How to Do CNC Milling Costing: A Detailed Step-by-Step Guide to Accurate Estimates

How to Calculate the Cost of a CNC Machined Component

Pricing a CNC machined component can feel like a puzzle, but it’s simpler than it seems when broken into steps. Here’s how you can calculate costs with ease:

1. Choose the Right CNC Machine: Match the machine to the geometry, tolerances, and surface finish requirements of the component.
2. Define Raw Material Shape and Size: Select the most appropriate material shape and standard size to minimize waste.
3. Add Allowances: Include machining allowances for achieving the desired surface finish and accuracy.
4. Plan Setups and Operations: Identify the number of setups and the sequence of operations for efficient machining.
5. Select Tools and Parameters: Choose tools and cutting parameters based on material and features to be machined.
6. Account for Auxiliary Tasks: Factor in time for tool changes, work offsets, and idle movements.
7. Calculate Total Cost: Combine material costs, machine hourly rates, and machining times to estimate the final cost.

This guide will walk you through these steps in detail, helping you understand CNC costing and how to optimize it.

1. Selecting the Right CNC Machine

Choosing the appropriate machine is the foundation of CNC costing. A single part can often be machined using multiple types of machines, but the goal is to select the optimum machine that:

• Minimizes manufacturing cost.
• Maintains ease of machining.
• Produces quality aligned with the tolerances and surface finish defined in the drawing.
  
  

Understanding Machine Classifications

• Turning Machines (Lathes): Best suited for parts with multiple coaxial circular features, such as shafts and cylindrical components.
• Milling Machines: Ideal for parts with features spread across multiple planes, such as holes, pockets, or complex profiles.
• Turn-Mill Machines: Combine turning and milling operations in one setup, ideal for parts with a mix of features.
• Advanced Machines (4-axis, 5-axis): Handle intricate geometries requiring machining from multiple angles in a single setup, often used for aerospace and precision engineering parts.
• Sliding Head Machines: Effective for machining small and long parts with tight tolerances.
  
  Part Feature Analysis
  ‍Analyse the part’s geometry and tolerances to decide the machine type:

• For coaxial features, a lathe is the primary choice.
• For features on different faces, a milling machine is generally better.
• Consider tolerances and surface finish requirements specified in the drawing. Tight tolerances may necessitate higher-precision machines.
  Example: A part with threaded holes on a cylindrical face might require a turn-mill machine. However, if the tolerances for the threaded hole are stringent, a separate milling operation may be needed for better accuracy.
  
  

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Machine Hourly Rates

The hourly rate for a machine is influenced by several factors:

• Capital Investment and Complexity: Machines with advanced features or precision capabilities command higher rates. Machines like simultaneous 5-axis machining have higher hourly rates due to their capabilities.
• Size: Larger tables or spindles increase the cost due to higher capital investment.
• Precision: Machines delivering finer surface finishes and tighter tolerances incur higher costs.
• Operating Costs: Power consumption, maintenance, and tooling expenses contribute to the rate.
• Market Availability: Machines with limited market presence tend to have higher service costs.

Pro Tip: Match the machine's capabilities to the part’s requirements to optimize CNC machining cost.

2. Defining Raw Material Shape and Size

After selecting the machine, the next step is to define the raw material's shape and size. This is critical for controlling both material costs and machining costs, while minimizing waste.

Available Raw Material Shapes

• Cylindrical Rods
• Rectangular Blocks or Plates
• Hollow Pipes: Useful for parts with internal voids.
• Hexagonal Rods: Often chosen for specific geometries like hexagonal bolts.

Selecting the appropriate raw material shape directly impacts the machining cost breakdown.
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3. Calculating Material Dimensions and Stock Allowances

The dimensions of the raw material depend on the finish size of the part, with added machining allowances to account for irregularities.

Why Allowances Are Necessary

Raw materials from rolling, extrusion, forging, or casting processes often have:

• Rough surface finishes.
• Dimensional variations outside drawing tolerances.

Formula for Material Cost:
Material Cost = Weight (kg) × Rate (₹/kg)

Tip: Use a machining cost calculator to quickly estimate material costs.

4. Determining Setups and Sequence of Operations

The first step is to figure out how many setups are required and the sequence in which features will be machined.

Setup Planning

• For small rectangular blocks, use a standard vice for clamping.
• For larger materials, consider clamping arrangements directly on the machining bed.
• For higher production volumes, custom jigs and fixtures may be used to reduce setup time.
  
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5. Tool Selection

Tool Selection Criteria

• Type of Cut: Example: The first operation is often facing, requiring a face mill cutter.
• Feature Size: The tool diameter should be smaller than the width of the cut to ensure continuous tool engagement and reduce wear.
• Material Compatibility: Use carbide for hard metals, HSS for softer materials.

Choosing the right tools is critical for managing CNC machining time estimation.

6. Machining Parameters and Time Calculations

Once the tools are selected, determine the cutting parameters and calculate machining times.

Parameters to Consider

• Cutting Speed: Determined by the material and tool combination.
• Feed Rate: The speed at which the tool advances into the material.
• Depth of Cut: Based on the feature dimensions and tool capability.
  
  

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7. Auxiliary Operations

Auxiliary operations contribute significantly to the overall machining time and must be accounted for.

Key Auxiliary Operations

• Work and Tool Offsets: Aligning the raw material on the machine to ensure proper positioning.
• Non-Cutting Movements: Includes tool reorientation and positioning.
• Loading and Unloading Time: Time taken to mount and dismount the raw material from the machine.

What if You Could Use AI for CNC Costing?

What if calculating CNC machining costs didn’t involve juggling Excel sheets and endless formulas? What if AI in CNC costing could simplify the entire process, saving you time and effort?

Enter ACE (Automated Costing Engine)—a revolutionary tool that takes the guesswork out of CNC costing. With ACE, you can upload your CAD files and receive detailed cost estimates for machining time, setup costs, and material allowances in just seconds.

ACE doesn’t just provide you with instant quotes; it generates a detailed CNC costing report that breaks down material costs, machining parameters, auxiliary operations, and more. To understand how this report can provide actionable insights and help optimize your manufacturing process, explore our detailed guide to CNC costing reports and see the value it offers.

FAQ Section

How to calculate raw material costs for CNC parts?

Calculate the raw material volume, multiply by density to get weight, and multiply the weight by the material’s rate.

What influences CNC machining costs the most?

Machine hourly rates, material costs, setup times, and auxiliary operations are the primary cost drivers.

What is a CNC costing report?

A CNC costing report details material costs, machining time, setup costs, and auxiliary tasks for accurate cost estimation.

‍Final Thoughts

CNC costing is an iterative process that balances machine capabilities, raw material selection, and machining allowances to achieve cost-effective and high-quality production. By analyzing part geometry, understanding material availability, and considering the drawing’s tolerances, you can optimize the costing process and improve overall efficiency.

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