Drilling Stainless Steel

[Ian_Edwards]

I've read the advice on drilling SS posted on the forum and followed it. Using a drill press with a sharp drill bit, low speed, lots of axial force and well lubricated.

I bought two lots of SS from different suppliers, a sheet of 6mm x 270mm x 250mm and a length of 6mm x 25mm flat bar.

I've successfully drilled the sheet material, starting with a 3mm pilot hole and then opening out to 8mm. The swarf came off in a nice continuous spiral and the process was relatively quick .... no problem.

However, drilling the flat bar is much more difficult, using exactly the same technique, the swarf comes off in chips and the drill bit chatters, progress is slow and the drill bits get damaged. It looks like chips are breaking off the cutting edge of the drill bit.

The drill bits are 4% cobalt and I keep them sharp by re-grinding them. I can take a drill bit that works well on the plate and it struggles on the flat bar.

I guess they are different grades of 316 and the flat bar may be work hardened.

I've got about 25 holes to drill in the flat bar, is there anything I can do to make life easier?

Would annealing the flat bar work? And to do that, would I heat it to cheery red and then allow it to cool slowly? Like I do for mild steel. Would that make the SS more prone to corrosion?

Is the a better type of drill bit I could buy, which would cut the through the flat bar?

Or anything else??

 

[Norman_E]

When you try to drill the flat bar is it the 3mm pilot drill that is failing or the 8mm? If the former is struggling to cut I would try drilling first with a new drill of 4 to 5mm as a 3mm drill is smaller that I use as a pilot drill in stainless. I think your flat bar may have been work hardened in the rolling process, in which case you might do better to buy another piece from a different supplier. I have drilled stainless steel of various grades and from a number of sources and have never had a piece that resisted drilling as you describe. 316 stainless is actually not a particularly hard grade and I am wondering if you have been supplied something different by mistake.

Edit: I have recently drilled 304 grade and 430 grade stainless using some cheap HSS drills from Lidl with no difficulty.

 

[d4dunmor]

Probably different grades of s/s
If u are drilling a pilot hole the 2 flute drill might be chattering as it's trying to centre itself
Personally I would try and centre drill and drill through clamping bar and a bit of cutting fluid
And of course safety glasses
Next look at your drill bit drills from diy stores are OK but struggle under pressure from harder tougher materials titex or ghuring drills i would use
Speeds and feeds next
Peripheral speed is what burns out drills so as slow as a standard pillar will run
U will know yourself when the feeds are good it will feel right and cut right
Drill safe !!

 

[Ian_Edwards]

When you try to drill the flat bar is it the 3mm pilot drill that is failing or the 8mm?

The answer is both, I'm actually using a 3.9mm bit as a pilot, some time ago, I bought 10 @ 3.9mm drill bits by mistake from RS Components, I thought I'd ordered 1, I didn't realise that they came in packs of 10!

I'll try a slightly bigger pilot drill and see if that helps and carefully centering the 8mm bit in the pilot hole.

Is there any possibility of annealing the SS bar?

 

[earlybird]

.
Is there any possibility of annealing the SS bar?

Annealing 304/316 needs over 1000 deg. C, Very bright red and really needs a furnace.
A different pilot size might be of benefit, but I sometimes find that the work hardening from a larger pilot is unhelpful.
Stub drills and bigger point angle are better than standard drills.
Add; are you sure its stainless steel and not some exotic heat-resisting alloy?

 

[Sammy Lou]

this thread and the other recent thread on drilling stainless recommend using 'sufficient lubrication' but when drilling 304 or 316 stainless I've always used trefolex cutting compound which makes a significant difference to the cutting ability of HSS drill bits.

 

[vyv_cox]

300 series stainless steel can be strengthened by cold rolling. It is possible that your flat bar has been produced by this method, can you check with the supplier? Heating to cherry red (although I like the sound of cheery red!) should commence the recrystallisation process but you may need to hold it there for a while. Air cooling should be sufficient. This should not reduce corrosion resistance at all but of course some strength will be lost.

 

[Graham_Wright]

Does the strip have round edges or cut (guillotined) edges?

I've never had a problem drilling all grades of stainless with HSS drills unless I have pushed too hard and work-hardened it. Once it has work-hardened, you're on a hiding to nothing and the only chance is to start again from the other side. If you are using a drill press, you receive feedback from the handle and you can tell when things rare going well.
The point of the lubricant is to remove the heat. Oil works for me (although I do have a can or Tufcut - do not breathe the fumes!).

Just take a deep breath before the start to prevent choking!

Bags of pressure and low speed as others have said.

