How can OEM buyers select the appropriate manufacturing process based on tolerance requirements, part structure, materials, and production quantity?

Ago 24, 2026

How can OEM buyers select the appropriate manufacturing process

based on tolerance requirements, part structure,

materials, and production quantity?

 

Dimensional tolerances in manufacturing are often one of the technical requirements that OEM buyers and engineers are most concerned about when sourcing custom metal parts. But an often overlooked question is: does the smaller the tolerance mean the better the product?

The answer is not necessarily. Different manufacturing processes have different dimensional accuracy and repeatability capabilities. For example, sand casting is suitable for large, complex structural parts, but more CNC machining allowances are usually required; investment casting can obtain castings closer to the final size; forging can provide excellent mechanical properties and structural integrity, but there are significant differences in dimensional accuracy between different forging methods; and CNC machining can usually achieve the highest dimensional accuracy, but it also means higher processing time, equipment and inspection costs.

Therefore, what is really important for OEM buyers is not simply to find the “highest precision” manufacturing process, but to find: a process that can meet the actual use requirements of the product while taking into account manufacturing costs and delivery efficiency.

1. What are dimensional tolerances in manufacturing?

Dimensional tolerances refer to the allowable deviation of a part’s actual dimensions from its theoretical dimensions. For example, a drawing specifying 50 ±0.05 mm means the actual dimensions of the part typically need to be controlled within the range of 49.95–50.05 mm.

This may seem like just a numerical change, but for manufacturers, the production complexity can change significantly.

Therefore, strict tolerances are not a free performance upgrade. If the actual product only requires ±0.05 mm, but ±0.01 mm is required across the entire part, the end result is likely to be: no significant performance improvement, but a significant increase in manufacturing costs. This is a key consideration for OEM buyers when selecting manufacturing processes.

2. Why is the highest precision process not necessarily the best manufacturing solution?And how to choose the right manufacturing process?

In the selection process for customized metal parts, most OEM buyers hold a misconception: higher manufacturing precision equates to a superior process selection solution.

In reality, process selection should not be based solely on precision. It requires a comprehensive assessment across six core dimensions: part size and structural complexity, material utilization rate, mechanical performance parameters, annual procurement volume, subsequent deep processing requirements, overall manufacturing cost, and delivery cycle.

Taking core shaft components of heavy-duty transmission systems as an example: while directly using a single alloy steel bar for full CNC machining can achieve extremely high dimensional accuracy, it has significant drawbacks: extremely high raw material cutting losses, cumbersome machining processes, high tool wear costs, persistently high overall cost per unit, and a significantly extended production cycle.

For these core load-bearing components that must withstand high loads, impact loads, and long-term cyclic operation, the industry’s optimal and mature solution is: forged blank + heat treatment + CNC precision machining.

This composite process ensures both the core mechanical performance and assembly precision of the part while greatly reducing material waste and production costs, making it the standardized and preferred solution for heavy-duty shaft components.

For complex cavity-type parts such as valve bodies, pump bodies, and irregularly shaped connectors, full Mecanizado CNC is not the optimal solution.

If these parts were directly milled from a single piece of steel, most of the redundant substrate would need to be removed, resulting in long machining times, extremely low material utilization, and very high mass production costs.

However, the combination of precision casting and CNC finishing perfectly meets the production requirements of these products: this approach ensures core assembly accuracy and performance while maximizing material utilization, reducing production time, and enabling efficient mass production of complex parts.

Longway’s production philosophy is application-scenario-centric, customizing the optimal manufacturing solution.

3. High precision does not necessarily mean lower manufacturing costs.

This is a point easily overlooked in OEM procurement. Suppose a part has 100 dimensions. If engineers require all dimensions to have very strict tolerances, then high-precision machining equipment, upgraded tooling and cutting tools, and additional finishing processes are needed during the manufacturing stage. However, in actual use, perhaps only the bearing mounting position, sealing surface, and mating dimensions truly require strict control.

A more reasonable design approach is to set strict tolerances for critical functional dimensions and use standard universal tolerances for non-critical dimensions. While fully ensuring product performance and assembly requirements, this effectively reduces machining, inspection, and scrap losses, significantly controlling overall manufacturing costs and balancing design requirements with actual manufacturability.

