# Fixture plates

Fixture-plate grids, locating bores and threads, mounted geometry, material choices and the limits of pitch, tolerance and torque specifications.

Reviewed: 2026-11-02

Tags: tooling, workholding, fixture-plates, locating

Over a free **300 mm** locating span heated uniformly by **10 K**, an assumed expansion coefficient of **23×10⁻⁶/K** gives **69 µm** growth. A second component at **12×10⁻⁶/K** grows **36 µm**, a **33 µm** difference. This modeled mismatch compares free expansion. Restraint can change geometry or add stress, and the coefficients are illustrative.

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

Fixture plates provide mounting and locating features for clamps, vises and fixtures. The machine table supports the plate.

## Locating and fastening have different jobs

Carr Lane's modular system combines a precision alignment bushing above a threaded insert in each multipurpose hole. Ordinary screws, studs and special locating screws can use the same hole; the locating screws intentionally combine position and fastening functions. Its grid addresses identify component positions. This is one system's construction, not proof that every threaded grid has precision locating bores. [Carr Lane, modular fixturing and multipurpose holes](https://www.carrlane.com/engineering-resources/technical-information/manual-workholding/modular-fixturing)

| Feature | Primary function | Separate uncertainty |
| --- | --- | --- |
| Threaded hole | Receives a screw or stud for restraint | Thread clearance and screw-head seating do not establish precision location |
| Precision bore or bushing | Receives a locating pin or fitted feature | Fit, positional tolerance and wear affect location |
| Grid pitch | Repeated nominal spacing | Does not itself specify positional accuracy |
| Mounting hole or slot | Connects plate to its support | May have a different locating role from the workholding grid |

Dowels locate while tightened screws provide preload. Check where cutting reactions pass through the assembly.

## One manufacturer's grid examples

Saunders Machine Works gives the following current general specifications:

| SMW family | Thread | Locating bore | Grid |
| --- | --- | --- | --- |
| 1/2 inch | 1/2-13 UNC | Ø0.5005 ±0.0002 inch | 1.25 inch centers; second pattern offset 0.625 inch in X and Y |
| M12 | M12×1.75 | Ø12.013 ±0.005 mm | 30 mm centers; second pattern offset 15 mm in X and Y |

It distinguishes M6 and 1/4-inch families, which lack those precision counterbores. Thread designation therefore cannot stand in for locating-bore geometry. [SMW, hole-spacing and bore specifications](https://saundersmachineworks.com/blogs/knowledge-base/smw-fixture-plate-tolerances)

Two offset patterns do not establish compatibility with every finer-pitch component; the actual hole coordinates and locating features remain relevant.

## Mounting and installed geometry

Carr Lane describes locating standard/heavy plates on T-slotted tables with keys and fastening them with screws and T-nuts. Its quick-change subplate arrangement uses a round and relieved diamond key together to avoid binding. Its pallet example instead locates from two ground reference edges. These are different locating schemes, even when the top surface serves the same fixturing purpose. [Carr Lane, table, subplate and pallet mounting](https://www.carrlane.com/engineering-resources/technical-information/manual-workholding/modular-fixturing)

SMW's published table gives **0.0005 inch parallelism for 4140 plates with longest edges up to 20 inches**, versus **0.001 inch for its 6061/7075 examples**. It does not qualify unconstrained flatness; mounted results also depend on the table. These are product-specific conditions. [SMW, parallelism and flatness](https://saundersmachineworks.com/blogs/knowledge-base/smw-fixture-plate-tolerances)

**Engineering synthesis:** parallelism relates surfaces, while flatness concerns a surface's form. Neither is identical to alignment with the machine axes. Dirt, burrs, local support and mounting distortion can change the installed surface. A nominal thickness or bore tolerance does not establish the fixture's complete machining error budget.

## Material and restraint limits

SMW positions its 4140 steel plates for demanding high-cycle work and its aluminum plates for lower weight and easier handling. It identifies anodized 7075 and bare 6061 as separate product choices. These descriptions do not supply a universal load capacity for a material name. [SMW, steel versus aluminum](https://saundersmachineworks.com/blogs/knowledge-base/steel-vs-aluminum-which-fixture-plate-is-right)

Its fixturing-hole torque table varies with material and thread engagement. For **1/2-inch/M12** threads at **1.0×diameter engagement**, it lists **68 N·m for steel** and **61 N·m for aluminum**. The page separately notes that the counterbore reduces threaded engagement. These are specified screw-torque conditions, not cutting-force or payload ratings; its plate-installation hardware has separate specifications. [SMW, fixture-plate torque specifications](https://saundersmachineworks.com/blogs/knowledge-base/fixture-plate-torque-specs)

As engineering synthesis, plate thickness, unsupported span, thread engagement, clamp geometry and mounting affect strength and stiffness. A lighter plate can reduce handling mass without establishing an equivalent deflection under load. A precise hole pattern does not eliminate those structural constraints.

## Worked example: thermal change across a locating span

**Original hypothetical model:** assume a free, uniformly heated 300 mm span with constant linear expansion coefficient α = 23×10⁻⁶/K and temperature increase ΔT = 10 K:

```text
Length change = α × L × ΔT
              = 23×10⁻⁶/K × 300 mm × 10 K
              = 0.069 mm = 69 µm
```

The coefficient is an illustrative input, not a certified value for a cited plate. A hypothetical second component at α = 12×10⁻⁶/K changes by **36 µm**, giving **33 µm differential free expansion** over that span.

This illustrates why temperature and material pairing affect locating geometry. It does not predict constrained plate motion, bow or stress. Actual gradients, mounting constraints and component temperatures matter; a reference-condition bore tolerance is not a guarantee of unchanged spacing during machining.

## Coverage and source dates

SMW's tolerance and material articles display **December 17, 2024**; its torque article displays **December 18, 2024**. No issue date is established for Carr Lane's technical page. Sources were inspected on November 2, 2026; copyright and search dates are not publication dates.

This reference does not establish universal grid compatibility, flatness, repeatability, tightening torque, payload or cutting-force capacity. Plate-to-machine and fixture-to-plate connections both matter. Manufacturer examples do not establish a shop's installed tooling, measured condition or approved workholding loads.

## Related reference

[Workholding and fixture references](index.md)