Making Model Steam Boats E-Book

Stephen Bodiley

• Herausgeber: The Crowood Press
• Kategorie: Lebensstil
• Sprache: Englisch

Seitenzahl: 307

Veröffentlichungsjahr: 2022

Beschreibung

Making model steam boats provides everything you need to design, build and sail your own steam launch. Learn about all aspects of boat design, from the inner workings of the engine to the finishing coat of paint. Explore topics such as hull dynamics, propeller selection and boiler capability, along with information on engine performance, boiler safety and hull design. Follow the plans to build a simple screw-driven launch, a fast patrol boat or a twin cylinder paddle steamer. The technical information and theoretical calculations provided explain how boats are designed and allow builders to progress to creating their own models. Making a live steam model boat requires a broad range of skills including machining, silver-soldering, sheet metal work, woodwork and finishing. Part of the enjoyment of model boats comes from this variety of skills and all the required techniques are explained in full.

Leseprobe

First published in 2022 byThe Crowood Press LtdRamsbury, MarlboroughWiltshire SN8 2HR

[email protected]

www.crowood.com

This e-book first published in 2022

© Stephen Bodiley 2022

All rights reserved. This e-book is copyright material and must not be copied, reproduced, transferred, distributed, leased, licensed or publicly performed or used in any way except as specifically permitted in writing by the publishers, as allowed under the terms and conditions under which it was purchased or as strictly permitted by applicable copyright law. Any unauthorised distribution or use of this text may be a direct infringement of the author’s and publisher’s rights, and those responsible may be liable in law accordingly.

British Library Cataloguing-in-Publication DataA catalogue record for this book is available from the British Library.

ISBN 978 0 7198 4132 3

DisclaimerSafety is of the utmost importance in every aspect of metalworking and model engineering. When using tools, always follow closely the manufacturer’s recommended procedures. However, the author and publisher cannot accept responsibility for any accident or injury caused by following the advice given in this book.

Cover design by Sergey Tsvetkov

CONTENTS

PREFACE AND ACKNOWLEDGEMENTS

INTRODUCTION AND UNITS

1WORKSHOP EQUIPMENT AND SAFETY

2THE POND RUNNER: A MODEL OF THE SIMPLEST DESIGN

3HARBOUR PILOT: THE ENGINE

4HARBOUR PILOT: THE BOILER AND FIREBOX

5HARBOUR PILOT: HULL CONSTRUCTION

6PADDLE STEAMER: A VINTAGE VESSEL

7MORE IDEAS FOR MORE BOATS

FURTHER READING AND INFORMATION

INDEX

PREFACE AND ACKNOWLEDGEMENTS

Looking back, it was clear at an early age that I was interested in mechanical things. Childhood memories include taking mechanical clocks apart to see how they worked, and fixing the family car with my father. Christmas lists included Meccano, LEGO Technic, model steam engines and, one year, a radio-controlled car.

Later I studied mechanical engineering at college, and when we bought our first house, not only was there a single garage, but also a small square of land behind it, which was turned into a workshop space. Precious funds were put towards a Myford lathe – and a long period of self-teaching and learning was under way.

Many years later the workshop still holds my interest. It is used to keep a classic car and motorbike on the road, as well as doing jobs for neighbours and friends, and of course for model engineering.

Model engineering provides an interesting mix of mechanical principles and practical skills that come together to form a creative and rewarding hobby. Steam engines have long been a favourite model engineering topic, and model boats are particularly suited to steam propulsion, as even the simplest of engines can be made to move a model boat, so modest are the power requirements.

When I was asked to write a book on model boats, the aim was twofold. First, to present some model boat designs fully explained and with clear drawings, to enable builders of all abilities to make them; and second, to include information on the design aspects of steam engines and boats such that readers can create their own designs and experiment.

For me, workshop time is about learning new skills, self-improvement and enjoyment. With this in mind, here are some live steam model boats of my own design, which I hope people will enjoy building and sailing.

ACKNOWLEDGEMENTS

Much gratitude goes to the following people who have contributed to the writing of this book:

Alan Fisher

Alison Brown

Arthur Ganson

Chris Lloyd – Nexus Special Interest Model Books

Eric Baird – Brighton Toy and Model Museum

John Bodiley

Marvin Klotz

Mohammud Hanif Dewan

Odilon Marcenaro

Stan Bray

Tubal Cain

INTRODUCTION AND UNITS

This book contains the plans and building instructions for three live steam model boats.

