Centre Punch

Lathe Introduction

The metal lathe is designed to support and rotate a piece of material as it is being shaped by a cutting tool. The lathe can produce cylindrical, conical, helical and spherical shapes that are coaxial with the centre line of the lathe.

This type of lathe is often called a centre lathe. Small machines are usually mounted on a bench or a stand and sometimes called bench lathes. Bench lathes are available in various sizes, capable of turning lengths between about 600mm and 1000mm. Larger, heavy duty lathes can turn lengths up to 3 metres or more.

There are two main types of centre lathe. Lathes in which turning speeds are varied using step-cone pulleys are belt driven and are usually called belt-drive head lathes. Lathes that are driven through a gear box are usually called geared head lathes. The drive mechanism of a lathe is contained within the headstock as shown in the diagram below.

In the headstock, a spindle or main shaft is mounted in robust bearings that must are capable of withstanding heavy loads imposed by turning operations and maintaining the accuracy required for the finished job. The main functions of the headstock are to:

Support and rotate the workpieace.
House the speed changing mechanism.
Enable the work holding devices (Chucks) to be attached to the spindle.

The tailstock is made up of two castings. One rests on the guide ways and is clamped to the bed to suit the length of the work. The tailstock spindle or quill is housed in the upper casting which has a sideways adjustment. The tailstock spindle is bored to a standard morse taper to take centres, drill chucks and other attachments.

The carriage is the part of the lathe that supports and controls the movement of the cutting tool. It is comprised of the saddle which fits over and slides on the guide ways and the apron which is fastened to the saddle and hangs in front of the lathe bed. The apron carries the handwheel mechanism for the hand feed as well as the mechanisms for engaging the automatic feed and the lead screw.

Chucks

Lathe chucks are fitted to the spindle and are used to grip the work. Generally, two sets of jaws are provided with lathe chucks. One set for holding work externally, the other internally. Jaws for a self centering chuck must be fitted with identification amrks corresponding to those on the chuck. They must also be fitted in correct sequence.

The three jaw self centering chuck is made so that the three jaws all move together and are always the same distance from the centre. The three jaw chuck is gennerally used to hold round or hexagonal work.

Important safety tip: Once the chuck has been tightened to secure the work, ensure the chuck key must be removed from the chuck prior to starting turning on the lathe.

Cutting Speed & Machine RPM

Cutting speed is defined as the number of metres of material that passes the lathe tool in one minute, measured around the circumference of the work. Recommended cutting speeds depend on the type of material you are turning and the properties of the cutting tool. For example, Bronze has a faster cutting speed than steel because it is a much softer material. A tungsten carbide cutting tool will enable even higher cutting speeds on the same material compared to using high-speed steel cutting tools.

To achieve the required cutting speed the lathe has to be set to the correct RPM for the diameter of the material. This information can be read from a chart; however, it is useful to know how to calculate RPM when a chart is not available.

Example: Calculate the lathe RPM for rough turning a mild steel bar of 42mm diameter with a high-speed steel cutting tool.

The basic formula for calculating revolutions per minute is: RPM = Cutting Speed/Circumference.

When using a high-speed cutting tool, the cutting speed for rough turning mild steel is 30 metres per minute. Circumference of the bar is calculated as follows:

Circumference of the bar = ℼ x diameter of the bar

= (22/7) x 42mm

= 132mm

RPM = Cutting Speed/Circumference

= (30m x 1000) / 132mm (Note that the cutting speed 30m per minute is multiplied by 1000 to convert to mm)

= 230 approximately

Parallel Turning

Parallel turning or straight turning is a process used to cut the metal parallel to the axis. Parallel turning is done to decrease the diameter of the metal. When performing parallel turning, the cutting tool must always be set to the correct height (centre line of the work), just the same as when performing the facing process. Most turning is done from the tailstock toward the headstock using a right-hand tool. To go in the opposite direction, from the headstock toward the tailstock, use a left-hand tool.

It is good practice to set the tool during turning for safety in case cutting forces push the tool away from the workpiece. Keep the tool short for rigidity and be sure the tip is on centre. Then position the tool tip near the workpiece and move the carriage toward the headstock to the required location.

