Pilot Holes

cringeDo you know what it’s like to watch someone doing something terribly wrong and yet you can’t look away?

That “I can’t believe he did that” moment?

Well let me tell you more.
A friend was going to mount a metal bracket on his boat.
No big’gy there BUT he was having to drill into some serious thickness of mild steel (8mm) AND make a precision job of it.

We’re talking a 12 mm hole with a tolerance of accuracy of only 0.5 mm or 20 thou (thousands of an inch).

Tools he had to hand.
A Battery powered hand drill.
Variable speed on the trigger 0-300 rpm.
And a small set of HSS drills.

That’s it, nothing else in his tool kit.
First I knew about it was he came a calling for some oil to lubricate the drill bit. So I sort of invited myself along to see how he was going to achieve the task.

His intention was to slap the pre-drilled bracket onto the metal, hold it by hand, and prepare to drill with the 12 mm drill bit fitted.  (Cringe)

What about marking out the holes first?
Nope, happy to drill using the bracket as his template.

Aren’t you going to clamp the bracket?
Nope because I’m only using it as a guide and not drilling into it.

There was more, well worthy of a shed load of cringing, but by now I’m gently stepping backwards as he revved the hand drill and was positioning himself to start drilling in one go with the 12mm (½”) drill bit.

Umm, spectacular would be one word to describe what happened next as the drill bit into the base metal, skated a little, which caused it to bite into pre-drilled bracket AND SPUN EVERYTHING.

Meanwhile I’m taking two more steps backwards stopping just in time as I’d forgotten I was on a pontoon and not dry land aka next step, I get wet!

OK, no one got hurt much i.e. he needed a plaster or two but having had my full years quota of “Cringing” in one hit, and remembering SWMBO had just increased our life insurance cover, I gently took the drill off him i.e. “Give me that sodding thing before you kill someone!” and sent him off to first aid his hand.

Remember this is no big task so I got my tool kit (still retaining a VERY tight hold on his drill) and marked out with one of those things called a pencil the position of the bracket and the holes.

The problem is the hand drill though as every drilled holes ends up slightly larger than the diameter of the drill’s size. For instance, a l/4-inch twist drill will produce a hole that may be several thousandths of an inch larger than l/4-inch. this difference in a workshop is caused by a few things:

  • The size of the twist drill
  • The manufacturing quality of the drill bit
  • The accuracy of the drill chuck (build and when spinning)
  • The drilling speed
  • The accuracy and rigidity of the drilling machine,
  • And how well the work is clamped.

Only we’ve got a hand drill, a swaying boat, and not a lot of other equipment.

So I auto-punched the center of the desired holes, removed everything and drilled my first of 4 pilot holes.
Only why use pilot holes and what size would you have drilled your pilot holes?

Look at these drill bit points.

A lot of the cheaper drill sets come with the “bland” cutting face on the left. The split point has two tiny cutting edges in the center.
The standard 118 deg drill has a non cutting “flat” called the chisel edge and rather than drilling into metal, it more wears it away until the finer cutting edge just off the center is reached. On the other hand the split point (135 deg drill) eliminates almost all of the chisel point thus drills from the word “GO”. They also seem to self-center better including not skipping across the metal as they try to gain purchase.

Brain fried yet?
Bottom line, cheaper drills don’t like starting on flat metal and will nearly always “walk” off the pencil or even pop mark you’ve marked to pinpoint the desired center of the hole.

Cheap drills or not, the smaller the tip, the less of a chisel point is present so it follows that if you can drill a small hole ACCURATELY, then gently increase that hole by using drill diameters that are larger than the next size drill bits “chisel” tip, the bigger drill cuts its way in easier and with little “skip”.

There is a general rule. In fact there are a lot of general rules, but the one that has served me well is to use the “quarters” gambit. Eh?

Simple really.
End drill is 12 mm, divide that by four.
Thus my pilot holes start off with 1.5 mm, then 3 mm, 6 mm, and finally the grand daddy i.e the 12 mm. See, not hard at all.

The experts talk about selecting drills slightly bigger than the ‘web’ (the chisel in the red rings bit) thus the drill cutting surface engages quickly. They are right of course but my eyes are old and tired so I K.I.S.S. (keep it stupidly simple) it.

Anyway to cut the story short, with the hand drill virtually on tick over, as slow is good the larger the drill bit you use, the holes were drilled. Best bit about it? All in the right place.

Except the next bit of metal was to be self tapped onto the bracket.
There are quite a few different types of self tappers, Some are tapered and self-starting, some need a pilot hole. Standard cop-out quote “consult your manufacturers literature”.
bsbullFor the real world you need a basic figure to work to as if you’re anything like me, I use what I’ve got.

I’m a fan of stainless steel self tappers.

They are a bit softer than the usual hard steel types but the benefit is they only rust a bit in general use. Now before anyone says stainless doesn’t rust, that ain’t so and all metal corrodes. It’s just some takes longer than others.

Now ordinarily I just measure the “flats” on a thread and drill my pilot hole to that size when working in sheet metal.


Note:- Always measure at
the “fat bit” not at the point

Yet, for the more accurate people in the world, I found this little table for drilling a pilot hole a bit more accurately than me and my ruler (and dodgy eyes).

Screw   Nominal     General
Size          Dia.           Pilot
6           3.50 mm    2.90 mm
8           4.17 mm    3.50 mm
10          4.71 mm    3.85 mm
12          5.38 mm    4.30 mm
14          6.14 mm    5.20 mm

When it matters always refer to the manufacturers info.

p.s. Always clamp your work! (pretty please).

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2 Responses to Pilot Holes

  1. DM says:

    love the practical application!

  2. Ah, never get between a man and his tool box (something about it being a Man Cave………… us women have bottomless handbags)

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