Drilling stainless steel

[2383]

I seem to be blunting drills very rapidly drilling in 316 stainless (5mm thick). I am using a bench press drill, HSS bits, broaching oil, using 3-4mm pilot hole then 8mm bit.
Still, in spite of this blunting drill bits+++

Any suggestions?

 

[Steve_N]

Is it possible that you're letting too much heat build and therefore work hardening the steel? SS should be pretty soft.

I've been drilling a lot of 13mm holes in 5mm 304 this week without problems. Same type of gear as you - I go very slowly with light pressure and go up in about 2mm drill increments, the latter only really to prevent the wandering that you can get if you go straight in with the big drill bit. The bits are still sharp so far..

 

[bugs]

Are you using the correct speed?
I cant remember the calculations but it matters.
You will notice that when you have the correct speed the swarf will be one long continuous clean spiral with lots of progress. A bigger bit will need to be run slower, peripheral speed etc

Quality of the bits also matters. You could try tungsten carbide.

 

[oldharry]

slow drill with light pressure and 'let the drill do the work'

 

[FlyingDutchman]

Make sure to apply enough pressure to allow the drill to cut so that spiral chippings come out.
Drill at low speed.
A pilot hole is not good as it allows the oil to flow away instead of lubricating the drill.
Good Luck!

 

[2383]

Thanks, I'll try those suggestions

 

[LORDNELSON]

I changed to using cobalt tipped drills when drilling stainless steel, some years ago, and have had no problems since then. They go through stainless steel, including 316, like a knife through butter. They cost a bit more, an 8mm cobalt tipped drill from Axminster Power Tools (0800 371822) costs £4.90 but are well worth while, they last a long time without sharpening.

 

[tcm]

agree with below. Also, it's lubricant but also *coolant* that is needed so use lighter stuff but quite a lot of it if poss.

 

[jkim1]

Cutting lube for SS and low speed are the answers, if you are going to drill big holes low speed is even more important. I have wrecked cobalt, HSS, jobbers, and TN coated ones. Dont be upset if your break 3 drills on one hole stainless is not always homogenous, it sometimes has very hard spots 6mm away it could go through like a knife through butter.

 

[doug748]

I find cutting compound helps, the same stuff you use for thread cutting. Rocol is one trade name, I think. Brian

 

[angelsson]

Hi,
A toolmaker friend of mine gave me a tip when he helped me drill some stainless, he used his spit, and it worked very well, as it got hot clean shiny swarf wound up from the piece being drilled, it works well with mild steel also.
Mike

 

[pappaecho]

I have done a lot of 8 mm holes in stainless. I use 300 rpm, and quite lot of pressure, and the metal comes out as a continuous length of swarf. My drill was from a boot fair and was from Portsmouth dockyard and is hard silver steel. It gets sharpened very occasionally. Both the part and the drill bit should not get hot. If it does the drill needs sharpening!
If it gets hot the metal case- hardens... which is bad news

 

[stevesales]

Some grades of stainless (not so much real 303 or 316 but cheapo copies) work harden. This means if you let the drill rub you are done for.
Don't worry about cutting speed - we drill 10mm holes in 316 at 5,000 rpm.
Keep the drill sharp and keep the pressure on the cut.
Learn to sharpen drills yourself - apply the cutting edge to the grinding wheel to generate the cutting angle then apply the back of the 'edge' and rotate the drill anticlockwise while rocking it to generate clearance up to the cutting edge.
You don't need special materials to drill ss, ordinary sharp HSS drills are fine.

 

[Birdseye]

There's some nice bits of knowledge in here - mostly right but some wrong.

Austenitic stainless work hardens easily, so you must avoid any skidding and you must ensure that the drill cuts deep enough into fresh metal. So cut slow, with sharp drills and a fair weight on the drill bit. The harder the cutting edge of the drill, the slower it will blunt, so go for good quality drills. Using a coolant will help avoid heating up the tip of the drill and softening it / bluntening it.

The worst approach is to start with a dull drill, dont put on enough pressure, get the tip heating up and softening, then effectively have the drill bit rotating but not cutting. You will have created a real hard spot.

The type of swarf is as much a reflection of the material composition as anything else. There are free machining versions of the common stainless grades that will give a nice swarf. And stainless cannot vary within 6mm as described unless it is grossly faulty or has been previously processed wrongly. In which case it should be thrown.

The only other possibility is if you have managed to get a piece of cold processed plate - cold rolled say, or surface ground. If you have, then the surface will already be work hardened to some degree and you will have problems of the sort you describe.. What you need for an easy life is some annealed plate. Is your piece dull and matt or shiny?

 

[pandos]

Plenty of pressure. slowish speed cool with water or spit. Simple, works everytime.

 

[stevesales]

Birdseye makes a good point about the 'previous history' of the material. Many fittings may have been rolled or bent in a way which, as he says, gives you a pre hardened surface and lots of bother.
A normal twist drill always rubs a bit in the centre where there really isn't a cutting edge. That's why a pilot drill (smaller cutting edge and higher pressure when hand operated) can help.
At the sort of feed rates you will get from a hand drill I don't think coolant will make a lot of difference.
Obviously consider good quality hole saws for anything bigger than 1/2" in thin plate.

 

[2383]

Thanks for all the very helpful posts.

 

[martinb]

I agree, and drilling a pilot hole could be compounding the problem by work hardening the area around the first hole, causing problems when drilling the second hole and thus blunting the bits cutting the second hole.

Martin

 

[Avocet]

If you get really stuck - say because you've overheated and work-hardened it at a point where you want to drill, you could try using a masonry drill. They have extremely hard tips but the rake angles are all wrong for drilling metal. If you re-grind the tips so that they have a similar sort of angle to an ordinary HSS drill (not in itself an easy task because the tips are nearly as hard as some grind stones!) you will then be able to drill pretty much anything!