# Magnetic chucks

Magnetic workholding circuits, switching systems, contact limitations and the distinction between pull-off force and cutting-load resistance.

Reviewed: 2026-11-02

Tags: tooling, workholding, machining

In a friction-only example, **4,000 N** of normal magnetic attraction gives **800 N** lateral resistance at an assumed **μ = 0.20**, or **400 N** at **μ = 0.10**. The calculation excludes uplift, overturning, vibration, stops and loss of contact. For its AX milling chucks, Eclipse gives approximate lateral resistance of one fifth of downward pull and calls for side and end stops. [Eclipse, AX milling guidance, undated](https://www.eclipsemagnetics.fr/products/workholding-systems/useful-workholding-tips/).

Use [Choosing workholding](../workholding-and-fixtures.md) to compare location, support and restraint across fixture types.

## What the chuck holds

Eclipse locates work over opposite poles so the workpiece closes the magnetic circuit, including when pole blocks reach irregular surfaces. Stops establish position and react machining loads. [Eclipse, useful workholding tips, undated](https://www.eclipsemagnetics.fr/products/workholding-systems/useful-workholding-tips/)

## Three switching systems

| System | Field and switching | Practical distinction |
| --- | --- | --- |
| Permanent | A lever mechanically changes the arrangement of the internal permanent-magnet circuit. | Holding needs no electrical supply. Mechanical switching affects automation and access. |
| Electromagnetic | Current through coils magnetizes their ferromagnetic cores. | Continuous power maintains attraction; interruption loses holding force. Coil heating can affect accuracy. |
| Electropermanent | A short electrical pulse changes the state or polarity of permanent magnets. | Holding persists without continuous current; electrical power is still required for switching. |

WALMAG distinguishes these switching mechanisms. Verify variable force, demagnetization and integration against the chuck and controller. Retained attraction during a power interruption leaves machine operation to assess separately. [WALMAG, technology comparison, undated](https://eshop.walmagmagnetics.com/Page/en-US/13)

## Material, thickness and contact

The workpiece is part of the magnetic circuit. Eclipse identifies material permeability, heat treatment, thickness, flatness and finish as contributors to holding performance. A thin section cannot carry the same flux as a sufficiently thick section on the same chuck. Fine pole pitch shortens the flux path and is useful for small or thin work, rather than providing a universal minimum thickness for all chucks. Roughness and unevenness create air gaps that reduce attraction. [Eclipse, selection guide, 1 February 2021](https://www.eclipsemagnetics.com/en-us/resources/guides/factors-to-consider-when-choosing-a-magnetic-chuck/)

Contact area means effective magnetic contact, not simply length multiplied by width. Suburban Tool notes reduced holding at frame regions without magnets and identifies coatings, oil, rust and dirt as gap sources. Its published comparison measurements use a ground 1 × 1 × 1 inch cold-rolled steel cube near the chuck center under near-ideal conditions. Such a test does not describe a rough casting, pocketed plate or partly supported ring. [Suburban Tool, magnetic holding power, undated](https://www.subtool.com/pages/magnetic-holding-power)

## Pull-off force and lateral resistance

Normal pull-off force acts away from the face. Cutting can instead produce sliding, overturning and intermittent loads. For milling on its mechanically switched AX chucks, Eclipse describes lateral resistance as approximately one fifth of downward pull and calls for side and end stops. That approximate relationship is specific guidance for that application, not a friction coefficient for every chuck, coolant and surface combination. The same guide emphasizes varying cutting-force directions and positioning the cut so forces act toward the stops. [Eclipse, AX milling guidance, undated](https://www.eclipsemagnetics.fr/products/workholding-systems/useful-workholding-tips/)

**Illustrative arithmetic:** suppose an application-specific assessment establishes 4,000 N of normal attraction. A simple friction-only model with an assumed coefficient of 0.20 gives lateral resistance `0.20 × 4,000 = 800 N`. At an assumed coefficient of 0.10 it gives only 400 N. These are model results, without a safety allowance, not permissible cutting forces. The calculation excludes stops, uplift, moments, vibration and loss of contact; it explains why a normal-force rating alone cannot specify a milling load.

## Named product boundaries

FLAIG's RS 70 permanent circular chuck, product 2103 70, has radial poles and a 700 mm outside diameter. Its individual technical-data table lists 30 poles, 140 N/cm² nominal holding force and 400 rpm maximum speed. The same page's general design description says approximately 160 N/cm², so the page contains an unresolved force-rating discrepancy. Neither figure establishes force on a particular part. The 400 rpm entry is for RS 70, not a general magnetic-chuck speed rating or an approval for an arbitrary rotating assembly. [FLAIG, RS 70 product data, undated](https://www.flaig-te.de/en/RS-70/2103-70)

## Datum, distortion and release

A magnetic face is also a support interface. Industrial Magnetics describes FLAIG fixed pole extensions establishing three-point support and mobile extensions rising to the workpiece level without pulling it down. This makes a useful distinction between establishing a datum and forcing a nonflat part onto a plane. As an engineering consequence, an elastically flattened part can change shape after release; flatness measured while clamped need not equal free-state flatness. Extensions also change the contact arrangement and cannot be treated as an unchanged bare-face setup. [Industrial Magnetics, FLAIG pole extensions, undated](https://www.magnetics.com/products/flaig-chuck-pole-extensions)

Switching off does not necessarily remove magnetism from the workpiece. Eclipse notes that hardened material can retain magnetism and be difficult to release, and describes separate demagnetization after removal. Residual magnetism is therefore a release and downstream cleanliness consideration distinct from the chuck's ON/OFF state. [Eclipse, selection guide, 1 February 2021](https://www.eclipsemagnetics.com/en-us/resources/guides/factors-to-consider-when-choosing-a-magnetic-chuck/)

## Scope and related reference

This reference covers machining workholding, not magnetic lifting capacity. The sources do not establish a universal alloy suitability, minimum thickness, derating curve, friction coefficient or rotating speed. Named product data and explanatory arithmetic remain separate from a validated machining setup.

See [Workholding and fixture references](index.md) for the broader relationship between location, support and restraint.