Speeds and Feeds

Cutting Metal

  • How fast can you cut metal?  Well, it depends on two major factors:
    • The type of metal to be cut.
    • The material the cutting tool is made from.
  • Normally harder metals are cut more slowly than softer metals; for example, aluminum.

What are speeds and feeds?

  • The speed refers to how quickly the cutting edge(s) of the tool pass through the metal.
  • The feed refers to the rate at which the tool is traversed over the work.

Why are speeds & feeds important?

  • If you try to cut metal too quickly you will:
    • Burn up the cutting edge of the tool;
    • Cause the surface of the work to become work hardened;
    • Waste time changing or sharpening tools;
    • Shorten the life of cutting tools.  The useful life of a cutting tool depends first on the cutting speed, then the feed rate, and lastly the depth of cut.

Rate of metal removal

  • The rate at which metal is removed from a workpiece is dependent upon how fast the tool cutting edges are moving through the metal (the speed) and how fast the tool is traversed over the work piece (the feed).  Let us start with the speed.  (Speed is normally expressed in Surface Feet per Minute) – sfpm or fpm

How do we know how fast to cut metal? (What speed to use.)

  • The cutting speed of all commonly used metals is freely available in manufacturers technical sheets and in the Machinery’s Handbook.
  • Some metals are so common that we remember their cutting speeds without consulting any reference.

Where can you find cutting speeds?

  • Students at the Centre may choose to consult the Machinery’s Handbook or their textbook for the cutting speeds they will require to complete their projects in the machine shop.
  • We should practice using both resources.
How do we convert the cutting speeds into RPM?
  • RPM is short for Revolutions Per Minute.
  • The cutting speed assumes the tool cutting edges are moving in a straight line.  Normally they are not.
  • nWe need to convert the cutting speed into RPM.
  • RPM = 12 x CS / π x Dia.

Applications

  • When drilling a hole, use the diameter of the drill in the formula not the size of the workpiece.
  • When milling, use the diameter of the cutter.
  • When turning on a lathe, use the diameter you are cutting.
Other factors affecting cutting speed.
  • Condition and heat treated state of the workpiece material.
  • Use of coolant, how it is mixed and how it is applied.
  • Type and grade of tooling material.
  • Operation being performed.  Feed rate and depth of cut.
  • Condition of machine and workpiece clamping.
  • Desired tool life.

Rough Shop Formula

  • RPM = 4 x CS / Dia

How do we know how fast to feed the cutter across or through the workpiece? (What feed to use.)

  • The feed rate for all commonly used metals is freely available in manufacturers technical sheets and in the Machinery’s Handbook.
  • Some cutting operations are so common that we remember the feed rates without consulting any reference.

What Determines the Feed Rate?

  • How much material each cutting edge can remove in one pass or revolution determines the feed rate.
  • In the case of a standard twist (jobber) drill, the larger the drill the greater the feed rate.  (When used in the same material.)

Help for distance calculation

  • distance traveled = time(min.) x feed/tooth x # teeth x rpm
  • Calculate how far a 2 flute drill will travel into a workpiece in 3 minutes if the feed rate is 0.002 per tooth and the rpm is 580.
  • Dist. Trav. = 2 x 0.002 x 3 x 580 = 6.96

Help with rpm question

Help with rpm question

Help with rpm question

Using Feedrates

  • Feedrates on drilling machines and lathes are set using feed per revolution.
  • Feedrates on milling machines are set using feed per minute.  To convert feed per rev. to feed per minute multiply the feed per rev. by the rpm of the cutter.

Theoretical Surface Finish

  • The feedrate has a direct impact on surface finish; so does any radius on the tool nose.  Consider the lathe example here.

 

 

 

Steel Making

What is steel?

  • Steel is iron that contains carbon and some other alloying elements.
  • Steel remains one of the most important of the materials used in engineering and manufacturing.

What is iron?

