How to make a counterbore question

No, not if you want it to cut. If you managed to achieve a flat end, it wouldn't cut because the drill wouldn't have the necessary clearance or rake angle in order to do so. You can alter a typical twist drill to be flat with almost no effort, by hand, but you have to understand how drills cut if you expect to achieve success. The web must be ground such that the flat bottom drill becomes center cutting, and the rake angle of the drill must be established when so grinding. Twist drills have considerable positive rake to begin with (the angle of the flute), but you grind on that surface when making the conversion to flat bottom, so you must be careful to not grind it negative. It's not hard, and no special equipment is required, although you can achieve better results with a cutter grinder. If you have a pedestal or bench grinder, a flat wheel with a sharp right corner is adequate, along with a small square to determine when you're flat and square.

Harold

No, but you can sharpen a drill bit with a flat end by hand on a bench grinder, like so...

The technique is not too different from regular bit sharpening.

The bit won't cut to center, so you need to drill the clearance hole for your screw first. Unless you get the grind just right and have a rigid setup, the drill will tend to skate, so it helps to start the c'bore with a drill and use the flatted drill to square up the bottom where the screw head seats.

Ned Simmons

Alex Are you talking about a hand held stone or a rotating grinding wheel? A stationary stone is not going to do it. It may be possible to reduce the diameter with a a grinder of some kind but it is not going to be very accurate and not centered very well. Plus you would have to sharpen the end afterwards. Again possible but its not easy. A better way would be to chuck in a lathe and use a toolpost grinder. In the long run it would be easier to buy a cheap set or individual c'bores or just use a drill and endmill. I used to run a tool grinding cell, I really miss the Brierley drill grinder we had (sigh :-) it would do this job in a couple of minutes. also do of course drill, taps, countersinks etc.

I found these at enco

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What size are you needing?

Do it the easy way and buy an inexpensive endmill of the right diameter.

Only time you you should ever use an endmill in a drill chuck is when you are using it like a drill, i.e. axial feed only.

Wholesale Tool has the best prices and widest range of sizes of endmill (fractional by 32s, decimal and metric) that I have found. For counterbore use you won't need center cutting endmills.

see:

Rig up a depth stop. IMNSHO the only thing that looks worse than random or LAR [looks about right] depth counterbores is random/LAR depth countersinks.

Unka' George (George McDuffee) .............................. Only in Britain could it be thought a defect to be "too clever by half." The probability is that too many people are too stupid by three-quarters.

John Major (b. 1943), British Conservative politician, prime minister. Quoted in: Observer (London, 7 July 1991).

I made a counter bore (#10 SH screw) in a lathe by grinding down an appropriate drill bit with a tool post grinder equipped with a VERY fine stone. It worked - but - only for aluminum (6-61 - T6) and then only in the mill with considerable pressure. If you wanted to 'counter bore' in wood or aluminum you "might" be able to grind down a drill bit as you suggested but NOT with a hand-held stone. Possibly with a dremel - and only then if you have Supreme hand-held control. As others have suggested - an end mill would work - or - bite the bullet and buy the appropriate counter bore tool.

Regards. Ken.

Having created counterbores by pretty much any method available, I'm not a fan of using end mills, particularly if they are used in a drill chuck, and worse of all, in a drill press. End mills are ground to be side cutting (we all know that) and will readily cut oversized. Not only does one risk ruining parts, but they're considerably more dangerous to use than other cutting tools, due in part to the sharpened sides and their willingness to hog.

Quills in a drill press tend to be, at best, sloppy---so if the end mill wings ever so slightly, it's inclined to pivot about the short side, cutting well oversized. Uneven chip loading further aggravates the condition.

The use of either a piloted commercial counterbore (they tend to be troublesome with pilot breakage) or the use of a flat bottom drill, applied after the desired counterbore has been drilled to full diameter deep enough for the margin of the flat bottom drill to act as a guide on the pre-drilled hole, is the best method going for cutting a counterbore. Chip evacuation is not an issue, there are no pilots to break, and the flat bottom drill can be easily created by anyone with the most basic of equipment, i.e., a grinder that has a properly dressed wheel, a square, and enough sense to know how the drill should cut. It's the only method of counterboring that I endorse. As George suggested, do set a depth stop. This type of counterbore cuts so freely that it's dead easy to drill through your parts.

Note: This type of counterbore is useless for spot facing unless it is very short.

Harold

I'll be using this counterbore on hard wood like Ebony. I should have mentioned it before.

Harold and Susan Vordos wrote:

Yep----makes a big difference. If you go the route of drills, make them as short as possible for rigidity. Wood won't pilot them as well as metal, and drills often don't run very true. They'll still work, and probably very well, but they'll need to be well sharpened so they cut freely. They'll serve you best if the work is clamped-----unlike commercial counterbores with pilots. You can use them freehand in a drill press with good results. Broken pilots aren't likely to be a problem in wood, but they are limited to counterbore depth because of poor chip removal. They'll also be sharpened for cutting metals, so they'll have a lack of desirable rake and clearance. I've machined African blackwood (bagpipes) and know that sharp tools are important.

Harold

--Howzabout using a plunge router?

You would have to offset the drill bit in the drillpress chuck so you grind some clearance behind the cutting edge but it would work.

John

The way I always made counterbores in wood involved a forstner bit. The trick was to make the counterbore first:

1. Drill the pilot hole, using a twist-drill bit about 1/2 the size of the center spike on the larger forstner bit.
2. Drill the counterbore with the larger forstner bit.
3. Drill the clearance hole with an smaller forstner bit (or an ordinary twist drill bit), following the same pilot hole.

Joe Gwinn

I'd be keenly interested in hearing how you'd do that. Achieving relief (clearance) can't be accomplished that way unless I've missed something.

Harold