Sheet Metal Processing Tolerance Table

Sheet metal processing tolerance table

Inconsistent tolerance standards have caused many bosses to suffer losses. This makes sheet metal parts' dimensional deviations the most common quality dispute in factories. Mastering these basic data can help you pay less liquidated damages and accept more high-unit-price orders.

How to control linear size

The most basic and important parameter of sheet metal parts is the linear dimension, which will directly affect the assembly pass rate. The national standard divides the tolerance into four levels. The higher the level, the more stringent the precision. Precision level F is suitable for parts that require precision fit such as electronic shells and instrument panels.

The standard that most factories follow daily is the medium grade M, which can meet the relevant requirements of ordinary mechanical structural parts. The rough grade C is suitable for appearance parts or non-fitting parts. The coarsest grade V is usually used for prototypes or temporary fixtures. When selecting materials, decisions need to be made based on actual uses and excessive pursuit of accuracy will only increase costs in vain.

Be particular about chamfering and rounding corners

Although the rounding radius and chamfering height may seem like small details, they have a huge impact on part safety and assembly. Specifically, the precision-grade fillet tolerance is controlled within the range of ±0.2 to ±2 mm, which is more suitable for products with appearance requirements. As for the tolerances of medium and rough grades, they have been relaxed to ±0.4 to ±4 mm, which can fully meet the use needs of ordinary structural parts.

Chamfering is mainly used for deburring and preventing hand cuts. The roughness of the machined surface allows a deviation of ±1 to ±8 mm. Some customers will place special emphasis on chamfering specifications. Be sure to confirm clearly before placing an order to prevent rework after delivery.

Don’t ignore the bending angle

Among sheet metal parts, bending angle deviation is a common problem. In many cases, assembly failure is due to poor angle control. For the precision level, when the short side is less than or equal to 10 mm, an error of ±1 degree is allowed. As the side length increases, the relevant requirements become more stringent. Once it exceeds 400 mm, only an error of ±0 degrees and 5 minutes is allowed.

General structural parts are suitable for medium grade, and the allowable range of roughness grade C is from ±1 degree 30 minutes to ±0 degrees 10 minutes. The coarsest level V has the lowest accuracy, but it can basically meet the needs for modeling parts or brackets. When measuring, use the folded edge on the shorter side as a basis. This is an industry standard practice.

How to choose among the four levels

When choosing a tolerance level, you must not just rely on feeling, but rather determine which level it is based on the functional positioning of the part. For precision-fitting pin holes and guide rail surfaces, be sure to use grade F, otherwise they will not be installed. As for ordinary chassis shells and brackets, they can be M-level, so as to achieve the highest cost performance.

If the exterior sheet metal parts are covered by spray coating, then there is no problem in choosing grade C, which can save the finishing process. For rough-machined parts or blanks, if you directly use level V, you can reduce the number of cuts and improve efficiency. Many factories suffer losses because they use fine-grade products instead of coarse-grade products. This is obviously a waste of man-hours and materials.

Common mistakes

Many factories produce parts that are out of tolerance. This problem stems from the thickness of the material. Once the thickness of the plate is increased, the rebound amplitude during bending will be increased, and the angle created by the same mold will be larger. The hardness of different batches of steel plates lacks stability, and changes in the heat treatment state will affect the dimensional accuracy.

Poor shear section quality will have a consequential impact on subsequent bending, making it difficult to maintain a consistent angle where the burrs are large. Insufficient tonnage of the bending machine will cause the angle to fail to meet the requirements, and the amount of rebound will increase if the pressing force is insufficient. These issues need to be judged in advance during the process preparation stage.

Practical experience is key

During actual production, the tolerance table only serves as a reference and needs to be adjusted based on the capabilities of the equipment and the conditions of the mold. The equipment of those old sheet metal factories with a long history has been calibrated and operated, has good stability, and can be close to the F-level standard. However, the equipment of the new factory has deficiencies in terms of accuracy. If it is forced to undertake the production of precision parts, it will have to suffer losses silently.

When communicating with the customer, the tolerance standards must be written into the contract, and construction can only begin after both parties have confirmed it. If the other party's requirements exceed the normal standards, the additional costs and cycle time must be explained in advance. Only by doing things in a standardized manner can we establish a long-term cooperative relationship and avoid subsequent disputes.

I would like to ask, what level of accuracy do you think your factory can currently achieve? How will you respond when you encounter a situation where customers' tolerance requirements are too high? Feel free to leave a message to share your experience.