Morris Minor Owners Club

Excuse my ignorance; does anyone know what speed I should set my drill to for using cobalt spot weld drill bits? I have an old Black & Decker late 1960's drill with just two speeds- 1200rpm and 900rpm, and I suspect it will be too fast for them and cause them to blunt very quickly.

When using standard cobalt drill bits I've always used max speed - I've no idea if this is good but they didn't blunt nearly as quick as my HSS (rubbish) bits.

Hello Ray,

is it your drills that are rubbish or do you think that HSS is rubbish because it is not, as long as you buy good quality ones.

Mild steel machining speed is about 80ft\min which gives a 1\4" drill at 1200 rpm as a guide. I'm not sure what speed for cobalt tools, but are they solid cobalt or plated similar to tin plated drills?

Alec

Hi
I run a bodyshop and find the drill bits last longer at a slower speed and also you can be more accurate when drilling through the piece you are taking off and leaving the piece staying behind intact,at high speed you will be inclined to go through both pieces which you dont want to happen
Ian

Good advice Ian, you must be an engineer! Timber is best cut at high speed.

is it your drills that are rubbish or do you think that HSS is rubbish

The one's I've bought were rubbish, generally because they were cheap. The problem is knowing which are good and which are the same cheap stuff with a high price!

are they solid cobalt or plated

Cobalt steel was my understanding (as in steel with a small amount oif cobalt in the blend so they can give it a good surface hardness.

Hello Ray,

OK, I just had a quick look and cobalt is classed as the next step up in hardness from HSS. One point about drilling is that adequate pressure should be applied or else the drill just skids and overheats. I use HSS generally, even for stainless steel that I work with quite often, drills do need to be kept sharp though.

Alec

Many thanks all. I'll use the slower speed and apply adequate pressure. Better get started on my chassis leg then...

I use an engineering supply company, they have a website, send out a free catalogue, and offer next day service. They offer a range of different qualities for all their items, much easier than taking pot luck at your local supermarket! J&L industrial supplies, WWW.METALWORKING. MSCDIRECT.COM Should get you to them!

Thanks linearaudio. I had a look at their website and notice that they are based in the USA. Might be a problem returning something if it's the wrong item though.

Oh dear! Got the wrong address!!

UK branch is www.mscjlindustrial.co.uk

Remember having this trouble finding their web address when I first tried!!

Well worth a look at their catalogue!

Excellent! Thanks. Looks like they have everything. I'll give it a go.

Interesting, this thread may save me running through so many drill bits, I may go out and invest in a good quality drill ;-) I had never considered the speed of the drill as a factor in blunting bits..

Hello Nigel,

if you are talking about drilling thin metal the speed is not that important as the drilling time is short, and so the heat won't build up too much. Once a drill loses it's edge however, it will heat up and blunt even more. As I said earlier they do need a lot of pressure to work effectively.

Alec

They shouldn't have a lot of pressure put on them, that is what shortens their life. A good drill bit at the correct speed for its diameter and material should almost cut by itself! It can also be just as damaging to run a drill bit too slowly, as it will not cut quickly enough and therefore ends up generating more heat. The important thing is the cutting speed in feet/min, but having said that its not likely you will find that information at Halfords!

Hello Linearaudio,

I have to disagree, drills will not 'cut by themselves' except in some materials, particularly plastics where they need restraint in many instances particularly at breakthrough. We are generally talking about the first drill i.e not following a pilot drilled hole.
I did quote the cutting speed for steel in my earlier post and that and the one for Aluminium (1,000 ft\min) are the only ones I remember

Alec

Alec wrote:Hello Linearaudio,

I have to disagree, drills will not 'cut by themselves' except in some materials, particularly plastics where they need restraint in many instances particularly at breakthrough. We are generally talking about the first drill i.e not following a pilot drilled hole.
I did quote the cutting speed for steel in my earlier post and that and the one for Aluminium (1,000 ft\min) are the only ones I remember. Alec

They did when I did my apprenticeship. We were told never to apply force to any cutting operation. Mind you we knew how to sharpen drills in those days, and were in a fairly ideal workshop condition. Real life isn't like that!

Hello Linearaudio,

We will have to agree to disagree, but what does an auto feed drill do but apply pressure?
I have always sharpened my own drills, as it would be prohibitively expensive to discard them when blunt and a blunt drill is totally useless.

Alec

I think the word 'force' is misleading, it implies excess, I think you need to gently push a drill in order to progress!
The only time I have been aware of a drill cutting a hole without some form of pressure is when you open out an existing hole, then the drill bit tends to get pulled into the workpiece, especially in brass, then it's a job to hold it back!

les wrote:I think the word 'force' is misleading, it implies excess, I think you need to gently push a drill in order to progress!
The only time I have been aware of a drill cutting a hole without some form of pressure is when you open out an existing hole, then the drill bit tends to get pulled into the workpiece, especially in brass, then it's a job to hold it back!

Yes, thats the crux of the matter, on pondering, I may have been a bit pedantic- the original comment about having to use "a lot of pressure" to get a drill to cut seemed to me to imply excess, but I agree the term "force" means nothing without a qualifier. What I was driving at is that if everything is set up correctly, then surprisingly little pressure should be required ( though granted, stainless is a bit tough!). What a muddle! Let's leave that one to die

Thinks:
"Quality" is another word which means nothing without a qualifier eg- good, bad, yet if you buy a "quality" product (as all things seem to be advertised) then you immediately presume a "good" signifier! And are often disappointed!! Thats another gripe entirely though!