The first is the simplest, quickest build and is reminiscent of the toys from the 1920s and 1930s. Companies such as Bowman, Mamod and Bassett-Lowke all produced similar designs. This first model is a good introduction to making a live steamboat, and uses a number of purchased parts to assist further the build process. It is fired by solid fuel tablets, and the engine is a typical oscillating cylinder model powered by a boiler with the minimum of fittings. The hull is flat bottomed to simplify manufacture and to ensure that the final result is stable at sea. This model is the smallest of the three and is designed to be a free steamer, the rudder being set to sail a course, alone on the water.

The second model is the ‘Feature Build’ and provides a step up in skills and complexity. The model is designed to be interesting to build without being overly difficult. The engine is of a rotary valve design, the steam and exhaust controlled by ports on the main axle. The boiler has the essential fittings of a safety valve and steam take-off, but also an extra bush for filling either manually or from a pump, and a superheater and lubricator are included. The smokestack acts as an exhaust and also a condenser, to catch any oil in the steam and prevent it entering the lake. This time the hull has a more complex profile, and there is room to add radio control to the rudder for more adventurous navigation.

Included within the Feature Build chapters are some rudimentary calculations and considerations to allow builders to adapt the model to their own design. This includes some safety points regarding boilers, boat theory including water displacement, centre of gravity and hydrodynamics for the hull, and some information on measuring engine performance.

The third model is a paddle steamer. This uses the boiler from the Harbour Pilot, coupled to a twin-cylinder oscillating engine, and uses a reduction gear to power the paddles. The hull uses a different construction process, and once again there is the option to use radio control.

As model engineering projects there is an expectation that some workshop facilities are available. Both engines involve machining and require some accuracy in the moving parts to get a working result. Where possible, a number of different approaches to a particular build step are described, as options will depend on the equipment available; and an attempt has been made to keep tooling to a minimum.

The boilers, although not complex, require some copper smithing and sheet metal work. The pressure vessels are of silver-soldered construction as is proper these days to ensure a safe outcome, and they should be pressure tested before steaming.

Finally, the hulls are woodwork projects. A bandsaw and sanding disc help make this part of the project easier, but hand tools will also do the job.

Altogether, the variety of skills is what makes live steam boats interesting. For newcomers to the hobby the build instructions are described step by step, and for experienced modellers the chapters on designs and theory should expand on practical skills and enable creativity.

For inspiration, Fig. 1.1 is a picture of a finished model out on the water. It is to be hoped that building and sailing your home-built model will give you many hours of enjoyment.

Fig. 1.1  The Harbour Pilot maiden voyage at The Lake Grounds.

UNITS

Metric units are used wherever possible in this book. Being based on the number ten, they hold a certain logic and there is less of a requirement to know how many of one unit go into another; the answer is usually ten or one thousand.

Compare this to the imperial system, which has, over centuries, evolved to be quite complex. Length for example can be in ‘thou’, which is 1/1000th of an inch but is sometimes also (confusingly) called ‘a mil’. There are 333⅓ thou in a Barleycorn, and three Barleycorns to the inch; 12in to the foot, and 6.0761ft in a fathom. It is not a particularly straightforward system.

However, steam engines come from a time when imperial units were the norm. Many of the books, and much of the wisdom on steam engines and boilers, is therefore in old units. Sometimes a metric conversion still makes sense, but there are other units, pressure for example, where pounds per square inch (psi) is more common and easier to relate to, than the metric equivalent. So apologies in advance in the use of mixed units throughout the book, but an effort has been made always to pick the most logical and straightforward units for a particular task or calculation.

The drawings in this book are in metric for the most part, but again, some imperial sizes are used where necessary. Copper pipe, for example, is usually sold in imperial units, so steam unions must be drilled to suit. You will also see bar stock sizes specified in imperial sizes, as this is what most model engineering suppliers stock.

The same mixture occurs with threads. Threads are a combination of British Association (BA) and Mechanical Engineering (ME) pitch. BA is, in fact, a metric standard and has its origins in the world of horology and scientific instrumentation. As a result, many small fasteners particularly suited to model engineering are sold in BA sizes.

ME threads are imperial but are classified by their pitch. So all the diameters have the same pitch, which is very useful for steam fittings where a large diameter fine pitch thread is called for.

