Fixed size or tolerance – how do I order correctly?

The decision whether aluminium sheets are cut to an exact fixed size or whether a certain tolerance range is accepted is crucial for many applications. Even a few millimetres can determine whether a component fits or not in the technical or industrial sector. This applies above all to constructions with tight fits, prefabricated housings or precision machine components.

Dimensional accuracy also plays a central role in further processing – for example when drilling, milling or joining. Anyone ordering imprecisely here risks not only scrap, but also unnecessary costs through reworking or reordering.

What does "fixed size" mean for aluminium sheets?

A fixed size denotes an exactly specified final dimension that is to be supplied without deviation. With an ordered fixed size of e.g. 200×300 mm, it is expected that the sheet has exactly these dimensions – without falling short or exceeding them. This differs fundamentally from a nominal size, in which a certain tolerance – i.e. a margin for deviations – is factored in.

To guarantee a fixed size, particularly precise machining is necessary. Modern cutting methods such as CNC saws, milling machines or laser technology are used for this. These enable dimensional accuracy in the tenth-of-a-millimetre range, but are associated with greater effort, longer processing time and a corresponding price. Fixed sizes should therefore be chosen deliberately and clearly communicated in the order.

What are typical tolerances in cutting?

In standard cutting, the usual manufacturing tolerances apply to aluminium sheets. Depending on the method and sheet thickness, these are typically:

• ±2 mm for length and width (cut with panel saw)
• ±0.5 mm for CNC machining (on request)
• ±0.2 mm for milled edges (involves effort)

These tolerances are sufficient for many applications, in particular if the sheets are still to be reworked or are not installed to an exact fit.

In which cases is a fixed size necessary?

An exact fixed size is always advisable when the sheet is to be inserted into an existing recess, when holes or milled features must subsequently be positioned exactly, or when there is no margin for installation clearance. A fixed size should also be used for direct mounting or further processing of the component as well as for technical drawings or standard dimension specifications. In all these cases, the extra effort for a fixed-size cut is worthwhile in order to guarantee fit accuracy and functionality.

When are tolerances sufficient?

In many situations, standard tolerances are entirely sufficient – in particular when high dimensional accuracy is not required. Provisional inserts, covers, large sheets for CNC blanks or installations with clearance are typical use cases. Even if the sheets are going to be further processed or trimmed later anyway, exact preparation in advance is usually not necessary. Here you save not only costs, but also time in production.

• provisional inserts or covers
• large sheets for CNC blanks
• installation with clearance or flexible mounting
• cuts that are further processed later

Here you save not only costs, but also time in production.

Risks with unclear dimension specifications

Wrong orders often arise from imprecise or contradictory specifications. If no fixed size is noted, production is generally carried out with standard tolerance. However, anyone expecting an exact dimension without indicating this risks deviations.

Equally problematic: wrong units, unclear orders (e.g. length x width), missing specifications for sheet thickness or confusion of internal and external dimensions.

Ordering notes: how to specify fixed size or tolerance correctly

• Note clearly whether it is a fixed size or a nominal size with tolerance
• Use technical drawings or sketches where possible
• Always specify order, unit and reference point (e.g. internal dimension, clear width)
• For tight tolerances: make direct contact or explicitly request precision

Recommendations from practice

For standard applications, a tolerance of ±2 mm is usually sufficient. For milled or visible parts, on the other hand, exact dimensions make sense, since optical and technical precision is decisive here. If you want to save time, you can also order a rough cut and rework it yourself.

For series production or the manufacture of prototypes, on the other hand, CNC cutting with a defined fixed size is advisable in order to guarantee repeat accuracy and minimise sources of error. If subsequent machining is planned, it can also make sense to allow for a machining allowance.

Cuts are then deliberately ordered 2–3 mm larger in order to mill them exactly to the final dimension later. This increases process reliability and avoids scrap.

Order to an exact fit and without reworking: choose fixed size or tolerance correctly and configure your aluminium cut directly online.

FAQ

What is the difference between fixed size and nominal size? A fixed size is an exact final dimension. A nominal size is a guide value for which tolerances are permissible.

Are fixed sizes always more precise than standard cuts? Yes, but they require more elaborate machining and are more cost-intensive.

How do I recognise whether I need a fixed size? If fit accuracy is decisive or subsequent machining requires exact references, you should request fixed sizes.

Can I also specify a precision with tolerances? Yes. You can, for example, specify a desired deviation of no more than ±1 mm – provided this is technically feasible.

What happens if I make no specification? In this case, the usual tolerances are applied by default. Therefore check in advance whether these are sufficient for your intended use – or whether a fixed size is required.