CNC Machining FAQ

CNC (Computer Numerical Control) machining is a subtractive manufacturing process that employs computerized controls and machine tools to remove layers of material from a stock piece (workpiece) to produce a custom-designed part. Unlike additive manufacturing (3D printing) or formative technologies (injection molding), CNC machining creates parts by cutting away material from a solid block. It is the primary manufacturing method for metal and plastic parts requiring high precision, tight tolerances, and excellent physical properties.

The technology originated in the 1940s and 1950s. The first numerical control (NC) machine was developed by John T. Parsons in collaboration with MIT in 1949, initially designed to manufacture complex helicopter blades. By the late 1950s, NC machines evolved into CNC (Computer Numerical Control) with the integration of digital computers. This evolution standardized the use of G-Code as the universal programming language for machine tools.

The CNC manufacturing workflow consists of three primary digital stages:

1. CAD (Computer-Aided Design): The creation of a 2D vector or 3D solid model of the part.

2. CAM (Computer-Aided Manufacturing): Software that converts the CAD model into toolpaths and generates the machine code (G-Code).

3. Setup and Machining: The code is uploaded to the CNC machine, which executes the programmed movements to cut the raw material.

These are the programming languages used to control CNC machines, standardized under ISO 6983.

• G-Code (Geometric Code): Instructs the machine how to move (e.g., G01 for linear interpolation, G02 for circular interpolation).

• M-Code (Miscellaneous Code): Controls auxiliary machine functions (e.g., M03 to start the spindle, M08 to turn on coolant).

Tolerance standards for CNC machining are typically defined by ISO 2768-1.

• Standard Tolerance (Medium – m): Generally ±0.125 mm (±0.005″). This is the industry standard for features without specific tolerance annotations.

• Precision Tolerance (Fine – f): For critical features (e.g., bearing fits), CNC machines can achieve ISO IT7 grades or tighter, ranging from ±0.005 mm to ±0.05 mm, depending on machine rigidity and thermal stability.

The difference lies in the Degrees of Freedom:

• 3-Axis: The cutting tool moves along the X, Y, and Z linear axes. It is suitable for planar milling and drilling.

• 5-Axis: In addition to X, Y, and Z, the machine adds two rotary axes (A and B axes). This allows the tool to approach the workpiece from any angle.

  • Advantage: 5-axis machining enables the production of complex geometries (like impellers) and reduces setup time by machining five sides of a part in a single operation (3+2 machining).

Machinability is a measure of how easily a material can be cut by a machine tool. It is often rated against a baseline steel (AISI B1112) which is assigned a rating of 100%.

• High Machinability: Aluminum 6061 and Brass C360 (Rating > 100%). These allow for high cutting speeds and produce excellent surface finishes.

• Low Machinability: Titanium Ti-6Al-4V and Inconel. These materials generate high heat and cause rapid tool wear, requiring specialized tooling and slower feeds.

• Climb Milling (Down Milling): The tool rotates in the same direction as the feed. The chip thickness starts at maximum and decreases to zero. This is the preferred method for CNC finishing as it reduces heat generation and improves surface quality.

• Conventional Milling (Up Milling): The tool rotates against the feed direction. The chip thickness starts at zero and increases. This is typically used only for machining rough castings or forgings to avoid surface hardening.

Surface roughness quantifies the texture of the machined surface. The most common metric is Ra (Roughness Average), measured in micrometers (μm).

• As-Machined Standard: Typically Ra 3.2 μm (125 μin). Visible tool marks are present.

• Fine Machined: Ra 1.6 μm to 0.8 μm (63-32 μin). Requires slower finishing passes.

• Polished/Ground: Ra < 0.4 μm. Usually requires post-processing operations like grinding or polishing.

While highly versatile, CNC machining has geometric constraints:

1. Undercuts: Standard tools cannot reach areas underneath a surface (overhangs) without special cutters (e.g., T-slot cutters) or multi-axis re-positioning.

2. Internal Corner Radii: Because cutting tools are round, CNC machining cannot produce perfectly sharp internal vertical corners. A radius matching the tool diameter will always remain.

