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Tool bit
From Wikipedia, the free encyclopedia
The term tool bit generally refers to a non-rotary cutting tool used in metal lathes, shapers, and planers. Such cutters are also often referred to by the set-phrase name of single-point cutting tool. The cutting edge is ground to suit a particular machining operation and may be resharpened or reshaped as needed. The ground tool bit is held rigidly by a tool holder while it is cutting.
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Originally, all tool bits were made of high carbon tool steels with the appropriate hardening and tempering. Since the introductions of high-speed steel (HSS) (early years of the 20th century), sintered carbide (1930s), and ceramic cutters, those materials have gradually replaced the earlier kinds of tool steel in almost all cutting applications. Most tool bits today are either HSS or carbide.
Carbide, ceramics (such as cubic boron nitride), and diamond, having higher hardness than HSS, all allow faster material removal than HSS in most cases. Because these materials are expensive and hard to work with, typically the body of the cutting tool is made of steel, and a small cutting edge made of the harder material is attached. The cutting edge is usually either screwed on (in this case it is called an insert), or brazed on to a steel shank (this is usually only done for carbide).
By confining the expensive hard cutting tip to the part doing the actual cutting, the cost of tooling is reduced. The supporting tool holder can then be made from a tougher steel, which besides being cheaper is also usually better suited to the task, being less brittle than the cutting-edge materials.
The tool holders may also be designed to introduce additional properties to the cutting action, such as
- Angular approach - direction of tool travel.
- Spring loading - deflection of the tool bit away from the material when excessive load is applied.
- Variable overhang - the tool bit may be extended or retracted as the job requires.
- Rigidity - the tool holder can be sized according to the work to be performed.
- Direct cutting fluid or coolant to the work area.
Note that since stiffness (rather than strength) is usually the design driver of a tool holder, the steel used doesn't need to be particularly hard or strong as there is relatively little difference between the stiffnesses of most steel alloys.
Almost all high-performance cutting tools use the insert method. There are several reasons for this. First of all, at the very high cutting speeds and feeds supported by these materials, the cutting tip can reach temperatures high enough to melt the brazing material holding it to the shank. Economics are also important; inserts are made symmetrically so that when the first cutting edge is dull they can be rotated, presenting a fresh cutting edge. Some inserts are even made so that they can be flipped over, giving as many as 8 cutting edges per insert. There are many types of inserts: some for roughing, some for finishing. Others are made for specialized jobs like cutting threads or grooves. The industry employs standardized nomenclature to describe inserts by shape, material, coating material, and size.
A form tool is precision-ground into a pattern that resembles the part to be formed. The form tool can be used as a single operation and therefore eliminate many other operations from the slides (front, rear and/or vertical) and the turret, such as boxtools. A form tool turns one or more diameters while feeding into the work. Before the use of forum tools, diameters were turned by multiple slide and turret operations, and thus more work to make the part. For example, a form tool can turn many diameters and in addition can also cutoff the part in a single operation and eliminate indexing the turret.
For single-spindle machines, bypassing indexing the machine can dramatically increase hourly part production. On long-running jobs it is common to use a ‘roughing tool’ tool on a different slide, or from the turret to remove the bulk of material to reduce wear on the form tool. There are also different types of form tools. Insert tools are the most common for short- to medium-range jobs (50 to 20,000 pcs). Circular form tools are usually for longer jobs, since the tool wear can be ground off the tool tip many times as the tool is rotated in its holder. There is also a skiving tool that can be used for light finishing cuts. Form tools can be made of cobalt, carbide, or high-speed steel. Carbide requires additional care because it is very brittle and will chip if chatter occurs.
A drawback when using form tools is that the feed into the work is usually slow, .0005" to .0012" per revolution depending on the width of the tool. Wide form tools create more heat and usually are problematic for chatter. Heat and chatter reduces tool life. Also, form tools wider than 2.5 times the smaller diameter of the part being turned have a greater risk of the part breaking off. When turning longer lengths, a support from the turret can be used to increase turning length from 2.5 times to 5 times the smallest diameter of the part being turned, and this also can help reduce chatter. Despite the drawbacks, the elimination of extra operations often makes using form tools the most efficient option.
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