Drilling stainless steel

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Bi111ion

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So I want to enlarge a hole for a bolt on a fitting made of fairly chunky stainless steel. I thought cobolt alloy drill bits would do it bit much heat is generated and very little hole. What speed should I use? What kind of lubricant or cutting paste? Do the bits need to be ground to a different angle?
 
You want to cut slowly with a lot of pressure. Light oil or cutting paste helps. If you go fast you just build up heat and null the cutting edge of the bit, whilst hardening the steel!
Ideally you should produce a continuous piece of swarf as you cut.
 
With care you can just use a standard HSS drill bit, slow speed, and some lube.

It is easier with cobalt drills, new sharp decent quality ones not cheap rubbish, and lube, at slow speed.

Keep plenty of pressure on as well.

I drill 1/2mm to 50mm stainless of lots of grades and with care and practice it is nicer to drill than mild steels.
 
I used cobalt drill bits and machine oil to lubricate and drilled slowly, as recommended by a marine machine shop in Trinidad. I don't know what cobalt alloy is I thought the bits are pure cobalt.
 
The biggest problem I have found drilling stainless is that small power drills say 750W either go to fast and work harden the hole or snatch and stall the drill. I have a 1400W drill that goes slow with lots of torque and gives me no problems in drilling stainless. Just a thought.
 
KellysEye said:
I used cobalt drill bits and machine oil to lubricate and drilled slowly, as recommended by a marine machine shop in Trinidad. I don't know what cobalt alloy is I thought the bits are pure cobalt.
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Cobalt drills have about the same composition as HSS steels but with the addition of between 5 and 8% cobalt. HSS or tool steels have high carbon contents, anything between 0.5 and 1.5%, with a variety of alloying elements that often include molybdenum and tungsten.
 
Bi111ion said:
So I want to enlarge a hole for a bolt on a fitting made of fairly chunky stainless steel. I thought cobolt alloy drill bits would do it bit much heat is generated and very little hole. What speed should I use? What kind of lubricant or cutting paste? Do the bits need to be ground to a different angle?
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You need to say what diameter hole because the speed is related to that......Don't dwell keep the pressure on & with a bit of oil as lubricant Bob's your uncle :encouragement:
 
Thanks everyone that is really helpful. To summaris Powerful drill at slow speed. Cobolt drill.

What sort of oil should I use? Will engine oil do or do I have to buy something specially?
 
One of the most useful tools I ever bought was a pillar drill. It's just the cheapest machine mart one, but excellent for drilling metal. 500 rpm on the slow setting, but plenty of torque and most importantly is lots of pressure applied vertically. I guarantee if you bought one to do this you'd end up using it all the time.
 
I used cobalt drill bits to drill out the SS machine screws holding my mast foot down, after that exercise they were pretty much all blunt. They cost me around £30 for the set, so it seemed a bit daft and wasteful to bin them. I purchased this http://www.axminster.co.uk/multi-sharp-multi-sharp-dual-purpose-drill-tool-sharpener and with very little practise you can sharpen ten drill bits in minutes, and boom, the difference is amazing between a 50% dulled bit and a 100% sharp one.

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I have a spray for just this job that is a veggie based oil. Far less toxic than mineral oil, but it's on the boat and I can't give you a brand name, but it is German. Bought from Toolstation IIRC.
 
Bi111ion said:
I thought cobolt alloy drill bits would do it bit much heat is generated and very little hole.
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If by 'bit' you meant 'but', are you saying that you've already tried to drill the hole and failed? If so, you've probably work-hardened it and will be very lucky to enlarge the hole by means of drilling in situ. Might be best to take the part to a machine-shop, assuming it can be removed.

As said, for drilling SS by hand: sharp bit (HSS is fine, cobalt not necessary), slow speed, keep the job wet with oil, don't let it get too hot, lots of pressure...and be patient. Drill press is ideal if practicable.

And for the bi111ionth time, do stop calling it 'cobolt' ;)
Good luck.
 
I believe that the key to it is to make sure that you are always creating swarf. If the drill starts to "skate" over the surface then it will blunt and the SS will work harden.
 
Kelpie said:
The one thing I forgot to say was, use a centre punch first.
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Although I always centre punch I did read in a guide to stainless steel fabrication that this should not be done as it can work harden the very point where you want to drill.

As a cutting fluid I always use 'Rocol RTD'
 
A triangular tipped punch is recommended to reduce work hardening, though I've not got one.
Rigidity, including stub drills, is needed for SS, more difficult by hand.
Also, a more obtuse drill point angle, 140 deg.
Recommendations from Dormer Twist Drill and Reamer Handbook.
 
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