Week 7: Computer-controlled machining

Assignment:

Group assignment: do your lab's safety training. test runout, alignment, fixturing, speeds, feeds, materials, and toolpaths for your machine.

Individual assignment: Make (design+mill+assemble) something big (~meter-scale)- Extra credit: don't use fasteners or glue - Extra credit: include curved surfaces

The planing of the week

Day	Activity
WED	Review and the class with Neil
THU	Group Assigment
FRI	Individual assignment
SAT	Group Assigment
SUN	Documentation
MON	Final Project
TUE	Documentation and details

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🔎 Group Assignment Summary

➡️ Read the Full Group Assignment

🔧 Lab Safety Training

Before operating the CNC machine, it was crucial to complete a safety training session. The CNC milling machine is a powerful tool, and improper use can lead to injuries, tool breakage, or material damage.

CNC Machine Overview & Controls

Router bit

The choice of router bit determines the cutting efficiency and final surface quality. We tested several types of bits: Common Router Bits & Their Applications

• Spiral Upcut BitThis bit is great for removing material quickly and keeping the cut cool. It pulls the chips upward, which helps clear debris but can also cause rough edges on the top surface. It works best for plastics, soft metals, and deep cuts, where chip evacuation is important.
• Spiral Downcut Bit Unlike the upcut bit, the downcut bit pushes chips downward, leaving a cleaner top surface. This is ideal for wood, plywood, and laminates, where you want smooth edges on the visible side. However, it can trap chips in deeper cuts, which might generate heat and affect performance.
• Straight BitA straight bit cuts evenly in all directions, making it perfect for pocketing and simple profile cuts. It doesn’t create a pulling effect like spiral bits, which helps prevent material tear-out. This bit works well for MDF, softwood, and plastics when a clean and stable cut is needed.
• Compression BitA combination of upcut and downcut geometry, this bit is ideal for plywood and veneered materials. It pulls up from the bottom and pushes down from the top, leaving both surfaces clean and smooth. This makes it perfect for materials that tend to chip or splinter. Each of these bits has its own strengths, and selecting the right one depends on the material, the type of cut, and the final finish you want. By understanding their behavior, we can achieve cleaner cuts, faster production, and better overall results in our CNC projects.
• End Mill Designed for precision and high accuracy, end mills are commonly used in metal, acrylic, and composite materials. They provide smooth, controlled cuts and are great for detailed profiles and deep slots. Unlike typical wood-cutting bits, end mills have sharper edges and can handle harder materials.

Source: ToolsToday Learning Desk

Ensuring CNC Precisiong

Runout: Checking Tool & Spindle Accuracy

Before diving into machining, we performed a runout test directly on the CNC router to check if the spindle and tool were properly aligned. Runout happens when the cutting tool doesn’t rotate perfectly centered, causing wobbling. This can lead to inconsistent cuts, uneven slot widths, rough edges, and faster tool wear.
We ran a test by cutting parallel slots into the material at different feed rates and speeds to observe any variations in cut accuracy and edge finish. The image below shows the results, where we measured slot widths and noted speed settings. Our findings confirmed that runout on this CNC machine was minimal, meaning the spindle and collet were well-calibrated.

Dial Indicator Test

Why Did We Also Use a Dial Indicator?

Even though the CNC’s runout was minimal, we wanted to better understand how misalignment impacts precision, so we performed a secondary test using a dial indicator on a smaller milling machine. This allowed us to visualize how small tool or spindle misalignments affect precision.
By rotating the spindle manually and observing the needle movement on the dial, we saw how even slight shifts in alignment could impact cut quality. This reinforced the importance of checking tool seating and spindle calibration before machining.

Speeds and Feeds

Understanding Feeds, Speeds, and Chip Load

One of the most crucial aspects of CNC machining is setting the correct feed rate, spindle speed, and chip load to achieve clean, efficient cuts while preventing tool damage.
These parameters determine how fast the tool moves through the material, how deep each pass should be, and how much material is removed per revolution.
We used the spreadsheet calculator from “Calculating Feeds and Speeds – A Practical Guide | Wood CNC Router” by Cutting It Close to determine the optimal settings for our tool and material.

Chip Load Calculation Formula

The chip load is the thickness of the material removed by each cutting edge of the tool. Proper chip load values help ensure that the tool doesn’t overheat or wear out too quickly. We calculated our feed rate using the following formulas:

Feed Rate Formula:

If you’d like to reference or modify the calculations, you can download the feed rate spreadsheet here:

• Download Feed Rate Calculator

Vacuum Table & Sacrificial Board

How the Vacuum Table Works

A vacuum table uses negative pressure to hold down the material during machining. The principle behind it is simple: atmospheric pressure pushes down on the material while the vacuum pump removes air from beneath it, creating a pressure difference that keeps the workpiece firmly in place.
How Strong is the Hold? The force holding the material depends on the vacuum pressure and the surface area of the material. A larger material has more surface area in contact with the vacuum, increasing the holding force.
See the diagram below for a visual explanation of how vacuum tables generate hold-down force
See the diagram below for an illustration of cutting forces acting on the material

Source: Mekanika.io

Factors Affecting Vacuum Strength:

• Material surface area Larger pieces experience stronger suction.
• Leaks or gaps in material Air leaks reduce the vacuum’s effectiveness, so materials with holes or rough surfaces may need additional sealing
• Vacuum pump power A higher-pressure vacuum pump creates a stronger hold.

