Drilling holes in metal

[brianhabby]

Hi there,

Does anyone know the best techniques/drills to use when drilling holes in metal?

I have some angle iron from an old bedstead that I want to use for making some adjustable legs for a project. I need to drill some 11mm holes so started with an 8mm HSS drill bit. the first three holes went without a hitch but I only got part way through the fourth hole before the drill gave up the ghost.

Frustrated at only getting three and a half holes out of a bit, I put a new one in but it just burnt out straight away without finishing the hole. So I bought yet another drill and this went the same way.

So, I've given up for now.

These holes are only a few inches apart. Can metal be much harder in one part than another? What am I doing wrong? Do I need to buy better drills, and if so which ones do I buy & from where? The last one I bought was a Rawlplug one so I thought that would be okay but it wasn't.

Any & all observations on this would be most welcome.

regards

Brian

 

[GCR]

Angle section from old bedsteads is notorious for hard "spots". It sounds to me as though you have work hardened/burnished the hole. You need decent metal cutting drill bits. If I were drilling that metal I would start with a smaller drill, say 4mm (after centre punching of course) and I would lubricate the drill bit with cutting oil, failing that 3in1. then step drill in 3 or 4mm increments.
Bob

 

[TonyW]

Brian
I suspect that you may have started with too large a drill bit. If you can, try starting with a smaller bit and working in stages up to 11mm. I also believe that speed of drill may be an issue - not sure but suggest that this should be somewhere in the 300-1000 range. Perhaps also use a cutting oil lubricant.

Cheers
Tony

 

[iajon69]

What type of drill are you using? If you are using a hand held drill then chances are that the cutting speed is way too fast.
As GCR has already mentioned, cutting oil or paste is a good idea - have found that engine oil is ok for a few holes.

A nice sharp drill bit, slow cutting speed and some lubrication and you can't go wrong

Ian

 

[brianhabby]

Thanks Tony & Bob for the quick replies.

I take the point about starting with a smaller drill bit and working up to the 11mm, but the hole that stopped me is only just under the 8mm (probably 7-7.5mm) so I would have though a brand new drill would have finished it of with no trouble.

It sounds to me as though you have work hardened/burnished the hole. You need decent metal cutting drill bits.
Bob

Bob, can you explain a bit more about "work hardened/burnished the hole". Have I hardened the metal more with my technique?

Also, you mention "decent metal cutting drill bits" but which ones and from where?

regards

Brian

 

[CHJ]

As the others have said Brian, you appear to have work hardened the metal by generating too much heat.

Normal HSS bits should be fine for that type of metal, the smaller the steps in drill size leading up to your finished size the better.

Although you can't see it the metal at the cutting edge is reaching red or even white heat, in worst case you will see sparks, the mass of the surrounding metal is then chilling it quickly resulting in the hardening due to the re-arrangement of the metals carbon structure.

See here for guide

But even these speeds may be too high if your drills are not ground with optimum angles etc.

basically as others have said drill much slower.

 

[9fingers]

Brian,

I know just how horrible bed angle iron can be to drill. Buy some new decent steel angle. try metal supermarkets* if you have one nearby or yellowpages.

Use low speed for drilling above 6mm and lubricate all sizes. If the oil is smoking you are going too fast and in danger of wrecking the drill bit.
Once a drill bit has lost its edge it will just get hot and be ruined. Sharpen it or use a new one.
Once a drill has been rubbing rather than cutting, the workpiece becomes locally hardened and is even worse to drill. It becomes 'work hardened' or burnished.
Good luck

Bob

*nearest metal supermarkets and brum and manchester so not much good.
I could not see you location when composing a reply.
Instead try Stockwell Steel

Bangor Station Yard, Caernarfon Rd, Bangor, Gwynedd LL57 4SU
Tel: 01248 364041

 

[TheTiddles]

Workhardening is what you get in paperclips and bits of garden wire, which is why you can bend them a few times before they break.

