Raw Material Inventory: Why We Keep the Shelves Stocked and the Cash Moving

The Two Failure Modes: Too Little and Too Much

In a machine shop, raw material inventory is a balancing act. Too little, and you miss delivery dates, pay expedite fees, or lose a customer. Too much, and you tie up cash, fill your racks with metal that may never be used, and watch your margins shrink under the weight of slow-moving stock.

Most shops oscillate between these two failures. When they’re busy, they buy everything they think they’ll need, and the shelves groan with bar stock. When cash gets tight, they stop buying and run the shelves down to nothing. Then a rush order comes in, and they’re scrambling. It’s a cycle that feels like firefighting, and it never ends.

We wanted to break the cycle. So we started by asking a simple question: what materials do we actually use, and how often?

The ABC of Shop Stock

We pulled three years of job data from our ERP system and sorted every material by usage frequency and volume. The result was a clear pattern—what inventory people call the ABC classification.

A items: The everyday metals. These are the materials we cut every week, sometimes every day. In our shop, that’s 6061-T6 aluminum in a few common sizes, 303 stainless, 304 stainless, 316L stainless, 1018 steel, and POM (acetal) rod. These are the bread and butter of our production. We keep these in stock at all times, in multiple diameters, because running out means stopping the shop.

B items: The regulars. These show up once or twice a month, or they’re used in specific recurring jobs. For us, that’s 7075 aluminum, 4140 steel, 17-4 PH stainless, brass, and nylon. We don’t stock every size, but we keep a moderate inventory of the most common diameters and shapes. When a job comes in, we check the shelf first and order only what we’re missing.

C items: The specialty and one-off materials. These are the exotic alloys, the large-diameter bars, the materials we order for a single job and may never use again. Hastelloy, Inconel, titanium, PEEK, tungsten, ceramics, and oversized aluminum plate. For these, we order per job, and we either consume the material completely or return the remnant to the supplier if the contract allows. We don’t stock them. The risk of dead inventory is too high.

Old Li, who has watched inventory come and go for decades, approved of the ABC approach with a simple nod. “A items keep the shop running. B items keep the schedule flexible. C items keep the customer happy but the shelves clean. Don’t mix them up.”

Setting the Minimum: Safety Stock That Actually Works

For A and B items, we set a minimum stock level—a reorder point—that triggers a purchase when the quantity on hand drops below that line. The reorder point is not a guess. It’s calculated from three numbers: the average daily usage, the supplier’s lead time, and a safety factor for variability.

Here’s the formula we use:

Reorder Point = (Average Daily Usage × Lead Time in Days) + Safety Stock

The safety stock is the buffer for the unexpected—a supplier delay, a sudden spike in demand, a batch of material that arrives out of spec. We set the safety stock at 20–30% of the lead-time usage for most A items, and higher for materials with longer or less reliable lead times.

For example, we use about 50 bars of 1-inch 6061 aluminum per week. Our supplier delivers in three days. Our safety stock is three days’ usage—about 30 bars. So our reorder point is (50 × 3/5) + 30, or roughly 60 bars. When the shelf hits 60 bars, the ERP system flags a purchase order. By the time the new material arrives, we’ve consumed about 30 bars and have 30 left—enough to keep running if the delivery is a day late.

This system isn’t perfect. It requires accurate usage data and honest lead times. But it’s far better than the “order when it looks low” method we used to use. The shelves are never empty, and we’re not sitting on a year’s worth of metal.

The Supplier Relationship: Why We Don’t Shop on Price Alone

Our inventory strategy depends on our suppliers. If they deliver late, our safety stock gets eaten. If they deliver out-of-spec material, we scrap parts and lose time. So we’ve built relationships with a small number of trusted suppliers—not the cheapest on the market, but the most reliable.

We share our usage forecasts with our main suppliers. We tell them what we expect to consume each month, and they hold stock for us. Some of them offer consignment—they store material in our shop, we pay only when we use it. That’s the ideal arrangement for A items. It keeps our cash free and our shelves full.

We also qualify a backup supplier for every A item. If our primary supplier has a strike, a shortage, or a quality problem, we can switch without stopping production. We learned that lesson during a titanium shortage a few years ago, when our only supplier was backordered for three months. Now we have two sources for every critical material.

Old Li’s supplier rule: “A good supplier is a partner, not a vending machine. You don’t just put in a coin and take out metal. You talk to them. You tell them what you need. You pay them on time. And when you’re in a bind, they remember you.”

