How to Order Magnesium Alloy Plates: Cutting Tolerances, Dimensions, and Machining Allowance

When ordering magnesium alloy plates for machining or fabrication, one common mistake is to order the material at exactly the same dimensions as the finished part.

For thin magnesium sheets, this approach can sometimes work. However, for medium and thick magnesium alloy plates, the material should generally be ordered with additional allowance for cutting, dimensional variation, surface condition, and subsequent machining.

Understanding these factors before placing an order can help avoid insufficient material and unnecessary delays during fabrication.

Thin Sheet and Medium-to-Thick Plate Are Different

Not all magnesium alloy sheet and plate should be treated in the same way.

Thin Magnesium Sheet

Thin magnesium sheets are generally produced with relatively flat surfaces and are often used close to their supplied thickness.

If the required thickness is known, customers will normally purchase the corresponding thickness rather than buying a substantially thicker material and machining it down.

For many applications, the surface does not need to be extensively machined. After cleaning and removing surface contamination, the material can proceed to subsequent surface treatment or fabrication.

For thin sheet, therefore, the focus is usually on selecting the correct alloy, thickness, length, and width rather than leaving a large machining allowance.

Medium and Thick Magnesium Alloy Plates

Medium and thick plates are different.

When a magnesium alloy plate is cut from larger material, its final dimensions are affected by the cutting method, material condition, plate size, and equipment.

The supplied plate should therefore not be considered a precision-finished component.

For parts that will eventually be machined to a drawing, the purchased plate should normally be larger than the finished component.

Why Cutting Dimensions Have Tolerance

Magnesium alloy plates are commonly cut using equipment such as horizontal or vertical saws.

The saw blade itself has a certain thickness, and cutting produces a visible saw-cut surface. The cutting process is therefore different from precision CNC machining.

A typical saw-cutting tolerance can be approximately 1–3 mm, depending on the material, equipment, cutting conditions, and dimensions involved.

For this reason, ordering a plate at exactly the finished drawing dimension can leave too little material for subsequent processing.

Why Larger Plates Can Be More Difficult to Cut

The larger the plate, the more factors can affect the cutting process.

Large-area or long magnesium alloy plates can experience deformation during cutting. A long cutting path can also make the cutting process more sensitive to the condition of the saw blade and the material itself.

One issue that can occur is commonly described as “blade wandering” or “running off the cutting line.”

A saw blade can also undergo permanent deformation during use. Once the blade is no longer perfectly straight, maintaining a precise cutting path becomes more difficult.

The result can be variation in the final cut dimensions, particularly when cutting large, long, or wide pieces.

This is one reason why saw cutting should generally be considered a material preparation process, rather than a final precision machining operation.

How Much Allowance Should Be Left?

For parts that will undergo further machining, the required allowance should be based on the final drawing and the intended machining process.

As a practical starting point, customers should not order the raw plate at exactly the final part dimensions.

For length and width, allowing approximately 5 mm on each side is often a reasonable starting point for saw-cut material.

For example, if the finished component requires a length of 200 mm and a width of 100 mm, a larger starting blank may be appropriate rather than ordering exactly 200 × 100 mm.

The exact allowance should still be determined according to the part design, machining method, required tolerance, and material condition.

Cutting Allowance and Machining Allowance Are Not the Same

It is useful to distinguish between two different types of allowance.

Cutting allowance is the additional material needed because the plate must first be cut from larger stock.

Machining allowance is the additional material required for subsequent operations such as milling, turning, grinding, or surface finishing.

If a component requires both saw cutting and CNC machining, both factors should be considered when determining the starting material dimensions.

The final part may have very tight dimensional requirements, while the raw magnesium alloy plate does not need to meet those same finished-part tolerances.

Why You Should Not Order Material “Exactly to Size”

A common assumption is:

Finished part: 200 × 100 mm
Therefore, raw material: 200 × 100 mm

For medium and thick magnesium alloy plates, this can be risky.

The raw material may have dimensional variation, surface irregularities, cutting marks, or slight deformation. Saw cutting also has its own tolerance.

If the raw material is ordered too close to the finished dimensions, there may not be enough material left to correct the edges or machine the part accurately.

Ordering a slightly larger blank provides room for:

• Saw-cut variation

• Edge cleanup

• Surface correction

• CNC machining

• Dimensional adjustment

• Final finishing

Thickness Should Also Be Considered

Thickness is another important consideration.

If the final component requires machining on one or both faces, the starting plate may need additional thickness to provide enough material for the machining process.

For applications where the supplied surface is already suitable and no significant thickness reduction is required, purchasing the required thickness directly may be more appropriate.

For thicker plates that require substantial machining, the required machining allowance should be determined from the drawing and manufacturing process rather than simply selecting the same thickness as the finished component.

Start With the Drawing, Then Work Back to the Raw Material

The best way to determine the required magnesium alloy plate size is to start with the finished component drawing.

Consider:

1. Final length and width

2. Final thickness

3. Required dimensional tolerances

4. Surface finish requirements

5. Saw-cutting allowance

6. CNC or other machining allowance

7. The size and geometry of the component

8. The cutting method used to prepare the blank

The raw material dimensions should then be determined by working backward from the finished component.

This is especially important for large or complex parts where cutting conditions can have a greater influence on the final blank dimensions.

Final Takeaway

Magnesium alloy plates are not always supplied as precision-finished blanks.

Thin sheets can often be purchased close to the required thickness and used with relatively little additional material removal. Medium and thick plates, however, generally require more consideration of cutting tolerance, flatness, dimensional variation, and machining allowance.

As a practical guideline, avoid ordering medium or thick magnesium alloy plates exactly to the final component dimensions. Saw-cutting can typically involve approximately 1–3 mm of dimensional variation, and allowing around 5 mm per side for length and width can provide useful room for subsequent processing. The actual allowance should always be determined according to the drawing and machining requirements.

When ordering magnesium alloy plates, providing the final part dimensions, required alloy, thickness, and intended machining process allows the starting material to be selected more appropriately.

For CNC machining and precision components, sufficient raw-material allowance is often the difference between a usable blank and a piece of material that is simply too small to finish.