Sharpening drill bits that have been abused

[MikeBs]

Hi,
I drill a lot of holes mainly in stainless and mild steel, anything from about 2mm to 4mm thick. They are predominantly 8mm, 10mm and 12mm holes for bolts to fit through.
Its not precision stuff and I use a battery drill with 0-400 RPM and 0-1400 RPM
Prior to reading some info on this site, I had the philosophy of "If its not cutting fast enough then oil it and increase the speed".
I was also unaware of the effects of heat on metal and tempering, hardening etc...
Hence I have abused my HSS drill bits and they now no longer cut well and some not at all. I have tried sharpening them on a grinder, with some success, but they seem to blunt again really quickly.

My first question is - If I have taken a HSS drill bit to a point where there is smoke coming from it, is it just wrecked or can it be resurrected?

I always drill a pilot hole of 3mm - 5mm first, depending on what size I have on the bench. But this size bits are fairly cheap and I treat them as throwaway items.
Its the bigger drills that start to get pricey to replace.

My second question is - Are the expensive carbide drill bits worth it or are they just as easy to wreck or harder to sharpen?

I like to have about 3 or 4 of the larger sizes so if 1 stops cutting I can reach for another one, but getting a stock of carbide drills is an expensive game.

Any advise would be appreciated.

 

[Maker]

HSS can be annealed apparently but I've never managed it, it seems to stay hard forever, so if a resharpened drill doesn't cut for long, you've probably not sharpened it very well (too steep a relief angle will blunt quickly).

Carbide drills are definitely not worth it in a hand drill, you'll snap them so quickly.

 

[MetalMonkey]

Usually the correct speed is much slower than you think. Pilot just needs to be big enough for the web of the bigger drill.

Regrind the HSS drills if they are decent quality and not bent, they should be fine. Hand sharpening is an art form, but a good skill to acquire.

As above, carbide in a hand drill is a recipe for disaster, even the cobalt ones are really best used in a pillar drill.

 

[Seadog]

The pilot should be 1.5 x the chisel edge length, I read somewhere.

 

[Erie Fred]

HSS = Straw colored chips. don't run them faster than that, blue chips
are for carbide only.

When you grind them, do you dip them frequently in water to keep them cool ?
If your drill bits are mangled/chipped up badly enough for major grinding back to good,
consider slicing them off with a cut-off wheel, will be less grinding/heat.

 

[Rrobson]

Only really an issue with stainless, especially the thinner plate as it’s easy to overheat it. less speed more pressure.
If your not confident sharpening, buy a sharpening jig or go on YouTube, plenty of helpful videos on there.

I wouldn’t bother with a pilot for the size of drill your using, I’d just thin the web a little or grind to something resembling a split point drill, will make life much easier.

 

[MikeBs]

HSS can be annealed apparently but I've never managed it, it seems to stay hard forever, so if a resharpened drill doesn't cut for long, you've probably not sharpened it very well (too steep a relief angle will blunt quickly).

Thanks for all the replies.
I’ll stick with HSS for now then.
I do dip in water during sharpening.
After sharpening they seem to cut, but not unless I have a pilot hole. It’s like the outer part of the drill is sharp, but the middle just doesn’t have an edge. To get to 12mm, I start with a pilot hole then 8mm and work up to the larger size.
Whereas a new drill bit seems to drill with only a decent punch mark.

 

[mike os]

For stainless I prefer 5-8% cobalt and a proper cutting compound like rocol or similar

Sharpening is a bit of an art, 2 tips:

1 dont let it get too blunt, its easier to tickle than take a significant amount off, as soon as performance drops off change it
2 it's just a lot of practice to get it right... like most hand skills

For a significant amount I would suggest a more controlled set up... pillar drill or mag drill... better control and better speed control

A decent drill sharpener can make it simpler but at those sizes it's not really necessary

 

[brightspark]

i generaly use a pilot up to the size of the center chisel of the larger drill as that just rubs when turning instead of cutting .just go directly to 12mm as long as the drilling machine will pull it over with a reasonable amount of pressure at slow speed and using cutting compound

 

[frank horton]

cobalt drill bits are really not much more expensive than HSS
I'm gradually replacing all my HSS.......
they will take diy abuse better.........

 

[optima21]

if you're drilling with a hand drill I'd drill a pilot hole as it then needs less pressure to drill the larger hole, if you're using a pillar drill that may not be necessary though. dont forget that a pilot hole in a pillar drill will generally allow the hole to be drilled slightly more accurately...... when drilling 0.6mm holes I've drilled pilot holes 0.4mm diameter (not in a hand drill though)
another option for you might to be to use step drills if you havnt tried then before like these

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[MikeBs]

Another option for you might to be to use step drills if you haven’t tried them before like these.

I have used step drills and find them OK, but need to switch to a corded drill to use them.
Although it’s very rare I need precision, I find these can stray even more than I allow for.