Unit 4 Comprehensive Quiz: CNC Routing (ShopBot)

Unit: 04 - CNC Routing (ShopBot) Duration: 30-45 minutes Passing Score: 70% Format: Multiple choice covering all modules Questions: 12

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Instructions

This comprehensive quiz covers all modules in the CNC Routing unit. You should complete all module assessments before attempting this unit quiz. The quiz tests both factual recall and application of concepts across modules.

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What does "CNC" stand for, and what is the fundamental advantage of CNC routing over manual routing?

Computer Numerical Coordinates — it uses GPS to position the router

Computer Numerical Control — a computer precisely controls the router's position and movement along programmed toolpaths, enabling repeatable, complex cuts impossible to achieve by hand

Calibrated Nesting Configuration — it automatically arranges parts on the material

Centralized Network Computing — it connects multiple routers to a single controller

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Explanation: CNC routing uses a computer to control stepper or servo motors that drive the router along three axes (X, Y, Z) according to programmed coordinates. This delivers sub-millimeter repeatability (typically ±0.1mm), the ability to execute complex curved toolpaths, and consistent results across hundreds of identical parts. The ShopBot is a popular gantry-style CNC router used in educational and small-production environments.

What is a "toolpath" in CNC routing, and what software generates it?

A toolpath is the physical groove cut by the router bit — it is created by the router itself

A toolpath is the path the operator walks while monitoring the machine

A toolpath is the precise set of coordinates and movements the router follows to cut, drill, or engrave — it is generated by CAM (Computer-Aided Manufacturing) software from a design file

A toolpath is another name for the G-code file format

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Explanation: The CAD-to-CNC workflow is: Design (CAD) → Toolpath generation (CAM) → Machine code (G-code) → Router executes. CAM software (VCarve, Fusion 360 CAM, Aspire) takes a 2D/3D design and calculates the exact router movements needed to reproduce it, accounting for bit diameter, depth of cut, feed rate, and plunge rate. The toolpath is exported as G-code — a line-by-line instruction set that the CNC controller interprets to drive the motors.

Why must you account for bit diameter when programming CNC toolpaths, and what happens if you don't?

Bit diameter only matters for aesthetic reasons — it affects the surface texture

Failing to account for bit diameter causes the router motor to overheat

The router bit removes material in a circle equal to its diameter — if the toolpath follows the design line exactly, the finished part will be undersized (for outside cuts) or oversized (for inside cuts) by half the bit diameter

Bit diameter does not need to be accounted for — the CAM software handles it automatically with no user input

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Explanation: A 1/4" (6.35mm) bit removes a 1/4" wide channel centered on its path. If cutting the outside of a 100mm square, the toolpath must be offset outward by 3.175mm (half the bit diameter) or the finished part will be 93.65mm — too small. CAM software provides "inside," "outside," and "on-line" offset options, but the operator must select the correct one for each feature. This is one of the most common beginner mistakes in CNC routing.

What is "climb milling" vs. "conventional milling" and when should each be used on a CNC router?

Climb milling moves the bit upward; conventional milling moves it downward

In climb milling, the bit rotation direction matches the feed direction (bit pulls into the material); in conventional milling, they oppose (bit pushes against the material). Climb milling produces a cleaner finish; conventional milling is safer for handheld routers but either works on CNC with rigid workholding

Climb milling is for hardwoods; conventional milling is for softwoods

They are two names for the same cutting technique

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Explanation: In climb milling, the cutting edge engages the material from the thick side of the chip to the thin side, producing a cleaner surface finish with less tear-out. However, the bit tends to pull itself into the material, requiring rigid workholding. Conventional milling engages thin-to-thick, which deflects the bit away from the material — safer for handheld routing but leaves a rougher edge. On CNC routers with proper clamping, climb milling is generally preferred for final passes.

What are "tabs" in CNC routing and why are they important?

Tabs are the software interface elements used to organize toolpaths

Tabs are metal brackets used to clamp the material to the bed

Tabs are small bridges of uncut material that hold the finished part in place within the stock sheet, preventing it from shifting or being thrown by the spinning bit during the final cut

Tabs are decorative notches added to the edges of CNC-cut parts

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Explanation: When a CNC router completes a profile cut, the finished part becomes a loose piece that can shift, vibrate, or be caught by the spinning bit — creating a dangerous projectile or ruining the cut. Tabs (typically 3-6mm wide, 1-2mm tall) are small bridges left at strategic points around the profile to hold the part in the stock. After the job is complete, tabs are cut with a hand tool and sanded flush. CAM software allows automatic tab placement with adjustable size and spacing.

What is the correct procedure for setting the Z-zero (height origin) on a ShopBot CNC router?

Visually estimate the surface height and enter it manually

Always set Z-zero to the machine's home position at the top of travel

Use the Z-zero touch plate (a conductive plate of known thickness) placed on the material surface — the bit lowers until it contacts the plate, completing a circuit, and the controller sets Z-zero to the material surface automatically

Set Z-zero to the spoilboard surface regardless of material thickness

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Explanation: Accurate Z-zero is critical — even 0.5mm error means cuts that don't go through or cuts that gouge the spoilboard. The touch plate method is the standard: the conductive plate (typically 12-15mm thick, exact thickness stored in the controller) is placed on the workpiece surface, and the bit is lowered at slow speed until electrical contact is detected. The controller then subtracts the plate thickness to calculate the true surface position. Always re-zero Z after changing bits, as bit lengths vary.

