Drilling a mild steel rod with a large diameter bit?

[sploo]

I have an application where I need to drill a ~30mm deep hole in the end of bright mild steel rod with a 14mm drill bit.

I've drilled such rod before (using my woodworking lathe; at low speeds and with plenty of oil). Usually I'd start with a smaller diameter bit and work my way up, but in this instance would it make any sense to go straight at it with the 14mm bit?

My question essentially relates to the risk of a larger bit not centring quite correctly on a previously drilled smaller hole vs the risk of using the 14mm bit only.

I'd use a HSS cobalt drill bit BTW.

 

[sunnybob]

from zero to 14 mm I would use at least 4 intermediate sizes.
Straight at it with 14 mm will overheat the metal and destroy the bit.

 

[sploo]

Thanks Bob. That was kinda where I was thinking. Slow and plenty of coolant may be ok for one bit, but it'd probably be a hard job. The rod is about 40cm long, so I'd use a steady-rest near the end. I guess I'll go through a set of sizes and just hope I can keep it centralised through the whole job.

 

[sunnybob]

We used to have to drill 15 mm holes through 15 mm mild steel plate, on site with hand held power tools.
First drill size was 4 mm, then we went up 2 mm at a time.

Get the first one spot on, and the rest will follow through.
Just be ultra cautious checking the drill stays vertical in both planes as you drill (assuming you dont have a drill press).

If youre new to that kind of drilling, get a friend to stand 90 degrees to you so he can check left right while you check front back.

 

[CHJ]

... I guess I'll go through a set of sizes and just hope I can keep it centralised through the whole job.

1. Start with a Centre Drill (To clear the web of your largest drill and reduce cutting forces)
2. Drill a couple of centimetres in with your largest drill. ( To provide an alignment guide for the flute lands on final sizing)
3. Open up hole in suitable steps.

 

[sploo]

I've just learned what a Centre Drill is, and from that, discovered a Spotting Drill

I think that'll provide me with an ideal starting point* - many thanks!

* No pun intended