" (also any tips on how I can keep a pilot hole drill-bit well-centred on a
slippery cylindrical brass shaft)"

File a small flat spot on the shaft and center punch the pilot spot.

For metric: Hole size = Nominal diameter - pitch.

Where pitch is the number of threads per mm.

For English: Hole size = ((1.299 x % / pitch)

Where % is expressed as a decimal and pitch is the number of threads per
inch.
In either case, use the nearest standard size drill.

Bob Swinney

Erratum:

Note English hole size is equal to:

Nominal diameter - ((1.299 x %) / pitch)

"Bob Swinney" wrote in message
newsS15b.332177$YN5.228732@sccrnsc01...
" (also any tips on how I can keep a pilot hole drill-bit well-centred on
a
slippery cylindrical brass shaft)"

File a small flat spot on the shaft and center punch the pilot spot.

For metric: Hole size = Nominal diameter - pitch.

Where pitch is the number of threads per mm.

For English: Hole size = ((1.299 x % / pitch)

Where % is expressed as a decimal and pitch is the number of threads per
inch.
In either case, use the nearest standard size drill.

Bob Swinney

On Tue, 02 Sep 2003 17:34:01 GMT, "Bob Swinney" wrote:

Erratum:

Note English hole size is equal to:

Nominal diameter - ((1.299 x %) / pitch)

much apprectiated!

"Bob Swinney" wrote in message
newsS15b.332177$YN5.228732@sccrnsc01...
" (also any tips on how I can keep a pilot hole drill-bit well-centred on
a
slippery cylindrical brass shaft)"

File a small flat spot on the shaft and center punch the pilot spot.

For metric: Hole size = Nominal diameter - pitch.

Where pitch is the number of threads per mm.

For English: Hole size = ((1.299 x % / pitch)

Where % is expressed as a decimal and pitch is the number of threads per
inch.
In either case, use the nearest standard size drill.

Bob Swinney

And lo, it came about, that on Tue, 02 Sep 2003 12:23:37 +0100 in
rec.crafts.metalworking , curiosity [email protected] was inspired to utter:

it's 45 years since I did metalwork at school so can anyone advise on this query? One of
the two tap-heads of an old-fashioned brass mixer-tap is missing a screw to secure it to
the square-headed shank (sorry if that's wrong terminology) it sits on. In addition the
hole in the shank still carries the end of the screw which was overtightened and thus
broke and fell out so I need to re-drill and tap a new thread 90 deg. away and would like
to know what the relationship is between the tap size and the drill size (obviously
smaller). Looking at the hot-tap head which still has an intact brass screw and
satisfactorily threaded hole, the screw appears to be 3mm (1/8") o.d.. Are threads
gauged by o.d or minimum diameter within the thread?

Screws are gauged by o.d. of the screw.

And is there a rule of thumb which
determines a suitable size drill-bit? Would this require a, say, 2.5 or 2 mm pre-drilled
hole? I'd guess as these taps are old that in any case the sizes are unlikely to be
metric (B.S.F? B.S.W.?).

"Assuming" you're in the UK and converting from an "older" british thread
to metric (simpler).

Fixing this is going to require
+ a left hand drill bit about 2.5 mm od,
+ an "ez out" (this is a tapered square punch. The idea is you drill a
pilot hole, and use the ez out to unscrew the broken off bit.)
+ a reversible drill

When drilling out the broken screw, one of two things will happen: you will
drill all the way through the screw, in which case you'll have a hole to use
the "ez out", or the drill will grab the screw and back it out of the ole. Rah
- either way, you're done with this part.
The good news, you may have something to check/test for proper size. WHo
knows, it could be metric.

If not, if this is a 1/8" hole. the nearest metric size is 3.10 mm, I'd
think one could get away with using a 3 mm drill to drill the old threads out.
At this point, the "simplest" (IMO) route would be to get a self tapping screw
and have it form threads as it goes in.
Otherwise, take a 3 mm drill, and find a screw of which the core is
slightly less than the diameter of the drill, then tap accordingly.

any guidance/help much appreciated.

I don't know what sort of help this is, and I'm trying to look up specifics
in the 1940 Handbook (metric data is rather sparse.) as my current copy is
locked away in the toolbox.

(also any tips on how I can keep a pilot hole drill-bit well-centred on a slippery
cylindrical brass shaft)

file a flat spot where you want the drill to start. that is the easy part.
You could use a center punch to make a crater for the drill bit to start in.



--
pyotr filipivich
"With Age comes Wisdom. Although more often, Age travels alone."