Material Requirements Planning (MRP) is one of those manufacturing concepts that every shop talks about, but few explain in terms that connect to what actually happens on the production floor. Most guides to MRP are written by software companies selling ERP systems. This one is written from the perspective of a precision machine shop that deal with customers' MRP systems every day.
At Spex, we manufacture precision machined components for OEMs across industries like power and electrical, fluid handling, measurement instrumentation, and agricultural equipment.
Our work sits in the middle of our customers' material requirements planning. We're the suppliers their MRP systems are scheduling, ordering from, and tracking. That gives us a different view of how MRP actually works in practice.
Here's what MRP looks like from the shop floor, and why it matters for any manufacturer sourcing precision machined parts.
Material Requirements Planning is an inventory and production planning system that helps manufacturers figure out three things:
The goal is to keep inventory levels as low as possible without running out of parts and causing production delays.
MRP systems take in data from three core sources: the Master Production Schedule (MPS), which defines what finished products need to be made and when; the Bill of Materials (BOM), which breaks down every component required to build those products; and current inventory records, which show what is already on hand or on order.
The system cross-references these inputs and generates purchase orders, work orders, and production schedules. When demand shifts (eg. a customer increases an order, a new contract comes in, or a forecast changes) the MRP system recalculates and adjusts the plan.
In a precision manufacturing environment, MRP determines when raw material gets ordered, when jobs hit the CNC machines, and when finished parts ship to the customer.
For example: an OEM that manufactures pressure gauges needs custom machined housings, fittings, and sensor bodies. Their MRP system has a Bill of Materials (BOM) for each gauge model that lists every component down to the individual O-ring. The machined parts from a supplier appear as line items in that BOM — each with a part number, quantity per assembly, and lead time.
When the OEM's MRP system runs its planning cycle, it looks at how many gauges are scheduled for production, checks current inventory of each component, and generates purchase orders for anything that's needed. Those purchase orders hit our desk as orders for specific part numbers, quantities, and delivery dates.
This is why accuracy in the BOM and realistic lead times are so critical. If the BOM has the wrong quantity per assembly, the OEM orders too many or too few parts. If the lead time in their system does not reflect actual machining and shipping time, parts arrive late and the production line stalls.
The Bill of Materials is the backbone of any MRP system, and it takes on added complexity when precision machined components are involved.
A simple consumer product might have a simple, single-level BOM. But most of the products our customers build have multi-level BOMs where machined parts feed into subassemblies, which then feed into higher-level assemblies.
For example, a valve manufacturer's BOM might look like this:
The machined valve body at Level 2 has its own set of specifications — material grade, dimensional tolerances, surface finish requirements, thread specifications, and any secondary operations like plating or heat treatment. All of this information needs to be accurately captured so the MRP system can plan procurement correctly.
One issue we see regularly is BOMs that do not account for material yield. If a machined part requires a 2-inch diameter bar but the BOM only lists the finished part, the MRP system cannot plan raw material purchases at the shop level.
For contract machine shops like Spex, this is handled on our side. We manage our own material procurement based on the part specifications. But for OEMs doing in-house machining, material yield and scrap rates need to be built into the BOM or the MRP calculations will consistently underestimate material requirements.
Lead time is the single biggest variable that separates a functional MRP system from one that produces plans nobody can actually execute.
In precision machining, lead time is not a single number. It is the sum of several stages: quoting and order processing, raw material procurement, machine setup and programming, machining operations, inspection and quality control, any secondary operations (anodizing, plating, heat treatment), and shipping.
For a standard repeat order on a part we have already programmed with material in stock, turnaround might be two to three weeks. A new part requiring first article inspection, exotic material procurement, and outside processing could take eight weeks or more.
The problem is that most MRP systems want a single lead time value per part. Some more advanced systems allow for separate manufacturing and procurement lead times, but many do not distinguish between a first-run order and a repeat order.
This is why maintaining close communication with your machining supplier matters — the lead time your MRP system uses should reflect current reality, not a number someone entered three years ago.
