I need to countersink two holes, 10mm hole and 20mm countersink
diameter, in unhardened flat stock. Buying a HSS 20mm countersink seems
to cost £20 or more, so does anyone have one I could borrow for beer
money, or alternatively, is there a special technique for countersinking
holes with ordinary tools which doesn't need a countersink?
-- Peter Fairbrother
What, and grind the cutting edges to 90 degrees?
Hmmm, cheaper than a countersink, and I can always grind it back to
about 130 degrees or whatever afterwards, almost all my large drills are
hand-reground by eye anyway. 
The other method I though of was using a say 6mm end mill with the mill
head at an angle and the work rotating on a table, but that would be a
real hassle to set up just now, the BCA is in bits (again).
Only I don't have a 20mm drill! Got 19 and 21 in the blacksmiths set,
and a few other loose large ones, but no 20 mm - I think the 21mm drill
may be getting a lot shorter ...
-- Peter Fairbrother
feeling grumpy today, broke a large carbide endmill and a couple of
expensive inserts yesterday, ouch.
 I can't do the points of big drills properly by hand on a grinder,
and I'm not making a Quorn anytime soon, so I don't bother with the
points, just the edges.
For the center of a large hole I use cheapo modern bullet-point drills
(but HSS, £1.50 for five). Incidentally these have some of the
properties of centre and spot drills, ie they don't wander much - though
you can't step drill sizes with them as they won't follow the center of
an existing hole.
But then I don't step the sizes anyway - after the first starter hole I
use the full size ordinary drill, even if it's something like a 4mm
starter and a 34.8 mm drill (the largest I have) - seems to work fine. :)
I've got one that large, how soon do you need it?. I might actually
get round to that little welding test tomorrow so could run it over when
done and bring the countersink if you can do it while I'm there.
Where do you live?
I have a couple (actually one is 19.5 mm and the other 20.5 mm) you are
welcome to borrow - but surely the 2X return postage (I wouldn't want
anything else) would go a good way towards buying one, then you are
sorted for next time. Or treat yourselves to this set, on special offer:
Alternatively, if you can set the work up in a lathe, it's pretty easy
to do an internal chamfering job with a boring tool
Essentially you want to cut a 45 degree valve seat just like I do on the
mill with homemade tooling for cylinder heads. Get a bit of scrap mild steel
bar, say 1 inch diameter and 2" long, turn a 45 degree chamfer on one end
about 1/2" long and turn the other end to whatever size you want to hold in
a chuck or collet. Set that up on the mill with the chamfer horizontal and
with a 3mm milling cutter mill a deep slot to take an ordinary 1/2" or 3/8"
square carbide milling insert such as these. Two passes because the inserts
are normally 3.2mm thick.
Drill and tap a couple of 5mm threads in the sides for small grubscrews
(2.5mm allen key size) to hold the insert and you've got yourself an
adjustable 45 degree seat cutter or chamfering tool. You can slide the
insert up and down in the slot to adjust maximum and minimum cutting size.
On small diameter bar it can be a bit of a mare finding enough space to get
the grub screws in to a decent thickness of metal but you'll work it out. On
the bigger sizes of tool you can fit in 6mm threads for 3mm allen key size
grubscrews quite easily. On very small bar you might need to go down to 4mm
grubscrews and smaller inserts.
Ideally you want the cutting edge of the insert about on the centreline of
the bar so you need to get that right for whatever cutting direction you
decide on before milling the slot but it's not hugely critical. You don't
want more than about 3mm of the insert sticking out if it's all to remain
nice and rigid and cut cleanly.
A range of those in different sizes and at 30, 45 and 70 degree angles is
all I've needed to cut many hundreds of 3 angled race engine valve seats
over the last 20 years on both cast iron and hardened steel unleaded petrol
seat inserts. Mine are drilled and reamed at 3/8" to run on a pilot fixed in
the valve guide for perfect concentricity but you won't be that fussed just
for chamfering things as long as you clock up the hole centre fairly
Variations on the theme can be used to bore flat bottomed holes or for
flycutting or even side milling. With a cheap diamond wheel from Cronos or
similar you can sharpen the inserts as required and even put fancy shapes on
them for special jobs or reduce their size for small work.
Damn those things are cheap now. £17 inc postage FFS. Mine is just like that
one shown and they were £40 when I needed one but I found one with a slight
mark in the aluminium (perfect on the actual diamond part) for £25 I think
it was and it'll sure as hell outlast me given it shows no wear at all after
grinding christ knows how many carbide inserts into dust over the years.
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