Important safety tip: Rotate the spindle by hand to check for clearance between the tool holder and compound rest and the chuck to make sure that there will be no collisions during machining. Repeat this process at the beginning point of the cut, especially if the workpiece is being supported by a tailstock mounted centre.

Watch this video to get a deeper understanding of parallel turning.

Centre Drilling

Work supported by a centre mounted in the tailstock must be drilled to fit the centre so a bearing surface is formed. Centre drilling is usually performed by mounting the workpiece in the lathe chuck and a centre drill in a drill chuck held in the tailstock.

The tailstock is positioned with the centre drill close to the workpiece. It is then locked to the guide ways and the centre drill is fed into the work using the tailstock hand wheel. The drill does not rotate but the work rotates as the drill is fed into it.

Important safety tip: The small pilots of centre drills are fragile, so apply adequate cutting fluid, use light feed and retract frequently to remove chips from the hole to prevent breakage.

Watch this video to get a deeper understanding of centre drilling.

Facing Off

Facing is the process of removing metal from the end of a workpiece to produce a flat surface. Most often, the workpiece is cylindrical, but using a 4-jaw chuck you can face rectangular or odd-shaped work to form cubes and other non-cylindrical shapes.

When a lathe cutting tool removes metal it applies considerable tangential (i.e. lateral or sideways) force to the workpiece. To safely perform a facing operation the end of the workpiece must be positioned close to the jaws of the chuck. The workpiece should not extend more than 2-3 times its diameter from the chuck jaws unless a steady rest is used to support the free end.

Choose a cutting tool with a slightly rounded tip. A tool with a sharp-pointed tip will cut little grooves across the face of the work and prevent you from getting a nice smooth surface. Clamp the cutting tool in the tool post and turn the tool post so that the tip of the cutting tool will meet the end of the workpiece at a slight angle. It is important that the tip of the cutting tool be right at the centre line of the lathe; if it is too high or too low you will be left with a little bump at the centre of the face.

Watch this video to get a deeper understanding of facing off.

2. Research - Subject Specific Terminology (SST)

IND4-9 identifies a range of technologies and their intended uses

Knurling Tool

What is Knurling?

Knurling allows hands or fingers to get a better grip on the knurled object than would be provided by the originally smooth metal surface. Occasionally, the knurled pattern is a series of straight ridges or a helix of "straight" ridges rather than the more-usual criss-cross pattern. Knurling doesn't add material to the grip. Rather, it creates the textured surface either through pressure or by cutting away some of the material.

What is a knurling tool used for?

A knurling tool is a manufacturing and repairing tool used on lathe machines. A knurling tool is used to give the metal a criss-cross pattern shape, which helps to hold the knurled tool more firmly. Knurling can be done either by machine or hand. Hand knurling uses a rolling roll that creates the desired pattern on the workpiece.

Lathe Cutting Tool

Lathe cutting tools are tools you can use with a lathe machine to produce turned parts and shape materials into the desired shape. There are three major classification categories for lathe cutting tools: material, uses, and method of applying feed.

Cutting edge material is commonly made from high speed steel, but sometimes carbide, tungsten or even diamond is used.

Your tool selection will be determined by the process you wish to complete such as, roughing, finishing, facing, boring, chamfering, knurling or thread cutting.

Emery Cloth

Emery cloth is a type of coated abrasive that has emery glued to a cloth backing. It is used for hand metalworking. It may be sold in sheets or in narrow rolls, typically 25 or 50 mm wide, often described as "emery tape". The cloth backing makes emery cloth stronger in tension than sandpaper, but still allows a sheet to be conveniently torn to size.

Emery was considered a suitable abrasive for fitting work and the final adjustment of steel parts for a perfect fit. It had the advantage that, unlike harder abrasives, it was not considered to embed abrasive traces in the polished components afterwards. Emery was also used for cleaning, as a means of removing rust from polished steel components.