To try to keep tooling investment to a minimum, the variety of thread sizes has been kept to the smallest range possible.

Finally, on the subject of units: the book sometimes talks about microns, which have the Greek symbol ‘Mu’ written as µ. This is 1000th of a millimetre, or 0.001mm. So 0.06mm can be written as 60µm, which means 60 microns.

CHAPTER ONE

WORKSHOP EQUIPMENT AND SAFETY

Every machine tool involves sharp edges or heat (or both), and as such it is impossible to write down every hazard. Machinists should get to know their tools and how to operate them safely so as to avoid injury. However, there are a few safety essentials, described below, that go a long way to ensuring you enjoy your workshop time.

Safety glasses: If you only follow one safety rule it should be this one: find some safety glasses that are comfortable, and wear them. You can get prescription ones if you need them, and once you have the habit of wearing them, it will feel wrong without.

Shoes: There is no need for safety boots, but avoid slippers and flip-flops.

Chuck keys: Don’t leave the chuck key in any chuck. As you release the chuck and walk away, take the chuck key with you.

Tidiness: It is important to make sure the floor is free of trip hazards, particularly cables. Keeping the floor swept also makes it possible to find small parts when you drop them.

Brazing/silver-soldering: Brazing work is hot – very hot. Keep a bucket of water to hand, and always be aware how you pick up the hot work, such that if you drop it, it doesn’t land on your arm or foot.

Pickling: Avoid serious acids such as hydrochloric or sulphuric, and work with citric acid crystals from a brewery shop. These are safe to work with and easy to dispose of.

Gloves: Gloves can be useful when brazing or pickling, but they have no place near machine tools, where they can snag and do more harm than good. Thin Nitrile or latex gloves are fine as they will tear if they get caught.

Hair: Long hair should be tied up or kept under a hat.

Ties: If you wear a tie, make it a safety tie, which will unclip if it gets caught.

WORKSHOP REQUIREMENTS

An effort has been made to keep the builds as uncomplicated as possible to enable more people to build them. However, as model engineering projects there is an expectation that some machine tools are available. As a minimum, a small lathe with a vertical slide is needed to produce the engine, and a drill press greatly helps speed things up when it comes to drilling and countersinking. A milling machine is not essential but is used in the book as a first choice for some of the operations, as it was available. Where possible, alternative methods such as using a D-bit instead of a reamer are suggested, to give the builder more options.

Taps and dies are needed – specifically, 8BA, in × 32tpi, ¼in × 32tpi, and in × 32tpi ME threads. The metric M4 thread is also used.

Silver-soldering equipment is required to build the boiler. A gas torch, along with Silver-Flo rod and Tenacity 5 flux, is recommended.

The completed pressure vessel needs to be hydraulically tested to twice the working pressure, and a method is described to approximate this without building a pump, but a pressure gauge is necessary. Similarly, the safety valve tension must be set for it to be effective.

For the hull, only hand tools are needed, but shapes can be cut on a bandsaw if one is available. Sanding is quicker with a power sander, but hand sanding will suffice, especially if the timber is soft.

As a shopping list, some of the more specialized parts needed are these:

8BA hex-head steel bolts

8BA C/sunk steel screws

8BA grub screws

M4 grub screws

Copper pipe in diameter

Copper pipe ⅛in diameter

in nuts and cones to make steam unions in bronze balls

Stainless spring length in OD, with a wire gauge of in

1¾in copper tube 20 or 22 gauge

Propeller shaft and propeller

Loctite 638 or similar, such as Truloc 268

A good model engineering supplier should be able to provide most of these parts, aside from the propeller and shaft, which are best sourced from a model shop.

Hex-head BA screws are sometimes available with a smaller head. This is a good option to take as they will suit the small scale of the engines in this book.

Also consider getting a box wrench for the hex screws and a small 0.9mm hex key for the 8BA grub screws.

Fig. 1.2  The author’s workshop with a Myford lathe as the centrepiece. Out of shot are a Dore-Westbury milling machine and bench grinder.

