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Magnetic Workholding by the Numbers

Holding, grinding and lifting steel with magnets: pull, air gaps, thin plates, cutting forces, chucks, lifting magnets, heat and residual magnetism

How magnets hold steel and how to prove they will: pull from flux density, air gaps, thin plates, milling and grinding forces, chucks, lifting magnets and demagnetising.

10 chapters · About 100 minutes · Free to read · Intermediate machining, tooling and fabrication engineering

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KEVOS® LibraryMANUFACTURINGMagneticWorkholding bythe NumbersKEVOS® ORIGINAL BOOK10 CHAPTERS

About this book

A magnetic chuck holds a steel plate with no clamps in the way of the cutter, and a lifting magnet picks up a plate with no slings or lugs. Both are quick, and both fail in ways that surprise people: a plate slides on a chuck that was holding it a minute ago, a thin part will not stay put however strong the chuck, a lifting magnet that easily held a test plate drops a rusty one. Each failure has a cause that can be calculated.

This book shows how. It starts from the flux density at the contact face, which sets the pull per unit area, and follows that pull through everything that reduces it: air gaps from scale, paint and chips, material that saturates or will not magnetise, plates too thin to carry the flux, parts too small to bridge the poles, and heat. It then compares the holding force with the forces that try to move the part, from milling and grinding, and with the weight and stiffness of plates on lifting magnets. The last chapters deal with distortion, residual magnetism and how to prove a set-up works.

It is for machinists, toolmakers, tooling and manufacturing engineers, fabrication supervisors and the people who plan lifts in a plate shop. It assumes school physics and algebra. Each chapter works the running case with real numbers, includes one plausible mistake and how it was found, links the KEVOS® tool that does the chapter’s calculation where one exists, and ends with an exercise and its worked answer.

The running case

Every chapter follows one fictional part and the shop that makes it. This is an illustration. A machining and fabrication shop in Newcastle, New South Wales, makes the BP-600 base plate, a 600 × 400 × 25 mm mild steel plate that carries two linear guide rails on a packaging machine.

  • Material: hot-rolled grade 250 steel plate, bought as 2,400 × 1,200 × 25 mm sheets with mill scale, flame cut to size.
  • Milling: both faces milled on a vertical machining centre fitted with an electro-permanent magnetic chuck, 800 × 500 mm, with square 50 mm poles on a 60 mm pitch.
  • Grinding: two 80 mm wide rail seats ground to a flatness of 0.01 mm on a surface grinder with an electromagnetic chuck.
  • Handling: sheets moved from the rack to the cutting table with two permanent lifting magnets, each with a working load limit of 500 kg, on a lifting beam under a 1 t crane.

Flux densities, forces, derating factors, prices and times in the book are invented for the case or calculated from stated assumptions. They do not describe KEVOS® work or any real business, product or manufacturer’s rating.

How to use the book

Read Chapter 1 first: it sets out the pull formula and the magnetic circuit that every later chapter uses. Chapters 2 to 4 cover what reduces the pull: the material, the gap and the size of the part. Chapters 5 and 6 compare the pull with milling and grinding forces, Chapter 7 compares types of chuck, and Chapter 8 deals with lifting. Chapter 9 covers heat and Chapter 10 residual magnetism and proving a set-up.

Every calculation in the book was checked by running it. The models are deliberately simple, and each chapter says where they stop being reliable. A maker’s tested rating for a particular chuck or lifting magnet always overrides an estimate from this book.

Source limits and professional responsibility

This book is educational. The pull formula and magnetic circuit models give estimates that explain behaviour and show which changes matter; they do not replace the maker’s ratings, instructions and derating tables, or a pull test on the real part. Lifting magnets are lifting equipment and must be selected, inspected, used and maintained as the model WHS laws, the relevant Australian Standards and the maker’s instructions require, by competent people. Strong magnetic fields can affect pacemakers and other active implants; people with implants should get medical advice before working near chucks and lifting magnets. Decisions about real lifts, machine set-ups and safety remain with the competent people responsible for them.

Who this book is for

Machinists, toolmakers, manufacturing and tooling engineers, fabrication supervisors and lifting coordinators who hold steel parts on magnetic chucks or move steel plate with lifting magnets.

What you will learn

  • Estimate the pull of a magnet on steel from the flux density at the contact, and explain why catalogue figures are ceilings rather than promises.
  • Predict how air gaps, surface condition, material, thickness and part size reduce the holding force, and choose pole pitch to suit the work.
  • Check that a part on a magnetic chuck will not slide under milling or grinding forces, and decide when stops, clamps or a lighter cut are needed.
  • Choose between permanent, electromagnetic and electro-permanent chucks, and plan a safe lift with lifting magnets derated for the real plate.
  • Control heat, distortion and residual magnetism, and prove a magnetic set-up with pull tests, gauss readings and written checks.

