Drilling Stainless Steel (Large DIA)

Dougal

Dougal

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I've recently received some conflicting advice about this, so thought I'd try here... I'm fed up with breaking and blunting all my HSS drill bits on the boat.

I need to drill a couple of 13mm holes through 4.5mm 316 stailess. So far, I've been told to use:

1) HSS with tungsten carbide tip
2) Cobalt bits
3) Titanium coated HSS

I have learnt about using slow speeds and lubricant :-)

Thanks
 
2nd vote for Cobalt. Would drill a 5mm pilot first and you will need a big slow drill. If you are not using a pillar drill you might end up with a 'trilobe' shaped hole similar shape to a rotary engine rotor.

If that appears to be happening I'd resort to a file :(
 
Thanks.

I can't use a pillar drill as it has to be done in situ. Not too worried about ending up with the trilobe, as it's just an oversize hole to pass a BIG coach bolt through. Just so long as the head will sit tight.
 
I have never used cobalt drills for stainless, always finding High Speed Steel adequate. If you decide to try HSS go for Dormer on the discount tools site.
Whichever, let us know how you get on.
 
These drills need quite a lot of pressure and low speed.Without a drill press I would start with a small size drill like 5mm and widen the hole in one or two mills increments so only the tips of the flukes bite into the metal.Always put as much pressure as you can and decrease speed with diameter.The smaller sizes however need a much faster speed despite what most people say.It has to do with the torque they can take.Too low a speed and they break.As you go up in size they get stronger and able to whitstand a higher loading.
If you can find cutting lubricant use it.
 
Dougal said:
I've recently received some conflicting advice about this, so thought I'd try here... I'm fed up with breaking and blunting all my HSS drill bits on the boat.

I need to drill a couple of 13mm holes through 4.5mm 316 stailess. So far, I've been told to use:

1) HSS with tungsten carbide tip
2) Cobalt bits
3) Titanium coated HSS

I have learnt about using slow speeds and lubricant :-)

Thanks
Click to expand...

A high speed steel drill should be perfectly alright but it is important not to dwell as this will harden the surface.
Plenty of lubricant & a couple of pilot holes & bobs your uncle.
 
i previously worked in a machine shop that only machined stainless.....so...

firstly get a decent brand HSS drill....search for Dormer as a make, you just dont need cobalt, the drill will not go blunt in 1 or 2 or 3 holes if used correctly, if you are doing 10+ maybe a tungsten carbide one, but they are big money....2 x HSS would be better...

use on the slowest speed you can, i mean really really slow, 200rpm max, will take a while.....the trick is to not get the tips of the drill hot, at all, so you need lots or cutting fluid, WD40 or similar is fine, but it need to be swimming in it, if you see any smoke it is getting too hot and the edges on the drill will go very very quickly, like 1 second....

then just go slow, really slow, should be fine, if done carefully (dont overheat drill) you will come out of it with a decent hole and bit still ready for another day.

You will need a pilot hole first, 4mm or 5mm, the speed on that is about 500rpm max no bigger.....otherwise it starts to "chatter" when you go with the bigger drill bit.

Finally, the stainnless work hardens, so if you pause, or push too slowly, it hardens, so keep a positive firm pressure so it is cutting at all times.
 
I've recently drilled a couple of dozen 0.5" holes in thick 316 and 304 - and re-sharpened only once.

I'd second the use of ordinary HSS bits. Use slow speed, and use pressure immediately to prevent skidding. Work up from 1/8" in steps.

If drilling with the workpiece at an angle, I use condensed milk for lube as thinner lubes won't stay in place. But when drilling vertically with a pillar drill, I like to plug the hole from below with grease and pour oil into the hole, so that drilling takes place in an oil bath.

When re-sharpening the bit, I use an increased rake angle - maybe that's technically wrong, but I find this encourages the bit to 'dig-in' more easily in soft s/s.
 
electrosys said:
I've recently drilled a couple of dozen 0.5" holes in thick 316 and 304 - and re-sharpened only once.

I'd second the use of ordinary HSS bits. Use slow speed, and use pressure immediately to prevent skidding. Work up from 1/8" in steps.

If drilling with the workpiece at an angle, I use condensed milk for lube as thinner lubes won't stay in place. But when drilling vertically with a pillar drill, I like to plug the hole from below with grease and pour oil into the hole, so that drilling takes place in an oil bath.

When re-sharpening the bit, I use an increased rake angle - maybe that's technically wrong, but I find this encourages the bit to 'dig-in' more easily in soft s/s.
Click to expand...
Yes it's wrong but it works!
 
I have always found that pressure (and slow speed) is the main factor in successful drilling of stainless. A bench drill is easy. If I have been unable to use a bench then the next best thing has been to arrange a wooden lever to press down on the drill. I've ended up with some fairly Heath Robinsonesque arrangements but it definitely works. It's surprising how much pressure it takes.
 
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