WHY THE TANG?

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Here’s a prequel to today’s discussion: **LINK**

https://www.model-engineer.co.uk/forums/postings.asp?th=140702&p=2

MichaelG.

A 2 morse taper tang is 13.5mm x6.5mm and this is found on drills up to 22mm, it would seem optimistic to expect this to drive a drill that size with a decent cut on it.

Mike

... then it's underspecified, or there wouldn't be so many twisted tangs in industrial workshops!

Can't we agree that the tang can serve both purposes. I was taught that the taper should be enough for drive, but clearly some machine tools use the tang (also).

Tapers are ejected by the screw on my mini lathe tail stock. In order to get maximum travel on it, I have cut the tangs off all the tail stock tooling. The mini mill/drill uses a 10mm drawbar and has been modified to be self ejecting. No tangs needed.

I’ve had three lathes, admittedly two of them WW lathes but none of them had a recess for the tang.

Of course but it would be good to know if they're deliberately designed that way.

Although I understand the case made by m'learned friends for the tang being a driver, it seems to fly in the face of having a taper at all. As slipping damages the taper and that of any machine it's stuck into later, I'm not sure having a belt-and-braces tang is a good idea. Maybe the purpose is to limit the amount of slip, and therefore the damage done to both male and female surfaces, and it isn't intended as permanent driver?

Quite interesting given the knowledge and wide experience of forum members that we can't answer such a simple question. The tang may be more subtle than it appears. Hidden depths perhaps!

Tried and failed to find the original US Patent, which might reveal all. Possibly Michael Gilligan could apply his famous search-engine super-powers and see if he can find it?

Dave

I dont think the tang was intended as a permanent driver. But it does temporarily stop the drill taper from spinning and allows it to regain its grip if the taper slips -- as can often happen. Possibly not the original primary intent of the tang in the patent but more of an added bonus.

Seems a pretty likely principle when you look how common woodruff keys are in sprockets and gears that mount on a taper on older machinery. Many of which are transmitting dozens of horsepower on vintage motorcycle engines etc.

But at the end of the day if your tailstock is in good condition you should not normally have to rely on the tang for drive. Providing the taper has been firmly tapped home with a brass hammer.

Myfords, Drummonds And many other lathes dont have any kind of slot or socket to engage with the tang and drive 100 per cent via the taper. No big problem there. But when the taper does let go it spins and can do damage.

Edited By Hopper on 16/09/2020 12:49:21

Edited By Hopper on 16/09/2020 12:53:06

Edited By Hopper on 16/09/2020 12:58:45

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Dave

Back in February 2019 [see my earlier link to the prequel thread] ... This was the best that I could offer in response to ndiy:

[quote]
Posted by not done it yet on 11/02/2019 08:26:08:

Look guys, I know the drive should be friction between the tapers, but answer me this: Why was a tang fitted ...

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The definitive source of information would be the original patent ... but I have yet to locate a copy.

What I have learned, however, is that the taper drive with tang was patented by Samuel Colt, before Stephen Morse patented his drill.

Ref: **LINK**

https://books.google.co.uk/books?id=mkoSDQAAQBAJ&pg=PT130&lpg=PT130&dq=morse+taper+shank+patent&source=bl&ots=oJYnUYVVTC&sig=ACfU3U1OYvbO47dqoJ1JUE_dl7bZF5XpIg&hl=en&sa=X&ved=2ahUKEwiitPDvrbPgAhWt6uAKHa2ADZsQ6AEwGXoECAkQAQ#v=onepage&q=morse%20taper%20shank%20patent&f=false

MichaelG.

[/quote]

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I’m not really all that interested, but it looks like a search naming Samuel Colt might be more fruitful.

MichaelG.

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Edit: But I have just found this reference:

[48] S. Colt, British Patent No. 861, 1854. 

Edited By Michael Gilligan on 16/09/2020 16:40:13

Patent documents can be a good starting point. Be aware, however, that whilst everything written in a patent document must be true, the whole truth is not a necessity. Likewise, the necessary features must be shown completely, but other related but inessential features may be omitted, to another scale, or adjusted to avoid disclosing helpful aspects unrelated to the invention as such. I would always take care, especially with seemingly detailed drawings of which the victorians were great exponents. (Speaking as a 40 year veteran of this stuff)

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Duly noted, john

The best I can offer is this [which may, or may not, be copied from the patent]:

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Note: I sincerely hope that this small extract counts as ‘fair quotation’ from the source document.

MichaelG.

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Ref. https://www.sciencedirect.com/science/article/pii/0378380489900752

Edited By Michael Gilligan on 16/09/2020 17:13:46

To amplify the suggestion of an extended plain diameter; Some of my MT shanks, with tappings have pieces of 3/8 BSW studding inserted so that they will, be ejected from the Tailstock on the lathe.

A taper drift is effective in breaking the taper in the Mill.

The main thing is that there is something there that can be subjected to forces to eject the Morse taper.

Howard

Thinking about this further. I think the Tang was intended to be for ejection purposes of a drill bit in a blind spindle or quill. The fact the tang engages with a “socket” is just a coincidence of the construction method employed in the construction of the quill.

