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A lathe bit is a tool used to cut steel from a workpiece being turned to size in a metal lathe. The metal lathe uses cutting tools called bits that are commonly manufactured from hardened and sharpened steel or ceramic. Unlike the hand tools used in wood lathe turning, a metal lathe bit is fastened into a holder affixed to the lathe and is adjusted by turning a crank in and out by hand or automatically by the lathe itself. The typical lathe bit is a small piece of square steel stock with a machined cutting edge offered in a variety of styles, each capable of being resharpened and designed to fulfill a specific purpose. Some of the higher-end computer numeric controlled(CNC) lathes use a disposable, triangle-shaped ceramic cutting bit that can be turned to provide three cutting edges before requiring replacement.
The primary cause of lathe bit failure is heat. Lathe operators will often attempt to force the lathe bit into the turning stock, hoping to speed the cutting process along. This generates a great deal of heat and can burn the cutting edge of the cutting tool or bit, effectively dulling it. Not providing the tool with a coolant or lubricating oil will also lead to premature failure even with proper speeds and feed rates being adhered to. Some of the steel cutting tools can be resharpened and heat treated to be reused — however, the ceramic bits are single-use only and are commonly more costly than their steel counterparts, making the triple cutting edge design a must.
A well-equipped metal shop will have several of each style lathe bit in stock to reduce the downtime associated with a dull cutting tool. For the shop with no reserve cutting bits in stock, a dull bit will necessitate shutting the lathe down, removing the bit and resharpening the tool prior to replacing it into the tool holder and resuming work. Extra bits allow a worker to replace dulled bits with a fresh bit and then resharpen all dulled bits at the end of the day to resupply the stock of sharp bits for the next shift.
Some experienced machinists will store and transport their lathe bit collection in a soft-padded box to protect the cutting edges from being chipped or nicked while in transit. By controlling the storage and transportation of their lathe bit collection, they assure a sharp and clean edge on the cutting tools they begin their shift with. When using a common sourced lathe bit from a company tool room, the operator is typically forced to begin each shift sharpening the cutting tools.
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