LATHES

A lathe is one of the most useful engineering tools. It can make a multitude of cylindrical components such as axles, wheels, pulleys, bushes, washers and spacers. Less obvious is that you can also turn castings and odd-shaped components with the four-jaw chuck, and even create square components using facing operations on a workpiece. With additional attachments you can turn tapers, spheres, cut slots, hob gears and cut threads. For the models in this book only a simple lathe is needed, and in case the reader is in the process of sourcing a machine, the following considerations should be borne in mind:

Speeds: High speeds are the realm of woodworking; for metalworking, being able to turn at a slower speed is more useful. A lathe with a range of 25rpm to 750rpm will be more useful for metalwork than one that runs at 3,000rpm but only goes down to 500rpm. Some lathes advertise variable speed control, which sounds nice, but the lower speeds are also lower power. More useful is a lathe with a ‘back gear’, which is a reduction gear that not only reduces speeds, but increases torque, which is very useful for large parts.

Thread cutting: The back gear is also desirable for thread cutting, where the tool will move relatively quickly towards the chuck – if the chuck speed is really fast you won’t be able to stop it at the correct point every time.

Gearboxes: Gearboxes are useful if you do a lot of thread cutting, but other than that, they are quite a luxury. Thread cutting is still possible by using change wheels – not as convenient, but perfectly workable with a little patience.

Capacity: The size of the lathe is defined by the length and swing over the bed. The length is the distance between the chuck and tailstock (known as the ‘length between centres’), and the ‘swing’ is the maximum radius/diameter that will fit over the bed. However, be aware that diameter over the cross-slide will be much less and is a more useful number to reference.

Overall, the size you need will be dictated by the size of the projects you are interested in making. Some lathes have a gap in the bed, which is a taller area near the headstock that can be used to turn large diameter items that are not too long. This is a useful feature.

Accessories: The list of accessories available for a good lathe is almost limitless, but a lot can be done with a three-jaw chuck, a four-jaw chuck, a vertical slide, a few tools and a set of drills. Other fixtures and fittings can be sourced as needed, or often made to some of the many plans published by other engineers.

Having said all this, any lathe is better than no lathe, and the budget might mean that a small hobby machine is the order of the day, in which case, so be it. It will have limitations, but also massive possibilities and many uses.

Finally, it should be said that not everyone has a large, dedicated workspace. This is always nice, because you don’t have to tidy up at the end of the day, but plenty of people work in the garden shed or spare room – I have even seen plans for a small workshop situated in a reading desk. My first workshop was in the attic of our house, with dim light, low headroom and cobwebs in every corner, but it was a workshop nonetheless and was enough to produce some successful working models.

TOOLS, TIPS AND TECHNIQUES

Whole books can be written on the vast number of set-ups and machining operations that can be done in the home workshop, but a few tips and techniques that are relevant to steam launch builds are described here.

USING A LATHE

To get the best from any lathe there are some guiding principles that are worth thinking about. Rigidity is one of the main concerns. The more rigid the set-up, the better the results. You should get an improved surface finish, less noise, less tool chatter and better accuracy because there will be less springing of the tool and workpiece.

With this in mind, try to minimize tool overhang, minimize workpiece protrusion from the chuck, and ensure that the lathe slideways are smooth with no free play. If the tool is chattering, try slowing the spindle speed and/or adding some light oil or cutting fluid to the cut.

If the workpiece is long and thin, then use the tailstock to support the far end.

If the tool requires a high cutting force or is rubbing, check that the tool is sharp and on centre height; and check the tool geometry for the material you are cutting.

MARKING OUT

Marking out is an important step to ensure work is produced to the required accuracy. A selection of marking-out tools is shown in Fig. 1.3. It is not necessary to have all of these, but a steel rule, a small square, some dividers, a scriber and a centre punch are the minimum requirements. A protractor has its uses, and layout fluid greatly improves visibility of the markings.

Fig. 1.3  A selection of marking-out tools. Digital calipers are sometimes (unfairly) called ‘guessing sticks’, but they are quick to use and provide internal, external and depth measurements. They are accurate enough for most model engineering purposes.

Odd-leg or Jenny calipers are useful. Unlike normal calipers, these have one sharp point and one contact edge, which can be used to mark out lines parallel to the edge of the workpiece. Many drawings reference dimensions from the edge of the part, so these calipers can be used to transfer such dimensions directly. Jenny calipers can be accurately set against a steel rule by placing the blunt leg on the end of the rule and then ‘feeling’ the point drop into the correct graduation on the scale. Similarly, the points on standard dividers can locate on the rule markings, but it is easier not to start from the zero. For example, to set the dividers to 20mm, put one point on 10mm and adjust until the other point drops into 30mm. This is more accurate than having one point on the zero, which is off the end of the rule.

