Make a Spade Bit of Any Size Out of Scrap

I needed a 40mm wide hole. The shop didn't have a 40mm bit. Necessity is mother of invention so I invented my own hole maker. Here's how I did it.

Of course I could have bought an adjustable bit for the princely sum of 5000 yen but I only needed two holes. Again, necessity is the mother of invention.

One fine day I was building a foot powered lathe inspired by Don Weber. I couldn't find any cheek block bearings anywhere around south Yokohama so I caved and bought some big sealed roller bearings (similar to but larger than those in rollerblades or skateboard wheels). This presented a new problem: I needed to mount the bearings in wood. So why not drill a hole the same size as the bearings? Because I couldn't find an appropriate bit, that's why. Ah yes, the plot sickens.

So naturally I found my solution where I find all my answers: a dead bicycle; the frame of a bike to be precise.

Danger, warning, achtung! Danger, warning, achtung! Danger, warning, achtung!
Danger, warning, achtung! Danger, warning, achtung! Danger, warning, achtung!

This tool is risky. It has a chance of shooting a sharpened hunk of metal at high velocity in a random direction and hurting someone badly. Please be very careful if you use this!

Danger, warning, achtung! Danger, warning, achtung! Danger, warning, achtung!
Danger, warning, achtung! Danger, warning, achtung! Danger, warning, achtung!

Step 1: Parts and Tools

Ingredients:
-One thin-walled steel pipe ~3 inches long x an arbitrary width (mine was from a bike frame)
-a screwdriver bit with a hexagonal butt end (this is really just an adapter to mount the bit in a drill).
-two short machine screws and nuts. I used 1/4 inch screws.
-and the secret ingredient: love.

Tools:
-drill
-drill bit the same size as your two machine screws.
-bench grinder (or a file and patience).
-hack saw
-a big flat screwdriver
-vice
-hammer
-measuring tool (vernier calipers recommended)
-love

Step 2: Make the Adapter

Here's how to mount a pipe into a drill.

Clamp the ends of your pipe flat in the vice around the screwdriver bit (see picture).

Now drill a hole on either side of the screwdriver bit. Put your machine screws through each hole. Screw the nuts on tight. These are to clamp the pipe down tight on to the bit. Otherwise the flattened pipe will not grip tightly enough and hence will spin on the screwdriver bit.

Step 3: The Business End

Now let's turn our attention to the other end of the pipe. Flatten it preliminarily and check the width. Is it the correct size or slightly larger?

If yes, lucky you; proceed to the next step.

If no, try this: flatten the end some more to get it as wide as possible. Any luck? Me neither, it's still not wide enough. Don't worry though, I've got a fix for you.

Open up the end of the pipe again with the help of a hammer, vice, and flat screwdriver. Get the end back to a roughly round shape and try to smooth out the kinked edges.

And now, the real magic: we are going to make the pipe wider.

-Make a cut down one side of the pipe about 1 inch (3 cm) long (see picture).
-Widen the cut. A cold chisel worked for me but your flat screwdriver could work here instead.
-Keep the cut splayed wide apart whilst flattening the pipe again. This is tricky because the cut gap will want to close up. I kept flat a screwdriver lodged in the cut whilst clamping the pipe flat again.

Is it the flattened pipe the right size now? Great!

Step 4: Shaping and Sharpening

Here's where the tool becomes more than the sum of its parts.

Look at a proper spade bit. Notice how it has three points: one on each edge and a long central one. I tried to duplicate this shape approximately though in retrospect I should have tried a conical shape first. More about this in the next step.

Also notice how the spade bit is sharpened on one side of each face. Duplicate this.

Grind your pipe into shape on a bench grinder. My central point was slightly off-axis because the adapter end is imprecise but the bit functions just fine.

Step 5: Some Thoughts on Shape

If I were to do this over again I would try to make the whole bit come to a single point at an obtuse angle. I think this would make the point stronger but still functional and for my application it would help my roller bearings have a nice unobstructed movement.

Step 6: Drill Holes!

Yay, I just saved a whack of cash and justified keeping things like pieces of bike frames around the garage. Oh yes, also I got a hole of exactly the right size. This bit is made of mild steel so it won't keep an edge for long but it will work fine in soft wood for a few holes.