hole in the middle?

over the holiday I had cause to make a 1/4" whitworth knob.
1" diameter, 1/4" thick, 1/4 whit hole in the middle.
Simple enough job, but I only had some 1" x 1/4" Alu flatstock
(actually an old shelf bracket!!).
I cut off a slice, centre popped, marked a circle,
and attempted to drill the tap hole for 1/4"
I was off the mark by 1.5mm
As a "proof of concept" I then hacksawed the square
piece octagonal (remove corners) and filed to a good
circle. The hole (of course) was still off centre.
At this point I should point out that I am a woodworker
with a few files and a hacksaw. I don't have a lathe,
or even a drill press.
It's all hand work.
What techniques could I use to get an accurately placed
hole?
BugBear
Reply to
bugbear
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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.
Reply to
Norman Billingham
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?
Reply to
Austin Shackles
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
Reply to
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
Reply to
MikeH_QB
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.
Reply to
the wizard
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.
Reply to
Suzy
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.
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I may try and buy something like this:
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BugBear
Reply to
bugbear
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....
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Cheers Trevor Jones
Reply to
Trevor Jones
Duly Noted ;-)
BugBear
Reply to
bugbear
Just drill the hole first and mark the outer circle afterwards using a paper template if you're only making a knob.
Allan
Reply to
Allan Waterfall
Aah, but I wanted the largest possible knob from limited stock...
BugBear
Reply to
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
Reply to
Peter Fairbrother

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