Cobalt drills

[vyv_cox]

I agree you cant harden the structure but do you not get a formation of chrome oxide on the surface which is very hard?

All stainless steel relies on a surface film of chromium oxide to provide its corrosion resistance. It forms within seconds of being cut or machined but is very thin.

 

[coopec]

>
, I have never been able to drill Stainless with HSS bit designed for metal only Cobalt has been successful for 316. Next time you do it please can you post a video of it.

 

[KellysEye]

>Next time you do it please can you post a video of it.

Firstly it's very thin secondly is it 316 marine grade?

 

[Scillyboy]

FWIW the best set of drills I've ever had are from Lidl's. They claim to be cobalt and suitable for SS and I've certainly had more luck with them than "genuine" cobalt drills bought individually from a local engineering supplier. At £5 or £6 pounds a set they've been excellent. Probably not as good as a £45 set, but I'm not prepared to spend that much on drills.

+1. I bought a second set for the boat. I've used them on S/S very successfully with a spot of cutting oil. They come in 0.5mm size increments, which I find useful.

 

[Nigel-in-Oxford]

Last week I got a set of cobalt drills from screwfix for drilling my prop shaft. I was surprised at how easily they went in. Made it as easy as drilling mild steel, light pressure with a dewalt 12v drill on low speed.

 

[coopec]

This article is worth bookmarking

"Stainless steel material is however susceptible to a phenomenon called work hardening which is one of the reasons this material is difficult to cut and drill. Work hardening is the result of the action of cutting edges deforming or compressing the surface of the work piece, causing a change in the worked material's molecular structure that increases its surface hardness. .........."

The full article Tool Tip: Drilling in Stainless Steel
http://blogwithtravers.blogspot.com.au/2013/02/tool-tip-drilling-in-stainless-steel.html

 

[sailorman]

"Work hardening " who would have thought it :disgust:

 

[vyv_cox]

This article is worth bookmarking

"Stainless steel material is however susceptible to a phenomenon called work hardening which is one of the reasons this material is difficult to cut and drill. Work hardening is the result of the action of cutting edges deforming or compressing the surface of the work piece, causing a change in the worked material's molecular structure that increases its surface hardness. .........."

The full article Tool Tip: Drilling in Stainless Steel
http://blogwithtravers.blogspot.com.au/2013/02/tool-tip-drilling-in-stainless-steel.html

However, we can also make work hardening work for us, rather than against us. Very many boat fittings such as bottle screws, tangs, chain plates, bolts and many others are cold stamped, which work hardens them and increases their strength considerably. Perhaps the best example is stainless steel wire, which is cold extruded. A reduction in surface are of only 3% in this process multiplies the strength of the annealed metal by three or four.

Incidentally, beware of statements like: 'causing a change in the worked material's molecular structure'. Work hardening can be seen in a normal metallographic microscope, it is due to the accumulation of dislocations in the crystal structure. Molecules are about a billion times smaller.

 

[nigelmercier]

They come in 0.5mm size increments, which I find useful.

I've still got my 1/2" HSS drill set my dad bought me 40 years ago (well, most of them), increment is 1/64"

I had a rule: only drill the initial hole with multiples of 1/8", then you rarely need to sharpen the intermediates.

Also got an AF socket set from the same era, but the case is almost rusted away. Used it last week.

Now I feel old.

 

[Sarnia]

Personally I just buy cobalt drills in the sizes I need.
There are a few good sellers on eBay.

Me to, it always depends on what I am drilling, cobalt not always needed.

 

[Sarnia]

I'll leave Sailorman to explain! Drill sharpening is quite an art (and one I have to say, I've only ever had limited success with). There's the angle of the "cone" as you look at it from the side, but there's also the "rake" angle on each of the flutes. At almost microscopic level, (easier to see, the larger the drill is), when you look at each flute, it obviously needs to dig into the metal that you're drilling and "shave" a piece off, so the angle if you look at a cross section through each flute from the side is also important. That said, all other things being equal, cobalt drills are just "better" in my experience.

