Metal vs. Plastic CNC Machining: How to Choose Between Turning & Milling

First: Metal vs. Plastic CNC Machining—The Big Differences That Matter

Before diving into turning and milling, let’s start with the basics: Metal and plastic don’t just look different—they react differently to being cut, shaped, and finished. These differences change everything from the tools we use to the speed of the process.

1. Material Hardness & Tool Wear

Metals (like aluminum, stainless steel, or brass) are harder and more abrasive than plastics (like ABS, PEEK, or nylon). This means:

• Metal needs tougher tools: We use carbide or high-speed steel (HSS) tools for metal—they hold up against the friction. For example, milling aluminum requires a carbide endmill that can withstand 300–400°C (572–752°F) heat.
• Plastic needs sharper, gentler tools: Dull tools can “smear” plastic instead of cutting it (think of trying to cut butter with a blunt knife). We use polished carbide tools for plastic—they slice cleanly without melting or warping the material.

Tool wear is a big cost factor too. A carbide tool for stainless steel might last 500 parts before needing replacement; the same tool for ABS plastic could last 2,000 parts. If you use a metal tool on plastic without adjusting, you’re wasting money on premature tool changes.

A client once came to us with a problem: They’d used a stainless steel tool to mill ABS plastic sensor housings, and the tools were wearing out in 300 parts (instead of 2,000). We switched them to a polished plastic-specific carbide tool—their tool life jumped 6x, and they saved $800/month on tool costs.

2. Heat & Warping Risks

Cutting generates heat—and metal and plastic handle heat in opposite ways:

• Metal dissipates heat (mostly): Aluminum and steel can handle high temperatures, so we can run CNC machines faster (e.g., 10,000 RPM for aluminum milling) without warping the part. We still use coolant to prevent tool overheating, but the part itself stays stable.
• Plastic melts or warps easily: ABS starts to soften at 105°C (221°F)—way lower than metal’s tolerance. If we run the machine too fast, the tool’s friction melts the plastic, leaving rough edges or distorted shapes. For plastic, we slow down (e.g., 5,000–7,000 RPM for ABS milling) and use air cooling (not liquid coolant, which can soak into plastic and cause swelling).

One auto parts client tried to mill plastic clips at the same speed as aluminum—80% of the clips warped, and they had to scrap the whole batch. We adjusted the speed and switched to air cooling—their next run had a 99.5% pass rate.

3. Surface Finish Needs

The finish you want often depends on the material:

• Metal often needs post-processing: Milled aluminum might have a matte finish (Ra 1.6μm) right off the machine, but if you need it shiny (Ra 0.8μm) for a visible part, you’ll need polishing or anodizing. Stainless steel might need passivation to prevent rust.
• Plastic can get smooth finishes direct from the machine: Polished tools and slower speeds let us get a smooth finish (Ra 0.8μm) on ABS or PEEK without extra work. This saves time—no need for sanding or painting unless you want a custom color.

4. Tolerance Limits

Both materials can hit tight tolerances, but plastic has a catch:

• Metal holds tolerances better long-term: An aluminum bracket with ±0.01mm tolerance will stay that way for years—metal doesn’t expand or contract much with temperature changes.
• Plastic shifts with temperature/humidity: ABS can expand 0.2–0.4% if it absorbs moisture, which means a ±0.01mm tolerance might drift to ±0.02mm in humid conditions. For plastic parts that need tight tolerances (like sensor housings), we dry the material before machining and use heat-treated plastics (like glass-filled PEEK) that are more stable.

CNC Turning vs. Milling: When to Pick Which Process

Now that you know how metal and plastic differ, let’s talk about the two most common CNC processes: turning and milling. The key difference comes down to shape—what does your part look like?

What Is CNC Turning? (Best for Rotational, Symmetrical Parts)

Turning uses a rotating tool and a stationary cutting tool (or vice versa) to shape parts that are round or symmetrical around an axis. Think of a lathe: the part spins, and the tool cuts away material to make shapes like shafts, sleeves, or bolts.

When to Use Turning:

• Parts that are round or cylindrical: Shafts for car transmissions, plastic bushings for door hinges, metal sleeves for wiring.
• Simple, symmetrical features: Grooves, threads, or tapers (e.g., a tapered metal pin for aligning two parts).
• Fast, low-cost for rotational parts: Turning is faster than milling for round parts—you can make a metal shaft in 2 minutes, vs. 5 minutes with milling.

Material Notes for Turning:

• Metal turning: Works great for aluminum, brass, and stainless steel. We use coolant to keep tools cool and parts smooth.
• Plastic turning: Possible, but trickier. We need slow speeds (3,000–5,000 RPM) and sharp tools to avoid melting. Turning is good for plastic bushings or knobs, but not for complex shapes.

A car repair shop needed 500 metal shafts for a transmission repair kit. We used turning—each shaft took 2.5 minutes to make, and we delivered all 500 in 3 days. If we’d used milling, it would have taken 6 days and cost 30% more.

