Well if the hole was already at the reamer size then something made it drill over-size. You should have had 16thou left for reaming.
I suspect it’s a bad drill. Do you have a letter A drill that you can try in a piece of scrap?

I would go for (the fairly new dormer), in my experience their sharpening is a bit random to say the least and depending how new where was is made they haven’t been made in this ciuntry for some time. And really it should be drill, bore, ream.

Quote
“And really it should be drill, bore, ream.”

Thats what my dad always taught me to do. Nobody seems to bother doing it anymore though.

Phil P

Never had a problem with the Dormer drills that I use all from Brazil, always have enough material to ream at 0.2mm smaller >6mm and 0.3mm smaller <6mm. These are the split point A002 and A022 range.

At 6mm and under I don’t usually bother to bore but do on certain larger sizes however I don’t see much point in then getting the reamer out. may as well bore to the fit I want as a bearing will want a bit more clearance on a shaft than a flywheel and I don’t have over and under reamers.

On 28 December 2025 at 16:54 Bazyle Said:

…until he found one that gave the hole size they wanted for a particular fit.

In the engineering literature, has anyone seen an account of honing (or perhaps grinding) the flutes of a drill bit to reduce its cutting diameter?

I guess because of the helix, you would need a hone with a long contact area.

In a similar manner, is there any guidance on how much to deliberately grind a drill off-centre to produce an oversize hole?

Are both of these a suck-it-and-see process?

On 28 December 2025 at 15:33 JasonB Said:

Never had a problem with the Dormer drills that I use all from Brazil, always have enough material to ream at 0.2mm smaller >6mm and 0.3mm smaller <6mm. These are the split point A002 and A022 range.

At 6mm and under I don’t usually bother to bore but do on certain larger sizes however I don’t see much point in then getting the reamer out. may as well bore to the fit I want as a bearing will want a bit more clearance on a shaft than a flywheel and I don’t have over and under reamers.

You can make a reamer cut a little oversize by striking hard down the front of the cutting edges at the cutting angle with a HSS blank or an old brazed carbide lathe tool. It raises a minute burr on the cutting edge and shaves more metal out of the previously reamed, undersized hole. Trial and error of course to get the right amount of pressure to get the right amount of oversize.

I am surprised no one has asked what machine you used to drill the hole? An old clapped out drill press with 0.2mm runout at the chuck is obviously gonna produce a terrible hole! For example.

When reaming any hole I always start with a drill which is 0.5 mm smaller than the finished hole. The pre-Reaming drill which is usually 0.1 to 0.3 mm smaller (the amount depends on the finished hole size), than the finished hole. This means the pre-reaming drill has less work to do and is following a pre-existing hole. The use of a slot-drill just to start the hole will ensure the hole is in the right place, but I would never try this on a drilling machine. (Slot drills are usually undersize anyway).

In my 40 years of toolmaking I have never had a failure when following this procedure.

Regards

Gray,

Interesting that you reamed in the mill (under power??), what sort of reamer and what speed/feed?

In that case if the reamer cut metal all the way round the hole the reamer is oversize or the holder/collet etc is not running true. You could try an experiment with a piece of scrap in the lathe, that will take the mill out of the equation. Best of luck.

Steve, you don’t say what if anything you used to centre the drill as it started the second bearing. If you just let the drill bit find it’s own start then it could have wandered slightly which could possibly have caused both holes to cut oversize as the edge of the drill would have been pushed against the side of the hole. For jobs like this I use a long series ctr drill to get a good start on the far side bearing.

You mention a pilot drill befor ethe 15/64th reaming size drill. If the 15/64th had been ground off ctr then that should not really make much difference as it should cut onsize if a pilot is used as per Graham’s method.