flat/spade drill bits

[Stephen]

Hello.

One of the faqs (I can't remember whether it was the original or the wiki) mentions that spade bits can be resharpened. How is this done, and does anyone do it? Or are they so cheap they are considered disposable?

It also mentions that by grinding one side of the spade, you can drill bigger holes. I don't understand this; I would have thought the hole will only be as big as the largest (unground) radius? Is it that the bit is unbalanced so jumps around to make a wider hole? If it jumps around so much, it must be difficult to drill precisely?

Thanks.

[Harry Bloomfield]

It happens that Stephen formulated :

Disposable yes, but I have reground them.

I don't understand it either - true if you off set the centre it will drill a larger hole, but it will also be off centre with the shank. You can certainly re-grind them to make them drill a smaller custom size hole.

[meow2222]

> mentions that spade bits can be resharpened. How is this done,

I'm wondering what it is that is an unknown. If you look at the bit you can see which edge is sharp, and at what angle.

cheap, but easy to grind when needed. A lot of people throw out blunted and damaged drill bits, but a quick estimate of the =A3 per hour value of regrinding shows its still worthwhile. And especially when you can regrind them to work much better.

Yes, just regrind it asymmetrically. One side now sticks out further, and that side will do the cutting. And yes, it wobbles. Since spade bits will work at low speed its not too much of an issue.

If you need a wobble free bit, whack the bit so the shaft is just slightly bent, then do an initial regrind with it running in the drill. This establishes the new centre, then you can give it its cutting edge angles.

NT

[John Rumm]

Very easy to resharpen on a bench grinder. Just try to take about the same amount of each shoulder, and keep the angle similar to that originally ground.

Can you remember where you read that? It does not make much sense, unless it is just suggesting that if you ground one side of the spike at the end of the spade you would move the point a little off centre - that would cause the bit to rotate through an eccentric path, and would cause one shoulder of the spade to cut a slightly wider path than normal (and the other to trace a narrower one than normal). It would make the hole a little wider (by a couple of mm perhaps), although it would drill even slower and with more vibration.

[Dave Liquorice]

And when you consider that drills are rarely truely sharp out of the packet.

[meow2222]

Its a very old bodge, and is mentioned in the wiki article. Spade bits are deep enough that you can grind a new point as far off centre as you like, giving anything upto twice the original hole width. Its only used as low speeds, making its poor centering a minor issue. Its not going to see much use today, but is an effective method where you cant find the size of bit you want, and when you're on a site far from the nearest town.

NT

[Stephen]

Thank you. As you said, it was on the wiki.

I'm sorry, I misunderstood it. I thought it meant that if you have a say, 22mm bit, you have: 11mm then a point then 11mm. I thought it was saying that if you grind it so that there's 11mm, a point, 3mm, you get a hole bigger than 22mm.

What (I think) it actually means is that you grind the point off and grind a new point out of the spade. This would enable you to have say,

15mm then a point, then 7mm, to drill a 30mm (2x15) hole.

I bought a stubby set of spades off ebay but they have little "ears" on each end of the spade. I guess I would have to sacrifice them if I ground the bit. The 16mm is blunt; it's also discoloured. I guess I overheated it.

In the short term it would be cheaper to buy a new set as I don't have a grinder; watch out for a new recommend a grinder thread ;)

[meow2222]

that's it.

In practice, since many bit sizes are available, one only generally wantes to enlarge the hole size a little. This can be done without grinding the original point off, the point is just ground away on one side, and ends up smaller.

If you're only adjusting it a bit you can retain one ear.

Well, I'd really recommend getting one, you can make your wood drilling bits many times as effective. And the avoided cost of replacing your bits (pardon me) comes off the cost of the grinder.

An angle grinder is needed to do the fast cutting wood bit conversion, and this will recut and sharpen drill bits - but I'm well aware this will trigger a torrent of protest. Works fine for me though (with some important precautions).

NT