Drilling 17mm hole in mild st

[pgrbff]

I need to drill some 17mm diameter holes through mild steel, 20mm thick.
How many steps tdo I have to take before drilling with the 17mm bit and what size hole do I start with?

 

[Sideways]

3 > 6 > 17 should do

The 3 only halfway through just to get everything on centre.
Use a drill press if you can.
This is the sort of job magnetic drills are made for. So easy with a rotabroach cutter.

 

[deema]

The only core drill that is required is a hole to clear the centre of the 17mm drill bit. If you look at the centre where the two flukes come together you will see an area that cannot drill. You just need a pilot hole to clear it. The second consideration is tge power if your drill. If it’s powerful enough, drilling after a pilot with a 17mm isn’t an issue. If it’s a little feeble, the steps that Sideways suggests is a good plan, except take the 3mm all tge way through. Again to clear the none drilling centre of the 6mm drill.

 

[pgrbff]

Thank you both. I'm going to use a friend's mill so very pleased to hear I don't have to buy any more drill bits. Only 4 holes, cheapest 17mm I could find was Bosch HSS on Amazon.it.