Drilling, Boring and Reaming: Three Tools, Three Different Jobs

Face the end, then start with a centre drill. Its short stiff body will not wander, and the small cone it leaves gives the twist drill something to sit in. A twist drill started on a bare or domed surface walks. Its chisel edge at the very centre does not cut so much as extrude, and until the full lips engage the drill is free to skate — which is how a hole ends up a millimetre off centre on a part that was set up perfectly. On the lathe the drill is in the tailstock and the work turns, so a wandering start also means the hole is not on the axis. Everything downstream inherits that, and no later operation removes it except boring.

Drill a hole with a 10 mm drill in a scrap of the material you are using, then measure it with a telescoping gauge and a micrometer, at two depths and two angles. It will be oversize — commonly by five to fifteen hundredths of a millimetre — and it will usually be slightly oval or bell-mouthed at the entry. A drill is supported only by the hole it is making, so any asymmetry in its two lips steers it, and the hole comes out larger than the drill by roughly twice that error. Do this once for each drill you rely on and write the number on the case. That single measurement turns the drill from an unknown into a tool with a known behaviour, which is the whole theme of this ladder.

If the hole must be round, straight and on the axis, BORE it. A boring bar carries one cutting tip and is fed by the slide, so the hole it cuts is defined by the machine's own geometry — it will cut a crooked hole straight and an oval hole round, because it takes no notice of what was there. If the hole must be an exact size with a good finish, REAM it. A reamer has many flutes and floats in the hole, so it cannot correct position — it follows. What it does superbly is take a controlled small amount off the whole surface at once. Bore first, ream second, and understand what each is for. A part reamed without boring is a correctly sized hole in the wrong place; a part bored without reaming is in the right place with a dubious finish.

Once the size is proved on the first part, check the rest with a go/no-go plug gauge rather than measuring each one. Reaming is a repeatable operation — the same reamer in the same material at the same speed gives the same hole — so what you need from part two onward is a yes or a no, not a number. The GO end enters full depth, the NO-GO end does not, and it takes two seconds. The number still matters when something changes: a new reamer, a different material, or the first part after a long gap. Measure then, gauge afterwards. That is the division of labour the go/no-go rung sets out, and holes are where it earns the most.