You can buy drill sharpening machines (of various quality). They usually have a flat grinding wheel and a holder that holds the drill bit at the correct angle to it. Most have more than one operation because there's more than one angle to get right.

The rake angle is very important, different angles for different materials, which also have recommended cutting speeds depending on the material, there are engineering tables for this. Some materials require a negative rake for best results, such as brass, cast iron, negative rake is achieved by grinding the cutting edge of the drill approximately at 90 deg. to the edge, just a touch on the grindstone is sufficient to produce the negative rake required. Similarly when turning on a lathe the cutting tool is ground negatively for those metals, quite different for s/s and mild steel.
Lubricants are important for both cooling and cutting efficiency, engineering shops still use a milky coloured oily 'mistic' as it used to be called anyway.
If you want to help cutting s/s try some spit, works well, also with other steels, some on your finger applied to the drill

 

[pete]

I recently made a few exhaust flanges out of 8mm thick 304 Stainless steel, I cut the holes using a 35mm Starret hole saw at slow speed using my bench drill and a bit of lubrication. I make 5 of these plates with a lot less trouble than I expected. Slow speed, and make sure the cutter keeps cutting and not just skidding on the surface.

View attachment 40256



View attachment 40257

Pete

 

[huldah]

I like to check my drills with a powerful magnifying glass before drilling stainless. It must cut immediately to avoid the risk work hardening. Usually a quick rub with a Easy Lap diamond sharpener is sufficient.

I have not needed to do this, but if work hardening has occurred, it may be possible to remove it with a Bremmel fitted with a diamond tip.

 

[coopec]

Others use masonry drills to drill stainless steel too.

http://bbs.homeshopmachinist.net/threads/29579-Carbide-tipped-masonry-bit-in-metal

 

[sailorman]

Others use masonry drills to drill stainless steel too.

http://bbs.homeshopmachinist.net/threads/29579-Carbide-tipped-masonry-bit-in-metal

you are joking

 

[BarryH]

I use these from Screwfix http://www.screwfix.com/p/erbauer-ground-hss-drill-bit-5mm-pack-of-5/65352 They also do a bulk pack of 150 assorted sizes for about 50 quid. I use them in my day job and find they last as long as more expensive bits and I drill a lot of holes.

Drilling stainless is more about technique than anything. I've drilled stainless with cheapo "Jobber" bits. Keep it slow and don't overheat the bit.

 

[lw395]

Others use masonry drills to drill stainless steel too.

http://bbs.homeshopmachinist.net/threads/29579-Carbide-tipped-masonry-bit-in-metal

The spade type carbide bits sold for drilling glass and tiles work well.
Particularly for enlarging a hole where the stainless is already work hardened.
I have a solid carbide end mill that goes in my dremel, very effective!

But a cobalt twist drill is first choice IMHO.

 

[sailvayu]

I use a regular drill bit when working SS. The trick is to run the bit at a low speed 1500 rpm around that, keep a steady firm pressure and hers the real trick, keep a steady flow of water to cool the bit. Oil will not work as it burns off. I use a little squirt bottle with water in it to shoot water at the bit as I am drilling. Do this and you will think you are drill mild steel and your bit will last for a surprisingly long time before needed sharpening. One of these days I need to make a video of this. This does work
Capt. Wayne

 

[nigelmercier]

Others use masonry drills to drill stainless steel too...

I occasionally watch the TV shopping channels. There's an advert for drills that look like masonry drills, but they use them to cut through several materials, including a steel file. Always been a bit sceptical myself.

 

[vyv_cox]

I occasionally watch the TV shopping channels. There's an advert for drills that look like masonry drills, but they use them to cut through several materials, including a steel file. Always been a bit sceptical myself.

There was always a man at LIBS selling drills that would cut files and almost everything else. They always looked pretty impressive but when I finally went to buy some he was no longer exhibiting. I understand he retired a few years ago.