LATHE TOOL CUTTER BITS Lathe bits can be either some high speed steel alloy or carbide alloy or other special very hard alloy. These are carefully ground to various special shapes for the proper cutting operations performed on the lathe. A very sharply pointed bit is used for roughing out the piece being turn. A more rounded tiped bit is used for the finish cuts. A special shape(s) is used for cutting threads. There are special shapes for right hand and left hand cuts. And a special shape is used for turning different type of metals, for example brass uses a special shape that is ground a lot different than one you would use on steel or aluminum.
These bits are commonly ground with a certain amount of what is called "rake". Rake is simalar to the angle or bevel, "draft", on a pattern only its purpose is far different.
A must buy for anyone new to lathes and tool bit making is the book by South Bend Lathe Works, tittled "How To Run A Lathe". Buy this book if for nothing else it describes in detail how to grind a lathe tool bit and the various shapes. I believe the only one publishing this book today is Lindsay Publications Inc. They have a web site on the net. See the links section.
TIP: GRINDING TOOL BIT. Not a how to but a word of caution. When grinding a lathe tool bit, or for that matter any hardened and tempered tool, never over heat it. This will remove the temper and ruin the bit. This is especially true for lathe tool bits. If you over heat one the area over heated must be ground off. IF you are grinding a tool you forged, hardened, and tempered your self you can reharden and retemper it most good quality steel and be annealed, hardened and tempered many times without hurting the steel or the quality. I've never tried rehardening a lathe tool bit, but most of the book I've read say don't, carefully grind off the heated part or throw it away. It has to do with evenly heating the whole bit for hardening and tempering, if you don't get it even it will stress it when in used and break. When grinding a lathe tool bit work slowly and cool the bit often, water will work fine. Grind it bare handed if it gets to hot to comfortably hold you are getting it to hot. work even more slowly and carefully as you near the final shape and cutting edge, you never want any part to change colors or blue due the heat or you ruin it by to much heat.
LATHE BORING BAR TOOL BITS
The boring bars below are forged from drill rod. This was from one of gingery's books, although he could have gone into more and better detail on how to grind the shape on them. I think the rake on several of them is to much, leaving too little metal to support the cutting edge. I'll try them, they may work ok on aluminum, but may break on steel. There are three sizes here two each of 1/4, 5/16, & 3/8, plus the bar holders that fit into my homemade tool post. Forging, annealing, hardening, and tempering. Forging is the art of heating steel red hot and bending, hammering and shaping it into what you want. Forging is usually done in a forge either coal, charcoal, wood, or gas fired. But a lot can be done with an Oxyacetylene torch.
Annealing is heating a piece of steel to a bright red and allowing it to cool slowly. Annealing removes the hardness and temper, thus making the steel as soft and possible.
Hardening is done by heating the steel to various shades of red and cooling it quickly (quenching). Thus making it as hard as possible or harder than annealed. when I say various shades of red I mean heating it to a bright red and quenching it will make it very hard. Heating to a dull red will make it quite hard, but not as hard as heating to a bright red. Harding makes the metal harder, but also more brittle, more subject to break or shatter when stressed.
Tempering is reheating hardened metal to the point it changes colors then at the proper color quenching it (cooling it quickly). Tempering is to remove some of the hardness making the metal less brittle, and less likely to break. It will still be hard but not so brittle. The higher the temp it gets before quenching the more hardness is removed. For most thing you heat to a straw color then quench.
TIP: QUENCHING, or cooling metal quickly is done by heating the metal to the desired color then cooling in some liquid usually oil or water. I've done a lot of reaserch, and have had some experience. What I can tell you is, to many it is an art form, and everyone has an opinion on the subject, I can only tell you what worked for me. One book I read the author made a point out of making the point the only way to quench metal for harding and tempering was to use boiling oil. A whole chapter was devoted to making a vat to hold the oil and a burner to keep it near boiling. Others say to use ice water. The point is in my opinion find out what the metal manufacturer recommends and go with that. With todays alloys it may be the best option. The boring bars above where called oil hardening drill rod. When I first hardened them I quenched them in tap water, see I had my own opinion. This did not work, they got so hard and cooled so quickly they cracked, and most had to be ground off and reforged. I then tried harding them in oil, which worked great. Water will cool faster than oil. Oil forms a sort of thin ash like insulation on the surface when first submerged and cools more slowly.
TIP: HARDENING & TEMPERING. What can I say in a home shop and as a hobbiest and tinkerer, with limited resources you do what you can with what you have and can make. For some tools it would be nice to harden and temper the whole tool. This is just not possible most of the time and requires special equipment to evenly heat the metal. In most cases it is only the part that works that needs to be hardened and tempered. The working part is that part of the tool that does the work, in the case of the boring bars, it is the part that comes into contact with the metal being bored. Thus only the tip need be hardened. Tempering is a little tricker to do. Do NOT try to temper the cuting (working) area by heating it directly. The metal will heat to fast and change colors to quickly to catch the right color at the right time to quench it in time. The best approach is the heat it a little below the working area, in the case of the boring bars around an inch or so below the working area, that way you can watch the metal change in color moving up toward the tip, when it get near remove the heat. The color will continue to move up, when the tip reaches the desired color, in this case straw like, imediatly quench it. Before tempering polish off the discoloration from the harding proccess with emery cloth or other means so long as you have the shine of metal, so you can see the color changes.
Spindle driven Boring bar that come out of the temporary headstock. This is what is used to bore out the lathes finished headstock. 
This is the adjustable gauge for setting the cutting depth of the boring bar cutter.
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