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MAKING YOUR OWN BOWL GOUGES by Grant Marshall
(this article first appeared in "TURNAROUND", the newsletter of the Association of Woodturners of South Africa)

Part 1

Warning! Although the procedures described below have been used successfully by blacksmiths all over the world for centuries, some of the operations involved are potentially very dangerous. Unless you are completely confident in your practical abilities and are prepared to take full responsibility for ensuring your own safety, do not try this at home! I have endeavoured to describe this process as clearly as possible and to point out the dangers where possible but I accept no responsibility for the outcome if you try it yourself.

With the current high cost of imported tools, buying a range of different sizes of gouges is very expensive and having additional specialised gouges with different bevel angles for special cuts is a luxury few can afford. While many turners are happy to make scrapers or thin parting tools, most probably imagine that bowl gouges would be too difficult to make in a home workshop. The fact is that with simple equipment and a basic grasp of the principles, making a usable gouge is not difficult at all.

Here follows a distillation of my own explorations into the field of home-made gouges. My knowledge of the materials and processes involved is very limited indeed, assembled from scraps of information from various books and articles on blacksmithing, knife making and model engineering. I therefore welcome any feedback from anyone who has any comment or anything to add.

This article describes the process of making a gouge in the following steps:
Basic metallurgy (the minimum we need to know about tool steels, simplified to the extreme)
Selecting suitable steel
Grinding a flute
Hardening the steel to its maximum possible hardness (which is too hard to be useful)
Tempering (softening it very slightly to a practical hardness)

The finished product is by no means the ultimate gouge. You will not be throwing out your Robert Sorbys or Titanium coated tools in favour of these, but they work well enough to be in everyday use filling the gaps in your range of gouges. If, for example, you already have a high quality 12mm HSS bowl gouge, try supplementing it with a 6mm gouge for detail work, a 10mm gouge with a long pointed bevel for delicate finishing cuts, and a 16 or 18mm heavy gouge with a very steep blunt bevel for working across the bottom inside deep bowls.

The main advantage of home-made gouges is that they cost next to nothing. This means you can make as many as you like and modify flute shapes, bevel angles and grinds to your heart's content because they are almost expendable. The main disadvantage is that you will almost certainly have to sharpen more often, particularly on hard, abrasive woods where they are unlikely to hold an edge as well as High Speed Steel. More sharpening means they won't last as long, but who cares? On wet wood they perform well, you may not even notice any difference between these and your expensive gouges. The other limitation is that they are vulnerable to heat (the section on tempering explains why). This does mean you need to keep them cool while sharpening, particularly with smaller tools. Just dip them in water frequently while sharpening and don't ever let the tip get hot enough to burn your fingers.

(VERY) BASIC METALLURGY

To make cutting tools we need steel which can be hardened sufficiently to hold a durable edge. Ordinary steel, usually called Mild Steel can't be hardened because it lacks sufficient carbon. Our focus is therefore on Carbon Steel (also called High Carbon Steel or Tool Steel) which contains between 0.8% and 1.2% carbon. For now we can forget about High Speed Steel, I'll explain why later.

When heated to above its transformation temperature ("Cherry Red" in blacksmith's terms, or about 800 degrees C), steel has an affinity for carbon, combining with it to form a carbon-iron compound called iron carbide. Below that temperature it doesn't like carbon. Allowing it to cool very slowly from red hot will cause it to dissociate into its constituent components of iron and carbon. This process is called annealing and will cause a softening of the steel. Cooling it very rapidly, on the other hand, (by quenching it in oil or water) gives it no time to separate and the carbon atoms get trapped inside the iron crystal structure. This yields a very hard and very brittle steel measuring 65 - 66 on the Rockwell C hardness scale. Unfortunately this too hard to be useful because the micro edge chips off instantly in use.

To be useful we need to temper it, in other words to soften it very slightly by heating it just enough to turn a precise amount of the very hard stuff back into carbon and iron which will make it more flexible and tougher. How hot we make it determines how much turns back. Too much heat causes too much softening and the hardness is lost.

That's pretty much all we need to know about this vast and complex field. To summarize: We take a piece of carbon steel, heat it red hot and quench it very quickly to harden it as much as possible. Then in a process called tempering we soften it just a little to achieve a durable hardness.

The exact temperatures involved vary depending on the precise content of the steel, but for our purposes transformation temperature is around 780 - 820 degrees C (cherry red when viewed in your workshop in daylight - in pitch darkness or direct sunlight what looks like cherry red will be a very different temperature). Conveniently for us, this is also exactly the temperature at which carbon steel becomes non magnetic, so when a magnet won't stick to your red hot gouge you're at the right temperature. For tempering we use much lower temperatures, around 150 to 350 degrees C and here we can make use of the colours of the oxides released onto the surface during heating. (Click here to see the table of temperatures and colours.) We've all seen a piece of clean shiny steel change colour as it is heated, going from pale yellow, through straw and brown to purple to blue (as novice turners we may have seen the tips of our gouges go those same pretty colours during overzealous sharpening). That first pale yellow appears at 200 and that's about right to temper the steel to the hardness we want (about 62 Rockwell C). The purples and blues mean a higher temp has been reached and the steel has been softened too much for our purpose.

MATERIAL

There is no shortage of sources of carbon steel suitable for tool making. The ideal would be to buy a rod of the right diameter from a specialist steel supplier. The advantage here is you get a description of its exact composition and you get precise instructions for hardening and tempering so there's no guessing. Whether this will yield a better tool given our crude home-workshop methods is debatable. Different steel manufacturers will have different names for their steels but you will need to ask for a cold work carbon tool steel that can be hardened and tempered in a home workshop. Bohler Steel makes one called "K460", other names are "Oil hardening drill rod", "O1", BO1", and "Silver steel". These steels may have small amounts of other alloys added to them to give them particular properties.

But we don't need to buy fancy steel to make a usable gouge. There's plenty of useful junk steel available. Almost any steel harder than mild steel will probably work, and anything that was springy in its previous life will do fine. A motor stripper or a junk shop should yield plenty of different sizes.
Here are some examples:
Any round spring steel
Torsion Bars and anti Roll Bars from cars suspension
Torsion spring rods used in some car boots
McPherson strut or shock absorber shaft
"W"- shaped camp bed leg
Bakkie (pickup) tonneau cover support rod
Any round tool steel
Long screwdriver

Note that some things like shafts and tools may have been casehardened, which means the outside is hard (and hardenable) but the middle isn't. For gouges we need to harden all the way through because our cutting edge is located somewhere near the middle. Steel used in things like garden tools and crowbars may not contain enough carbon to be hardened enough.

To test the suitability of steel on the spot try punching, filing or hacksawing it, you'll soon know if it is hard enough because the file will skate off it without cutting it. Back in your workshop you can use a spark test to get an idea of the composition of the steel. Touch a piece of mild steel on your bench grinder so you get a stream of sparks out into space. Take a good look at the sparks and compare them to the sparks you get from the tip of a file or an allen key (both are high carbon steel). Notice how carbon steel produces a lot more sparks than mild steel and they are much finer and whiter and they explode much more in flight. The more little explosions, the more carbon in the steel.

Basically, any steel you want to use for tool making must make sparks exactly like those made by the allen key. Just for fun check out the sparks from the back end of a drill bit or from your best gouge (high speed steel). HSS produces very few sparks, they are deep red in colour, they don't explode at all and they fly in curving paths.

Okay, so now you have a suitable length of carbon steel rod and you want to make a gouge.

To Part 2 >>

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