3. Material Waste: As a subtractive process, it generates significantly more waste (chips/swarf) compared to additive manufacturing or casting.

Machining stainless steel typically costs 20% to 100% more than aluminum (like 6061) due to three main factors:

1. Machinability: Stainless steel is harder and tougher. CNC machines must run at slower RPMs and feed rates to cut it effectively, meaning the same part takes longer to produce (higher machine time costs).

2. Tool Wear: It is abrasive and generates high heat, causing cutting tools (end mills/drills) to wear out much faster, increasing tooling expenses.

3. Raw Material: The raw material cost of stainless steel (especially 316 grade) is generally higher than standard aluminum alloys.

• Switch Grades: If corrosion resistance isn’t critical, switch from 304/316 to Stainless Steel 303. It contains sulfur for better machinability, which can reduce machining time by up to 25%.

• Relax Tolerances: Avoid specifying tight tolerances (e.g., +/- 0.01mm) unless absolutely necessary, as hard metals are time-consuming to finish precisely.

• Avoid Deep Pockets: Deep cavities in stainless steel require long tools prone to vibration (chatter), slowing down the process significantly.

• SS 303 (The Machinist’s Choice): Best for non-critical parts like fittings, shafts, and nuts. It is the easiest to machine but has lower corrosion resistance and toughness.

• SS 304 (The Standard): The most common “18/8” stainless steel. Excellent corrosion resistance and weldability. Ideal for kitchenware, automotive, and general industrial parts. It is tougher to machine than 303.

• SS 316 (The Marine Grade): Contains Molybdenum for superior resistance to chlorides and acids. Essential for marine environments, chemical processing, and medical devices. It is the hardest to machine of the three.

Yes, we specialize in machining 17-4 PH (Precipitation Hardening) stainless steel.

• Why use it: It offers a unique combination of high yield strength (up to 1100-1300 MPa) and good corrosion resistance (comparable to 304).

• Heat Treatment: We usually machine it in the annealed (Condition A) state and can then heat treat it (e.g., H900) to achieve maximum hardness (HRC 40-47). It is widely used in aerospace components and medical instruments.

The “L” stands for “Low Carbon”.

• SS 316: Standard carbon content. Good for general use.

• SS 316L: Lower carbon content (<0.03%). The lower carbon prevents harmful carbide precipitation during welding. Choose 316L if your parts will be welded to ensure maximum corrosion resistance at the weld joints.

It depends on the crystal structure:

• Austenitic Grades (304, 316): generally Non-Magnetic in their annealed state. However, the CNC machining process (cold working) can induce slight magnetism in 304.

• Martensitic/Ferritic Grades (400 series, 17-4 PH): These are Strongly Magnetic.

• Tip: If you require a strictly non-magnetic part (e.g., for MRI equipment), please specify this so we can perform post-machining annealing.

Stainless steel has a tendency to harden instantly when it is cut or deformed, making the surface harder for the next cutter pass. To prevent this, our CNC machinists use:

• Sharp Tooling: We change tools frequently to avoid rubbing.

• Constant Feed: We never let the tool dwell in one spot.

• Flood Coolant: High-pressure coolant keeps the temperature down to prevent thermal hardening.

Passivation is a chemical process (using nitric or citric acid) that removes free iron from the surface of machined stainless steel parts.

• Why do it: Although stainless steel is corrosion-resistant, machining can embed microscopic iron particles from the cutting tools into the surface, which can rust. Passivation removes these particles to restore the material’s natural corrosion resistance.

• Recommendation: Highly recommended for medical, food, and marine parts.

Yes. Electropolishing is an electrochemical process that removes surface material on a microscopic level. It smoothens the surface peaks and valleys, creating a bright, mirror-like finish (Ra < 0.4μm) that is ultraclean and resistant to bacterial growth. It is the gold standard for pharmaceutical and semiconductor components.

Absolutely. SS 304 is the industry standard for food contact surfaces (“18/8” stainless). For salty or acidic foods (like tomato sauce or brine), we recommend SS 316 for its superior resistance to pitting corrosion. We provide material certifications to prove the chemical composition meets compliance.

generally, No. Stainless steel requires high torque, low RPM, and extreme machine rigidity to cut effectively. Most desktop routers are designed for wood or soft aluminum and will suffer from “chatter” (vibration) or stall when cutting steel. Why choose Boona: Our industrial 3-axis and 5-axis CNC mills are built to handle the high cutting forces of stainless steel, ensuring precise tolerances and smooth finishes that desktop machines cannot achieve.