  • Iron is a magnetic metal, extracted from iron ore.  Metal alloys containing iron are referred to as ferrous metals.
  • Approximately 5% of the earth’s crust is made up of iron found in chemical combination with oxygen, sulfur, silicon and other elements. (Iron Ore)

Why are alloying elements added to steel?

  • Alloying –  Adding other elements to steel to change its properties.  For example:
    • Stainless steel contains the elements chromium and nickel, to make it rust resistant for use in corrosive environments. i.e. kitchens.
    • Manganese is added to increase toughness, while steel for cutting tools have tungsten and cobalt to keep them hard, even when hot.
Why is carbon the most important of the alloying elements?
  • Carbon gives a steel the ability to be hardened by heating and quenching.  This makes the finished product stronger and more wear resistant.
  • Carbon content influences machinability.
  • Carbon is the major alloying element in plain carbon steels.

What influences our choice of steel?

  • Physical characteristics.
  • Ability and ease of hardening.
  • Cost.
  • Availability.
  • Machinability.

Threads

What is a thread?

  • A helical groove formed on the outside or the inside of a cylinder.
  • Derived from the inclined plane.

What are threads used for?

  • Fastening one part to another part.
  • Adjustment.
  • Measurement.
  • Transmission of power

What are some common thread types?

  • ‘V’ thread: 60º for a unified or metric thread
  • Square thread, Modified Square Thread 5º
  • Acme: 29º for a standard ACME Thread
  • Buttress: 33, 45 AND 50 degree thread angles are common.
  • Rolled Threads: For screw shels of electric sockets and lamp bases.

The Unified Thread Form

American Standard B1.1-1949 complied with the agreement between the USA, Canada and England for the development of an interchangeable thread form.  This form is ‘V’ shaped and has a 60 degree thread angle.

Unified thread series (‘V’ thread)

  • UNC – Unified National Coarse
  • UNF – Unified National Fine
  • UNS – Unified National Special
  • UNEF – Unified National Extra Fine
  • UNR – Unified National Radius

Unified Thread Form

Unified Thread Form

Unified Thread Form

Size Details

Size Details

How are threads designated?

1/4 (Nomimal Size) – 20 (Threads per inch) UNC (Thread series)

When would I use a UNF thread?

  • On thin-walled material.
  • When resistance to loosening is important.
  • For fine adjustment.
  • For short lengths of engagement.
  • To increase the shear strength of the bolt.

When would I use a UNC thread?

  • On ductile, soft or brittle materials.  The thread bites deeply into the bolt.  Resists stripping.
  • When fast assembly is required.
  • Definition:  Ductile materials are able to withstand sudden impact and can be drawn out into thin wire.  Copper is a good example of a ductile metal;  lead is not.  Lead can withstand sudden impact but cannot easily be drawn into wire.

To calculate pitch from TPI

  • To find the pitch of a thread when the TPI is known just divide one by the TPI.
  • Pitch = 1 / TPI

Class of fit

  • Threads, when assembled, may have a loose or a tight fit.  The degree of looseness or tightness is referred to as the class of fit.
  • There are three basic classes of fit: 1 2 & 3 – Class 3 is the tightest fit, class 1 is the loosest.

Example

  • 7/8 – 9 UNC 1A has considerable clearance between the nut and bolt.
  • 7/8 – 9 UNC 3A has very little clearance between the nut and bolt.
  • The letter ‘A’ means that this is an external thread.
  • The letter ‘B’ would indicate an internal thread.
  • If a class of fit is not specified then class 2 is assumed.

What is the lead of a thread?

  • The lead of a thread is the distance a nut would move along the thread in one complete revolution.
  • The pitch of a thread is the distance from one thread to the next thread.  Therefore, the lead of a thread is equal to the pitch multiplied by the number of starts.
  • The lead of a single start thread is equal to the pitch.

Formula for “Lead”

  • Lead = Number of starts x pitch

Find the lead of the following thread: 

Find the lead of the following thread:

Find the lead of the following thread:

What about metric threads?

