Why did I break two drill bits?

[drummerdimitri]

I have to admit I am a newbie when it comes to drilling and today I had to drill a couple of holes in 4 mm thick steel bar but unfortunately ended up breaking a 8 and a 8.5 mm drill.

Not sure why that has happened and I want to understand why it did so that I won't repeat the same mistake in the future.

I was using a mag drill with twist drills, everything was clamped down firmly, used very high quality cobalt drills, light pressure on the feed, used cutting oil, started the hole with a center drill so it seems I did things right except I wasn't otherwise I wouldn't have broken the head off two drills .

That bein said however, I have no clue how deep to drill with the centering drill and whether or not I should have pre-drilled with a smaller diameter drill prior.

Also, the mag drill only goes up to 520 RPM but that should be an issue as I have successfully drilled multiple 2.5 mm holes with no issues in the past.

The first drill broke because the piece slipped out of the vise so that was understandable but the second one I'm not sure.

Could it be that I drilled too deep/wide with the center drill and that caused the drill to grab and break?

What is the right diameter starting drill relative to the drill to be used?

Any general advise that would prevent such future failures would be really helpful as this can become a nuisance given the cost of the drills (10-15$ each).

 

[DeeKay]

Where in the cut did the bits break? Towards the end of the hole or right when you started drilling? Most bits that I've broken happen at the end of the hole when they poke through and grab those two knarly looking bits.
I'm not a machinist by any means but from my experience Cobalt bits like higher speed and a fairly rigid set up. I've actually had better luck with HSS for hand and mag drilling, they're not as brittle and tend to be more forgiving... plus they're about half the cost.

 

[drummerdimitri]

Where in the cut did the bits break? Towards the end of the hole or right when you started drilling? Most bits that I've broken happen at the end of the hole when they poke through and grab those two knarly looking bits.
I'm not a machinist by any means but from my experience Cobalt bits like higher speed and a fairly rigid set up. I've actually had better luck with HSS for hand and mag drilling, they're not as brittle and tend to be more forgiving... plus they're about half the cost.

I'd say they broke halfway but it is hard to say as I had pre center drilled prior so not sure about that.

The only reason I bought cobalt bits is because I heard they would last a lot longer and be cheaper on the wallet in the long run as I tend to take good care of my tools. I've invested 200 $ in a set from 1-16 mm in 0.5 mm increments so I'm in too deep to switch to HSS stuff at this point.

 

[foghorn1966]

It could be a hard spot in your 4mmm steel bar that your bits are grabbing as well.

 

[drummerdimitri]

It could be a hard spot in your 4mmm steel bar that your bits are grabbing as well.

So how should I deal with that?

 

[Farleyfan]

is it actually braking the drill in half or chipping the leading edges of the drill ?

 

[drummerdimitri]

is it actually braking the drill in half or chipping the leading edges of the drill ?

Broke the head off completely

 

[SILVERPLATE]

Don’t know if I am right or wrong, but I always use a cutting oil when drilling metal.

 

[Farleyfan]

If the center drilled hole is bigger than the drill bit the tips on the edges could be grabbing and pulling in, the magnetic drill wont stall. Just a guess.

 

[drummerdimitri]

Don’t know if I am right or wrong, but I always use a cutting oil when drilling metal.

If you'd had read my initial post, you'd know that I had done that.

 

[drummerdimitri]

If the center drilled hole is bigger than the drill bit the tips on the edges could be grabbing and pulling in, the magnetic drill wont stall. Just a guess.

I think that's what happened as the hole was probably too large.

 

[GrantCee]

Generally, a pilot hole should be no bigger than the thickness of the web of the final drill.

However, an 8mm hole should not need a pilot hole. If positional accuracy is needed, a spotting drill could be used, but there's no need for a through-hole at that size, particularly given the relative thickness of the material.

 

[larry_g]

A lot of things here that could be the problem.

550 rpm is on the slow side. This allows you to get quite a big 'bite' and hang up the bit causing breakage.

4mm thick piece being drilled. Your almost through the piece before the flutes engage the hole. Again this allows the bit to grab.

A mag drill usually allows you quite a bit of down force giving to big of a feed rate. Again grabbing.

Cobalt bits are a bit more brittle so grabbing will allow them to shatter easier.

