Saturday, September 30, 2006

Internal threading on a lathe, part 1

I was right, the internal threading tool was the hardest to grind of all. I could have ground all of the other tools I have made before in the time it took me on just this one. Of course, the first time doing something is always the longest. I sacrificed a broken parting tool to make it.

The difficulty stems from a few things:

First, you have to remove a gargantuan amount of metal. The most, easily, out of any tool. Grinding dust gets everywhere. I also had to dress the grinding wheel several times during the operation to bring back its cutting ability with the diamond tip dressing tool.

Second, because the tool is thin and not much metal is left, the tip heats up very rapidly. So you must be more mindful of cooling it. This is less of a burn hazard and more of loss of temper/hardness problem. I once read somewhere that modern tool steels don't suffer from hardness loss after excessive heating from prolonged grinding. This statement assumes one thing: that we are grinding a normal tool with plenty of surrounding metal, which will conduct it to your fingers and become unbearably hot, thus requiring a trip to the water cup. A tool with a thin shaft will NOT do this fast enough and it is easy to get the tip of the tool orange hot, with NO HEAT INDICATION on your fingers. This will ruin the temper on the tool tip, as indicated by an extreme softness of the steel even after it has been cooled in the water cup. If this is the case you must grind off the soft spot until hard metal is again reached.

Third, grinding the inside angle of the tip is not easy as it requires holding the tool at an extremely awkward angle that puts your hand frighteningly close to the spinning wheel. I have not yet come into contact with a spinning grinding wheel, and I don't ever want to suffer the experience.

The end result:

It definitely aint pretty, but it works.

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