At Spex, we work with our customers to establish realistic lead times and update them when conditions change. If a material supplier extends delivery from four weeks to six, that affects every part we machine from that material, and every OEM whose MRP system depends on our quoted lead time.
Two MRP concepts that are especially relevant when sourcing precision machined components are safety stock and lot sizing.
Safety stock is the buffer inventory kept on hand to protect against variability in demand or supply. For machined parts with longer lead times, maintaining some safety stock prevents a demand spike or a supplier delay from shutting down the assembly line. The tradeoff is the carrying cost. Machined parts are typically higher-value components, so excess inventory ties up more capital per unit than commodity fasteners or raw materials.
Lot sizing determines how many parts to order at a time. MRP systems offer various lot sizing methods: lot-for-lot (order exactly what is needed), fixed order quantity, economic order quantity, and period order quantity, among others.
For precision machined parts, lot sizing involves a practical consideration that pure MRP math does not always capture: setup costs. CNC machining involves significant setup time for each production run — loading programs, setting tools, fixturing, and running first article inspection. Ordering 50 parts versus 500 parts doesn't require 10x the setup effort. This means there is a natural economy of scale that favors larger order quantities, even if the MRP system's lot-for-lot calculation says to order exactly 47 pieces.
This is one reason we encourage our customers to discuss order quantities with us during the quoting process. We can often identify a quantity break point where the per-unit cost drops significantly, and that info should be fed back into the MRP system's lot sizing parameters.
MRP systems are powerful, but they depend on good data and realistic assumptions.
Here are the challenges we see most often from the supplier side of the equation.
Inaccurate inventory records. If the MRP system thinks there are 200 parts on hand but the actual count is 150, it will under-order. Physical counts, cycle counting programs, and disciplined inventory management are essential.
Outdated lead times. Supply chains are not static. Material availability, supplier capacity, and transportation timelines change. Lead times in the MRP system need to be reviewed and updated regularly.
Ignoring supplier capacity. Traditional MRP assumes infinite capacity. It will generate purchase orders without considering whether the supplier can actually produce that volume in the required timeframe. This is why blanket orders, forecasting agreements, and regular communication with key suppliers are so important. When we know a customer's quarterly forecast, we can plan capacity and stage material in advance.
BOM errors. A wrong revision level, an incorrect quantity per assembly, or a missing component can cascade through the entire MRP plan. Engineering change management and BOM version control are not glamorous, but they are critical.
Rigid planning horizons. MRP runs on defined planning periods: weekly, monthly, or otherwise. Real demand does not follow neat planning periods. Some manufacturers address this with more frequent MRP runs or by supplementing MRP with Kanban systems for high-volume, repetitive parts.
MRP focuses specifically on material and production planning, making sure the right parts are available at the right time to meet the production schedule. Enterprise Resource Planning (ERP) is a broader system that includes MRP as one module alongside financial management, human resources, customer relationship management, quality management, and more.
In practice, most manufacturers today use an ERP system that includes MRP functionality rather than a standalone MRP system. The advantage is that data flows between departments — a sales order automatically triggers MRP calculations, which generate purchase orders, which update financial projections, all within one system.
For a machine shop like Spex, our own production planning system manages job scheduling, material procurement, machine utilization, and quality records. On the customer side, our parts and lead times are data points in their ERP/MRP system. The better aligned those data points are with reality, the smoother the entire supply chain runs.
Precision machining is not just about hitting tight tolerances on a CNC machine. It's about being a reliable link in your supply chain — the kind of supplier whose lead times you can trust, whose quality is consistent, and whose communication keeps your MRP system aligned with reality.
At Spex, we support our customers' material planning in several ways: maintaining consistent lead times through disciplined scheduling and material management, providing accurate and detailed quoting that includes lead time and lot size recommendations, flagging material availability issues early so customers can adjust their MRP parameters, and delivering to schedule so downstream production stays on track.
We are an ISO 9001:2015 certified precision machine shop specializing in CNC milling and turning for OEMs across power and electrical, fluid handling, instrumentation, agricultural equipment, and chemical processing industries.
If you are sourcing precision machined components and want a supplier who understands how your MRP system works, reach out to our team for a quote.
