How (not) to drill metal sheet?

I was faced recently with trying to drill an 8mm diameter hole in a piece of sheet metal 2-3mm thick. The sheet was a bit from an old lawnmower blade. Sawing that off made me think this metal sheet might be particularly hard. I tried to drill a small diameter hole and gradually increase the diameter with successive drillings. I was using a tower drill and bits which had scarcely ever been used before. The drill was set at medium speed of rotation. It worked for the first 2 diameters, 3 & 5mm I think. But at 7mm the bit just refused to bite. I tried another similar sized bit from a different set, still no good. It was as if either the bits were blunt or the metal suddenly much harder. I have tried sharpening the 7mm bit and added a drop of oil to the cutting surface - less noise until the oil was gone then shrieking loadly before I stopped drilling.

Am I doing something (possibly everything) fundamentally the wrong way? Any adivice would be much appreciated.

TIA Frank Stacey

Reply to
Frank Stacey
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how did you sharpen the 7mm drill bit? although HSS drills are not difficult to sharpen there is a knack and if not just right the cutting edge does not touch

You are using HSS drills I assume

Try the 7 mm drill in a scrap bit of mild steel and see if that works you will soon know if it is properly sharp

alos be aware that the drilling speed are much lower for 7 mm than 3 or 5 mm typically 2000rpm for 3mm and 900 for 7 mm

If you have overheated the bit you may have ruined it

some info here

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Reply to
TMC

In message , TMC writes

I would expect mower blades to be made from a hardenable steel, hardened and then tempered back to a compromise between retaining a sharp edge and too brittle/subject to fracture.

When I had a proper job, odd bar ends of bearing steel (SKF3) used to find their way to my home workshop. I quickly found unlubricated turning swarf self hardens to little springs.

Red heat and slow cooling would solve your problem but may spoil the blade for the purpose you intend.

An expert will be along shortly:-)

regards

>
Reply to
Tim Lamb

Some more infor here:

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Reply to
John Rumm

Steel doesnt 'work harden', it has to be heat treated. Something like a rotary mower blade would not be hardened all over, as a little bit of flex would be good in such a situation. Case hardening of the blade edges would be possible, though unlikely for the usual back-garden mower.

I'm not so sure. My Hayter rotary blade is pretty soft, and has to be changed/sharpened once a year. If hardened, I'd expect it to last a lot longer.

Alan.

Reply to
A.Lee

Sorry. Try

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many others.

Reply to
1501

Next time drill first then cut, and the metal won't get so hot, and is easier to hold. Drilling with the work immersed in water or oil may help too. Once it gets hard: it's time to anneal or start again if you wanted to preserve the original qualities of the metal. At a pinch you can try a glass drill, but the tungsten tips of those get very hot and soon fall out or snap. Then you could try enlarging your pilot holes with diamond burrs. Better to think of the proper approach in advance.

S
Reply to
Spamlet

Generally agreed but for relatively thin metal the tendency is for big drills to get half way through then rip things to bits. At faster speeds this tendency is less likely, but you harden the metal and blunt the drill more quickly. All a bit trial and error. Possibly best get as far as you can, then sharpen, and turn piece over and try and cut through quick at speed from the other side, before the metal guesses what you are up to ;-) And see my other note.

S
Reply to
Spamlet

In article , Frank Stacey writes

Metal cutting drill bits just aren't meant to cut that way, they are meant to start a cut near the (centre) point then propagate and maintain that cut to remove material. By all means drill a pilot but then let the correct size bit start and maintain the cut.

I can see why you went that way but a drill bit is not the tool for gradually increasing a hole. Arguably, 8mm in a 3mm depth is probably enough for the bit to be self centring and avoid it counting as 'sheet'. In a borderline case, I'd be inclined drill the largest hole that could be maintained as self centring then enlarge it with a conecut or tapered reamer.

I don't do a lot of this kind of thing but the above was recounted to me by one who did.

Reply to
fred

Try some Bosch or Dormer HSS-G drill bits, you can pick them up cheap on fleabay.

Managed to get a Bosch white holesaw "bi-metal" to throw yellow metal today on a backbox. So old it was undecided until the last moment whether to friction weld, eject its teeth or actually achieve some cutting before the universe goes cold.

Quality bits and jobber bits can vary quite considerably, and plenty of bent half-blunt stuff kicking about. I binned a lot of old bits and just bought HSS-G, whilst tired a few nights ago I found I had drilled quite hard brick with one quicker than the Bosch Multi-Construction bit. I have found even those branded drill bits to vary a little more than I would like.