I have no evidence for suggesting this, and at great risk of starting a hare, maybe the Tang evolved originally to drive the BLANK rather than the finished DRILL or Reamer. With centres in both ends and a Tang you have a sensible way of mounting the blank between centres and driving via some kind of two finger socket. This gives access to the whole of the blank for machining the taper and the drill diameter at one setting. Once the existance of a tang on the end of drill as a result of manufacture has occurred, consequent utility for ejection and as a rotational limiter adds a subsequent benifit possibly sufficient to ensure that the Tang stays even if the manufacturing process changes with the advent modern collets for example.

regards Martin

I think this subject has already been done to death, but, before this thread dies...

What Hopper has written is undoubtedly true. Apart from anything else, in industry, it's too much to expect that every drill taper will be un-bruised and clinically clean, and every socket will be cleaned before the bit is inserted, so the tang is a 'safety' device, taking over driving duty, should the taper friction fail, and thereby stopping relative movement between socket and bit. A scored MT socket is a sad sight, and would most likely not have been produced had there been a back-up tang to prevent the rotation of tool within socket. You tend to see them on milling machines, where there is no provision for locating a tang, because a draw-bar is expected.

Of course, if there's a foul-up of sufficient violence to twist the tang to bu**ery, all bets are off, but we'd never do that, would we?

There's also the 'Use-M-Up' (or something like that) sockets that used to be / still are available to drive damaged MT drill bits (the end has to be modified). Evidence that a tang-like end can drive sufficiently well to be useful.

Edited By Kiwi Bloke on 17/09/2020 11:10:46

Hi, if the tang was designed to be driven or even partly driven, why is it not tough enough to resist twisting and wringing completely off? I don't believe it's there as a warning or any kind of safe guard, as by the time it's wrung off and the drill bit stops turning, it is already starting to do damage inside the quill and this will accumulate the more times it happens.

They are not particularly hard as you can use a file on them without any real trouble and who would design a drive dog with such a sloppy fit, as the chances are if it started to turn in the quill, you would get an impact load onto the two opposite corners of the tang and that to me is not good engineering design.

The drill makers tell you that the tang is for ejection purposes only, so why don't people believe them, and I'm sure manufacturers don't make their machines with abuse as a given thing to do by not keeping tools in good order and not discarding damaged ones.

Regards Nick.

Does no-one have an opinion on this even to tell me I'm barking up the wrong tree.

Someone must have an idea of how taper drills were manufactured in the early days surely.

regards Martin

It sounds a reasonable theory to me

Rob

Damaged tangs suggest to me heavy-handed use - a machine-shop operator who is drilling holes and very likely wishes he wasn't but has to make a living somehow, is less likely to treat the company's tools and machines as gently and lovingly as we in our home workshops who have had to pay for the equipment. Blame ye not the tool, but the user.

That's an interesting suggestion, Martin, that the tang aids making the drill itself. I must admit I have never seen any description of how that is done, but you might be right. Nevertheless, we are considering the use of the tool, not its manufacture.

I have augmented my library of model-engineering books with ones written for the engineering trade - very much harder working conditions for the tools. This quote is from one of them, a text-book for engineering college students and reference for professional machinists:

The action of the taper is to wedge itself into the drilling-machine spindle, which has a taper to suit... The greater the pressure the tighter the drill, but even so, it is necessary to provide a positive drive in the form of a "tang" which fits into a corresponding slot in the machine spindle.

Sandy, A.H., A.M.I.P.E., "Cutting Tools For Engineers" , Crosby, Lockwood & Son Ltd, London {date not stated], Chap VI "Drills & Drilling" pp 56-70.

The title page informs us that Mr. Sandy, a "Silver Medallist, City & Guilds", was an "Instructor and Lecturer, Mechanical Engineering Department, Borough Polytechnic, London." I think we can, and I certainly do, accept that he knew his subject.

Also, I pointed out that if the tang was purely for ejecting the tool from the spindle it would not need to be made so elaborately, to fine limits prescribed in the official, industrial standards.

The quote itself is from p.57; but even given that the pages are slightly smaller than modern A5, that page range shows there is more to drills and drilling than might be obvious. My omission ("to suit..." is of just a reference to adaptors.

Why lathe tailstocks do not normally have tang-slots, is another matter, and I have often wondered that. It is a nuisance when using my tailstock die-holder in the ML7, as the taper does not provide sufficient grip. On the other hand, it does not fit the Harrison L5 tailstock as the tang hits something - the end of the barrel screw, probably.

I can only suppose that lathe designers assume a hole too large for drilling from a Jacobs-style chuck that fits the tailstock, will be bored to size from a pilot hole.

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Martin

I have tried, and failed, to find a copy of Colt’s patent ... and I can’t justify the expense of trying the British Library [which should hold it]

The paper that I quoted earlier does mention that Forsyth used a threaded drawbar, and Colt introduced the tang.

This would imply, but certainly does not prove, that your suggestion is erroneous.

MichaelG.