Why is the pilot diameter of a counterbore so large?

[HOWARDTParticipant @howardt]

If it is a morse taper and you use a drift it should have a tang. Different drift for larger tapers.

[Ian PParticipant @ianp]

Posted by HOWARDT on 03/02/2017 18:37:06:

If it is a morse taper and you use a drift it should have a tang. Different drift for larger tapers.

Howard

No 'should' about it. Some tooling with MT shanks have tangs, some don't.

I have never see a revolving centre with a tang.

Ian P

[loco manParticipant @locoman67579]

I believe that Mike Poole made a very accurate comment re the huge size of the original Whitworth size heads. After those standards were set it – and possibly late 1939 – early 1940? – it seems as though there was a re-think and sizes were dropped – for instance the hex size of a 5/16 BSW bolt hexagon dropped down to the size of a 1/4" head size. Along with the introduction of BSF – ie British Standard Fine threads. Maybe this was why the dual marking of some spanners was introduced – for instance – 3/16"BSW / 1/4"BSF.

Some of the older fitters that I worked with used to refer to the down-sizing of hexagon dimensions as 'war standards' — maybe a way of conserving / making better use of steel supplies??

Maybe someone else has some more accurate information over the change – I await with interest.

Ian

[Neil WyattModerator @neilwyatt]

Posted by loco man on 03/02/2017 22:04:19:

Some of the older fitters that I worked with used to refer to the down-sizing of hexagon dimensions as 'war standards' — maybe a way of conserving / making better use of steel supplies??

It was indeed a way of conserving steel – and speeding up production.

Consider that many fixings will have been turned from hex bar.

Neil

[Tim StevensParticipant @timstevens64731]

A little bit of history:

BSF was in regular use before 1910 – I have before me a book from 1907-8 with a table of sizes from 1/4 to 6 inches. Oddly, the 1/4 BSF has 25 tpi – although this may be a typesetting error.

The head sizes for finished bolts and nuts are;

1/4 – 33/64

5/16 – 19/32

3/8 – 45/64 etc.

You might like to compare these sizes with the modern accepted decimal dimensions of Whitworth spanners.

And I suggest that one reason for a large diameter of countersink was because they are intended for use with cheese heads, or fillister, or round, or hexagon heads. It may be that there is a Standard in existence that has not been updated to suit socket head screws. What are commonly called 'Allen Screws' were very rare until about WWII – and are referred to as hexagon socket head cap screws. My understanding of the term 'cap screw' refers to the use of the screw to hold the cylinder head or cap on a steam cylinder.

I'm don't mind anyone raising an objection to my comments, especially if they can add more up to date or accurate facts, but I would be obliged if I could be told of any deletions of my contributions in future. Thanks.

Tim

[Wayne WardParticipant @wayneward51978]

I’d always recommend a chamfer at the entry to a screw clearance hole. If the underhead radius interferes with a sharp edge, you will damage that critical fillet and possibly find the head falling off your screw when you would really prefer it not to. I speak from experience here as a design engineer rather than as a machinist. Having too tight a clearance hole is asking for trouble when a hole pattern is concerned. You can’t be sure of holding the position of a tapped hole to much better than within a diameter of 0.25 or 0.3mm of true position and even with accurate machining of the set of clearance holes, you will probably have some interference with the underhead fillets. It’s possibly not a worry with relatively unstressed applications, but if you have something with a lot of cyclic stresses, this short of thing can catch you out. Small clearances don’t do much for you as your screw holes won’t be good enough to provide accurate and predictable location. You would do better to rely on dowels, spigots etc, or possibly shoulder screws to locate.

[HOWARDTParticipant @howardt]

Morse tapers without tangs are driven out of a taper by striking the small end down its length, or driven out by a retracting tail stock. Morse tapers with tangs are driven out with a drift. That is ow I were taut, otherwise damage will occur.

[Nick HulmeParticipant @nickhulme30114]

I have a set which consists of an arbour with interchangeable cutters and pilots.

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