Chapter 16: Machining Operations and machine tools

What makes each machining operation produce unique part geometries?

1) Generating shape- part geometry determined by feed and trajectory of cutting tool 2) Forming shape - part geometry created by the shape of the cutting tool

What is an example of how a part can be made in a six spindle bar machine (know each step for test)?

1) feed stock to stop, (2) turn main diameter, (3) form second diameter and spotface, (4) drill, (5) chamfer, and (6) cutoff Refer to slides/ textbook for picture

When are 3 jaw chucks used vs. 4 jaw chucks?

3 jaw chucks (self-centering) are used for clamping round objects (move simultaneously), where 4 jaw chucks take longer to center (jaws move independently) but can clamp rectangular shapes and is suitable for eccentric turning. The jaws are moved with chuck wrenches.

What are examples of forming to create a shape?

A form tool can be used in form turning, drilling and broaching

How are holes produced in drilling?

A hole cutter is used, which is thin walled with multiple tooth cutter saw teeth. They are often called hole saws.

What is turning?

A single point cutting tool removes material from a rotating workpiece to generate a cylindrical shape. It is performed on a machine tool called a lathe. Can be used for facing, contour turning, chamfering, cutoff and threading.

What is threading?

A pointed tool is fed linearly across the outside surface of the rotating workpart in a direction parallel to the axis of rotation at a large effective feed rate, thus creating threads in the cylinder.

What is boring?

A single-point tool is fed linearly, parallel to the axis of rotation, on the inside diameter of an existing hole in the part. Boring is like turning but inside of an existing hole. Can be horizontal or vertical.

What is tapping?

This operation is performed by a tap and is used to provide internal screw threads on an existing hole.

What machines can files be used in?

Band file: like band saw, but file segments joined to create the band (saves money by replacing one part at a time) Disk type filing: files in disk form

What is the difference between shaping and planing?

Both use a single point cutting tool moved linearly relative to the workpart, however, in shaping the cutting speed (speed motion) is caused by moving the tool where in planing the cutting speed (speed motion) is caused by moving the workpiece

How are the costs of broaching minimized?

Broaches can be built in separate sections that are joined together. So when one section breaks, like one shell on a shell construction pull broach, then only that piece needs to be replaced.

What is a though hole vs a blind hole?

A through hole is when the drill exits the opposite side of the work and a blind hole is when the drill does not exit the opposite side

How can you avoid chatter?

Add stiffness and/or damping setup Operate at speeds that avoid natural frequencies Reduce feeds and depths to reduce forces Change cutter design to reduce forces Use a cutting fluid

What sequence of steps can a mill-turn center perform?

An example: a) Turning b) Milling c) Drilling d) Cutoff

What is broaching?

Broaching is a multiple tooth cutting tool moved linearly relative to the work at slow speeds. This is very expensive (from complicated and custom-shaped geometry), but provides a good surface finish, close tolerances and a variety of work shapes. The cutting tool, called a "broach", will have different sections like the shank, roughing teeth, semi-finishing teeth, and finishing teeth. The teeth are are progressively larger and take more of the workpiece with each tooth. Broaching puts tools in tension, where steel is better than carbide.

What are Burrs used for?

Burrs are files that help reach internal areas and file them where a workpiece can't be filed by hand. Types: HSS burr, carbide burr, rotary file

How can you make sawing less expensive?

By using HSS, tungsten-carbide, or other inserts (electron-beam welded), you can replace the small parts instead of the whole blade.

How are chips removed in broaching?

Chips are allowed to exit through the gullets, which are rounded spaces between each tooth.

What is counterboring?

Counterboring provides a stepped hole, in which a larger diameter follows a smaller diameter partially into the hole. A counterbored hole is used to seat bolt heads into a hole so the heads do not protrude above the surface.

What is high speed machining?

Cutting at speeds significantly higher than in conventional operations (up to 6 times faster). It is always valued to produce faster and more economically. In order to perform at high speeds, the material must have good toughness for the first impact, high hot hardness, use rubber for damping vibrations and fluids to lubricate and cool.

What is the difference between climb cut (down milling) versus conventional cut (up milling)?

Down milling: takes large thickness at the entry point, because the blade rotates in the direction of feed. This method can break the machine because of the stresses involved with making first contact with a thick contact. Up milling is preferred for safety, but takes away small chips with an abrupt exit. The thickest part of the chip, then, is at the end of the chip.

Drill press vs. radial drill press

Drill presses are usually mounted straight up and down, standing on floor and having a bench but radial drill presses are lifted from the side to accommodate larger pieces.

What is drilling?

Drilling can be performed on a lathe by feeding the drill into the rotating work along its axis. It creates a round hole in a work part (boring only enlarges an existing hole). The cutting tool is called a drill or drill bit and the machine tool is called a drill press.

What is filing?

Filing is material removal with fine wide teeth. It has the same cutting action as sawing and broaching (one direction). It is easy to control because it produces small chips and is used for finishing and deburring operations.

What factors play into the surface finish of machining?

