Why is the pilot diameter of a counterbore so large?

I just checked my M5 counterbore. It has a 4.2mm pilot, and a 10mm cutter diameter. Go figure!

It's stamped M5 10 4.2 on the skank, so it's not altered just made for a special purpose.

My first thought is that it's for counterboring a threaded hole, but why would you do that!?

For making the counterbore before you tap M5 fine.

Well as its the tapping drill size for M5 I suppose its possible to counterbore whilst say a cover whilst it is in place, then do the threading and cover clearance hole later, but the 10mm diameter still does not make sense.for a caphead.

Maybe these counterbores are not intended for capheads? I have just measured an M5 Pan head fixing and its 9.65mm OD so would be a nice fit in the 10mm hole.

Ian P

Thanks, Locoman,

Your message made me wonder if I could get a neater result by substituting imperial counterbores for the metric ones. Here are my calculations, giving first the metric screw size and maximum recommended cap diameter for a socket cap screw, then the nearest imperial screw size, imperial counterbore designation and diameter of counterbore in millimetres, and finally the clearing drill recommended for normal and close tolerances with the imperial counterbore tool.

M3 5.5mm cap Imp screw #4 Cbore diam. 5.56mm (7/32) clearance 3.2mm (close), 3.45mm (standard)

M4 7.0mm cap Imp screw #6 Cbore diam. 7.14mm (9/32) clearance 3.9-4.3 mm

M5 8.5 mm cap Imp screw #10 Cbore diam. 9.52mm (3/8) clearance 5.2-5.6 mm

M6 10 mm cap Imp screw 1/4" Cbore diam. 11mm clearance 6.8-7.2mm

I got my figures for maximum metric screw head sizes from roymech.co.uk, my US hole and counterbore sizes from rorty.net/holes-and-counterbore-sizes and I converted the US drill sizes using CSGnetwork.com drill size conversions. Before I spend money on any imperial counterbores I would be most grateful to hear from anyone who has tried these substitutions with metric cap head screws. I am very reluctant to let imperial units into my workshop, but the calculations suggest that these counterbores might be worth trying, using the normal clearance drills recommended for the metric screws in my Zeus tables.

M3 into 5.5 will go though I counterbored these with a reground drill with almost a flat end and no pilot, clearance hole is 3.0

Of course you may need a counterbore for a hex head and a socket, not just for hex drive keys.

I don't follow

He means a hex head as in bolt. Yes there are standards for those, and clearances for spanners.

And why is the shank of live centres so short? one always has to insert something into the sleeve for the drift

to get to it

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

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

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

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

Neil

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

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.

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