table together based on Tubal Cain's recommendations in "Drills Taps and Dies" using only metric drills in 0.1mm steps, since this is what I have. I went for 60-65% engagement. 8BA though is awkward since 1.8mm gives 75% engagement and 1.9mm is somewhat less than 60%. I plump for 1.9mm. I've just tapped these 16 holes 8BA in ally and an 8BA screw doesn't feel loose.

Cheers,

Rod

Mike:

In fairness to Zeus, the booklet has become easier to read over the years.

BTW, I bought your book and am looking forward to reading it. May be a subsequent edition will modify the advice that the Zeus booklet has the best set of tables?

I have occasionally wished that someone would market a set of stub drills with, say, three sizes for each thread so that the important question of percentage thread depth is automatically catered for.

ega, I am hugely flattered that you have found a copy of my book, which came out in two editions. The first in, 1987, was entitled "The Complete Guide to Metalwork" - the publisher's choice, which made me a bit cross because it wasn't. The second edition, which came out a couple of years later, carried some minor corrections and - thank goodness - was re-titled "Metalworking - a Manual of Techniques." Alas, there is no chance of a third edition - I'm now 75 and a stroke survivor, and I don't know an artist with Andrew Mackintosh's skills. All I can say is that it was great fun writing the first edition. And, perhaps, I should treat myself to a new Zeus book - mine was purchased around 1975, is much thumb-printed, and stinks of soluble oil!

A reasonably priced set of stub drill in 0.1mm steps would be a good start. I've looked everywhere for a set, even Poundland

Perhaps we should lobby Ketan.

Rod

I have bought the small size select boxes with several of each size starting at 0.4mm and going up to 3.2mm by 0.1mm. Off ebay. Not the greatest drills but ok. They are in a yellow tray in a clear box. If the hole size is important I always measure them.

When they get broken it's down to me and always in an electric hand drill that has rather a lot of torque. Only 3/8" fitted with a 1/2" chuck but made by Elu before B&D started making the stuff elsewhere and not so well.

I bought them some time ago and they were cheap. When a size gets used up I'll just order more from somewhere like drillservices.

I think in terms of thread engagement too Rod and being well clear of root rads which is what causes the problems. Goes back to training.

John

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They are still about but slightly different selection

**LINK**

There is a hong kong seller offering 10's in all sizes in a tube as well. Bright so might even be ground end. Going on bright ones from super markets etc though I think many are too hard along the shank.

John

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Well No. 49 worked much better, so I'm amending my copy of TC's handbook and laminated card in my workshop.

Neil

hi Neil,

i had Tubal Cain's tapping tables marked up with many alterations on enlarged photocopied sheets. they are IMHO very poor.

they were hanging off a bulldog clip on my workshop wall and got accidentally incinerated when i was making my last boiler. i havent missed them!

cheers,

julian

Wasn't there a thread a few months ago about a chap having problems with a larger tap, perhaps M8.

It is worth thinking about the material. Aluminium or brass is going to be easier than steel so it may need a bigger hole. Also there is a big difference in taping a hole though sheet with only say 3 turns of thread and a deep hole where the poor tap has to be working on 6-8 threads at once.

A trick if you need high engagement in a deep hole is to proceed with the taper tap until stiff, then take the second tap in until it is stiff, which may only be a turn from first cutting, then back to taper and so on, slow but it works. You are opening up the part formed threads so the taper tap is only having to wok with its initial taper part not the whole length.

There's a handy tabulation on this page

... Although I'm struggling to correlate the 'Rule of Four' mentioned in the text.

MichaelG.

Mike:

I have PMd you.

I agree - why is it that if I want to drill a 25mm diameter hole, I need to buy a drill 2.5 metres long?

A set of decent HSS stub drills 1-10mm in 0.1 increments would be much more useful than jobbers. Or at least 1-6mm set. Jobbers cost a lot of height on small machines when the depth of the hole is usually small.

Steve

Steve

Machinery's Screw Thread Book has for 8BA:

1.70 (96%), 1.75 (87%), 1.80 (77%)

No 50 is about half way between 1.8 and 1.75

I have always taken the view that a deep thread can get away with a smaller %age engagement, although I remember reading somewhere that the first three threads do all the work ...

Cheers, Tim

From the listing of his publications at https://en.wikipedia.org/wiki/Tom_Walshaw I'd guess I'd have the 1st or 2nd edition Model Engineers Handbook from 1981 or 1986, most likely the latter.

Nearly 30 years on one thing I recall is opening the parcel and not only receiving the book but also a huge number of die cut sticky corrections, possibly extending to two or three sheets of A5 that were sized to overlay dozens of printing errors.

I've certainly broken BA taps in the past, but I can't blame Tubal Cain as I usually used books & tables from Moore & Wright, Dormer, Presto or Starrett.

Edited By Martin 100 on 06/03/2016 13:48:27

I think cheap and a lot of them is the better option as sizes get smaller and smaller - 'cause when they show signs of blunting they can be thrown away. Some toolrooms that work with certain low temperature hardening steels only use drills once. I have a number of dormers in the range of 1/4 to 3/8" but odd sizes that one gave me some time ago.

Might be worth checking drillservices for stub drills. I think Brian managed to get a set of Ebay.Oz - the sellers ther must be a bit more aware of what people might want.

John

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Edited By Ajohnw on 06/03/2016 14:10:33