Countersinking large holes

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?
Thanks,
-- Peter Fairbrother
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Can you get away with a 20mm drill? Peter
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Drawfiler wrote:

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. [1]
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.
[1] 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. :)
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Peter Fairbrother wrote:

Peter,
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.
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Stick it in the 4 jaw and turn a taper?
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Peter,
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:
http://www.greenwood-tools.co.uk/ishop/728/shopscr72.html
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
Regards,
David
--
David Littlewood

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OK so you donrt want to use a drill so make a D bit out of a bit of hardenable steel and use that. Peter
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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.
http://cgi.ebay.co.uk/SPG-423-GRADE-C6-SQUARE-CARBIDE-INSERTS-BOX-10-/130362099913?pt=LH_DefaultDomain_0&hash=item1e5a2fc8c9
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 closely.
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.
http://cgi.ebay.co.uk/DIAMOND-GRINDING-WHEEL-75MM-CUP-TOOL-CUTTER-GRINDER-/260440865466?pt=LH_DefaultDomain_3&hash=item3ca37c3aba
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.
--
Dave Baker



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Not sure if this will help, but page 51 here
http://www.glostertooling.co.uk/Download/Engineers%20tool%20catalogue.pdf
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