I've seen the same pattern hundreds of times. A drawing lands on my desk. The material is specified as 316L stainless. I look at the part — it's a bracket. No corrosion requirement. No temperature requirement. It could be 6061 aluminum and work fine for half the cost and half the lead time.
Material choice is the single biggest lever you can pull on CNC machining cost and delivery. Not tolerances. Not surface finish. Material. And most engineers don't spend enough time on it before they hit "send" on the RFQ.
The three ways material drives cost
Raw material price. This is the obvious one. A 100mm x 100mm x 25mm block of PEEK costs roughly 30 times what the same size block of 6061 aluminum costs. PTFE is cheaper than PEEK but still 5-8x aluminum. Stainless 316 is about 2x 6061. These differences hit hardest on large parts where material is most of the total.
Machinability — cycle time and tool wear. This is the one that surprises people. A part that takes 20 minutes in 6061 aluminum might take 35 minutes in 316L stainless and 50 minutes in PEEK. Why? Because you have to slow down.
Aluminum 6061 machines fast. You can run aggressive feeds and speeds (300+ SFM with carbide). Tool life is long. Cycle times are short.
Stainless 304 and 316L work-harden. If the tool rubs instead of cutting, the surface gets harder by the second. You have to keep the feed rate up and the speed down — typically 100-150 SFM. Tool pressure is higher so you take lighter cuts. More passes, more time.
PEEK and other engineering plastics have their own problems. They're soft but abrasive. Glass-filled grades eat carbide. The real challenge is heat. Plastics don't conduct heat away through the chip the way metals do — the heat stays in the part. If the part gets too hot, PEEK can anneal and lose mechanical properties. PTFE creeps under cutting pressure. You run conservative speeds, take light finish passes, and watch your coolant strategy.
A complex PEEK part can easily take 3x the machine time of the same geometry in aluminum. Most shops price by machine time. That 3x goes straight to your quote.
Material availability. 6061 aluminum? Any shop has it on the shelf or can get it tomorrow. 7075-T6 in a specific thickness? Maybe a few days. PEEK in CA30 grade (30% carbon fiber)? Could be two weeks. PAI (Torlon) 4203 in 50mm plate? Could be four weeks if the distributor is out.
When the material isn't in stock, two things happen: your lead time stretches, and the shop may charge a premium for expedited sourcing. I've seen projects delayed three weeks because someone specified an exotic grade that nobody stocks, when a standard grade would have worked.
Real numbers from real parts
Here's a comparison I ran recently on a medium complexity housing — roughly 150mm x 80mm x 40mm with pockets, threaded holes, and a sealing surface. Quantity 50. Same geometry, different materials:
| Material | Material Cost/Part | Machine Time | Total/Part | Lead Time |
|---|---|---|---|---|
| 6061-T6 Aluminum | $4.20 | 22 min | $28 | 10 days |
| 7075-T6 Aluminum | $7.80 | 25 min | $34 | 12 days |
| 304 Stainless | $9.50 | 38 min | $52 | 14 days |
| 316L Stainless | $12.00 | 40 min | $56 | 14 days |
| PEEK (unfilled) | $42.00 | 48 min | $98 | 18 days |
| PEEK (30% GF) | $55.00 | 55 min | $125 | 21 days |
| PTFE | $18.00 | 35 min | $60 | 14 days |
| PAI (Torlon 4203) | $85.00 | 52 min | $155 | 28 days |
The spread from cheapest to most expensive is over 5x — same part, same quantity, different material. That's before you factor in surface treatment or special handling.
What you can do about it
Question the material spec before it's locked. Ask: what does this part actually need? Not what the last engineer specified on a similar part. Not what the datasheet says is "best." What are the real requirements?
Start with temperature. If the part never sees 100C, you don't need PEEK. If it never sees 200C, you don't need PI. Aluminum 6061 is good to about 150C for most applications.
Then chemical exposure. No acids, no solvents, no cleaning agents? You probably don't need stainless. Mild corrosion environment? 6061 with Type II anodize is cheaper than 316L and often works fine.
Then mechanical loads. If the stress is low, standard grades work. Save the high-strength alloys for where they earn their cost.
Ask about stock availability before finalizing. When I quote a job, one of the first things I check is whether the material is in stock locally. If the spec calls for 12mm PEEK CA30 and nobody within 500km has it, I call the customer and ask if unfilled PEEK would work, or if 15mm stock is acceptable. Half the time, it is — and the lead time drops from three weeks to three days.
Design for the material you choose. Aluminum can have thinner walls than stainless because it cuts easier and doesn't work-harden. Plastics need generous internal radii because sharp corners create stress concentrations that PEEK and PTFE don't handle well. If you're switching materials on an existing design, expect to adjust the DFM.
When expensive materials are worth it
None of this means you should always choose the cheapest material. If you're making semiconductor wafer handling components, the outgassing spec demands PEEK or better — and the $98/part is justified because the alternative is yield loss worth thousands per wafer.
If you're making surgical instruments that go through 500 autoclave cycles, PEEK or PAI is non-negotiable. The material cost vanishes compared to the validation cost of switching materials later.
If you're making aerospace brackets where every gram matters, 7075-T6 aluminum (lighter and stronger than 6061) or even titanium becomes cost-effective when you account for fuel savings over the life of the aircraft.
The point isn't "cheap material good." It's "right material for what the part actually does."
Bottom line
Before you send your next RFQ, spend fifteen minutes on the material call. Check what's in stock locally. Ask if the spec from the last revision still makes sense. If you're not sure, send the drawing and ask the shop for a material recommendation — we do this every day and can usually suggest alternatives that save time and money without touching performance.
That housing example? The customer started with PEEK 30% GF ($125/part, 21 days). After a conversation, we landed on unfilled PEEK with a hard anodized aluminum alternative for non-critical areas ($98/part for the PEEK version, $28 for the aluminum). Mix of both cut the total BOM by 40% and knocked a week off delivery.