CNC Tool Path Design: Principles, Strategies, and Best Practices for Precision Machining

Efficient and precise tool path design is fundamental to modern machining, ensuring both productivity and high-quality surface finishes.

From roughing to finishing, selecting the appropriate tools, feed rates, cutting depths, and clamping methods directly impacts machining accuracy, tool life, and workpiece integrity.

 Proper planning of roughing toolpaths, medium finishing, contouring, and corner clearance allows manufacturers to optimize material removal while minimizing tool breakage and surface defects.

Additionally, integrating specialized techniques such as CNC-guided copper electrode machining enhances the precision of complex or high-allowance workpieces, making tool path strategy essential in advanced manufacturing.

General Principles for Tool Paths

• Roughing

Under maximum machine load, operators should select the largest possible tool, maximum feed rate, and fastest feed speed in most cases.

With a given tool, feed speed is inversely proportional to feed rate.

Generally, machine load is not a limiting factor; tool selection primarily depends on whether the product’s two-dimensional angles and three-dimensional arcs are excessively small.

After selecting the tool, determine the tool length. The principle is that the tool length should exceed the machining depth.

For large workpieces, consider whether the tool holder will interfere.

• Finishing Cut

The purpose of the finishing cut is to achieve the required surface finish on the workpiece while leaving an appropriate allowance.

Similarly, select the largest possible tool for finishing operations and complete them as quickly as feasible, as precision finishing requires extended processing time.

Employ the most suitable cutting depth and feed rate. Under identical feed conditions, a larger lateral feed rate yields faster processing.

The cutting depth for curved surfaces correlates with the desired surface finish quality, while the feed rate depends on the external shape of the surface.

Maintain the smallest possible stock allowance without damaging the surface, utilizing the largest tool, highest spindle speed, and appropriate feed rate.

Clamping Method

All clamping is horizontal long and vertical short.

• Vise Clamping

The clamping height must not be less than 10 millimeters. When machining workpieces, operators must specify the clamping height and machining height.

The machining height should be approximately 5 millimeters above the vise surface to ensure stability while preventing damage to the vise.

This constitutes a standard clamping method, where the clamping height also depends on the workpiece size—larger workpieces require correspondingly greater clamping heights.

• Clamping with Clamping Plates

Clamping pins secure the clamping plates to the workbench, and screws fasten the workpiece to the plates.

This method is suitable for workpieces requiring insufficient clamping height or subject to high machining forces, generally yielding good results for medium to large workpieces.

• Support Block Clamping

When the workpiece is large, the clamping height is insufficient, and bottom screw fastening is prohibited, operators use support blocks.

This method requires secondary clamping: first secure the four corners, machine other sections, then secure the four sides and machine the corners.

During secondary clamping, prevent workpiece movement by securing before releasing. Alternatively, secure two sides first, then machine the other two.

• Tool clamping

For tools over 10mm diameter, clamping length must be no less than 30mm; for tools under 10mm diameter, clamping length must be no less than 20mm.

Operators must securely clamp tools to prevent collisions and direct insertion into the workpiece.

Classification of Cutting Tools and Their Applications

• By Material

White Steel Tools: Prone to wear, used for roughing copper molds and small steel materials.

Tungsten Steel Tools: Used for corner cleaning (especially steel materials) and finishing.

Alloy Tools: Similar to tungsten steel tools.

Purple Tools: Used for high-speed cutting, resistant to wear.

• By Cutter Type

Flat-bottomed cutter: Used for flat surfaces and straight side walls, clearing flat corners.

Ball-nose cutter: Used for finishing and polishing various curved surfaces.

Bullnose cutter (single-sided, double-sided, and five-sided): Used for roughing steel stock (R0.8, R0.3, R0.5, R0.4).

Roughing cutter: Used for roughing operations; note the required allowance (0.3).

• By Tool Shank Type

Straight Shank Tools: Suitable for a wide range of applications.

Angled Shank Tools: Not suitable for straight surfaces or surfaces with a slope less than the shank angle.

• By blade configuration

Double-edged, triple-edged, quadruple-edged.

More blades yield better results but require greater work input, necessitating corresponding adjustments to rotational speed and feed rate.

Higher blade counts extend tool life.

• Differences Between Ball Cutters and Flying Cutters

Ball Cutters:When the concave gauge is smaller than the ball gauge and the flat gauge is smaller than the ball radius, the tool cannot fully clear the bottom corners.

Flying Cutter: Its advantage lies in clearing bottom corners.

Comparison with identical parameters: V = R * ω. Significantly faster rotational speed (flying cutter), greater force produces brighter polished surfaces.

Flying cutters are more commonly used for contouring profiles, sometimes eliminating the need for intermediate polishing.

Disadvantage:When the concave dimension or flat dimension is smaller than the flying cutter diameter, the process does not achieve polishing.

