hole in the middle?

Give yourself a chance by not trying to drill the tapping size hole in one go. Either buy a 1/8" stub drill or (better) a BS3 centre drill. Either gives you a short stiff drill which will locate much better in your centre pop and give a start for the bigger drill.

You can drill aluminium with wood drills - if you had a brad point wood drill of the right size (1/4 BSW = 5.3mm) that would work too - what you need is a positive centre location.

On or around Mon, 07 Jan 2008 15:57:18 +0000, bugbear enlightened us thusly:

I assume you got your centre punch in the middle, by the expedient of drawing diagonals on the square bit first?

Yes, and I even marked a 7mm circle (as well as the outer 25mm circle for the final shape of the knob) from the centre "dot".

So I can see and measure my error quite well :-(

BugBear

As Norman B suggested, if you 'center popped' first it will only be quite a shallow indentation, use a small drill first (like 1/8") as that will locate in the mark. Then use the 1/4" tapping drill. Alu is obviously quite soft, so a 'big' drill will tend to grab into the metal only approximately where u first place it - hence (probably) your 1.5mm error. Mike

Hi BugBear, When centre punching Alloy a pronounced burr is raised, it is all to easy for a drill to slip down the outside of this burr, leading to a misplaced hole. You could try filing the alloy flat, this would leave just a dimple for the drill to start in, ideally this drill should be a centre drill, which "does what it say on the can". Centre drill till the hole is wide enough for the chisel point to fit in. Better still is to completely drill through with a drill about the size of the chisel point, then follow up with the final size. I understand that Cherry Hill uses an archimedian drill, to open up a centre popped location,before drilling to size. If it is good enough for her, it certainly good enough for anyone in this group.. T.W.

As Norman B suggested, if you 'center popped' first it will only be quite a shallow indentation, use a small drill first (like 1/8") as that will locate in the mark. Then use the 1/4" tapping drill. Alu is obviously quite soft, so a 'big' drill will tend to grab into the metal only approximately where u first place it - hence (probably) your 1.5mm error. Mike

Plus, I would add, as they are so cheap buy yourself a drill-press. It will revolutionise (ouch!) all your work. And if you've got away without before ALWAYS wear safety spectacles especially now that drilling is nearer eye level.

Actually, I am desperately short of workshop space, the issue with a drill press is not cost, but space. I also strongly prefer (just as a personal quirk) not using electric tools.

Take a look at the Cole Drill, as well, then. At least, know what one looks like, so as to recognise it as the bargain it is, if you see one at a car boot sale.

They take up little space, and will drill holes that would make many other drills give up. Portable, too! :-)

Lesse....

Duly Noted ;-)

BugBear

Just drill the hole first and mark the outer circle afterwards using a paper template if you're only making a knob.

Allan

Aah, but I wanted the largest possible knob from limited stock...

BugBear

That's where you went wrong

Drill into the pop correctly - keep the drill vertical (assuming it isn't in a drill stand) - start with a small drill then enlarge - or use center drills, or better, spot drills to start off with.

A zillion RPM also helps sometimes, if under control.

-- Peter Fairbrother