CNC Guide

What CNC Actually Is

  • CNC (Computer Numerical Control):

    • A machine that manages the following processes:

    • Takes a CAD model

    • Uses CAM software to generate toolpaths

    • Cuts material using a spinning endmill

    • In FTC (FIRST Tech Challenge), it is primarily a CNC mill rather than a lathe.

    • Materials commonly cut include:

    • Aluminum

    • Polycarbonate

    • Delrin (acetal)

    • Sometimes wood or HDPE.

Why CNC Is Huge in FTC

  • CNC provides several critical advantages:

    • Precise hole alignment: Essential for shafts and bearings.

    • Custom plates: Upgrading standard components.

    • Weight pocketing: Reducing material for weight efficiency.

    • Clean edges: Enhancing the aesthetic and functional properties.

    • Perfect motor mounts: Ensuring reliability and performance.

  • Elite FTC teams typically CNC their side plates, distinguishing high-quality builds from ones simply bolted together.

CNC Workflow (Step by Step)

  • The following steps outline the CNC milling process:

    1. CAD the part:

    • Ensure full constraints are applied.

    • Maintain accurate dimensions and hole spacing.

    1. Export the Design:

    • Save in formats such as .STEP or .DXF.

    1. CAM Setup: In CAM software (usually Fusion 360):

    • Select tool (determining endmill size).

    • Set spindle speed (speed of cutter rotation).

    • Set feed rate (speed of tool movement through material).

    • Define cutting depth.

    • Generate toolpaths that guide cutting.

    1. Machine Setup:

    • Clamp material securely to prevent movement during cutting.

    • Zero the machine in X, Y, Z dimensions.

    • Install the correct endmill based on design needs.

    1. Run the Job:

    • Monitor the cut process closely.

    • Clear chips to maintain visibility and prevent clogs.

    • Listen for chatter, adjusting settings as necessary.

Common CNC Tools (You Should Know These)

  • Endmill: The principal tool used for cutting.

    • Types of endmills include:

    • Flat endmill: Most commonly used for standard cuts.

    • Ball endmill: Ideal for creating curves and rounded edges.

    • Chamfer bit: Used for creating angled edges.

    • Typical sizes in FTC:

    • 1/8 inch

    • 1/4 inch

    • Notably, a smaller bit allows for more detail but results in a slower cutting speed.

Key CNC Concepts (They Might Ask These)

  • What is a toolpath?

    • The programmed path that the cutter follows during machining.

  • What is the feed rate?

    • The speed at which the tool moves horizontally through the material.

  • What is spindle speed?

    • The rate at which the cutter spins, measured in RPM (revolutions per minute).

Effects of Incorrect Settings

  • If the feed rate is too fast:

    • Leads to tool chatter, resulting in poor finish quality.

    • Increases the chance of a broken bit.

  • If the spindle speed is too low:

    • Produces rough cuts and can prematurely wear the tool.

    • Results in an unfavorable surface finish.

  • What is step-down?

    • Refers to how deep the tool cuts per pass.

    • Smaller step-down values lead to safer cuts with less risk of damaging the piece.

Workholding (Very Important)

  • Important considerations related to workholding include:

    • Material must be:

    • Clamped securely to ensure stability.

    • Flat against the spoilboard to provide a solid foundation.

    • Maintained without vibration to prevent compromised cuts.

  • Movement of the material can result in a ruined part, emphasizing the necessity of proper setup.

CNC Design Rules for FTC

  • Best practices for designing CNC parts include:

    • Avoid super thin sections: These may compromise structural integrity.

    • Avoid sharp internal corners: Endmills typically have rounded edges which cannot cut sharp angles.

    • Add fillets wherever possible to reduce stress concentrations.

    • Make holes slightly oversized for clearance to accommodate tool size and material expansion.

    • Think about tool accessibility: Ensure tools can reach all parts of the design to prevent uncut sections.

Pocketing

  • Pocketing: The process of removing material inside a plate to reduce weight without sacrificing strength.

    • Reasons for pocketing:

    • Weight reduction for improved efficiency.

    • Maintain structural strength.

    • Achieve a clean appearance.

    • Areas to avoid pocketing include:

    • Around bearing mounts.

    • Around motor mounts.

    • Near areas experiencing high torque pivoting.

Common FTC CNC Parts

  • Typical parts cut using CNC in FTC include:

    • Drive side plates

    • Arm plates

    • Motor mounts

    • Bearing plates

    • Intake plates

  • High-end teams may also CNC their drivetrain rails for enhanced performance.

If They Ask: Why CNC Instead of 3D Print?

  • CNC advantages over 3D printing:

    • CNC aluminum is stronger, more rigid, better for high torque applications, and efficacious in heat management.

  • 3D printing advantages:

    • Faster prototyping, handling complex shapes, and achieving lightweight designs.

  • The best teams typically utilize both methods to capitalize on their respective strengths.

Common CNC Mistakes

  • Frequent errors encountered during CNC processes include:

    • Not accounting for tool diameter in designs.

    • Forgetting to zero the Z-axis correctly before cutting.

    • Cutting too deep too quickly, leading to desired part dimensions being compromised.

    • Designing holes that are too tight, resulting in poor fit.

    • Weakening parts through excessive pocketing, leading to failure under stress.

What You Actually Need to Understand about CNC

  • Essential knowledge areas for operating CNC machinery include:

    • Understanding why precision matters in manufacturing.

    • Familiarity with basic terminology related to CNC.

    • Safety awareness regarding machine operation.

    • Knowledge of when CNC processes are appropriate, as well as how to effectively design parts for it.

  • When providing advice, consider the following questions relevant to CNCing a drivetrain plate:

    • What is needed for hole alignment for shafts?

    • How can bearing support strength be ensured?

    • What methods will allow for weight reduction without weakening load zones?

    • What is the appropriate material thickness for durability?

    • How does tool accessibility affect design viability?

  • The depth of understanding demonstrated indicates proficiency in mechanical design principles.