  • Metric threads have a 60 degree thread angle, the same as a Unified thread.
  • Metric threads are designated by their nominal outside diameter and the distance from one thread to the next.  This distance is known as the pitch of the thread.  For example:
Metric threads

Metric threads

 When Choosing a Metric Thread

  • Where possible, choose threads from the Standard M Profile Screw “Coarse Thread  Series”.  These coarse threads are the preferred choice when selecting metric threads for general purpose fastening

The lead of the following thread: 

  • M20 x L5 – P2.5 – 6g  (TWO START)
  • Pitch = 2.5 mm, Lead = # starts x pitch = 5.0 mm

Designation for a “Rounded Root”

  • M42 x 4.5 – 6g – 0.63R:  A rounded root is desirable on any metric thread and should be at least 0.125 x P

Grinding Machines and Equipment

Horizontal Spindle Surface Grinder

Horizontal Spindle Surface Grinder

Horizontal Spindle Surface Grinder

Horizontal Spindle Surface Grinder

Horizontal Spindle Surface Grinder

  • Surface grinder with direction and control of movements indicated by arrows. Wheel “A” controls downfeed “A”.  Large wheel “B” controls table traverse “B”. Wheel “C” controls crossfeed “C”

Electro-magnetic chuck for surface grinder. The stop strips at the back and left side are adjustable for height and can be used to align work.

  • Top surface of chuck must be reground when remounted.  N.B. Chuck must be switched on.

Periodic deburring of the chuck with a fine grit oilstone, is a good practice.

Vacuum chucks such as this one are considered good for thin and non-ferrous work.

Using a laminated vee-block to grind the angular surfaces on a Vee-block.

The Coolant

  • The coolant used for grinding is often water based.
  • Coolant must be kept clean with ‘cyclonic’ or other filtering equipment.  Dirty coolant may produce ‘fish-tail’ marks on the ground surface.
  • Coolant must be kept free of hazardous bacteria.

Vertical Spindle or “Blanchard” Grinder.

Vertical Spindle or “Blanchard” Grinder.

Vertical Spindle or “Blanchard” Grinder.

Cylindrical Grinder

  • Two dead centers are  better than revolving or live centers when precision cylindrical grinding.
Cylindrical Grinder

Cylindrical Grinder

Cylindrical form grinding.

  • Typical application of the angular center-type grinding machine, showing angled grinding wheel finishing both diameter and shoulder.
  • Sketch of traverse grinding with interrupted surfaces. Wheel should always be wide enough to span two surfaces or more at once.
  • Straight plunge grinding, where the wheel is usually wider than the length of the workpiece feature.
  • Taper grinding with the workpiece swiveled to the desired angle.
  • Angular plunge grinding with shoulder grinding. Note the dressing of the grinding wheel.

 Principle of internal cylindrical grinding.

internal cylindrical grinding

internal cylindrical grinding

 How do we grind round parts without centres?

  • The Process of ‘Centerless Grinding’ allows us to grind cylindrical workpieces without centres.  The workpiece revolves between two wheels.  One wheel is called the ‘regulating wheel’ the other is the ‘grinding wheel’.
  • Principle of the centerless grinder – The grinding wheel travels at normal speeds, and the regulating wheel travels at a slower speed to control the rate of spin of the workpiece.
centerless grinder

centerless grinder

3 Basic Methods of Centerless Grinding

  • Through-feed – Long bars
  • In-feed – Parts with shoulders
  • End-feed – Tapers

Through-feeding of long bars: The regulating wheel is set at a small tilt angle to feed the bar between the wheels.

In-feeding in the centerless grinder: The regulating wheel is also set at a small tilt angle to keep the workpiece against the end stop

Principle of internal centerless grinding.

Principle of internal centerless grinding.

Principle of internal centerless grinding.

Universal tool and cutter grinder.

