UKC

I’m looking for advice on drilling 8mm holes through angle iron whose hardness has taken me by surprise. I have managed to get through with a 4mm cobalt tipped bit, using a pillar drill, but am so far defeated in enlarging these holes. I have bought several different bits of various specs including cobalt tipped but got nowhere. A friend suggested a tungsten tipped drill, but when I search these on the net they just look like masonry bits. The large cobalt bits have a sort of two-in-one tip which has made me wonder if they’re designed to be used without a pilot hole. 
Any recommendations or advice, including things like drill speeds and the need for cutting fluids would be most welcome. I have been warned that there’s a lot of cheap rubbish on the net.

oddly enough, having that 4mm hole through may make the job of getting an 8mm hole that bit more difficult, if you have further holes don't put in the 4mm again, the issue is that the 8mm drill bit is now only drilling/cutting along half of its cutting edge and will be prone to blunting/ruining. 

slower drill speeds, oily cooling/lubricating of any type you can, more pressure and avoid that 4mm hole

I'm surprised your cobalt bits won't do it. I've had good success drilling stainless with Bosch cobalt bits (although probably not as thick as your angle iron).

I'm lazy with speed and can't be bothered changing the belt on my pillar drill so drill everything at a medium speed (not sure what rpm). For the bigger stuff like an 8mm hole I'm definitely going too fast.

I use plenty machine oil to try and get as much life as I can out of my bits and reduce the pressure to avoid heat build up, but struggle to step up sizes as the bits tend to bite and chew the metal up. I therefore normally only do a very small pilot, and go straight with the bit size I want for the final hole. And just take my time. More oil.

I've been blow away with these stepper bits however, even cheep Chinese ones seem to take loads of abuse: https://www.amazon.co.uk/Blue-Spot-Tools-32050-Bronze/dp/B074ZR6RTY/ref=asc...

If your angle iron is thick you may need to use the stepper bit from both sides, but that would be my suggestion in your position.

That possibly explains why I couldn't drill the heads of some Torx screws recently. 

It's not just me that struggles to step up a size then. Good to know.

Your standard bits should be fine, if by two in one you mean they're split point then they're better at not walking than 'standard' ground but a small pilot hole or centre punch still worthwhile. 4mm pilot might be a bit on the big side but shouldn't cause huge issues.

Maybe drop the speed (500rpm?) with some decent cutting fluid, make sure you're feeding hard enough, if it's squealing you're being too light and will blunt the drill.

At the end of the day the right speed, feed, fluid and whatever steel blend the bit is are all just to extend the life of the bit, and if you've blunted it already you won't get much further until you're using a sharp one.

As other post, stepper bits can be very useful, probably unnecessary here but an option, I'd sooner spend the money on decent jobber drills though (Bosch, Rucko, not Amazon specials).

Friendly warning that steel can really catch when redrilling holes so make sure you're not standing the wrong side of a homemade propellor.

I’ve drilled hundreds of holes in angle iron and similar material, usually on site so using a hand drill. I would use two or three pilot bits to step up to 8mm. Sometimes but not very often used oil as lubricant as a lot of jobs are vertical.

I'm not surprised you've had so much trouble as I've occasionally come across the same thing - a seemingly ordinary piece of mild steel that's almost untouchable with regular drill bits. I've wondered if it's a fault at the time of manufacture, maybe to much carbon or the wrong temperature. You could try grinding the holes out bigger with something like a Dremel with a slim and/or pointed grinding stone? Finishing off with a small round file?

Hard materials/cheap chinese junk steel (notorious for hard spots).

Slow speed high feed pressure, plenty of coolant. Cobalt or quality HSS (Dormer) will do it. 

8mm with a split point shouldn't need a pilot hole but it shouldn't hurt either. 

Thaks for all the advice. Should be able to sort something out!

There's a lot of mis-labelled/mis-sold crap out there drill wise.

That said, any old HSS jobber drill from the builder's merchant should do just fine used right in mild steel. 8mm holes in angle iron using a drill press should be no match for a nice Cobalt drill. No point persisting with a blunted drill, get a new one and use some oil (3in1 is much better than nothing, specialist cutting oils work like magic) and and make sure the speed is appropriate. I'd not be tempted to step up in small size increments if you can avoid it, you just end up chipping or overheating the corners of your drills.

You could try cutting through the mill scale with a grinder, the bright metal below it will be softer.

Unfortunately, the act of failing to drill a hole can itself create a nasty hard spot. The solution is usually a better, sharper drill, oil/coolant and more care with speed and feed.

jk

I finally lashed out on a Dormer tungsten carbide bit. It works. I used my 18volt drill/driver on slow speed and as little pressure on the button as possible. I reckon that was about 100 rpm. I used a teflon spray for cooling. The drill never got too hot to touch. I learned a lot. Thanks for all the advice.