Geometric factors: type of operation, tool geometry like nose radius, feed speed Work material factors: built-up edge effects, damage to surface caused by chip, tearing of surface when machining ductile materials, cracks when machining brittle materials, friction between tool flank and new work surface Vibrations: chatter in machine tool or cutting tool, deflection of fixtures, backlash in feed mechanism (gets caught). If chatter can be eliminated then the only factors of roughness are geometric and work material factors

What is form turning?

In this operation, sometimes called forming, the tool has a shape that is imparted to the work by plunging the tool radially into the work.

In turning, what are the cutting edges made of? How are they dealt with if they get defects?

In turning carbide cutting inserts are used, and if one side of the insert gets a defect you can switch to the other side. If the insert is unusable you can replace it entirely, but switching the tool is expensive.

What is contour turning?

Instead of feeding the tool along a straight line parallel to the axis of rotation as in turning, the tool follows a contour that is other than straight, thus creating a contoured form in the turned part.

What is taper turning?

Instead of feeding the tool parallel to the axis of rotation of the work, the tool is fed at an angle, thus creating a tapered cylinder or conical shape.

What is a machining center?

It is a highly automated machine tool that performs multiple machining operations under CNC control, one setup, and minimal human attention. Usually used for milling and drilling, has up to 5 axes of movement, automatic tool changing, positioning, etc.

What is a bar machine?

It is a machine similar to the chucking machine but it uses a collet for long pieces to be fed through the headstock. It is highly automated and has a better quality than chucking machines.

What is an automatic screw machine?

It is a variation of the bar machine but it is smaller, and is able to produce a high quantity of screws for small hardware items.

What is a vertical boring mill and why is it used?

It is used on large, heavy work parts with low Lenght/Diameter ratio

What is a chucking machine?

It uses a chuck in the spindle to hold the work part and the tailstock is absent so parts cannot be mounted between the centers. It is used for short and light parts and the cutting is automatic.

What are drawbacks of planing?

It uses a reciprocating motion which is not efficient or economical. The production of flat surfaces, grooves and notches are made on long workpieces (10m+) and to to their length chip breakers are necessary. Chip breakers break long chips to reduce friction and vibration.

What is a Turret Lathe?

It's a conventional lathe equipped with a six-sided tool holder called a turret on the tailstock. This is helpful if there are several processes to be done on a part so that the machine doesn't need to be stopped to change tools.

What are lathe centers?

Lathe centers are used to support long jobs at the tailstock end. They can be fixed ("dead center") or rotating with the job ("revolving center"). Dead centers provide higher quality because revolving centers can become loose while spinning.

What are HSM applications?

Machining large airframe components from large aluminum blocks Multiple machining operations on aluminum to produce automotive, computer and medical parts (quick tool changes and tool path control important) Die and mold industry (fabricating complex geometry from hard materials)

What are the tolerances like for machining?

Machining provides high accuracy relative to most other shape-making processes. Closer tolerances means higher costs.

What are Mill-turn centers?

Mill-turn centers are highly automated machine tools that do turning, milling and drilling operations. It can also stop the workpart at a defined angular position so a rotating tool can machine features into the outside surface of the part. Both of these are impossible on conventional turning centers.

What is milling?

Milling is when work is fed past a rotating tool with multiple cutting edges. The tool is perpendicular to the feed. Cutting tool: milling cutter Machine tool: milling machine. Creates planar surface. Can move in 3 axes, where lathes move in 2 axes

What are the two forms of milling?

Peripheral milling: horizontal spindle, parallel to surface and cutting edges on outside periphery of cutter Face milling: vertical spindle (perpendicular to surface), more accurate, cutting edges both end and outside periphery

What are the different types of saws?

Power hacksaw: Linear reciprocating motion of hacksaw blade against work Band Saw: bandsaw blade forms endless flexible loop with teeth on one end, which is better for complex shapes because it's flexible, but not good for hard materials Circular saw: rotating saw blade provides continuous motion of tool past workpiece (stiffest blades for very hard materials like ceramic)

How should products be designed?

Products should be designed to use the least amount of machining as possible. Heavily utilize net shape processes like casting, forging or plastic molding or near net shape. Avoid sharp corners, which are hard to machine and creates burrs. Avoid undercuts which causes stress concentrations and fractures. Design parts with features that can be produced with a minimum amount of setups. Avoid unusual hole sizes, threads, and features requiring special form tools (minimize amount of cutting tools required).

What is reaming?

Reaming is used to slightly enlarge a hole, to provide a better tolerance on its diameter, and to improve its surface finish. The tool is called a reamer, and it usually has straight flutes.

What are lathe steadies/rests and why are they used?

Rests are fixtures used to support long jobs. They can be steady rests which supports all around with three "fingers" or follower rests which support from two sides (two fingers) for machining on the other sides.

What are the different sawing operations?

Ripping (cut in half along long edge, like for wood), internal cuts (cutting circles, holes), angular cuts (cutting circles out of sides of a pipe), contour cutting (cutting shape on inside of workpiece), stack cutting (cutting a stack of workpieces at same time), and wafer slicing (using a donut-shaped saw to slice an ingot into thin pieces.