Inside this book

Chapters

  1. 01Why a magnet holds steelFlux, flux density and saturation in steel, the pull per unit area of a pole face, the magnetic circuit, and how to turn a chuck's pole layout into an honest holding estimate.
  2. 02Steels that hold and materials that do notWhich materials a magnet can hold and how well: saturation and permeability, carbon and alloy steels, hardened steels, cast iron, stainless grades, and how to derate for each.
  3. 03Air gaps and contactWhy a few hundredths of a millimetre of gap cost a large share of the holding force: a gap model fitted from two pull tests, surface finish, scale, paint, bowed plates and swarf.
  4. 04Thin plates and small partsWhy thin parts saturate and small parts fail to bridge the poles: a minimum thickness from pole width and saturation, fine-pole chucks, pole extensions and how to choose pole pitch.
  5. 05Holding against cutting forcesCompare a magnet's sliding resistance with milling forces: cutting force from power and speed, peak factors, friction with and without coolant, stops, and vacuum and clamp alternatives.
  6. 06Grinding on a magnetic chuckSurface grinding forces against magnetic holding, why a bowed part ground flat on the chuck springs back, thermal bow from grinding heat, chuck heating, and measuring at the right temperature.
  7. 07Choosing a magnetic chuckPermanent, electromagnetic and electro-permanent chucks compared: how each makes and switches its flux, heat, energy, power failure, interlocks, pole patterns and the cost of ownership.
  8. 08Lifting magnetsPlan a lift with lifting magnets: rated loads and their conditions, derating for thickness, surface and material, plate mass and centre of gravity, lifting beams, plate flex and safe use.
  9. 09Heat, hot parts and magnet temperatureHow temperature changes magnetic holding: steel's magnetisation and Curie point, reversible and irreversible losses in permanent magnets, hot plates on lifting magnets and chucks, and limits to set.
  10. 10Residual magnetism and proving the holdWhy parts leave a chuck magnetised and what that costs, measuring and specifying residual fields, demagnetising thick parts, and a plan of tests and records that proves a magnetic set-up.

Apply This in Practice

Use these KEVOS® tools to try the method from this reading with your own numbers.

References and source notes

The manuscript and worked examples are original KEVOS® writing. References inform particular concepts; their authors do not endorse this book. Web references were consulted on 31 October 2026.

  • E. M. Lifshitz and L. P. Pitaevskii, translated by J. B. Sykes and M. J. Kearsley, Pergamon Press, Oxford. Statistical Physics, Part 2: Theory of the Condensed State (Course of Theoretical Physics, Volume 9) (1980). In copyright. Used only as a map for the physics in Chapters 1, 2 and 9: the chapter on magnetism, which treats the ferromagnet's spontaneous magnetisation, its spin waves and why magnetisation falls as temperature rises. No text, equations, figures or problems are reproduced, and all cases and calculations are KEVOS® originals.
  • F. Bloch, Zeitschrift für Physik 61, 206–219. Zur Theorie des Ferromagnetismus (1930). Named in Chapters 1 and 9 as the origin of the result that a ferromagnet's magnetisation falls as the three-halves power of temperature well below its Curie point.
  • B. D. Cullity and C. D. Graham, Wiley-IEEE Press. Introduction to Magnetic Materials, second edition (2009). Background for magnetic circuits, saturation, the Curie temperature of iron and demagnetisation in Chapters 1, 2, 9 and 10.
  • J. M. D. Coey, Cambridge University Press. Magnetism and Magnetic Materials (2010). Background for ferromagnetism, domains and the force between magnetised bodies in Chapters 1 and 2.
  • J. C. Maxwell, Clarendon Press, Oxford. A Treatise on Electricity and Magnetism (1873). Named in Chapter 1 as the origin of the magnetic stress that gives the pull per unit area of a pole face, B² ÷ 2μ₀.
  • Arnold Magnetic Technologies. Temperature Effects on Magnetic Output (TECHNotes TN 0303) (2015). Source of the typical reversible temperature coefficients and Curie temperatures of permanent magnet materials quoted in Chapter 9.
  • Health and Safety Executive (United Kingdom), web page accessed 31 October 2026. Magnetic lifting devices (2026). Public guidance used in Chapter 8 for the hazards, derating for thickness and air gap, power-failure warnings and checks of lifting magnets. It describes United Kingdom law, which does not apply in Australia; it is cited for its engineering content.
  • Standards Australia. AS 4991-2004 Lifting devices (2004). Named by number and scope only in Chapter 8: requirements for lifting devices connected to a crane hook, including magnet lifters. No clause or value from it is quoted.
  • Safe Work Australia. Model Code of Practice: Managing the risks of plant in the workplace (2022). Named in Chapters 7 and 8 for the duty to manage the risks of plant, including lifting equipment and machine workholding, under the model WHS laws.
  • International Organization for Standardization. ISO 9934-1 Non-destructive testing — Magnetic particle testing — Part 1: General principles (2016). Named by number and scope only in Chapter 10 as the general standard for magnetic particle testing, which also deals with demagnetisation after testing. No clause or value from it is quoted.

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