Assembly vs. Subassembly in Epicor

How to Structure Them for Better MRP, Costing, and Production Efficiency

In Part 1 of this series, we explored the functional differences between Assemblies and Subassemblies in Epicor ERP. Understanding the terminology is an important first step, but designing an effective Bill of Materials (BOM) requires more than knowing the definitions.

The way Assemblies and Subassemblies are structured has a direct impact on manufacturing efficiency, planning accuracy, inventory management, and product costing. A well-designed BOM helps Epicor generate more accurate planning recommendations while making production processes easier to manage and maintain.

In this article, we will look at practical guidelines for determining when a component should become a Subassembly and how those decisions influence Material Requirements Planning (MRP), costing, and overall manufacturing performance.

Why Assembly Structure Matters

Every level within a Bill of Materials serves a purpose. The structure you create influences how Epicor plans production, calculates costs, schedules jobs, and manages inventory.

An effective Assembly hierarchy directly affects:

• Material Requirements Planning (MRP)

• Job creation and production scheduling

• Cost roll-ups

• Inventory management

• Engineering maintenance

If the hierarchy is overly simplified, important manufacturing processes may be hidden. If it is overly complex, planning becomes more difficult, BOM maintenance increases, and troubleshooting production issues can become unnecessarily time consuming.

The goal is to create a structure that accurately reflects how the product is manufactured while remaining manageable over time.

When a Component Should Be a Subassembly

Although every manufacturing environment is different, several situations commonly justify creating a Subassembly.

1. The Component Has Its Own Manufacturing Process

If a component requires its own routing, operations, labor reporting, machine resources, or quality inspections, it is often a strong candidate for becoming a Subassembly.

Separating these manufacturing steps provides better production visibility and allows Epicor to schedule and track the work independently.

Example

A finished automobile may contain several Subassemblies, including:

• Engine Assembly

• Transmission Assembly

• Wheel Assembly (Rim and Tire)

• Brake System

Each of these can have its own operations, labor reporting, work centers, and production schedule before being incorporated into the final vehicle.

2. The Component Is Used in Multiple Products

Reusable Subassemblies reduce duplication throughout the Bill of Materials.

Rather than recreating the same group of components in every finished product, manufacturers can define the Subassembly once and reference it wherever it is needed. This simplifies engineering changes, improves consistency, and reduces maintenance effort.

Example

A Wheel Assembly may be used across several vehicle models. Updating the Subassembly automatically benefits every finished product that references it, eliminating the need to modify multiple Bills of Materials individually.

3. The Component Benefits from Being Stocked

Some manufactured components are produced repeatedly and consumed by many production jobs. In these situations, building them in batches and maintaining inventory can improve manufacturing efficiency.

Producing common Subassemblies ahead of demand can:

• Reduce lead times during final assembly

• Improve production flexibility

• Reduce machine setup frequency

• Support more predictable scheduling

This strategy is particularly valuable when the Subassembly has a relatively stable demand pattern.

When a Component Should Not Become a Subassembly

Creating additional levels within a Bill of Materials is not always beneficial.

In many situations, keeping components directly within the parent Assembly results in a simpler and more efficient structure.

A separate Subassembly may not be necessary when:

• The component is only used in one finished product.

• It does not require independent manufacturing operations.

• It does not need its own routing or labor tracking.

• Separating it would add unnecessary complexity without providing operational value.

Adding excessive Subassembly levels can increase MRP processing, complicate job structures, and make production planning and engineering maintenance more difficult.

As with most ERP design decisions, simplicity is often preferable when it accurately reflects the manufacturing process.

Make Direct vs. Make to Stock

One of the most important decisions when defining a Subassembly is determining how it should be supplied to production.

Make Direct

A Make Direct Subassembly is produced specifically for a parent production job. Inventory is generally not maintained because each Subassembly is manufactured only when needed.

This approach is commonly used for customized products, engineer-to-order environments, or components with unpredictable demand.

Make to Stock

A Make to Stock Subassembly is produced in advance and stored in inventory for future production.

This strategy is typically appropriate when demand is consistent, the component is reused frequently, or producing larger batches improves manufacturing efficiency.

Factors that typically influence this decision include:

• Demand stability

• Production lead time

• Inventory carrying costs

• Available production capacity

• Component reuse across multiple products

There is no universal answer. The appropriate strategy depends on each organization's manufacturing processes and operational goals.

A Practical Rule of Thumb

When evaluating whether a component should become a Subassembly, consider the following questions:

• Does it have its own manufacturing process?

• Does it require independent routing or labor reporting?

• Is it reused across multiple finished goods?

• Would stocking it improve production efficiency?

If the answer to several of these questions is yes, defining it as a Subassembly is often the better design choice.

If not, keeping the component directly within the parent Assembly may result in a cleaner and easier-to-maintain Bill of Materials.

Final Thoughts

Assemblies and Subassemblies are more than organizational elements within Epicor ERP. They influence how products are planned, manufactured, costed, and maintained throughout their lifecycle.

A thoughtfully designed Assembly hierarchy can provide significant operational benefits, including:

• More accurate Material Requirements Planning (MRP)

• Improved production scheduling

• Better cost visibility and roll-up accuracy

• Simpler engineering maintenance

• Greater consistency across Bills of Materials

Rather than focusing solely on how a product is assembled, manufacturers should design their BOMs to reflect how the product is planned, produced, and managed over time. Doing so creates a foundation for more efficient manufacturing operations and helps Epicor deliver more accurate planning and costing results.