How to sharpen a drill bit and when to just replace it

Why a sharp drill bit is worth ten minutes of your time

You know that moment when you are halfway through a job and the drill starts screaming, the tip of the bit goes blue, and the hole is barely started. That is a dull bit, and it is not just annoying, because a bit that will not cut makes you lean on the drill, and a drill you are leaning on is the one that slips.

Most of us chuck the bit in the bottom of the drawer and buy another one, and I am as guilty of that as anybody. But a standard high speed steel bit will take a sharpen, and once you have done a couple you will wonder why you ever bought a whole new set for the sake of one size.

What a dull bit actually costs you

• It works the drill harder. The motor draws more and the bit heats up, and heat is what takes the temper out of the steel in the first place.
• It wanders. A sharp bit bites where you put it, and a blunt one skates across the surface until you end up with a hole a few millimetres off the mark.
• It tears the exit. Blunt bits blow out the back of the timber, so the screw sits in a mess instead of a clean hole.
• It is the one that slips. Which matters more than the other three put together.

The angle is the whole job

A standard twist bit is ground to 118 degrees at the point, which is 59 degrees each side of the centre line, and if you get both sides the same you have done it. If one side ends up longer than the other then the bit cuts a hole bigger than itself and wobbles the whole way down, so the thing to watch is not how sharp each edge looks, it is whether the two edges match each other.

Hold the bit up against a new one out of the packet and compare them. That is the cheapest gauge there is.

How people actually do it

• On a bench grinder. The fastest way once you have the feel for it. Rest the bit on the tool rest, bring the cutting edge square to the wheel, and roll it gently while you hold that angle. Dip it in water every few seconds, because if the tip goes blue you have cooked the steel and no amount of grinding brings it back.
• With a hand file. Slower, and much easier to get right the first few times. A second cut flat file or a diamond file gives you far more control than a grinder does, and for small bits it is genuinely the better tool.
• With a sharpening jig. These hold the angle for you, which is the part everybody gets wrong, so if you drill a lot they earn their keep pretty quickly.
• Hand it over. We run a tool sharpening depot at the shop, so if you would rather not have a go yourself, bring the gear in and ask us whether it is worth doing.

The bits you do not sharpen

Masonry bits have a tungsten carbide tip brazed onto the steel, and that tip wants a green silicon carbide wheel rather than the grey one on most bench grinders, so unless you have the right wheel you are only polishing it. Spade bits and hole saws are a different job again. And any bit that is bent, or worn back so far the flutes have gone, that one is finished and no amount of grinding is getting it back.

The thing that saves more bits than sharpening does

Punch your mark first. A centre punch puts a little dimple in the surface for the tip to sit in, so the bit starts where you want it instead of walking off across the timber, and a bit that is not fighting a sideways load stays sharp a lot longer. It is the cheapest thing in this whole post and it is the one I would actually bother with.

The other one is speed. Big bits want slow and small bits want fast, and nearly everybody runs a big bit far too fast and then wonders why the tip has gone blue. If you are seeing smoke, back the speed off before you do anything else.

Shop related products

• Sutton XP Bullet Jobber Drill Bit 9/32in HSS. A standard high speed steel twist bit, which is the kind that takes a sharpen.
• Supercraft Flat File, Second Cut 200mm. For sharpening by hand when you want the control.
• Supercraft Centre Punch. Punch the mark first and the bit stops walking.
• Sutton Masonry Drill Bit 6.5mm x 400mm. Tungsten tipped for brick, and one you replace rather than sharpen.

Did you know we also stock HSS jobber drill bit sets in store, in 13, 19 and 21 piece boxes, along with chainsaw files, sharpening kits, a 12 volt chain sharpener and a diamond sharpening steel. They are not all on the website, so give us a ring on (08) 8536 4333 to check what is on the shelf.

The rest of the hand tool range is at hand tools, and there is more about what we do at the shop on our essential services page.

More from the blog

• How to Store and Maintain Power Tools for Long-Lasting Performance
• Top 3 ways to use a reciprocating saw
• How to use a tape measure

I still hold every bit I sharpen up against a new one out of the packet, because I do not trust my own eye for an angle and I am not going to pretend otherwise.

Cheers,
Candeece

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