Sacrificial Board

• Protecting the CNC Table: Since CNC operations often involve cutting all the way through the material, the bit can cut slightly into the machine’s surface. A sacrificial board absorbs these cuts, preventing permanent damage to the actual bed.
• Enhancing Vacuum Hold Some materials, like plywood, have tiny air gaps that weaken the vacuum suction. A sacrificial board—typically MDF—creates a better seal between the vacuum table and the material, preventing air leaks and ensuring a stronger hold.
• Allowing Full-Depth Cuts Without Worry With a sacrificial board in place, users can confidently set their toolpaths to cut slightly beyond the actual material thickness, ensuring clean cuts without worrying about damaging the machine

Source: Cutting It Close "Calculating Feeds and Speeds A Practical Guide | Wood CNC Router"

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Individual assignment

Introduction:

For this week’s assignment, I designed, fabricated, and assembled a coffee table using a parametric design approach and CNC milling. The challenge was to create a structure that requires no screws or glue, relying entirely on press-fit joints for stability.

Designing the Table in Fusion 360

I started by designing the table in Fusion 360, ensuring that the model was parametric, meaning that the slot dimensions could be adjusted based on the actual thickness of the material.

• Base Structure: The table consists of interlocking legs that slot into each other, forming a stable base.
• Top Surface: A circular tabletop with precisely positioned slots that fit into the legs.
• Parametric Adjustments: The slot widths were defined using user parameters, so if the material thickness changed, the design could be updated instantly without manually modifying each slot.

To prepare the design for CNC milling, I applied Dogbones to all internal corners, allowing the parts to fit properly when cut with a round-end milling bit.
Tool Used: Nifty Dogbones for Fusion 360 is a Fusion 360 add-on that automates this process, allowing quick and precise application of dogbones across all necessary corners.

📝 Takeaway: Why Dogbones?
When CNC cutting, internal corners need to match the radius of the tool, or the pieces won’t fit together. The dogbone fillets add small circular extensions at the corners, making space for the joining pieces. This guarantees a perfect press-fit without the need for manual adjustments.

Finally, after reviewing the model in different perspectives to check for misalignments. Once the design was finalized, I exported the files and imported them into the CNC software to generate the toolpaths.

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🛠️ Parametric Values and How They Were Defined

Here's a breakdown of each parameter used:

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Preparing for CNC Milling

Before generating the toolpaths, I first set up the material dimensions and positioning in the CNC software.

📝 Takeaway: Why is this important? Having the correct material dimensions ensures that all toolpaths are accurately positioned and that cuts will be at the right depth without going too deep or leaving material uncut.

I arranged the parts to maximize material efficiency while ensuring enough spacing for the tool to pass. To ensure clean cuts and a precise fit, I selected the appropriate end mill and cutting parameters. To ensure clean, efficient, and safe machining, I adjusted the spindle speed, feed rate, and depth of cut.

Previewing & Simulating the Cut

Before starting the actual machining, I ran a toolpath simulation to verify:

• Cutting sequence – Ensuring the correct order of cuts.
• Depth accuracy – Confirming that cuts reached the correct final depth.
• No tool collisions – Checking that no unexpected cuts were made.

📝 Takeaway: Why?: A simulation helps catch mistakes before wasting material or damaging the CNC router.

CNC Milling Process

Assembly: Press-Fit Construction

It was time to assemble the table, ensuring that everything fit perfectly without the need for screws or glue. The design relied entirely on precision press-fit joints, meaning the connections had to be tight enough to hold securely but not so tight that they couldn’t be assembled.

The first step was to assemble the legs, which consisted of two interlocking pieces that slid into each other at a central slot, forming a sturdy X-shaped base.

Once the legs were assembled, we positioned the tabletop, aligning it with the slots in the legs. The final fit required the used of a rubber mallet, but once in place, the structure was sturdy and stable. The assembly was complete—resulting in a functional, strong, and elegant table without any fasteners or adhesives.

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Final Thoughts

This project was a great hands-on experience in designing, machining, and assembling a functional piece without screws or glue. The parametric approach in Fusion 360 made a huge difference, allowing me to easily adjust slot sizes to match the actual material thickness. Using Nifty Dogbones was also a game-changer, ensuring that all interlocking joints fit perfectly despite the round cuts made by the CNC. Setting up the toolpaths carefully—starting with inside cuts, adding ramp entries, and using holding tabs—helped keep everything stable and resulted in clean, precise cuts.

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Files

Here are the project files available for download:
Coffe table File Download .zip

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