Reply to
js.b1

I've just drilled 19mm holes in boxes made from 2mm mild steel.

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'm no expert but it seemed to work. My theory is to drill a small hole a bit wider than the central non-cutting part of the big drill, i.e. about 6mm in my case. Then I used the 19mm drill with a very slow speed, about 240 rpm using cutting fluid. The work has to be held very firmly. I don't think it pays to try to widen a hole a mm at a time with a twist drill. A reamer would be better, but might be wrecked on hardened steel.

Reply to
Matty F

It is possible to get tungsten carbide metal cutting drills (solid or inserts)for difficult steels .

Reply to
F Murtz

There is a drill grinding technique that involves reducing the cutting angle on the lips of the drill. It avoids the drill getting pulled into the metal. In effect you are giving it a negative rake

Reply to
John

When drilling, the steel will inherently be in the plastic limit, where as you correctly point out, Young's Modulus will be increased. But unless the steel is of type which can be heat treated, the steel will be pretty uniform, and so drilling one area shouldn't be very different to drilling in another.

I suspect it's more down to the drill geometry, though could be wrong.

My experience of lawnmower blades if that they're pretty soft where I've been able to file the blade to an edge. The last thing a manufacturer wants is a shattered blade. Plus mulching grass isn't such a bad thing!!

Reply to
Fredxx

I have used tungsten carbide masonry bits with steel since it's also easier to grind them to a sharp edge which cuts. However they are very unforgiving if the work pieces moves or get stuck, resulting in a chipped tungsten carbide bit.

Reply to
Fredxx

In message , Fredxx writes

Place isn't the issue. Quality of the steel is. If the blade were made of mild steel the OP should not have had a problem if using properly sharpened bits.

I think he has locally hardened the steel surrounding the initial hole. Annealing the work by taking up to red heat and then allowing to cool slowly should solve the problem. It is a very long time since I was an apprentice but quenching from red heat in water should produce brittle hardness and quenching in oil an intermediate level.

A pilot hole larger than the core diameter of the finishing drill should do the job. Low speed and ample coolant avoid localised work hardening.

Combine harvesters have serrated blade edges which self sharpen in work although impact with mole hills can have shattering effects:-)

regards

Reply to
Tim Lamb

The steel you're drilling has work hardened and no HHS bit will touch it. I don't know what you're using the steel for but heating it to red and cooling slowly (preferably under sand) will soften the steel and allow you to drill it. Alternatively, use one of the TCT wonder drills that you see advertised. I've seen them drill files, hacksaws and bearings and the do work. They look like masonry drill but take higher temperatures. The technique is to use max speed, max pressure and no coolant/lubricant. It sounds cruel but friction heats the steel to the point that it softens, then the drill cuts it. The swarf comes off red hot, sometimes incandescent. I've drilled out broken cap heads with these when no HSS drill would touch them. I now keep at least one set for getting out of trouble.

John

Reply to
John

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Reply to
zaax

Speaking as someone who has drilled many thousands of holes in all sorts of soft and hard metals...

1 The drill was not ground to the correct angles. I'll try and post some photos about this. 2 It was spun too fast and polished the metal, rather than work hardened it. 3 The twist drill was allowed to spin without pressure on the metal and it couldn't bite. 4 The twist drills were of very poor quality and couldn't keep an edge. It happens.

Dave

Reply to
Dave

The metal should not be even warm when drilling. If it is, the twist drill is not sharp.

If the twist drill is a good one, then you won't need a coolant.

For the metal to change state to hard, it would have to get to a very high temperature. I think you are confusing this with the bit polishing the metal because it can't bite and cut.

Don't even try this. An HSS twist drill, which has been ground correctly will tackle most steels, but the case hardened, or hardened ones.

Many years ago, I had to drill some incredibly hard steel on an aircraft. Twist drills come in several varieties.

1 These are made of something akin to copper. 2 HSS twist drills come in 2 types, those made of chalk and break every time you use one and those that are made of good metal that will take and keep an edge and drill a hole 3 You now move up to cobalt drills that are a lot harder that HSS. 4 Next comes what is known as a C1150, harder still. 5 Above this comes the D 200 which was what I was using to drill the aircraft steel. 3 by 2.5 mm holes and I had to go and grind a new cutting edge on it. Beyond this, you have to go to solid tungsten carbide drills.

Dave

Reply to
Dave

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