Hi everyone, and welcome to Bob's Wood Stuff. Today I'll be showing you how to sharpen a
spade bit, also sometmes called a paddle bit. If your bit is old, or if it's just really
cheap like mine, you probably need to sharpen it.
Let's go show you how. I'll be using this pocket microscope to check
on my progress as I go along. This will give me details about the edge of
the bit. This is the edge before doing any sharpening
at all. You can see that the edge is very dull, and
there are machining marks from making the bit that go all the way to the edge. Which
would prevent it from being sharp. And there's a nick in it.
The first step is to make sure the flat surfaces are actually flat, and to polish out the machining
marks. I'm starting with 220 grit sandpaper adhered
to a piece of glass. I can still see some machining marks on the
tip of the bit, so I'll keep going on the sandpaper until the sandpaper scratches go
all the way to the tip of the bit. Now the bit is looking pretty smooth, and
there's uniform scratched going all the way to the tip of the bit.
This is what you want to see on the last one inch of the bit.
The sandpaper is becoming clogged with steel particles, so we need to vaccuum it out so
the sandpaper doesn't stop cutting as it should. I also need to hone the narrow parts of the
bit, being very careful to maintain the same angle that they had, and to not remove too
much material. Because if I do, it will become slightly narrower, and it won't be the width
that it says it is. Now the side edge is starting to get pretty
sharp, so I'll move on to the 325 grit diamond stone.
I need to keep lapping on this stone until all the scratches from the sandpaper are removed.
And then I move on to the 600 grit diamond stone.
This is what the edge of the bit looks like right now. This is probably more than sharp
enough. But I know I can get it sharper, so I'm gonna keep going.
I'll move on to the 1200 grit diamond stone. At this point the bit is really sharp.
But to make it even sharper, I need to polish the curved parts of the bit.
And this can done using a slipstone. Mine are a Japanese water stone style, so
I need to soak them in water for a few minutes before using them.
Using the slip stone on the curves gave me a little bit of a burr on the flat side, so
I'm gonna go back to the 1200 grit stone real quick and polish that out.
This is what the edge looks like at this point. It's clearly much sharper than the previous
one, so let's go test it out. This is the one inch bit that was not sharpened.
It works, but it takes a lot of force and it cuts pretty slowly.
This is the seven eighths inch bit that I sharpened.
This one cuts through the wood very easily and I don't have to apply a lot of pressure.
It just goes right through it. Looking at the two holes side by side, you
can see that the hole made with the dull bit has tear out in the upper right and in the
lower left. This is why.
As the blade spins inside the wood, it is affected by the direction of the grain.
In this section, the blade is going downhill with the grain, or just "with the grain".
So it gives it a nice slice. But on this other section, the cutting edge
is going uphill with the grain, or "against the grain".
When this is done with a dull bit, it causes tearout.
But when done with a sharp bit, the amount of force required to slice the wood fibers
is less than the amount of force required to tear out the wood fibers, so you get a
good slice. You can see that this tear out happens all
the way down the wall of the hole, not just on the top.
So in conclusion, if you're going to be doing fine woodworking, it's worth sharpening your
spade bit, because you get a a really clean edge like a forstner bit.
If you're only going to be drilling through two by fours or something like that, it's
probably not worth sharpening.