The Remnant Problem: What to Do With Leftovers

Every machine shop has a remnant rack—the short bars, the offcuts, the “might need it someday” pieces that accumulate like dust. Remnants are a form of inventory, but they’re often unmanaged. They take up space, they get mixed up, and they’re rarely used because nobody knows what they are.

We now treat remnants as part of the inventory system. Every remnant gets a tag with the material grade, the heat number, the size, and the length. The tags are barcoded, and the remnants are stored in a designated area, sorted by material and size. The ERP system knows they exist, and when a job comes in that can use a remnant, the system flags it. We’ve saved thousands of dollars by using remnants instead of cutting into a new bar.

Remnants that are too short to be useful—under 6 inches for most materials—go into a scrap bin. We sell them to a recycling company. It’s not much money, but it’s better than letting them rust on a shelf.

Old Li, who has seen remnant racks grow into jungles, says: “A remnant without a tag is a mystery. A mystery is a waste. Tag it, measure it, and use it. If you can’t use it, sell it. Don’t let it sit there and taunt you.”

The Cash Flow Balance

Inventory is cash. Every bar on the shelf is money that could be in the bank, paying down a machine, or funding a new hire. So we watch our inventory turnover like a hawk. We want to turn our A items four to six times a year—meaning we sell through our stock every two to three months. B items can turn slower, maybe two to three times a year. C items are bought for specific jobs and should turn within the job’s production window.

We review inventory levels monthly. If an A item is turning too slowly, we reduce the reorder point. If a B item hasn’t moved in six months, we either use it, sell it, or write it off. We don’t let dead stock accumulate. A full shelf is not a sign of health. A moving shelf is.

Old Li’s cash rule: “Metal on the shelf is money sleeping. Money should be working. If it’s sleeping too long, wake it up or sell it.”

The Horror Shelf Gets an Empty Rack

On the horror shelf, we’ve mounted a small, framed photo of the empty 316L rack from that Friday afternoon. Next to it is a copy of the expedite fee we paid to a supplier who couldn’t deliver, and a handwritten note from the customer who had to wait until Tuesday. The label reads: “Empty shelf. Empty promise. The cost of no inventory.”

Below it, we’ve placed a small sample of each A item—a short bar of 6061, 303, 304, 316L, 1018, and POM—with a second label: “These stay on the shelf. Always.” Old Li arranged the display so the empty rack photo is at eye level. “It’s a reminder,” he says, “that the machines can’t cut air. They need metal. Keep the metal on the shelf, and the promises on the truck.”

A Short Guide to Raw Material Inventory for Machine Shops

If you’re building or rebuilding your inventory system, here’s what we’ve learned:

1. Classify your materials. A items are everyday, B items are regular, C items are specialty. Stock accordingly.

2. Set reorder points based on usage and lead time. Use real data, not gut feel. Add safety stock for variability.

3. Build supplier relationships. Reliable delivery beats the lowest price. Qualify a backup for every critical material.

4. Use consignment and forecasting. Share your usage with suppliers and ask them to hold stock. Free up your cash.

5. Manage remnants. Tag every offcut. Store it by grade and size. Use it before you cut a new bar.

6. Watch turnover. A items should turn four to six times a year. If they’re not moving, reduce the stock.

7. Don’t hoard C items. Order per job. Use it or return it. Don’t let specialty metal become a shelf ornament.

8. Review monthly. Inventory is not a one-time project. It’s a living system that needs attention.

The empty 316L rack is now a memory, but it’s a memory that shaped our system. We don’t run out of A items anymore. We don’t tie up cash in C items we’ll never use. We keep the shelves stocked with what we need, and we keep the cash moving. The machines stay busy, the customers get their parts, and Old Li doesn’t have to watch me beg for metal from a competitor again. That’s worth more than the cost of the inventory.

Old Li, after a recent inventory review, said: “A good shop has three things: good machines, good people, and good metal on the shelf. You can’t make parts without the metal. You can’t keep promises without the metal. And you can’t sleep at night if you don’t know where the metal is. Keep the shelves full, keep the tags straight, and keep the cash moving. That’s the whole secret.” He then went back to his lathe, where a bar of 6061 was waiting, already tagged, already measured, already ready to become a part.

How do you manage raw material inventory in your shop? Do you stock A items, B items, or just order per job? What’s your worst “out of stock” story, or your best inventory trick? I’d love to hear your material management strategies in the comments. The shelves tell the truth about a shop—let’s help each other keep them honest.