Metals are large regular structures of atoms (mostly, there are thousands of exceptions, but if you don't yet know about work hardening then this is a good place to start). However these regular structures are flawed, they are full of little discrepancies called dislocations, when you bend the metal you are moving these dislocations through the metal, till they reach a point that they can't move beyond (like the edge of the part). At which point if you try to bend the metal more you are not moving dislocations but crushing/stretching the actual metal matrix itself, which usually makes it break, or it just refuses to move.

When you are drilling the hole two things may be happening.
1) instead of cutting cleanly each bit of metal away, some is cutting and some is just deforming back from the bit, which is moving the dislocations away till they are all piled up around the hole and you are left with a very hard mantle around the hole.
2) when you heat some metals (especially steel) it can change its type, often to a harder form. The heat from the drilling may be locally hardening the material, which makes it more brittle but this isn't much use to you, it just makes it harder to drill through

So, does this help you, not really. There are ways to deal with these things but the best option is not to let them happen in the first place.

1) Get a decent, sharp drillbit. Anything that costs more than 10 pence is a good bet. HSS (high speed steel) is a standard material, it can mean a huge range of quality but a drill set that costs about £20+ is a decent investment, and you can sharpen then which cheap bits just aren't with the effort on. If your bit isn't sharp, sharpen it, you may need to do this every few holes.
2) Use a lubricant, 3-in-1, WD40, parafin, anything really, even soapy water would do, just use lots of it, this will absorb the heat, evaporate and keep the steel cool, keep it flowing.
3) Keep the speed down and the torque up, the bit needs to be cutting not flying over the surface creating loads of heat for nothing
4) Keep the bit straight, only the tip cuts, of the edge of the bit touches it creates lots of heat, vibration and load on the bit and drill for no benefit, with a hand drill this is difficult
5) Always centre punch and pilot, in any material, wood too.
6) A cone cutter might help you out, the advantage is you get a nice shearing cut and it will keep opening up the hole as you drill, finish with a straight bit for nice parallel sides, or better still ream, but those are expensive.

Alternatively, got any buddies with the right tools for the job??

Aidan

 

[jasonB]

If you have hardened the metal its not worth risking another bit on it.

You need to get rid of the hardness, this is known as annealing, with a blowlamp heat the metal to a dull cherry red and then allow to cool naturally DO NOT Quench.

Once cool you can try drilling again, at 8mm you can run around 800rpm dropping down to say 600rpm for the 11mm if using a drill press. If you are drilling with a handheld electric drill then 2/3rds of the speed will be better.

As has been said lubricate the bit with oil if you don't have cutting fluid.

A lot of the drills about are not fully ground as they are cheaper to make, try to find fully ground bits although they will cost more than DIY ones.

Jason

 

[brianhabby]

Thanks for all the comprehensive help guys, a great lesson in metalworking on a woodworking forum

I am using a drill press but set to the highest speed. I was under the impression that because these things are known as High Speed Steel drill bits then my drill should be set to a high speed. Obviously this is very wrong.

I think I'll dig out my blow torch and try Jason's method of softening the metal and then have one more go, using a coolant and slow speed. If that fails, then perhaps I'll get some new angle iron.

Bob,

Thanks for the info about Stockwell Steel in Bangor. I was aware of them, it's just that I have this stuff and wanted to do the job today (Sunday).

regards

Brian

 

[9fingers]

Brian,
The name High Speed Steel stems from the predecessor drill material which was carbon steel which had to be used with great care - generally slowly as it is very easy to overheat and soften.
HSS has things like cobalt added which makes it more tolerant but still has limits.

Reckon on using a cutting speed of say 15m/minute calculated using the radius of the drill bit so a 10mm drill is about right around 500 rpm.

Carbon steel still has its place in wood drills and many other wood cutting tools.

Bob

 

[brianhabby]

Well, I thought you all might like to know that your advice paid off

4mm then 6mm then 8mm - slow the drill down - and plenty of coolant - I used some sewing machine oil because that's all I had. The oil has another side effect that I discovered - instead of the dry swarf flying around the workshop, because it is now wetted with the oil, it tends to stay put which I feel happier about. I'll finish the larger holes again because I think I need a new chuck for my drill, the teeth have gone and I can't get it really tight.

Once again, thanks for all the help.

regards

Brian