CNC Combined with EDM: Copper Electrode Manufacturing

• When Copper Electrodes Are Required

1) When the tool cannot fully penetrate, a copper electrode is needed.

If penetration is still impossible within the electrode due to protruding features, further segmentation is required.

2) When penetration is possible but tool breakage is likely, a copper electrode is also needed. This decision depends on actual conditions.

3) Products requiring spark-textured finishes necessitate copper electrodes.

4) When excessively thin or tall core sections make copper electrodes prone to damage and deformation during machining and sparking, operators must use inserts.

5) Parts machined from copper electrodes exhibit exceptionally smooth and uniform surfaces (especially on curved surfaces), overcoming numerous issues encountered in precision milling and design drafting.

6) Use rough copper electrodes when precise external shapes or substantial allowances are required.

• Copper Mastering Process

Select the surfaces to be mastered, fill in missing surfaces or extend existing ones as needed.

Ensure all edges of the copper master are larger than the edges to be machined without damaging other product surfaces.

Remove unnecessary flat corners that the tool cannot clear (intersections with flat corners form deeper adhesive pockets), then fill them to create regular shapes.

Determine the maximum outer contour of the copper master, define a boundary, and project it onto the support surface.

Determine the reference frame dimensions, trim the support surface, completing the basic copper electrode drawing.

Material preparation: Length × Width × Height. Length and width must be ≥ Y_max and X_max of the reference frame.

Actual copper material dimensions must exceed the drawn reference frame.

Height must be ≥ theoretical copper electrode dimensions + reference frame height + clamping height.

Drawing Dimensioning Issues

1. When no existing machined surfaces are available, center the four sides of the plane with the center aligned to the origin and the top surface at zero.

For uneven top surfaces (in the case of copper electrodes), leave a 0.1 allowance.

This means during collision checking, the actual position is zero (Z), while the drawing shows a 0.1 offset downward.

2. When existing machined surfaces are present, align the drawing’s existing surfaces to zero (Z).

If the plane can be centered, center it; otherwise, use the existing edge for single-side touch probing.

For machined surfaces, verify the actual height, width, and length against the drawing specifications and program based on the actual material.

Generally, machine to the drawing dimensions first, then machine the drawing shape.

3. For multi-position machining, the first position (standard position) must establish reference surfaces for all subsequent positions.

Mill all reference surfaces (length, width, height) to ensure all future machining references align with previously machined surfaces.

4. Insert positioning: Place the insert within the workpiece body. Elevate it to a specific height, then raise the drawing by this height.

Center the plane relative to the workpiece body, and center the height according to the drawing. Secure it with screws at the bottom.

For square inserts, center directly. For rough positioning, center using the maximum outer contour. If using a fixture, center according to the fixture.

Determine the relative position between the insert drawing and the fixture, then place the drawing origin at the fixture’s center point.

Roughing Toolpath Selection

• Surface Grooving

1) Key considerations: area selection and surface selection

The machining area for toolpaths follows this principle: using the selected surfaces within the chosen area as termination surfaces, the toolpath covers all accessible regions from the highest to lowest points.

Selected surfaces should ideally encompass all surfaces, while boundaries should only define the machining area.

Extensions into non-surfaced areas should be less than half the tool diameter, as other surfaces provide sufficient allowance for automatic protection.

Extend to the lowest contour line whenever possible, as the lowest point may be inaccessible to an R-cutting tool.

2) Tool Selection: Seal off areas inaccessible to helical or oblique tool feeds, or regions unreachable during machining, reserving them for secondary roughing.

3) Before finishing, ensure all unroughened areas are fully machined—especially small corners (including 2D and 3D corners) and sealed regions—to prevent tool breakage.

Second Roughing: Typically select the area using 3D slot milling.

Use a flat-bottomed cutter; if feasible, employ planar slot milling with contour toolpaths.

Position the tool center to the selected boundary without damaging adjacent surfaces.

Generally avoid fine-tuning boundaries; use rapid bidirectional angles as needed.

Spiral feed with 1.5° angle and 1mm height. For linear slot shapes where spiral feed is impractical, use diagonal feed.

Enable filtering, especially for surface roughing. Keep the feed plane sufficiently high to prevent tool collisions; maintain adequate safety clearance.

4) Retraction: Generally use absolute retraction instead of relative retraction. Switch to relative retraction only when no islands remain.

• Flat Slot Milling

Mill various flat surfaces and recessed slots. When milling partially open flat surfaces, define boundaries to ensure the cutter can engage (with clearance greater than one cutter diameter).

For open areas, maintain clearance outward by more than half a cutter diameter and enclose the perimeter.

• Profile

When the selected plane is suitable for profile layering, use profile layering for tool lift (plane profile).

If the lift point coincides with the entry point, no Z-plane lift is required.

Generally perform a standard lift, avoiding relative height adjustments whenever possible.

The correction direction is typically right-hand correction (in-feed).