What Is CNC Milling? (Best for Complex, Irregular Shapes)

Milling uses a rotating cutting tool to remove material from a stationary part. It’s like using a router to carve wood—you can make flat surfaces, holes, slots, and complex shapes (like brackets with multiple holes or curved edges).

When to Use Milling:

• Parts with irregular shapes: Aluminum brackets for car engines, plastic sensor housings, metal mounting plates with multiple holes.
• Complex features: Slots, pockets (recessed areas), or non-symmetrical holes (e.g., a bracket with holes at different angles).
• Parts that need precision in multiple directions: Milling machines (especially 5-axis mills) can cut from multiple angles, making parts that fit into tight spaces (like under a car’s dashboard).

Material Notes for Milling:

• Metal milling: 3-axis mills work for simple brackets; 5-axis mills for complex parts (like aerospace components). We adjust speed based on metal hardness—slower for stainless steel, faster for aluminum.
• Plastic milling: Ideal for complex plastic parts (like ABS sensor housings). We use air cooling and sharp tools to prevent warping. Glass-filled plastics (like glass-filled nylon) are great for milling—they’re strong and hold tolerances well.

A consumer electronics client needed 1,000 plastic housings for a car GPS. The housing had a curved edge, two holes for mounting, and a recessed area for the screen—perfect for 3-axis milling. We delivered all 1,000 in 5 days, with zero warping.

The Gray Area: When to Use Both (Hybrid Processes)

Some parts need both turning and milling. For example:

• A metal bolt: Turn the cylindrical shaft (turning), then mill the hexagonal head (milling).
• A plastic knob: Turn the round base (turning), then mill a slot for a screwdriver (milling).

The key here is to do the processes in the right order—usually turning first (to make the base shape), then milling (to add complex features). This saves time and ensures precision.

Honscn handles hybrid parts in-house—no need to send your part to two different shops. We’ll turn the base shape, then move the part to a milling machine (often on the same production line) to add the final features.

How Honscn Helps You Pick (and Execute) the Right Process

The biggest mistake we see clients make is picking a process based on “what they’ve used before” instead of “what the part needs.” A client might try to mill a round shaft (wasting time) because they’ve never used turning, or use turning for a complex plastic housing (ending up with a lopsided part).

Honscn takes the guesswork out of this with three key advantages:

1. Material-Process Matching (We Don’t Force a One-Size-Fits-All)

We start with your part’s design and material, then recommend the best process—no bias toward one machine or another. Here’s how it works:

• Step 1: Free Design Review: You send us your CAD file (even a sketch works), and we look at:
  • Material (aluminum? ABS? PEEK?)
  • Shape (round? Irregular? Symmetrical?)
  • Tolerances (±0.01mm? ±0.1mm?)
  • Quantity (10 prototypes? 10,000 production units?)
• Step 2: Process Recommendation: We tell you exactly which process to use, and why. For example:
  • “Your aluminum shaft is round—turning will be 30% faster and cheaper than milling.”
  • “Your ABS housing has curved edges and multiple holes—3-axis milling is the way to go, with air cooling to prevent warping.”
• Step 3: Cost & Timeline Estimate: We give you a clear quote and timeline, so you know what to expect.

A startup making a car diagnostic tool came to us with a design for a metal probe. They wanted to mill it (since they’d used milling for plastic parts before), but the probe was a simple cylindrical shape. We recommended turning—they saved 25% on cost and got the parts 2 days earlier.

2. In-House Equipment for Both Processes (No Outsourcing)

We have 12 CNC machines in total—6 turning centers (for rotational parts) and 6 milling machines (3-axis and 5-axis for complex parts). This means:

• No handoffs to other shops: If your part needs both turning and milling, we do it all in-house. You don’t have to coordinate with two suppliers or worry about parts getting lost in transit.
• Consistent quality: The same team handles your part from start to finish. They know the material’s quirks (e.g., “this ABS needs 5,000 RPM”) and the process’s requirements (e.g., “this aluminum shaft needs coolant at 20°C”).
• Fast turnaround: We can switch between processes quickly. For example, we made 200 hybrid parts (turned shaft + milled head) in 4 days—if we’d outsourced one process, it would have taken 8 days.

A car parts distributor needed 500 hybrid metal bolts (turned shaft + milled hex head). We did the turning first (2 minutes per shaft), then moved the bolts to a 3-axis mill (1 minute per head). All 500 were done in 3 days, with 100% quality checks.

3. Expertise in Tricky Materials (Plastics, Heat-Treated Metals, etc.)

We don’t just work with “easy” materials—we specialize in the ones that other shops avoid:

• Heat-sensitive plastics: We’ve mastered machining PEEK (which melts at 343°C/649°F) and PVC (which releases toxic fumes if heated too much). We use specialized ventilation and temperature controls to keep parts and workers safe.
• Hard metals: Stainless steel (316L) and titanium are tough to machine, but our 5-axis mills and carbide tools handle them with ease. We’ve made titanium brackets for car exhaust systems that hit ±0.005mm tolerances.
• Composite materials: Glass-filled nylon or carbon-fiber reinforced plastics (CFRP) are great for car parts (strong, lightweight), but they’re abrasive. We use diamond-coated tools to prevent wear—something most shops don’t offer.