Universal tool and cutter grinder.

Universal tool and cutter grinder.

Grinding Wheels

We use grinding wheels to:

  • Size a part accurately;
  • Improve the surface finish;
  • Generate a surface with a specific shape.
  • Grinding operations are often performed on very hard metals.

What are they made from?

  • Grinding Wheels are normally hard and brittle.  They are made from one of these:
    • Aluminum Oxide: Used for grinding Steel
    • Silicon Carbide: Used for grinding cast iron, nonferrous and nonmetallic materials.
      • Friability: Aluminum oxide and silicon carbide are both very hard and brittle.  This “Friability” causes the grains to break easily.  During the grinding process, each broken grain reveals a new and very sharp cutting edge.
    • Diamond: Used for grinding cemented carbides, glass and ceramics.
    • Cubic Boron Nitride (CBN) (Borazon) (ABN):  Used for grinding hardened tool steels and superalloys.

Wheel Bonds

  • Grinding wheel abrasive is held together with one of the following bonding materials:
    • Vitrified – Inert and able to withstand high temperatures.
    • Resinoid – High operating speeds or loads.
    • Rubber – Flexible and for a high, burr free finish.
    • Metal – Base for the electro-deposition of diamond or CBN abrasive.

Grinding Wheel Shapes

  • Straight Wheel (Type 1)
  • Cylindrical Wheel (Type 2)
  • Flaring Cup Wheel (Type 11)
  • Dish or Type 12 Wheel
Grinding Wheel Shapes

Grinding Wheel Shapes

Wheel Identification

  • Five major factors are used to identify most grinding wheels:
    1. The type of abrasive
    2. The size of the grit
    3. The grade or hardness
    4. The Structure
    5. The Bond
  •  ANSI Standard B74.13-1990  Markings for Identifying Grinding Wheels & Other Bonded Abrasives
Markings for Identifying Grinding Wheels

Markings for Identifying Grinding Wheels

“Glazing” and “Loading”

  • A glazed wheel occurs when the abrasive grains are dull.  The cutting surface of the wheel appears shiny.
  • Loading occurs when foreign material becomes trapped in the voids (spaces) between the abrasive grit.
  • Dress the wheel when it becomes “loaded” or “glazed”.
  • The wheel at the top is “loaded”.  Bits of metal are embedded in its grinding face.  It is poor practice to off-hand grind soft metals like aluminum on a pedestal grinder. The same wheel, below, has been dressed to remove the “loading”.

Truing & Dressing

  • In a perfect world, a grinding wheel will be self-sharpening.  Dull grains will fracture or will be dislodged from the wheel surface, exposing new sharp cutting edges.  Unfortunately this is rarely possible.
  • Wheels need to be trued and dressed when mounted and must be dressed regularly thereafter.

Truing a Grinding Wheel

  • Truing a wheel ensures the outside cutting surface runs true with the machine spindle.
  • A wheel must always be trued after it has been mounted.
  • Truing a wheel on a precision surface grinder is normally accomplished with a single point diamond dresser.

Dressing a Grinding Wheel

  • Dressing a wheel exposes new cutting edges and improves the cutting action.
  • Wheels must be dressed regularly as required, using one of the following:
    • Rotating hand dressers
    • Abrasive sticks or wheels
    • Single point or cluster diamond dressers
    • Crush roll dressers

Single-point Diamond Dresser.

  • The most important precaution when using this dresser is to turn the diamond often to avoid grinding flats on it.  Take care not to subject the diamond to “Thermal Shock”.
  • One way of mounting a single-point dresser on a surface grinder. The dresser with its diamond is magnetically secured on a clean chuck. Note the diamond is slanted at a 15-degree angle and positioned slightly past the vertical centerline of the wheel.
Single-Point Diamond Dressor

Single-Point Diamond Dressor

Mounting Grinding Wheels

Mounting Grinding Wheels

Mounting Grinding Wheels