A centre punch is vital to locate holes with any accuracy. A good technique is to lightly punch the hole location and then inspect it: if it is slightly out of place then a second tap (a little harder this time) with the punch at an angle, can be used to move the first mark across.

MEASURING

A digital caliper is one of the most useful measuring tools in the workshop. It is easy and quick to use, and gives enough accuracy for most model engineering jobs. It also has the ability to measure inside, outside and depth dimensions, typically up to 150mm.

EDGE FINDING

An edge finder (sometimes called a wobbler) is useful when milling, to find the edge of the workpiece from which holes or other edges can then be placed. The edge finder is a mechanical tool placed in the chuck of the mill (or lathe with the vertical slide); it uses simple physics to highlight when the edge of the work has been found. Before using the tool it is first necessary to know the tip diameter.

To use the tool, it is placed in the chuck and turned at a moderately slow speed. The quill is adjusted and locked to set the ball of the tool level with the edge. The tool will waggle around wildly at the start but as the edge of the workpiece contacts it, the edge finder will start to run more and more true. Finally, as the edge is ‘found’, the edge finder will crawl up the side of the workpiece. In this precise position, the edge of the work is a distance from the quill axis, equal to the tip radius. For example, the wobbler shown has a ball diameter of 5mm. When the edge is detected, the centreline of the quill is therefore 2.5mm from the edge. The mill or lathe dials can be zeroed and then advanced 2.5mm where they are zeroed again, to pick up the edge of the workpiece as a datum.

Fig. 1.4  Edge-finder start position. A slow spindle speed (~250rpm) is best.

Fig. 1.5  As the edge finder approaches the target surface, the amount of runout will reduce until it is running exactly true.

Fig. 1.6  Finally, when the edge is located, the tool will be seen to step out as the ball end walks along the edge of the workpiece. At this point the edge of the work is off from the machine axis by the radius of the ball.

DRILLING AND REAMING

For simple plate parts, drilled holes should be centre-punched before they are drilled. If a hole is being drilled in the lathe, no punch mark is needed, but the hole should be centre-drilled first. A centre drill is just a short rigid drill that will pick up on the centre of the workpiece without wandering off course. Once the hole has been centre-drilled, progressively larger drills can be used in the normal way to get the hole to the final size.

For holes where an accurate diameter is needed, a reamer is the best tool for the job. This is effectively an accurately ground drill, but it should only be used to remove the minimum amount of material from the work. For a 6mm hole for example, you should drill to 5.8mm or 5.9mm before using the 6mm reamer to finish. Use a slow chuck speed for reaming, and feed it in with some sensitivity.

Reamers can be hand reamers or machine reamers. Hand reamers have a longer tapered section to help the operator align the reamer in the drilled hole, but for this reason they are only suitable for through holes. Machine reamers are parallel right up to the leading edge, aside from a small chamfer. These have more uses in model engineering, and can be used to make accurately sized holes using a range of machine tools.

D-BITS

D-bits are homemade reamers. They are simple to produce, work well, and allow the builder to ream holes in non-standard sizes. The material to use is a tool-steel or silver-steel rod. This can be turned to the final required diameter of the hole. Then it can be heated to red hot and quenched in water to give it a hardened working surface. Check with a small file, and if it skims over the surface without cutting, the steel is nice and hard.

Fig. 1.7  Suggested D-bit geometry. D-bits can be thought of as custom reamers, and as such should only be used to remove the smallest amount of material possible. When grinding the D-bit the remaining ‘D’ should be half the bar diameter or less to avoid rubbing.

Next, a bench grinder is used to create the shape shown in Fig. 1.7. The important things to note are that the remaining ‘D’-shaped part of the tool should be ground to just less than half the bar diameter – never more than half, or the tool will rub. The end can be left square to create a flat-bottomed hole, but relief is required on the non-cutting corner and under the cutting edge. A small flat is ground on to the leading edge of the tool to help it bite.

D-bits work well, but the less material they have to remove, the better. Aim to drill the hole to about 0.1mm undersize, and then finish with the D-bit.

LATHE TOOL CENTRE HEIGHT

To cut at its best, a lathe tool should be set at the centreline of the workpiece. A simple way to check and adjust this is to trap a short steel rule between the tool tip and a piece of rod held in the chuck. When the tool centre height is correct, the rule will sit vertical.

