Tool wear

Tool wear describes the gradual failure of cutting tools due to regular operation. It is a term often associated with tipped tools, tool bits, or drill bits that are used with machine tools.

Types of wear include:

• flank wear in which the portion of the tool in contact with the finished part erodes. Can be described using the Tool Life Expectancy equation.
• crater wear in which contact with chips erodes the rake face. This is somewhat normal for tool wear, and does not seriously degrade the use of a tool until it becomes serious enough to cause a cutting edge failure.

Can be caused by spindle speed that is too low or a feed rate that is too high. In orthogonal cutting this typically occurs where the tool temperature is highest. Crater wear occurs approximately at a height equaling the cutting depth of the material. Crater wear depth ~ t₀ t₀= cutting depth

• built-up edge in which material being machined builds up on the cutting edge. Some materials (notably aluminum and copper) have a tendency to anneal themselves to the cutting edge of a tool. It occurs most frequently on softer metals, with a lower melting point. It can be prevented by increasing cutting speeds and using lubricant. When drilling it can be noticed as alternating dark and shiny rings.
• glazing occurs on grinding wheels, and occurs when the exposed abrasive becomes dulled. It is noticeable as a sheen while the wheel is in motion.
• edge wear, in drills, refers to wear to the outer edge of a drill bit around the cutting face caused by excessive cutting speed. It extends down the drill flutes, and requires a large volume of material to be removed from the drill bit before it can be corrected.

Some General effects of tool wear include:

• increased cutting forces….
• increased cutting temperatures
• poor surface finish
• decreased accuracy of finished part
• May lead to tool breakage

Reduction in tool wear can be accomplished by using lubricants and coolants while machining. These reduce friction and temperature, thus reducing the tool wear.

Four-slide

A four-slide, also known as a multislide, multi-slide, or four-way, is a metalworking machine tool used in the high-volume manufacture of small stamped components from bar or wire stock. The press is most simply described as a horizontal stamping press that uses cams to control tools.The machine is used for progressive or transfer stamping operations.

A four-slide is much different than most other presses. The key of the machine is its moving slides that have tools attached, which strike the workpiece to form it. These slides are driven by four shafts that outline the machine. The shafts are connected by bevel gears so that one shaft is driven by an electric motor, and then that shaft’s motion drives the other three shafts. Each shaft then has cams which drive the slides, usually of a split-type. This shafting arrangement allows the workpiece to be worked for four sides, which makes this machine extremely versatile. A hole near the center of the machine is provided to expel the completed workpiece

The greatest advantage of the four-slide machine is its ability to complete all of the operations required to form the workpiece from start to finish. Moreover, it can handle certain parts that transfer or progressive dies cannot, because it can manipulate from four axes. Due to this flexibility it reduces the cost of the finished part because it requires less machines, setups, and handling. Also, because only one machine is required, less space is required for any given workpiece. As compared to standard stamping presses the tooling is usually inexpensive, due to the simplicity of the tools. A four-slide can usually produce 20,000 to 70,000 finished parts per 16-hour shift, depending on the number of operations per part; this speed usually results in a lower cost per part.^[2]

The biggest disadvantage is its size constraints. The largest machines can handle stock up to 3 in (76 mm) wide, 12.5 in (320 mm) long, and ³⁄₃₂ in (2.4 mm) thick. For wires the limit is ¹⁄₈ in (3.175 mm).^[3] Other limits are the travel on the slides, which maxes out at ³⁄₄ in (19.05 mm), and the throw of the forming cams, which is between ⁷⁄₈ and 2 in (22 and 51 mm). The machine is also limited to only shearing and bending operations. Extrusion and upsetting operations are impractical because it hinders the movement of the workpiece to the next station. Drawing and stretching require too much tonnage and the mechanisms required for the operations are space prohibitive. Finally, this machine is only feasible to use on high volume parts because of the long lead time required to set up the tooling

Machining vibrations

Machining vibrations, also called chatter, correspond to the relative movement between the workpiece and the cutting tool. The vibrations result in waves on the machinedsurface. This affects typical machining processes, such as turning, milling and drilling, and atypical machining processes, such as grinding.

A chatter mark is an irregular surface flaw left by a wheel that is out of true in grinding ^[1] or regular mark left when turning a long piece on a lathe, due to machining vibrations.

As early as 1907, Frederick W. Taylor described machining vibrations as the most obscure and delicate of all the problems facing the machinist, an observation still true today, as shown in many publications on machining.

Mathematical models make it possible to simulate machining vibration quite accurately, but in practice it is always difficult to avoid vibrations and there are basic rules for the machinist:

• Rigidify the workpiece, the tool and the machine as much as possible
• Choose the tool that will excite vibrations as little as possible (modifying angles, dimensions, surface treatment, etc.)
• Choose exciting frequencies that best limit the vibrations of the machining system (spindle speed, number of teeth and relative positions, etc.)

