Tool bit

Tool bit

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.


Back Rake is to help control the direction of the chip, which naturally curves into the work due to the difference in length from the outer and inner parts of the cut. It also helps counteract the pressure against the tool from the work by pulling the tool into the work.

Side Rake along with back rake controls the chip flow and partly counteracts the resistance of the work to the movement of the cutter and can be optimized to suit the particular material being cut. Brass for example requires a back and side rake of 0 degrees while aluminum uses a back rake of 35 degrees and a side rake of 15 degrees.

Nose Radius makes the finish of the cut smoother as it can overlap the previous cut and eliminate the peaks and valleys that a pointed tool produces. Having a radius also strengthens the tip, a sharp point being quite fragile.

All the other angles are for clearance in order that no part of the tool besides the actual cutting edge can touch the work. The front clearance angle is usually 8 degrees while the side clearance angle is 10-15 degrees and partly depends on the rate of feed expected.

Minimum angles which do the job required are advisable because the tool gets weaker as the edge gets keener due to the lessening support behind the edge and the reduced ability to absorb heat generated by cutting. The Rake angles on the top of the tool need not be precise in order to cut but to cut efficiently there will be an optimum angle for back and side rake.



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.

Carbides and ceramics

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 difficult 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).


Almost all high-performance cutting tools use indexable inserts. 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 16 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.

Form tools

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 box tools. A form tool turns one or more diameters while feeding into the work. Before the use of form tools, diameters were turned by multiple slide and turret operations, and thus took more work to make the part. For example, a form tool can turn many diameters and in addition can also cut off the part in a single operation and eliminate the need to index the turret. For single-spindle machines, bypassing the need to index the turret can dramatically increase hourly part production rates.

On long-running jobs it is common to use a "roughing tool" on a different slide or turret station to remove the bulk of the material, then finish with a finishing form tool. This increases the lifespan of the finishing tool's finely ground edge and ensures that the 5,000th part receives the same nice finish and dimensional precision that the first part received.

There are 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 steel, 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.


Overview of toolholder design

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.

Holders used on lathes

Bit holder and toolpost

The "toolpost" is the part of a metalworking lathe which either holds the tool bit directly or holds a toolholder which contains the tool bit. There are a great variety of designs for toolposts (including basic toolposts, rocker toolposts, quick-change toolposts, and toolpost turrets) and toolholders (with varying geometry and features.)

Box tool

A box tool is mounted on the turret of a turret lathe or screw machine. It is essentially a toolpost that brings its follower rest along with it. A tool bit (or several tool bits) and a compact (usually V-shaped) follower rest are mounted opposite each other in a body which surrounds the workpiece (forms a "box" around it). As the tool bit puts a lateral deflecting force on the workpiece, the follower rest opposes it, providing rigidity. Opposing tool bits may be used (instead of a rest) to cancel each other's deflecting forces, in which case the box tool begins to overlap in form, function, and identity with a hollow mill.

Holders used on shapers, slotters, and planers

Clapper box

Shapers, slotters, and planers often employ a kind of toolholder called a "clapper box" that swings freely on the return stroke of the ram or bed. On the next cutting stroke, it "claps" back into cutting position. Its movement is analogous to that of a butterfly-style check valve.

Holders used on milling machines

Fly cutters

Fly cutters are a type of milling cutter in which one or two tool bits are mounted. The bits spin around with the rotation of the spindle, taking facing cuts. Fly cutters are an application of tool bits where the bits are part of a rotary unit (whereas most other tool bit use is linear).

ee also

* Tool wear
* Diamond tools

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