THREE-JAW CHUCKS

Three-jaw chucks are a great time saver, clamping round or hex bar along the lathe centreline without the need to check alignment. However, they should be thought of as approximate devices. Modern chucks are actually very precise, but they are not exact. The only way to ensure two features machined on a bar are concentric is to do them in one setting. Once the part is repositioned or reversed in the chuck then the datum is lost and so features will not be exactly in line. For ornamental parts this doesn’t matter, nor for turning a bolt from hex bar, but it should be kept in mind when thinking through machining steps to ensure that accuracy is not lost where it is needed.

CROSS-DRILLING A WORKPIECE

Being able to accurately cross-drill a shaft or circular workpiece on the centreline can be useful. One of the simplest ways to do this is to create a drilling guide. This is turned to the same diameter as the workpiece and centre-drilled, then drilled to the final drill size on the lathe. On the drill press the workpiece and the drill guide can then be clamped in the vice, and by feeding the drill gently through the guide you should get an accurately placed hole in the part.

Fig. 1.8  Cross-drilling a round component using a guide. The guide piece is the same diameter as the workpiece so that both components can be clamped in the vice jaws simultaneously.

HONING SMALL CYLINDER BORES

For a small engine to run at its best the cylinder walls should be polished to remove any machining marks. In full sized engines there are hones to do this, but in a small engine we need a different solution.

A small bore can be polished using metal polish or toothpaste in combination with a wooden dowel turned to be a tight ‘wringing fit’ in the bore. The wooden dowel can then be turned at a slow speed in the lathe whilst the cylinder is moved up and down the dowel by hand. After a short while the bore should appear polished, without any tool marks. Protect the lathe bed from any polish or toothpaste during this process.

MACHINING A BAR TO LENGTH

Machining a piece of bar to length is a threestep process. First, both ends of the bar need to be faced off square in the lathe for a datum measurement. For this reason, the starting blank needs to be slightly over-long. With the bar ends square, the part can be removed from the chuck and the length measured with a vernier, caliper or micrometer. From this length an amount to be removed can be calculated from the specification.

To remove exactly this amount, set the top slide on the lathe to zero, taking out any backlash in the screw. Advance the lathe saddle until the tool just touches the workpiece, and lock the saddle firm. Then take progressive facing cuts, advancing the top slide the right number of graduations. If more than 3mm or 4mm needs to be removed, then it is worth getting close and then recalculating the final cuts to take out any error.

TURNING A SHOULDERED PART

The technique for this process is similar to turning a part to length, but the length and diameter have to be watched at the same time to get the part correct. The easiest method is to concentrate on the diameter and make the reduced shoulder over-long, and then turn the shoulder to final length using the process described to machine a part to length.

Measure the diameter regularly and calculate the number of graduations needed on the crossslide to get to final size. On the last cut up to the shoulder, stop the tool at the end and then extract it using the cross-slide to create a 90-degree shoulder. Now measure the length of the smaller diameter part and reduce as needed using the top slide.

CREATING A RADIUS ON A PART

When adding a radius around a hole, it is important to get it concentric; it is very easy to see when the two features are not co-axial. Filing the radius is fine, but a guide is needed to get it right, and filing buttons are commonly used to make this operation easier. A filing button is just a piece of tool steel turned with a shoulder, the small diameter being a close fit in the hole of the part and the larger diameter being the curve needed on the outside.

Fig. 1.9  Filing buttons are a useful tool to radius a component. Made from tool steel, they can be heated to red hot and quenched so that they become hard. Then the file will be found to skip over the work when the button is reached.

Ideally, the button is heated to red hot in the hearth and then quenched in water to make it hard. The button can now be held in the hole with a clamp or bolt, and a file used to file the corner flush to the button. If the button has been hardened the file should skim across the part once it is down to size. For wider parts a filing button each side is a good idea so that the radius doesn’t taper across the width.

MACHINING A FLYWHEEL

To look correct, the flywheel needs to run true when it is spinning on the crankshaft. Lathe chucks are very good at holding the workpiece central, but they are not perfect. If a flywheel is machined whilst held in the three-jaw chuck it is likely that it will wobble when fitted to the axle. The only way to get the flywheel to run perfectly true is to use an arbor to mount the flywheel for a finishing cut to remove any machining error.