Oxidation

All metals, with the exception of precious metals, will oxidize when exposed to oxygen and an electrolyte (i.e. atmospheric moisture). General corrosion occurs when most or all of the atoms on the same metal surface are oxidized, damaging the entire surface. Most metals are easily oxidized: they tend to lose electrons to oxygen (and other substances) in the air or in water. As oxygen is reduced (gains electrons), it forms an oxide with the metal. This is commonly known as surface rust. To minimise the oxidation process, metals can be coated with wax, oil or lacquer to protect the surface.

Warding File

A popular file with locksmiths, the Warding file was designed for filing or repairing "wards" in locks and keys. As the Warding file is thin, it is also suited for any application where the space is too narrow for other files to fit. This file tapers toward the end, in width towards the point. They are double cut on both faces and single cut on both edges. It is their tapered faces and thin profiles that make them ideal for finishing and deburring in the narrowest of spaces. Warding files are usually available in lengths from 100mm (4 inches) to 250mm (10 inches).

Oxy-Acetylene Torch

Oxy-fuel equipment is quite versatile, not only because it is preferred for some sorts of iron or steel welding but also because it lends itself to brazing, braze-welding, metal heating (for annealing or tempering, bending or forming), rust, or scale removal, the loosening of corroded nuts and bolts, and is a ubiquitous means of cutting ferrous metals.

The torch is the tool that the welder holds and manipulates to make the weld. It has a connection and valve for the fuel gas and a connection and valve for the oxygen, a handle for the welder to grasp, and a mixing chamber (set at an angle) where the fuel gas and oxygen mix, with a tip where the flame forms. The common types of torches are:

Welding
Cutting
Rose Bud
Injector

Air-acetylene produces a flame temperature of around 4000° F (2200° C). This is hot enough to solder aluminum work glass, repair radiators and braze plumbing fixtures. It is not hot enough to weld steel. When acetylene is burned in pure oxygen, the flame temperature may be as high as 5730° F (3166° C).

3. Processes & Techniques - Polishing & Buffing

IND4-3 identifies and uses a range of hand and machine tools to produce quality practical projects

Polishing and buffing are finishing processes for smoothing a workpiece's surface using an abrasive and a work wheel or a leather strop. Technically polishing refers to processes that use an abrasive that is glued to the work wheel, while buffing uses a loose abrasive applied to the work wheel. Polishing is a more aggressive process while buffing is less harsh, which leads to a smoother, brighter finish. A common misconception is that a polished surface has a mirror bright finish, however most mirror bright finishes are actually buffed.

Polishing is often used to enhance the appearance of an item, prevent contamination of instruments, remove oxidation, create a reflective surface, or prevent corrosion in pipes. In metallography and metallurgy, polishing is used to create a flat, defect-free surface for examination of a metal's microstructure under a microscope. Silicon-based polishing pads or a diamond solution can be used in the polishing process.

The removal of oxidization (tarnish) from metal objects is accomplished using a metal polish or tarnish remover; this is also called polishing. To prevent further unwanted oxidization, polished metal surfaces may be coated with wax, oil, or lacquer. This is of particular concern for copper alloy products such as brass and bronze.

While used less extensively than traditional mechanical polishing, electropolishing is an alternative form of polishing that uses the principles of electrochemistry to remove microscopic layers of metal from a base surface. This method of polishing can be fine-tuned to give a wide range of finishes, from matte to mirror-bright. Electropolishing also has an advantage over traditional manual polishing in that the finished product will not experience the compression and deformation traditionally associated with the polishing process.

4. Centre Punch Onshape Tutorials

IND4-5 selects and uses communication techniques when designing, making and evaluating projects and ideas

To design 3D model of your Metal work project you will be using Onshape, it is a professional-grade, cloud-based Computer-Aided Design (CAD) platform that students and teachers can access for FREE on any device, anywhere, anytime. Computer-aided design is the use of computers to aid in the creation, modification, analysis, or optimization of a design. CAD software is used to increase the productivity of the designer, improve the quality of design, improve communications through documentation, and to create a database for manufacturing.