A CNC routing job on 18mm plywood uses a 1/4" upcut spiral bit. The CAM program calls for a full-depth cut in one pass. Why is this dangerous?

The bit will overheat because plywood contains glue

The plywood will warp from the heat generated

Cutting full depth (18mm) in a single pass with a 1/4" bit exceeds the recommended depth-per-pass limit, overloading the bit with excessive lateral forces that can cause bit deflection, breakage, or the material being pulled from the clamps

Single-pass cuts are actually fine for plywood — this is not dangerous

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Explanation: The general rule is maximum depth-per-pass should not exceed the bit diameter (6.35mm for 1/4"). Cutting 18mm in one pass is ~3x too deep, creating extreme lateral forces on the bit. This causes bit deflection (inaccurate cuts), excessive heat (can burn the wood and dull or break the bit), and dangerously high forces on the workholding. The correct approach is 3 passes at 6mm each, or 4 passes at 4.5mm for a more conservative approach. Chip load (feed rate / RPM / number of flutes) must also be correct.

What is the difference between an upcut and a downcut spiral bit, and how do you choose between them?

Upcut bits are for wood; downcut bits are for plastic

They produce identical results and are interchangeable

Upcut bits eject chips upward (clean bottom surface, fuzzy top); downcut bits push chips down (clean top surface, may pack chips in deep cuts). Choose based on which surface needs to be cleanest

Upcut bits spin clockwise; downcut bits spin counterclockwise

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Explanation: The spiral flute direction determines chip ejection. Upcut spirals pull chips out of the cut — excellent for deep slots and pockets (prevents chip packing) but tends to lift and fray the top surface of the workpiece. Downcut spirals push chips into the cut — produces a clean top edge (ideal for laminated or veneered materials) but can pack chips in deep cuts, generating heat. Compression bits combine both: downcut on top, upcut on bottom, for clean surfaces on both sides of sheet goods.

What safety equipment is required when operating a CNC router?

Only safety glasses are needed since the machine does all the cutting

Safety glasses (for flying chips), hearing protection (CNC routers operate at 80-100 dB), dust collection system running, and no loose clothing, jewelry, or long untied hair near the spinning bit

A full face shield and welding apron

No safety equipment is needed because the CNC is enclosed

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Explanation: CNC routers present multiple hazards: (1) Flying chips and dust — the spinning bit ejects material at high velocity, requiring eye protection and dust collection. (2) Noise — router spindles at 10,000-24,000 RPM generate 80-100 dB, above the threshold for hearing damage. (3) Entanglement — the spinning bit can catch loose clothing, hair, or jewelry with severe consequences. (4) Pinch points — the moving gantry creates crush hazards. The dust collection system must be running before starting any cut to manage both visibility and respiratory exposure.

What is "workholding" and why is it the most common cause of CNC routing failures?

Workholding refers to pausing the machine to hold the emergency stop button

Workholding is the CAM software's ability to hold toolpath calculations in memory

Workholding is how the material is secured to the CNC bed — inadequate workholding allows the material to shift during cutting, ruining the part and potentially creating a dangerous situation

Workholding is the practice of holding finished parts while removing tabs

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Explanation: The router bit exerts significant cutting forces on the workpiece — both laterally (from the feed direction) and vertically (upcut bits pull upward). If the material is not securely fastened, it can shift, rotate, or lift during cutting. Common methods include: mechanical clamps (T-track + hold-down clamps), vacuum table (for flat sheet goods), double-sided tape (for small parts), and screw-down (screws in waste areas). Workholding must resist the maximum expected cutting force with a safety margin.

After a CNC routing job completes, what is the correct shutdown sequence?

Turn off the computer first, then the spindle will stop automatically

Just press the emergency stop and leave

Wait for the spindle to fully stop, raise the Z-axis clear of the workpiece, return the gantry to home position, turn off the spindle/router, turn off the dust collection, then remove the workpiece and clean the bed

Remove the workpiece immediately while the spindle is decelerating to save time

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Explanation: Proper shutdown prevents damage and injury. The spindle must be fully stopped before approaching the cutting area — a decelerating bit is still dangerous. Raising Z clears the bit from the work. Homing the gantry ensures a known starting position for the next job. Dust collection should run for 30-60 seconds after the last cut to clear residual dust from the ductwork. The spoilboard should be vacuumed and inspected for damage before the next job.

You are designing a box with finger joints to be cut on the CNC router. The design software shows perfect tight joints, but the test cut produces joints that are too tight to assemble. What went wrong?

The router bit is too sharp and cut more material than expected

The plywood thickness is not exactly as specified

The design did not account for the bit's cutting kerf — the actual slot width equals the programmed width minus the kerf compensation, making fingers too wide and slots too narrow for the specified material thickness

The feed rate was too slow, causing the bit to burn and swell the wood

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Explanation: Finger/box joints require precise fit, and CNC introduces several tolerance factors: (1) bit diameter offset — if the CAM offset is slightly wrong, all fingers shift by the error amount, (2) material thickness variation — nominal 18mm plywood may be 17.5-18.5mm, (3) kerf width vs. bit diameter — actual kerf may differ from nominal bit size due to runout or wear. The solution is to cut a test joint, measure the actual fit, and adjust the design by 0.1-0.2mm as needed. Professional CNC joiners always include a tolerance fit test in their workflow.

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Last Updated: 2026-03-19