4. Why do stricter manufacturing tolerances lead to higher procurement costs?

A simple and intuitive example: If the critical diameter tolerance of a shaft is increased from ±0.05mm to ±0.01mm, achieving stable compliance requires the manufacturer to upgrade the entire process and tooling, thus increasing additional costs. For OEM buyers, the more crucial question is whether tightening this tolerance truly improves the actual performance of the part, rather than simply asking: Can the supplier achieve higher tolerance precision?

5. How does LONGWAY help customers choose manufacturing processes?

Manufacturing process selection for OEM parts, LONGWAY typically doesn’t determine the manufacturing process solely based on a tolerance number on the drawing. Before providing a formal quote, we consider: Product application/Is the part subjected to static loads, impact loads, or long-term cyclic loads? Material requirements/Is it carbon steel, stainless steel, alloy steel, or other special materials? Dimensional tolerances/Which dimensions are critical functional dimensions? Which dimensions can use standard tolerances? Part structure/Is the product suitable for casting, forging, or CNC machining? Subsequent machining/Which parts require CNC machining? Is grinding required? Production quantity/Small batch or long-term large-volume production? Quality requirements/Is CMM, UT, hardness testing, or other special testing required?

By comprehensively assessing these factors, we can help customers choose a more reasonable manufacturing route.

Our goal is not simply to provide the “highest precision” solution, but to help customers find a solution that meets product performance requirements while maintaining reasonable manufacturing costs and delivery cycles.

Conclusión

 Correct tolerances are often more important than higher precision. For OEM custom parts, no single manufacturing process for custom parts can simultaneously provide: highest precision + lowest cost + fastest delivery in all situations.

Casting, forging, and CNC machining each have their own advantages. A truly reasonable manufacturing solution should be determined based on: part structure, material, mechanical properties, dimensional tolerances, production quantity, and end use.

For buyers, the most important question is not: which manufacturing process can achieve the highest precision?

but rather: what level of precision does their application actually require?

Only when design requirements and manufacturing processes truly match can unnecessary machining, excessive inspection requirements, and additional manufacturing costs be avoided.

If you are developing new custom castings, forgings, or CNC machined parts but are unsure which manufacturing process to use, LONGWAY can provide an initial assessment based on your drawings, materials, dimensional tolerances, production quantities, and application requirements, and help you determine a more suitable manufacturing route.

Submit your drawings and project requirements, and let us start with manufacturing process selection to find a more appropriate solution for your project.

PREGUNTAS FRECUENTES

Q1: What are the differences in precision and dimensional tolerances between different casting processes?

Different casting processes have different dimensional precision requirements. Generally, sand casting is suitable for large and complex structures, but dimensional tolerances are relatively loose; precision casting achieves better dimensional consistency and surface quality. For critical dimensions such as shaft holes and sealing surfaces, CNC machining is usually still required for final processing.

Q2: What are the differences in precision between different forging processes?

Die forging generally has better dimensional consistency and repeatability than free forging, making it more suitable for mass production and forgings with complex structures. Free forging is more suitable for large forgings, heavy shafts, and small-batch projects. For high-precision dimensions, CNC machining is usually still required after forging.

Q3: What level of precision can CNC machining achieve?

CNC machining can generally achieve higher dimensional precision than casting and forging, but the specific capability depends on the material, part structure, equipment, cutting tools, clamping method, machining process, and inspection methods. For parts with even higher precision requirements, grinding and CMM inspection may also be necessary.

Q4: What forging tolerance standards are commonly used in European OEM projects?

European OEM projects typically select appropriate forging tolerance standards based on DIN 7526,EN 10243,ISO 8062,ISO 2768. In actual projects, it’s not recommended to determine tolerances solely based on a single general standard. Instead, confirmation should be made in conjunction with specific part drawings, forging methods, and subsequent CNC machining requirements.

Q5: Does higher precision always mean better product quality?

Not necessarily. Overly strict tolerances may increase machining, inspection, and production costs, but they don’t necessarily improve actual product performance. A more reasonable approach is to identify which dimensions truly affect product functionality and set appropriate tolerances for these critical dimensions.

Q6: Why is CNC machining still needed after casting or forging?

Casting and forging primarily form the overall structure of the part, while CNC machining further controls critical dimensions, hole positions, mating surfaces, sealing surfaces, and surface roughness. Therefore, in many OEM projects, Casting + CNC Machining or Forging + CNC Machining is a more reasonable and complete manufacturing solution.