Hi, hoping somebody can point me in the right direction? I remember seeing an article ( MEW?) a while back regarding homemade counterbores which looked very similar to a wood drill. If I remember right it was basically a piece of silver steel with a spigot turned on the end and then either side of the spigot the bar was milled ( or ground) flat and the edge sharpened. You could then harden the cutting edges and away you go. I was wondering if anyone has a link to a web site with drawings/info on this type of counterbore?
Depending on the size you want it's much easier to make this sort of thing from a mild steel holder and a separate tool steel or carbide insert held in place with a grubscrew. You can then also have adjustability of cutting diameter too by moving the insert in a slot. I mill out all the the bores for valve seat inserts in the cylinder heads I modify with a variety of this type of home-made tool.
Conventional square carbide milling inserts are only a couple of quid each in a variety of sizes and can also be ground down smaller still if you have a diamond cup wheel like the excellent Chinese one from Chronos for £30 or so. The one I bought a few years ago has sharpened or modified umpteen of my carbide inserts without showing a trace of wear.
If you need a spigot in the tool then again the easiest way is bore a location hole in the tool and make the spigot separately to be held in place with another grub screw. Using stepped spigots you can have any size you like sticking out for a single bore size in the tool.
It does of course get tricky to fit all the separate bits in if you need cutting diameters less than about 3/4" and then a single piece HSS tool becomes more viable. Much more work to make it though and it can only do one specific job. The spigot is the worst offender for getting in the way so if you can manage without that you can go pretty small on diameter with tiny inserts.
I make most of my adjustable borers with a 3/4" shank to be held in an R8 collet. That's sturdy enough for most jobs.
For that purpose you could try drilling out to depth and then forming a flat base with a slot drill. I have done this lots of times for cap head screws as the commercial counterbores for this purpose are always way too big.
For a one off or a small number of holes it is very much quicker than faffing about making your own counterbores.