How are machined parts classified?

Rotational - cylindrical or disk-like shape Nonrotational - block-like or plate-like

What types of collets are there?

Round, square, hexagon and more. They are all split into three longitudinal segments.

What is sawing?

Sawing is cutting a narrow slit in the workpiece with a tool that has narrowly spaced teeth. The tool is a saw blade. This can separate a workpiece into two pieces, cut off unwanted parts or cut an outline of a flat part.

What are the different kinds of file teeth?

Single cut: used for most materials, rows of parallel teeth along whole file Double cut: two series of parallel teeth, forming a roughing end and a finishing end Rasp: Used to take off a lot of material, teeth short and raised Vixen: used to get a very high-quality finish, using parallel curved lines

What are spade drills?

Spade drills are used for making holes 1" or larger in diameter at low speeds or high feeds.

What is spot facing?

Spot facing is similar to milling. It is used to provide a flat machined surface on the workpart in a localized area.

What are the types of saw teeth?

Straight tooth: for copper, brass and plastic, alternating saw pattern Raker tooth: for steel and iron, includes spaces in between alternating saw pattern Wave tooth: for thin workpieces, pattern looks like a wave

What are examples of generating shape?

Straight turning (turn at constant depth), taper turning (turn at an angle), contour turning (vary shape with uneven profile), plain milling (reveal new surface with horizontal milling), profile milling (using end mill tool to take from bottom and side)

What is chamfering?

The cutting edge of the tool is used to cut an angle on the corner of the cylinder, forming what is called a ''chamfer.''

What is the most widely used shaper?

The horizontal push-cut, where the work is held in a vice or mounted while the shaping is done by HSS or carbide cutting tools held in a tool holder.

What is a knee-and-column milling machine?

The knee-and-column milling machine is the basic machine tool for milling. It derives its name from the fact that its two main components are a column that supports the spindle, and a knee (roughly resembling a human knee) that supports the worktable. Horizontal knee-and-column machines (a) only moves the table relative to the spindle where vertical ones move both the spindle and table

What is cutoff?

The tool is fed radially into the rotating work at some location along its length to cut off the end of the part. This operation is sometimes referred to as parting.

What is facing?

The tool is fed radially into the rotating work on one end (back side) to create a flat and smooth surface on the end

What are multiple spindle bar machines?

There are multiple spindles on the bar machine so multiple parts can be produced by multiple tools simultaneously. For example, six spindle automatic bar machines produce six parts at a time. After each cycle spindles are rotated (indexed) to the next position (one position for setting part, another for turning, etc.).

What is knurling?

This is not a machining operation because it does not involve cutting of material. Instead, it is a metal forming operation used to produce a regular cross- hatched pattern in the work surface.

What is countersinking?

This is similar to counterboring, except that the step in the hole is cone-shaped for flat head screws and bolts.

What is center drilling?

This operation drills a starting hole to accurately establish its location for subsequent drilling. The tool is called a center drill.

How are work parts held in drill presses?

Vise- work holder with two jaws Fixture - work holder that is custom-designed for particular workpart Drill jig - similar to fixture but also allows guiding the tool during drilling

Why would machining be required for a product?

When these matter: close tolerances (specify bare minimum to be functional, close tolerances may add cost but no value) good surface finish special geometric features: threads (low speed for accuracy), accurate holes, accurate cylindrical sections, flat and/or straight surfaces

Can you combine forming and generating?

Yes, for example when creating threads on a lathe or making slots in slot milling

What are the types of face milling?

a) Conventional face milling, b) partial face milling c) end milling, d) profile milling, e) Pocket milling and f) contour milling

What are the different methods of holding a workpiece in a lathe?

a) Holding work between centers with Dog plate (driven by lathe spindle) and Dog (clamped to work, driven by dog plate) and end held by a center at the tailstock for long workpieces b) A chuck for short workpieces with 3 or 4 jaws c) A collet with three slits that squeezes and work and is squeezed by a sleeve to hold a workbar d) A faceplate that fastens to the lathe spindle and is used to grasp parts with irregular shapes. These parts cannot be held by other methods. The faceplate is therefore equipped with the custom-designed clamps for the particular geometry of the part.

What are the types of peripheral milling?

a) Slab milling, b) slotting, c) side milling, d) straddle milling e) form milling

What is DN ratio?

bearing hole diameter (mm) * max spindle speed (rev/min) Typical DN ratio for HSM is 500,000 to 1,000,000 Allows larger diameter bearings to fall within HSM range, even though they operate at lower rotational speed than smaller bearings

What is HP/RPM ratio?

ratio of horsepower to maximum spindle speed conventional machine tools have higher ratio than HSM tools HSM includes ratios < 0.005 hp/rpm

What are requirements for high speed machining?

special bearings running at high rpm high feed rate capability CNC motion controls balanced cutting tools, toolholders, and spindles coolant delivery at high pressures chip control and removal systems (magnets for removal can be used)