You talk of a mag base drill and then the part slipped out of the vise? Can you show a picture of this setup? Can you show a picture of the broken bits? In most places there is not a part of the drill bit that is called the 'head' so the term is a bit confusing. Can you use this diagram, http://www.mitsubishicarbide.com/en...drills_technical_top/tec_drilling_terminology to identify what you are calling the head?

lg
no neat sig line

 

[lardy1]

As mentioned above, if your pilot is too large, it engages the bit further from center. When it "bites" there's nothing there to resist the bite.

 

[Downwindtracker 2]

Don't feel bad, breaking drill bits is fairly common. That's why they come in packs of 10. chuckle. It would be hard to know when it happened with just a 4mm thickness. With hand held drill motor ,I was taught to speed up as it broke through and back off the pressure. Basically spinning without cutting. It could also be a case of a bad batch of drill bits. I've seen that a few times as well.

 

[BillK]

That is not very thick, just over 1/8" and not a very large hole. I use that size bit almost daily removing broken 8mm exhaust manifold bolts and I do them with my 20v Dewalt hand held drill. I am thinking that the 520rpm is pretty slow especially if you are using a power feed. I never use a centering bit on that size hole. I buy the McMaster Carr bits in boxes of 10 and I wear them out before they break. I also do them dry.

I will say I never use the Cobalt drills, just the plain high speed steel. I think the cobalt ones are probably a lot more brittle.

Try a hand drill, those bits should cut through that in about 20 seconds ?

 

[seagull369]

As others mentioned, cobalts are typically more brittle than HSS. The greater the % of the metal the bit has in there, the bigger the issue becomes.

Perhaps it might help to not tighten down the chuck on your drill as much to allow for some slippage if the bit gets hung up.

 

[matt_i]

In only 4mm I bet the magnet drill itself is lifting or twisting and working your bit into a bind where it easily snaps the bit. Happens so fast you don't know its happening.

I wouldn't even bother sub 6mm thick (hand drill instead) and really it should be 1/2" or 12-13mm thick before you even break out a magnet drill.

A lot of people are going to get mad at me but the cobalt 135s I find are nearly worthless over the long haul. The 135 doesn't follow a plot hole very well - tons of rotational chatter and beating up the rim of the hole at entry, and you can never resharpen the split point on a grinding wheel in a pinch, and you get hit for major money up front. I keep 12pc packs of 1/8" dia 118s in my shop for all kinds of pilot drilling. 5/32" work their way in for milling machine drilling. Mostly Chicago Latrobe but Triumph and Precision Twist Drill are brands I recommend.

 

[drummerdimitri]

In only 4mm I bet the magnet drill itself is lifting or twisting and working your bit into a bind where it easily snaps the bit. Happens so fast you don't know its happening.

I wouldn't even bother sub 6mm thick (hand drill instead) and really it should be 1/2" or 12-13mm thick before you even break out a magnet drill.

A lot of people are going to get mad at me but the cobalt 135s I find are nearly worthless over the long haul. The 135 doesn't follow a plot hole very well - tons of rotational chatter and beating up the rim of the hole at entry, and you can never resharpen the split point on a grinding wheel in a pinch, and you get hit for major money up front. I keep 12pc packs of 1/8" dia 118s in my shop for all kinds of pilot drilling. 5/32" work their way in for milling machine drilling. Mostly Chicago Latrobe but Triumph and Precision Twist Drill are brands I recommend.

You're right, even if it lifts off a couple of mm, it wouldn't be noticeable and yes now that I think about it, I should have used a hand drill instead. Guess I was just trying to enjoy my new t̶o̶y̶ tool even though the application didn't really call for it.

 

[SILVERPLATE]

If you'd had read my initial post, you'd know that I had done that.

Missed it, sorry.

 

[GrayFlattop]

What do you know about the steel you are drilling into? If hardened or HSLA, that could pose as issue. Try drilling a smaller hole first - like 4 or 6 mm to see how that goes, then go to your final size.

 

[BukitCase]

I keep a scrap piece of 3/4x3" steel flat bar to place BELOW the thin piece I wanna cut with my mag drill - This improves "grab" enough that I've drlled nice clean 1" holes (annular cutter) in 16 ga. steel this way, no lift-off whatsoever. I also drilled a 1-1/2" hole near one end of a scrap piece of 1/2"x3" flat bar, it's big enough I can clamp the bar to .095" wall square tube. My drill has the swivel base, so I can just get it close , turn on the magnet and unlock the swivel and fine tune.

It's always nice to have options... Steve