The first step is to roughly machine the flywheel to size. In the case of a simple marine-style flywheel, the three-jaw chuck can be used to hold the flywheel blank and the axle hole can be centre-drilled, drilled and reamed to final size. Also in this setting, any mounting bosses or shoulders can be added. At this stage all the machined features will be perfectly concentric, but the outside edge may not be, depending on the condition of the chuck. To true up the outside face we need an arbor.

To create an arbor, a piece of scrap bar can be placed in the three-jaw chuck and turned to have an axle to fit the flywheel. Machining the arbor on the lathe in this way will give a reference that is exactly on the lathe centreline, removing any errors in the chuck. A thread may be added to the end of the axle to clamp the flywheel, or a flat can be filed on the axle if the flywheel has a grub screw.

With the flywheel on the arbor the runout from the chuck will be obvious and you can take light cuts until they go from intermittent to continuous. Now you have a perfectly true flywheel.

TRANSFER DRILLING

A useful process where aligned holes are needed is transfer drilling. This removes the risks of measurement error creating misalignment. The process is to mark out and drill one part and then use this as a template to make the holes in the second part. An engineer’s clamp is all that is needed to hold the parts together for drilling. Where one hole is tapped for a thread and the other is a clearance for the screw, concentration is needed to ensure that you drill the correct diameters in both parts.

ECCENTRIC OR OFF-SET MACHINING

The crank discs on these engines are examples of off-set or eccentric machining. Not only must the crank pin be the correct distance from the axle, it must also be parallel to it. Therefore, the hole for the crank pin is best drilled in the lathe. One process to achieve the required off-set is to pack out one jaw of the three-jaw chuck to put the workpiece off centre. So to start, the outside of the crank disc can be machined along with the centre hole and any shoulders. Now a packing piece must be calculated to give the correct throw on the crank pin. An approximate formula for this is:

This is all right for non-critical (cosmetic) eccentrics but unfortunately the formula gets less exact as the main bar diameter decreases and the throw increases.

If more accuracy is needed then it may be necessary to use this next formula, kindly shared by Marvin Klotz, which compensates for the width of the contact area of the jaws of the chuck. Marvin is a retired aerospace physicist and now spends his hobby time making engineering models and deriving formulae for model engineering activities. His more detailed formula is as follows:

where:

It’s quite a task keying this into your calculator.

Work out the brackets first, then do the multiplication steps, then the additions, and finally the subtractions. As an example, if the specification of an engine is for a crank throw of 6mm and the overall disc diameter is 19mm. Using the formula, that gives us:

So a packing piece of 8mm against one jaw of the chuck would give the required off-set with enough accuracy to create the eccentric.

The lathe can be used to create a small nugget of bar the correct length, which can then be used to space the workpiece from one of the jaws. In this setting the crank-pin hole can be drilled and tapped for the crank pin.

A word of caution should be given regarding safety with this set-up. It is clearly not how the three-jaw chuck was designed to be used. If in doubt, you can just use this method to mark a part by turning the lathe by hand and then drill it on the drill press; but with a slow spindle speed and light drill pressure, there should be little problem. The four-jaw chuck is another option, but the three-jaw method is a useful technique to be aware of even if it is seldom used.

CENTRING IN THE FOUR-JAW CHUCK

If the centre of the workpiece has been marked and centre-punched, then the easiest set-up on the four-jaw chuck can be done with a floating centre. Fig. 1.10 shows a piece of square bar being centred for drilling. The floating centre will waggle around as the chuck is turned by hand, and a DTI (dial test indicator) will show the runout. If the DTI is placed over the work, then when the dial runs fully clockwise the work is highest and the top chuck jaw should be tightened to push it down. When the DTI runs fully anti-clockwise the workpiece is low and the top jaw should be loosened. Working progressively round the chuck should soon centre the workpiece.

Fig. 1.10  Centring a workpiece in the four-jaw chuck. In this set-up the required centreline of the workpiece has been centre-punched. The floating centre can then pick up this centre mark, and the runout of the component is shown by the DTI (dial test indicator).

If you are trying to tighten a jaw to move the workpiece down but it is too tight, then try backing off the opposite jaw first. Also be aware that the jaws on either side may be gripping the work and may be working against you, so some sympathy for how the jaws work together is needed. Packing pieces of scrap metal can be used to allow the workpiece to be moved about between the jaws without scratching it.