Cobalt HSS drill bits - gone blunt drilling stainless steel

Yes, I guess the edges will not be work hardened.

Not teaching you to suck eggs, but it took me time to realise (and an old hand) that you must go really really slowly and lots of cutting fluid. You finally realise SS is actually quite soft, and not hard as you might think, any reasonable bit will do, but I also prefer Cobalt.
 
I used cutting fluid, and went really slowly, with brand-new bits. Didn't work though.

So, if work-hardened, what's the recommended next stage, if any.
I did think of getting some 7mm bits to "start cutting" the edges of the hardened bit and make the hole a bit wider, before upping to 8mm; would that help?

Thanks.
 
Carbide tip masonry drill really slow until you get a chip then back to the original bit sharpened. It probably isn't hardened all the way through so if you can get a chip out you should be ok.
 
Main tip thatI was given by a man who did lots of S/S work was to make sure that you are always producing swarf ....no skimming over the surface. That means slow speed and LOTS of pressure.
 
Slow tip speed, loads of pressure and no need for cutting fluid. Keep neat swarf coming off and SS will drill relatively easily. I always use drill press when I can and never use a pilot drill for any hole less than 12mm as the pilot can work harden the outer edge.
 
Robert Wilson said:
Pilot 6mm hole drilled ok, but 8mm bits went blunt.
Will the stainless steel (8mm thick) have work hardened?

What's the best course of action now, to increase from 6mm to 8mm?

Why do jobs go wrong right at the end of a project? :confused::livid:

TIA, as usual
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In addition to other comments and for future reference, a 6mm pilot hole for an 8mm hole is too big, should have gone with a 3mm pilot. IMO
 
All the above tallies with my experience. Very slow, use a pillar drill lots of cooling, lots of pressure and if the drill isn't producing swaf, stop dilling immediately, continuing only makes matter worse.

I'd add that not all cobalt drills are equal, you need at least 5% cobalt alloy, preferably more.

I found that Trefolex Cutting Compound, as recommended on this forum worked wonders, for both drilling and tapping SS.

When I got into a similar situation, using a 3mm pilot for a 7mm hole, where the pilot drill had obviously work hardened the edge of the hole, I needed to be really brutal, with a very low speed and all the axial pressure I could muster, until you are effectively stalling the drill motor.
 
I always use old oil or WD 40 for lubrication when drilling SS. They both give off lots of smoke if j gets too hot. Which means you are not cutting swarf and not enough pressure or drill is blunt. Not so hard to sharpen the drill. Use an oil stone with drill bit vertical in a vice or use a powered grinding wheel very carefully. Get a large good sharp drill bit and carefully study the angles to reproduce on your blunt drill. I would persevere with drilling even if it is work hardened. However I would not have used a pilot drill. olewill
 
Robert Wilson said:
I don't quite follow that, would you elaborate please?

Also, I don't have an oil stone; presumably a "fine" file wouldn't be suitable?

Finally, are the advertised "drill sharpening" gizmos worth getting?

Thanks
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Putting the flats of two nuts together gives an internal angle of 120 degrees. That's a guide to the angle you need at the drill point.

A file wouldn't work, the drill bit is too hard.

Sharpening machines vary from very good to useless, you get what you pay for.
 
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