Link between high-speed machining and vibrations

The use of high speed machining (HSM) has enabled an increase in productivity and the realization of workpieces that were impossible before, such as thin walled parts. Unfortunately, machine centers are less rigid because of the very high dynamic movements. In many applications, i.e. long tools, thin workpieces, the appearance of vibrations is the most limiting factor and compels machinist to reduce cutting speeds well below the capacities of machines or tools.

Different kinds of problems and their sources

Vibration problems generally result in noise, bad surface quality and sometimes tool breakage. The main sources are of two types: forced vibrations and self-generated vibrations.

• Forced vibrations are mainly generated by interrupted cutting (inherent to milling), runout, or vibrations from outside the machine.
• Self generated vibrations are related to the fact that the actual chip thickness depends also on the relative position between tool and workpiece during the previous tooth passage. Thus increasing vibrations may appear up to levels which can seriously degrade the machined surface quality.

Swarf

Swarf, also known as chips or by other process-specific names (such as turnings, filings, or shavings), is pieces of metal, wood, or plastic that are the debris or waste resulting from machining, woodworking, or similar subtractive (material-removing) manufacturing processes. Swarf or chips can be small particles (such as the gritty swarf from grinding metal or the sawdust from sawing or sanding wood); long, stringy tendrils (such as the springy chips from turning tough metals, or long shavings from whittling); slag-like waste (such as is produced within pipe during pipefitting work); or stone fragments and dust (as in masonry)

Some of these terms are mass nouns (such as swarf and sawdust) and some of them are count nouns (such as chips, filings, or shavings).

The rest of this article discusses metalworking swarf. Wood swarf is discussed at sawdust.

Cutting hazards and safety precautions

Chips can be extremely sharp, and this creates a safety problem, as they can cause serious injuries if not handled correctly. Depending on the composition of the material, it can persist in the environment for a long time before degrading. This, combined with the small size of some chips (e.g. those of brass or bronze), allows them to disperse widely by piggy-backing on soft materials and also to penetrate the skin as deep splinters.

It is standard training for machinists, and usually a standing workplace rule, to avoid handling swarf with bare hands. Similarly, it is also standard training for machinists, and usually a standing workplace rule, to minimize or entirely avoid handling swarf by blowing chips away with compressed air, but this practice is considered burdensome or impractical by some machinists. Some machine tool manuals proscribe this practice both for safety and for the preservation of way wipers and bearing seals. Alternatives to blowing chips away include vacuuming them away with an industrial vacuum (shop vacuum); gently washing them away with a coolant hose discharging at typical garden-hose pressure values; or preventing their generation in the first place (for example, forming threads instead of cutting them).

It is not uncommon for chips flying off the cutter to be ejected with great force and to fly several meters. These flying chips present a hazard that is deflected with safety glasses,face shields, and other personal protective equipment, as well as the sheet-metal enclosures (and polycarbonate windows) that surround most commercial CNC machine tools.

Due to its high surface area, swarf composed of some reactive metals can be highly flammable. Caution should be exercised to avoid ignition sources when handling or storing swarf in loose form, especially swarf of pure magnesium, magnesium alloy, pure titanium, titanium alloy, iron, and non-stainless steel.

Swarf stored in piles or bins may also spontaneously combust, especially if the swarf is coated with cutting oil

To extinguish swarf fires, a special Class D fire extinguisher optimized for metal fires is needed

Recycling

Metal swarf can usually be recycled, and this is the preferred method of disposal due to the environmental concerns regarding potential contamination with cutting fluid or tramp oil. The ideal way to remove these liquids is by the use of a centrifuge which will separate the fluids from the metal, allowing both to be reclaimed and prepared for further treatment. Small bundles of stainless steel or bronze swarf are sold as excellent scourers for dishwashing or cleaning encrustations of dirt. Recycling chips rather than putting them in the garbage stream (headed to landfilling or incineration) has various advantages:

• Environmental
  • As mentioned above, keep cutting fluid out of the waste stream
  • Reduce the amount of ore mining and metal refining that must be done to meet the annual global demand for metal stocks. For example, it takes at least 4 times the energy per unit of production to produce aluminum billet from mined ore—plus the environmental impact of the mining itself—as it does to produce it from recycled scrap metal stocks (such as chips and bar drops).
• Financial
  • There is almost always some money to be made from the stream of recycled chips, whether by the scrap processor, the machine shop, or often both. The scrap value (melt value) of the metal is a net gain beyond any costs of handling and transporting the chips.

Requirements

Machine shops are typically required by the scrap collector to:

• segregate metal types (e.g., aluminum chips in separate barrels from steel chips)
  • it is usually not required to segregate particular alloys (e.g., to keep 2024 aluminum separate from 6061)
• segregate solid chunks (such as bar end drops and scrapped-out parts) from chips (which are finer and are processed on different material handling equipment.
• drain and/or centrifuge all cutting fluid and way oil out of the chips (within reason)