Introduction: The Most Common Machining Mistake Buyers Make
The most common mistake buyers make when sourcing machined parts is assuming that all machining is the same. They send a drawing to a machine shop and ask for a price — without specifying or even thinking about which machining process is most appropriate for the part.
The result is that the manufacturer uses whatever process they have available, not necessarily the one best suited to the part. This can mean unnecessarily high cost (wrong process for the volume), poor dimensional accuracy (wrong process for the geometry), or inconsistent quality (wrong process for the material).
Nathan Engineering, as a full-capability manufacturer of machined parts in India, offers this guide to help buyers understand which machining process is right for their specific part — and why. The right process choice makes the difference between a good component at a good price and an expensive, marginal one.
The Four Main Machining Processes: When to Use Each
1. CNC Turning — for rotational parts
CNC turning is the correct process for any part that is fundamentally cylindrical: shafts, spindles, bushings, spacers, flanges, and housings with circular cross-sections. The workpiece rotates in a chuck while a stationary cutting tool moves along two axes (X for diameter, Z for length) to produce the required geometry.
Use CNC turning when:
- The part is fundamentally cylindrical or has multiple concentric diameters
- Internal and external threads are required
- Bores need to be precision-sized to a specific fit (H7, H6, or tighter)
- The diameter-to-length ratio exceeds 2:1 (long parts are more efficiently turned than milled)
- High-volume production of turned components justifies CNC automation
2. VMC Milling — for prismatic (non-round) parts
VMC (Vertical Machining Centre) milling is the correct process for parts that are not round: housings with flat faces, brackets, plates, manifold blocks, and any component with pockets, slots, angled surfaces, or hole patterns. The workpiece is held stationary while a rotating cutting tool moves in three or more axes.
Use VMC milling when:
- The part has flat faces, pockets, slots, or non-circular features
- Multiple holes need precise relative positioning (a bolt circle, for example)
- The part has features on multiple faces that must be in precise relationship
- Complex three-dimensional contours are required (using 4- or 5-axis VMC)
3. Grinding — for ultra-precision and hardened surfaces
Grinding uses abrasive wheels to remove very small amounts of material — achieving tolerances and surface finishes that no cutting operation can match. Grinding is applied after hardening to correct the distortion heat treatment causes, and to achieve the extreme precision required for bearing seats, gauge blocks, and precision instrument components.
Use grinding when:
- Tolerances tighter than ±0.005 mm are required on diameter or flatness
- Surface roughness below Ra 0.4 µm is required on a functional surface
- The part must be ground after heat treatment to restore dimensional accuracy
- Concentricity between multiple diameters must be within 0.002 mm or better
4. EDM (Electrical Discharge Machining) — for hardened and complex tool steels
EDM erodes material using electrical discharges rather than cutting. It can machine hardened tool steel, exotic alloys, and complex profiles that conventional cutting tools cannot reach. Nathan Engineering uses EDM for specific tooling and mould component applications where conventional machining is not feasible.
Process Selection by Part Type: Quick Reference
Shaft with multiple stepped diameters: CNC turning primary. Add grinding if bearing fits or tight concentricity are required.
Aluminium housing with pockets and bolt holes: VMC milling. CNC turning for any cylindrical boss features.
Stainless steel pump body with bored ports and threaded connections: VMC milling for body features. CNC turning for port bores if cylindrical.
Hardened steel gear: CNC turning for blank. Gear hobbing for teeth. Grinding after hardening for bore and datum face.
Thin aluminium plate with precision hole pattern: VMC milling. CMM inspection of hole position.
Plastic bushing for low-friction sliding application: CNC turning from POM rod stock.
Brass electrical connector pin: CNC turning from free-machining brass bar. High-volume CNC automatic lathe for quantities above 10,000.
Volume Thresholds: When CNC Automation Changes the Economics
Prototype and low volume (1–50 pieces)
Manual setup with standard CNC programmes. Per-piece cost is high due to setup time spread over few parts. Lead time is typically 5–10 working days.
Low-to-medium volume (50–500 pieces)
Optimised CNC programmes with work-holding fixtures for faster changeover. Per-piece cost drops significantly from prototype pricing. Lead time 5–15 working days.
Medium to high volume (500–10,000 pieces)
Dedicated tooling, optimised cutting parameters, and in-process gauging. Per-piece cost at its most competitive for batch machining. Bar-fed CNC lathes for turned components at this volume.
High volume (10,000+ pieces)
Fully automated CNC cells with robotic part loading and unloading. 24-hour production with minimal labour cost per part. Nathan Engineering discusses automation investment for specific high-volume programmes.
Common Machining Failures and How Nathan Engineering Prevents Them
Chatter marks on surface: Caused by insufficient rigidity in the machining setup. Prevented by proper workholding design, correct depth of cut and feed, and sharp tooling.
Dimensional drift across a batch: Caused by tool wear or thermal growth. Prevented by in-process measurement at defined intervals and tool wear offset corrections.
Burrs on machined edges: Caused by incorrect cutting edge geometry or worn tooling. Prevented by sharp tooling, correct cutting parameters, and deburring operations where required.
Poor surface finish on stainless steel: Caused by work-hardening from rubbing rather than cutting. Prevented by correct cutting speed, sharp inserts, and generous flood coolant.
Hole position out of tolerance: Caused by tool deflection or workpiece movement. Prevented by rigid workholding, pilot drilling before reaming, and CMM verification of hole positions.
Frequently Asked Questions
Q: Can you combine turning and milling on the same component? Yes. Nathan Engineering’s turn-mill capability (on machines with both rotating spindle and C-axis indexing) enables complex components with both turned and milled features to be produced in a single setup — eliminating repositioning errors.
Q: Do you offer reverse engineering from a sample? Yes. Send a sample part with any available documentation and Nathan Engineering will measure the component, generate a drawing if none exists, and provide a production quotation.
Q: What is the largest component you can machine? VMC work envelope: approximately 800 mm × 500 mm × 500 mm. CNC turning up to 400 mm diameter × 1,000 mm between centres. Larger components can be quoted on request.
Contact Nathan Engineering
- Email: nathan@nathanengineering.co.in
- Phone: +91 93601 75927
- Website: www.nathanengineering.in
Submit your drawing and annual volume for a process-appropriate quotation within 24–48 hours.