A luxury car manufacturer needed 100 carbon-fiber reinforced plastic (CFRP) brackets for their door panels. Other shops turned them down because CFRP is abrasive and hard to machine. We used diamond-coated milling tools and slow speeds—all 100 brackets passed their tolerance checks, and the client now orders 1,000 every quarter.

Real Client Stories: How Honscn Fixed Process & Material Mistakes

Let’s look at two clients who started with the wrong process or material—until we stepped in.

Story 1: Auto Repair Shop (Wrong Process for Metal Shafts)

A Texas-based auto repair shop needed 200 metal shafts for a batch of vintage car transmission repairs. They’d hired a shop that used milling (since the shop didn’t have turning equipment) to make the shafts.

The Problem:

• Milling took 6 minutes per shaft (vs. 2 minutes with turning), so the shop missed their 5-day deadline.
• 30% of the shafts were lopsided (milling can’t get the same symmetry as turning), so they couldn’t fit into the transmissions.
• The cost was $15 per shaft (vs. $10 per shaft with turning)—they overpaid by $1,000.

Honscn’s Solution:

• We recommended turning (since the shafts were cylindrical, no complex features).
• We used a carbide turning tool and coolant to keep the aluminum shafts smooth.
• We made 200 shafts in 3 days (2 minutes per shaft) and checked each one for symmetry.

Results:

• 100% of the shafts fit into the transmissions.
• The shop saved $1,000 (cost dropped to $10 per shaft).
• They now order all their metal shafts from us—we’ve done 5 more batches, with zero defects.

Story 2: Electronics Client (Wrong Process for Plastic Housings)

A California electronics company needed 500 ABS plastic housings for a car’s backup camera. They’d used a shop that tried to turn the housings (since turning is faster for round parts).

The Problem:

• The housing had a square recess for the camera lens—turning can’t make square features, so the shop had to mill the recess separately (adding 3 minutes per part).
• 25% of the housings warped during turning (the shop used too high a speed: 8,000 RPM instead of 5,000 RPM).
• The total time was 8 days (vs. the 5-day deadline they needed).

Honscn’s Solution:

• We recommended 3-axis milling (since the housing had square features and needed precision in multiple directions).
• We used a polished carbide tool and air cooling (5,000 RPM) to prevent warping.
• We milled the entire housing in one setup—no need for a second process.

Results:

• 500 housings delivered in 4 days (beating the deadline).
• Only 2 housings were rejected (0.4% defect rate).
• The client saved $800 (no extra milling step meant lower labor costs).

How to Choose: A Quick Checklist for Your Part

Not sure if you need turning, milling, metal, or plastic? Use this checklist to narrow it down:

1. What’s the Shape of Your Part?

• Round/cylindrical/symmetrical → Turning (faster, cheaper).
• Irregular/has holes/slots/curved edges → Milling (more flexible).
• Both round and irregular → Hybrid (turning first, then milling).

2. What Material Are You Using?

• Metal (aluminum, stainless steel, brass):
  • Round part → Turning (use coolant, carbide tools).
  • Complex part → Milling (3-axis for simple, 5-axis for tricky).
• Plastic (ABS, PEEK, nylon):
  • Round part → Turning (slow speed, air cooling, polished tools).
  • Complex part → Milling (slow speed, air cooling, avoid liquid coolant).
• Composite (CFRP, glass-filled nylon):
  • Complex part → Milling (diamond-coated tools, slow speed).

3. What’s Your Tolerance?

• Tight (±0.01mm to ±0.05mm):
  • Metal → Turning or 5-axis milling (metal holds tolerances better).
  • Plastic → 5-axis milling with heat-treated plastic (e.g., PEEK).
• Loose (±0.1mm to ±0.5mm):
  • Metal → 3-axis milling or turning (either works).
  • Plastic → 3-axis milling (ABS or nylon is fine).

4. How Many Parts Do You Need?

• 10–500 parts (small batch):
  • Turning or milling (either works—pick based on shape).
• 1,000+ parts (mass production):
  • Turning (faster for round parts) or automated milling (for complex parts).

Conclusion: The Right Process = Better Parts, Lower Costs

Picking between metal and plastic, turning and milling, doesn’t have to be a guess. It’s about matching the material’s properties to the process’s strengths:

• Use turning for round parts (metal or plastic) to save time and money.
• Use milling for complex parts (metal or plastic) to get the shape you need.
• Let material hardness and heat tolerance guide your speed and tool choices.

Honscn’s job is to make this easy for you. We don’t just machine parts—we help you pick the right path from day one. Whether you’re making 10 metal shafts for a car repair or 10,000 plastic housings for a new product, we’ll recommend the best material, process, and tools to get the job done right (and on budget).

Ready to stop guessing? Send us your part design (even a sketch) and tell us what you need it for. We’ll give you a free process recommendation, a clear quote, and a timeline—no strings attached. Let’s make sure your next CNC project is a success.