Galling (or “cold welding”) is a common issue where stainless steel threads seize up during assembly. To prevent this, Boona takes specific precautions:

• Material Pairing: We advise using dissimilar grades for mating parts (e.g., a 316 bolt with a 304 nut) to reduce friction compatibility.

• Surface Finish: We ensure threads are cut cleanly with sharp tools to minimize surface roughness.

• Hardness: Using harder grades (like 17-4 PH) or specialized coatings can also reduce the risk.

Deformation in thin walls (especially Aluminum & Delrin) is caused by residual stress release. To prevent this, we use a specific technical workflow:

• Roughing & Resting: We remove the bulk of material and let the part “rest” for 12-24 hours to naturally release stress.

• Low Clamping Force: Using soft jaws or vacuum fixtures to avoid squeezing the part out of shape.

• Stress Relief Annealing: For critical parts, we perform heat treatment before final finishing passes.

For standard CNC drilling, we recommend a depth-to-diameter ratio of 10:1 or less to ensure straightness and prevent drill breakage.

• If your design requires deeper holes (e.g., 20:1 or 30:1), we utilize Gun Drilling techniques or EDM (Electrical Discharge Machining) to maintain accuracy and concentricity.

Anodizing adds material to the surface.

• Type II (Standard): Adds approx. 5-10µm (0.005-0.01mm) per side.

• Type III (Hard): Adds approx. 25-50µm (0.025-0.05mm) per side.

• The Boona Solution: If you have tight tolerances (e.g., H7 hole), you must inform us. We will machine the feature slightly oversized (compensation) so that after anodizing, it shrinks back to the perfect dimension.

CNC milling tools are round and rotate. They inevitably leave a radius in internal corners matching the tool’s size.

• DFM Tip: Design internal corners with a radius (e.g., R=1mm or larger).

• Workaround: If a perfectly square corner is required for a mating part, we can use a “Dog-bone” fillet (over-cutting the corner) or use EDM machining to square it off.

• Cut Threads: Traditional method where material is removed. Good for most applications but creates weaker threads in soft metals.

• Form (Roll) Threads: Material is displaced/compressed rather than cut. This creates much stronger threads (hardening the material) and produces no chips. We highly recommend Form Taps for small threads (M2-M6) in Aluminum to prevent stripping.

Our standard turnaround for prototyping (1-10 parts) is 3-5 business days. For low-volume production (100-1,000 parts), it typically takes 10-15 days. We also offer an Expedited Service (Rush Order) where simple parts can be shipped in as fast as 24 hours. (👉 For detailed shipping options, please check our full Delivery FAQ.)

We have No MOQ requirements. Whether you need a single prototype to verify your design or 1,000+ production units, Boona Prototypes is equipped to handle it. We specialize in “High-Mix, Low-Volume” manufacturing, making us the ideal partner for startups and R&D verification.

Yes. Quality transparency is vital. Upon request, we provide full documentation packages with your shipment, including:

• Material Certificates (COA): Proving the chemical composition of the metal or plastic.

• Dimensional Inspection Reports: Using our in-house CMM to verify critical tolerances.

• First Article Inspection (FAI): Provided for production orders before the full batch run.

Yes. To deliver “production-ready” parts, we perform post-machining operations including:

• Insert Installation: Installing stainless steel Helicoils or Keenserts into aluminum/plastic parts to reinforce threads.

• Sub-Assembly: Fitting pins, bushings, or mating parts together.

• Kitting: Packaging sets of matched parts for your assembly line convenience.

CNC is the most cost-effective method for quantities under 500-1,000 units. If your volume exceeds 1,000-2,000 parts, we recommend switching to Pressure Die Casting (for aluminum/zinc). While Die Casting has higher upfront mold costs, the per-unit price is significantly lower (often saving 60-80%). We can guide you through this transition as your product scales.