 

[KellysEye]

>I've got about 25 holes to drill in the flat bar, is there anything I can do to make life easier?

I had to dril holes in a half inch thick SS bar and used a cobalt drill bit and three in one machine oil, drilling slowly. There are different grades od Cobalt drill bit the highest is 8% M42 Cobalt which I used . Cobalt drill bits are not made of pure cobalt, but rather a steel alloy with 5 to 8 percent cobalt. The 5-percent alloy is known as M35 grade, and the 8-percent alloy is M42. Hope that helps.

 

[colhel]

The problem with these threads is someone can advise the complete opposite to another, and still be right. My advice is NOT to use a pilot unless you can find one about 1.5 mm.. Holding the job secure, and keeping vibration down is the key (IMO)

 

[colhel]

Just to add, with the flat only being 6mm thick it won't dissipate the heat very well so try and keep the drill as cool as possible.

 

[doug748]

As others have suggested, you will make better progress if you clamp the workpiece down hard.
This stuff is used extensively in aerospace on hard alloys:

http://www.ebay.co.uk/itm/Rocol-RTD...a8a9279&pid=100005&rk=3&rkt=6&sd=142281867212

 

[Ian_Edwards]

The bar has rounded edges, the plate (which drills relatively easily) is laser cut and has square edges.

I've ordered a tin of trefolex cutting compound, I'll see if that makes any difference.

I google searched "annealing stainless steel 316" and came up with this page from the British Stainless Steel Association.

http://www.bssa.org.uk/topics.php?article=76

It seems to suggest both slow cooling and rapid cooling depending on the spec' of the SS, but since I don't know that, it seems like a good idea to go back to the supplier and ask.

I'll try annealing an offcut, and try "Cherry" Red, (spelling was never my strong point!) a hot as I can with a gas torch.

Thanks for all the input, I feel like I have a few options to chase down now, rather than carrying on and knackering more drill bits.

 

[lw395]

Carbide milling cutter.
At a push, carbide masonry drill sharpened with a diamond disc or dremeloid tool.

 

[C08]

I have drilled lots of stainless and at times had difficulty. My(cheap) pillar drill although on slow speed was not slow enough and just work hardened the hole. The only way I found was to use a dremel carbide grinder and grind away the work hardened skin and then start to drill again. My ordinary hand drills (700W) do not have the torque at low speed to drill stainless well but my 1400W hand drill is slow and fine. Lots of holes good luck with that.

 

[Poignard]

I would use a spotting drill.

 

[Ian_Edwards]

Some new "Jobbing Drills" I ordered arrived through the post this afternoon. They go through the flat bar, like hot knife through butter, the only other thing I'm doing differently is firmly clamping the the work to the drill table.

The bar is 304 SS, but I have been able to find out what grade of 304 it is.

So I'm now making progress.

 

[wotayottie]

When annealing austenitic stainless steel we used to use a temnperature of either 1050C or 1100 C depending oin grade. Both were iun an atmosphere controlled furnace - doing it in the open air, even if you can get the temperature that high, will cause surface oxidation and reduction in surface corrosion resistance.
The machineability of stainless steels will vary even within the same grade because the way in which chips break off depends on the internal structure. To make free machining grades we used to add sulphur to the composition or even copper with the former reducing corrosion resistance a bit. Trouble is that most stainless steel is made from scrap in an arc furnace so you do get far more tramp elements in the finished product than you see in steels made in the BOS route and this can make for greater variability.

Lastly the reason you drill slowly and with pressure is that austentitic stainless work hardens. So if your bar has been cold rolled, its surface will be hard and it will be hard to machine. As annealed austenitics will be around 35TT but with work hardening they can easily go up to 80 TT or higher. Thats a big difference

There is no quick fix. You are tackling the job correctly using sharp drills and high pressures to make sure you cut and dont skid since the latter work hardens.

 

[vyv_cox]

As annealed austenitics will be around 35TT but with work hardening they can easily go up to 80 TT or higher. .

What are these values (TT) please?

 

[jon and michie]

In answer to the difference in drilling the flat bar would have been slightly more tempered/work hardened during its forging as opposed to your sheet plate - the heat affected zone in this case in the flat bar is more spread over a small area i.e a piece of flat bar 40mm wide is more affected than a sheet 1200 wide. even though the grade of them both is the same i.e 316 or 304.

make sure that the drill speed is lower than what you would drill for mild steel - also worth looking at the pitch of the drill bit although you may of sharpened them i.e the leading and trailing edges - for stainless the actual pitch is a little less.

Hope this helps

jon