GETTING STARTED WITH ONSHAPE

Copy this URL and paste it into your browser: https://www.onshape.com/en/education/
Click on the green "For Students" button
Fill out your details as prompted NOTE CAREFULLY: Sign up with your education email, the one that is your.name@det.nsw.edu.au. Not your private email. See the picture opposite for how to fill out the second page. It will ask for the URL for the school - copy and paste this into the space: jamison-h.schools.nsw.gov.au
Make sure you remember your password - if you forget, you will have to go through the password recovery process. Your teacher cannot fix this for you. DO NOT SHARE THIS PASSWORD WITH OTHERS.
Log in

Tutorial 1

Centre Punch Onshape Tutorial 1.mp4

If you are at home, you can access the youtube video here (9:11 minutes)

Tutorial 1 Key Lessons:

Sketching geometry
Using construction lines
Extruding part using revolve

Tutorial 2

Centre Punch Onshape Tutorial 2.mp4

If you are at home, you can access the youtube video here (10:46 minutes)

Tutorial 2 Key Lessons:

Sketching geometry
Creating a helix
Sweeping a path
Editing sketch geometry
Creating a circular pattern
Combining multiple parts using a Boolean

5. Working Drawing

IND4-5 selects and uses communication techniques when designing, making and evaluating projects and ideas

6. Steps of Construction

IND4-2 applies a design process in the modification of projects

Start by cutting a piece of 12mm diameter Bright Steel to a length of 105mm (this allows a small amount of extra material, which will be machined off during the turning processes.

Knurling

Set the metal Lathe, to the following settings:

Speed 770rpm
Feed direction is away from headstock.
Feed rate = B,C,U,Z = 0.75mm

Place the Bright Steel into a 3 Jaw Self-Centring Chuck on the metal Lathe, so that about 80mm protrudes out of the chuck.
Tighten the Chuck with the Chuck Key. Remove the Chuck Key from the chuck immediately.
Place a Knurling Tool into the Tool Post and tighten the 3 holding bolts down.
Position the Knurling Tool so that it will cut along the length of the Tool Steel (about 10mm away from the chuck)
Ensure that the Knurling Tool is tightened onto the Steel
Ensure that the coolant can flow easily and that it will flow over the Knurling Tool.
Use the Self-feed mechanism to produce an even knurl along about 70mm of the Steel. DO NOT run off the end of your job.
Remove Knurling Tool and replace with a standard Lathe Tool in its holder, ensure that it is set to the correct centre height.

Facing the job

Without removing the Tool Steel, engage the lathe tool so that it begins to cut and by rotating the Cross Slide Handle slowly move the tool out towards the inside of the job. Repeat until the end is flat.

Taper Turning

Leave the job in this position. Adjust the Compound Slide to an angle of 3 degrees to the right of zero position.
Start from the faced-off end of the job. Ensure that the lathe tool just touches the end of the job. Rotate the Compound Slide Handwheel to start the taper. Using both the compound slide and the cross slide continue to create a taper that is 10mm long and has a diameter of 10mm (see diagram).
Using a file and emery cloth polish this end of your project.
Take your project out of the chuck and turn it around (protect the knurl using a sheet metal sleeve) retighten your project so that 60mm is protruding from the chuck.
Using the same taper turning technique, create a 45mm long taper with a tip of 6mm in diameter
Using a file and emery cloth again polish the taper
Leave the job in this position. Adjust the Compound Slide to an angle of 45 degrees to the right of zero position.
Use the same taper turning technique as before to create a 45-degree point on your project.
Your project is now ready for Heat treatment.

Heat Treatment

The job point must be Hardened & Tempered before it can be used.

Hardening Process

The point of the job is heated to Cherry Red in colour by using the LPG (Liquid Petroleum Gas) Torch. It is then Quenched into water to cool it.

Tempering Process

The point is made shiny again with the Wet & Dry.
Slowly wave the LPG torch over the point of the Centre Punch until the point becomes yellow in colour. Quickly quench in the oil.
If the colour of the point goes blue in colour the point must be rehardened.

Polishing

Reclean the entire job with the Wet & Dry so that it is shiny.
Buff the job on the Buffing Machine.

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