Cobalt drills

[Neil]

My drill bit sets are sadly depleted and since I have need of drilling SS shortly, I'd thought of investing in a set of cobalt drills. However, prices seem to vary widely and closer examination sometimes reveal that the real cheepies are cobalt-coated. Some state a cobalt content (3%-5%). Pukka makes are expensive, but how much is in the name? Any opinions amongst the cognoscenti as to the best compromise between cost and quality, and source?

 

[nigelmercier]

I've got a set of these, very pleased:

http://www.machinemart.co.uk/shop/product/details/cht384-25pce-cobalt-steel-drill-bit-se

 

[Neil]

I think the 19 pc set would be plenty for me - but there's a dearth of detail; what content the cobalt?

 

[lw395]

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

 

[Rigger Mortice]

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.

 

[sailorman]

i sharpen my own HSS twist drill bits & have never ever used anything else

 

[Avocet]

I find them very handy when using a hand drill in situ on a job. HSS works fine if you can get enough pressure on the bit to make it bite (e.g. in a drill press or pillar drill) but n the job, you don't always have that luxury.

 

[PCUK]

i sharpen my own HSS twist drill bits & have never ever used anything else

Me too!

 

[coopec]

I was buying cobalt drills as I thought I needed them to drill SS but an old engineer (with stacks of experience) pointed out, as sailorman says, HSS are perfectly alright to drill SS.

Actually I consider myself a bit of an expert on drilling SS because when I was drilling my chain-plates I typically took a day to drill a hole (if I did that I thought it was acceptable as at least I was making some progress). I drilled small pilot holes first and that was quite a help but when I tried to drill the main hole the drill just spun uselessly. I went back to the engineering supply crowd who were quite helpful and they supplied "Trefolex" and that did help a bit but more importantly they put me on to a guy who they claimed was an expert. When I phoned this chap the first question he asked was "has it been welded near where you are drilling?" I answered in the affirmative and he said "You are going to have a hell of a job because it is heat hardened". He advised when drilling SS you need very low 110 RPM and very large pressure. Furthermore he said if the drill doesn't "bite" straight away the SS will also "work harden"..
Fortunately my B-I-L came along and examined my predicament and he said when they were repairing Caterpillar Dozer engines and they had to drill out a broken engine stud they used sharpened masonary bits. We sharpened up a masonary bit, I made up a "stirrup" (threaded rod, a steel loop on one end and nut on the other) and he got his very old/large (aluminum cased) drill. There were plenty of holes in the deck for stanchions etc so I went below and put a nut and washer on the threaded rod that he had passed through. Then I went back up on deck, put a 6 foot length of 3" X 2" timber (75mm X 50mm) through the loop and across the back of the drill and put all my weight on it while he worked the drill. We stalled the drill plenty of time but each hole took about 2 minutes!!

 

[alahol2]

Yes, the wooden lever across the back of the drill to apply extra pressure works well if you can arrange it. I managed to drill a 6mm hole across my bow roller fitting using a battery drill and normal HSS bits by this method. Cutting grease helps too.

 

[sailorman]

I find them very handy when using a hand drill in situ on a job. HSS works fine if you can get enough pressure on the bit to make it bite (e.g. in a drill press or pillar drill) but n the job, you don't always have that luxury.

all depend`s on the angles one sharpens them with

 

[coopec]

sailorman

Without having a drawing it is difficult. By angle do you mean how pointed it looks from side on or do you mean how agressive the cutting edge is?

 

[vyv_cox]

I went back to the engineering supply crowd who were quite helpful and they supplied "Trefolex" and that did help a bit but more importantly they put me on to a guy who they claimed was an expert. When I phoned this chap the first question he asked was "has it been welded near where you are drilling?" I answered in the affirmative and he said "You are going to have a hell of a job because it is heat hardened". He advised when drilling SS you need very low 110 RPM and very large pressure. Furthermore he said if the drill doesn't "bite" straight away the SS will also "work harden"..

No argument with his work hardening advice but the 'heat hardening' is nonsense. It is not possible to harden a 300 series stainless steel by heat treatment, which includes welding. It could only happen if the rods used were of a completely different material. The only exception to this would be if the material was cold rolled, making it harder, in which case welding would soften it.

 

[sailorman]

sailorman

Without having a drawing it is difficult. By angle do you mean how pointed it looks from side on or do you mean how agressive the cutting edge is?

both, by experience for the material to be drilled eg timber

 

[HunterWanderer]

No argument with his work hardening advice but the 'heat hardening' is nonsense. It is not possible to harden a 300 series stainless steel by heat treatment, which includes welding. It could only happen if the rods used were of a completely different material. The only exception to this would be if the material was cold rolled, making it harder, in which case welding would soften it.

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

 

[Avocet]

sailorman

Without having a drawing it is difficult. By angle do you mean how pointed it looks from side on or do you mean how agressive the cutting edge is?

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.

 

[coopec]

vyv_cox

You sound as though you know what you are talking about and I am right out of my depth. When I look at this I say you are right

3.7.2 Heat Treating Stainless Steel | Forging Industry Association
https://www.forging.org/design/372-heat-treating-stainless-steel‎
Austenitic Stainless Steels — Conventional austenitic stainless steels will work harden with cold working, but cannot be hardened by heat treatment. Annealing ..

But I've drilled plenty of stainless steel and never experienced the trouble I had with the chain-plates.

Suppose they used a guillotine to cut the s/s strip

"COLD WORKING Definition: Altering the shape or size of a metal by plastic deformation. Processes include rolling, drawing, pressing, spinning, extruding and heading, it is carried out below the recrystallisation point usually at room temperature. Hardness and tensile strength are increased with the degree of cold work whilst ductility and impact values are lowered. The cold rolling and cold drawing of steel significantly

As I say I'm out of my depth,

 

[KellysEye]

>i sharpen my own HSS twist drill bits & have never ever used anything else

You don't mention Stainless but I assume that's what you imply, 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.

 

[Neil]

This is all very fascinating, but what's the best cobalt drill to buy?

 

[coopec]

vyv_cox

I don't know why it was so hard. Apparently guillotining and punching will work harden metal. Maybe after welding the s/s he used heavy force to bend it flat? After looking at his work (which was good) that is likely.