• Machine-Compensated Toolpath Settings

Compensation code 21: Switch from computer compensation to machine compensation. Use vertical feed for tool approach.

Where tool clearance is insufficient, increase radius (R) without leaving material allowance.

• Contouring

Use this method for machining enclosed surfaces. For open surfaces that require four passes, operators must enclose the top surface.

If within four passes or non-four-pass operations, select the range and height (always roughing with curved feed).

For roughing scenarios: the machining distance within any plane must be less than one tool diameter.

If exceeding one tool diameter, use a larger tool or perform two contouring passes.

• Curved Streamlines

Offering the best uniformity and crispness, they are suitable for light cutting and can often replace contour profiles.

• Radius Toolpath

Suitable for workpieces with large central holes (rarely used).

Precautions: When tool deflection occurs, the tool is dull, the tool is too long, or the workpiece is too deep, use a circular path instead of vertical up-and-down movements.

For sharp corners within the workpiece, split the machining into two separate toolpaths; do not attempt to cross over.

When finishing edges, extend the toolpath slightly (using curved retraction/approach movements).

Clearing Corners

1. Here, “clearing corners” refers to removing two-dimensional dead corners—areas not reached by previous machining steps.

For instance, if a finishing cutter must access these areas, clear the corners first before finishing.

For corners that are too small or deep, use multiple cutters to clear them; avoid using a small cutter to clear too many areas.

2. Clearing three-dimensional corners: Create small grooves and clear areas at three-dimensional transitions.

3. Tool breakage is common. Consider factors like using fine tools, excessive tool length, and excessive machining volume (primarily in the Z-axis/depth direction).

4. Toolpath selection:  For 2D contouring: Only suitable for clearing small corners (R0.8) and 2D planar corners.

Use parallel toolpaths. Employ contouring at constant height.

For inaccessible curved surfaces or dead corners unreachable by standard toolpaths: Seal the area first to initiate tooling.

Clear the corners last. Operators should generally seal small gaps within large surfaces first.

Medium Finishing

1. Medium Finishing: Refining steel surfaces with fine tools after roughing.

2. Principle: A process applied after roughing with large tools when significant layer-to-layer allowances remain, to achieve better results during final finishing.

3. Characteristics: Rapid material removal; suitable for large tools and fly cutters; high feed rates and wide spacing; surface quality is not a primary concern.

Flat workpieces may omit intermediate roughing; workpieces with uniform-height contours do not require intermediate roughing.

For uniform-height roughing, a finer approach may combine both operations, where “finer” refers to reduced surface allowance and layer spacing.

Material hardness is another critical factor—harder materials warrant consideration of intermediate roughing.

For optimal results and uniformity, the machining direction of intermediate roughing should ideally be opposite that of finishing roughing.

Finishing Toolpath

Designers must meticulously plan finishing toolpaths to meet the assembly requirements of various products and molds.

Operators should apply different toolpath configurations and parameter settings based on specific requirements.

1. Set both the initial and final cutting heights to 0.000 mm. Tolerance should be designed within 0.01 mm.

No filtering is required (smaller workpieces require tighter tolerances, as tolerance directly affects the final shape).

2. The front mold and parting line must achieve the highest surface finish.

The rear mold may be secondary, while non-interfering and clearance areas can be rougher.

3.The following factors determine toolpath design:

1) Specific geometry (e.g., planes vs. other surfaces), steep faces vs. flat curves.

2) Whether sharp angles exist between surfaces (separate if present).

3) Different requirements for two sections (whether to leave allowance, amount of allowance, differing surface finish requirements).

4) Surface protection during finishing is critical.

Operators must account for machining errors on protected surfaces and safeguard them according to protection requirements.

This includes range protection (zero tolerance for errors), height range, plane range, and surface protection.

5) Toolpath extension: When toolpaths approach edges during finishing, use circular retract/approach movements or slightly extend the surface beforehand.

6) Tool lift issues:Tool lifts waste time, so operators should minimize them.

Method 1: Set a lift gap (small gap);

Method 2: Cover the lift area (small gap);

Method 3: Avoid gaps (large gaps);

Method 4: For contouring at constant height, extend to the same height.

7) Tool approach issues in finishing: The first tool approach must start from outside the workpiece to prevent vibration and damage.

All finishing operations must include a defined approach.

8) Tool wear issues: For large workpieces, use multiple tools to complete finishing on the same part.

Adhering to structured tool path principles—from roughing and medium finishing to precise finishing operations—ensures consistent product quality, reduced machining time, and extended tool life.

Proper tool selection, clamping methods, and thoughtful path planning prevent workpiece deformation, collisions, and corner imperfections while supporting complex geometries and high-precision requirements.

By systematically addressing feed rates, cutting depths, tool approaches, and surface protection, engineers can achieve efficient, accurate, and reliable machining outcomes across a wide range of materials and applications.