It happens that Stephen formulated :
> 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?

Disposable yes, but I have reground them.

>
> 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?

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.

--
Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk

Harry Bloomfield wrote:
> It happens that Stephen formulated :
> > Hello.
> >
> > One of the faqs (I can't remember whether it was the original or the
> > wiki)

http://www.wiki.diyfaq.org.uk/index....tle=Drill_Bits

> > mentions that spade bits can be resharpened. How is this done,
> > and does anyone do it?

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.


> > Or are they so cheap they are considered
> > disposable?

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


> Disposable yes, but I have reground them.
>
> >
> > 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?
>
> 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.

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

On Thu, 8 Jan 2009 12:14:23 -0800 (PST), (E-Mail Removed) wrote:

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

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

--
Cheers
Dave.