it depends on how accurate you need the position, the diameters, and the surface finish. If some or all are less critical it is possible to grind the working end of a conventional twist drill – but you will need to be able to do the grinding – or perhaps find a local engineering firm to do it. If I was to have a go, I would first drill a smaller hole in the cast iron as a pilot for the drill – but of course you may have such a hole in the design anyway. This makes the likely accuracy better as you can gring your 10.5mm drill with an extension of the pilot size, and this will help to hold the drill central while cutting.

The shaping could be done with a lathe, if you soften the drill first – by getting it red hot and leaving it in the ashes to cool slowly. If you start with an old, well used drill bit, there will be much less overhang in the turning and you can hold the bit in a collet on the plain shank. Turn the pilot to a plain round shape – don't worry if there are still traces of the flutes. Then with the drill soft you can file the cutting edges to resemble a milling cutter, making sure that the outer corners remain good and sharp, and really opposite each other. Then heat to redness again, and plunge cutting end first into a large dish of water, stirring it round to maximise the cooling. This will (should) make the cutting end dead hard – and you could try the effect of using it on a sample of your cast iron. As long as the iron is 'grey' – not 'white' and glass-hard because the casting was chilled – this should work. The bottom of the counterbore may not be wonderfully flat, and you will get much better results with a column drill or a mill, than hand held.

It might help to use another standard drill to start the cuts and take off any skin, then use the counterbore to get the depth.

Hope this helps. For mass production you might be better with a properly tempered tool – I'm sure there is a posting here which will explain exactly how. Good luck

Tim

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A piloted counterbore should be fine in a pillar drill … to all practical purposes it is just another drilling operation: there are no nasty side-loads to worry about.

MichaelG

And maybe knock the hard skin off the top of the cast iron with an angle grinder , dremel grinder etc before using your HSS counterbore tool on it.