Panel through-cut, both faces visible
Start by checking a compression cutter. It only works as intended when the compression transition is correctly positioned in the material.
Technical guide · CNC router bits
CNC Router Bit Selection Guide
Choose by machine, material, operation, dimensions and required finish—then verify the exact cutter against manufacturer data before machining.
Quick choice
Start with the cut—not the product name
The same material may need a different cutter when the visible face, hold-down method, depth, spindle or operation changes. These are direction rules, not automatic product approvals.
Deep slot or pocket with chip buildup
Start by checking an upcut spiral because it evacuates chips upward. Protect the top surface if appearance matters.
Top face is the priority
Check a downcut spiral, but confirm that chips have somewhere to go and that the workpiece is securely supported.
Acrylic or compatible plastic
Check a polished plastic-specific O-flute. The process must form and remove chips; dust, melting or re-welding means stop and correct the setup.
Visual guide
Upcut, downcut or compression?
Use this infographic to compare flute direction, edge behavior and the critical compression-transition condition. It is a selection aid, not a universal speeds-and-feeds chart.
Selection workflow
Five gates before a recommendation
A router-bit recommendation is valid only after every gate passes. The selector should refuse a precise answer whenever a critical input is missing.
Decision matrix
What each geometry is actually for
“Best” means best matched to the stated constraint. It does not mean the geometry is universally safer, faster or cleaner.
| Task / constraint | Geometry to investigate | Main advantage | Main trade-off | Compatibility gate |
|---|---|---|---|---|
| Slots, pockets, general sheet cutting | Upcut spiral | Strong upward chip evacuation | Can lift fibers or veneer at the top surface | Hold-down must resist upward cutting force; confirm the tool is listed for the material. |
| Clean top face on plywood or panels | Downcut spiral | Pushes cutting action toward the top face | Can pack chips into a closed slot and add heat | Provide chip space or extraction and avoid an unsupported or poorly held part. |
| Through-cut with clean top and bottom faces | Compression spiral | Opposing helix zones control both faces | Fails if the transition zone is placed incorrectly | The first cutting depth and material thickness must engage both zones as specified. |
| Acrylic and compatible thermoplastics | Polished single-flute O-flute | Large flute space and plastic-specific chip formation | Wrong heat/load balance can melt and weld chips back | Use an exact manufacturer recommendation, rigid fixturing, chip evacuation and a non-dwelling entry. |
| ACM through-cut or trimming | Manufacturer-listed non-ferrous or ACM cutter | Geometry is matched to aluminum skins and core | Generic wood tooling may load, burr or wear rapidly | Confirm the cutter is explicitly listed for ACM and the required operation. |
| ACM fold line | Flat-bottom V-groove rated for ACM | Removes the top skin and part of the core for controlled folding | Cut depth is critical; it is not a through-cut tool choice | Match groove angle and residual skin or core to the panel maker’s folding method. |
| Lettering and shallow V-carving | V-bit or engraving cutter | Cut width varies predictably with depth and included angle | Small tips are fragile; depth changes alter line width | Confirm tip diameter, included angle, runout and programmed depth. |
| 3D relief finishing | Ball nose or tapered ball nose | Smooth 3D toolpaths and fine finishing options | Slow finishing and limited center cutting speed | Select ball radius, stepover, reach and taper for the model and machine rigidity. |
| Spoilboard flattening | Surfacing cutter | Wide path for leveling a spoilboard | Large diameter raises load and RPM constraints | Verify spindle power, shank and collet, manufacturer maximum RPM and safe toolpath clearance. |
Dimensions & feeds
The checks that prevent false compatibility
A correct geometry can still be the wrong tool if its shank, usable cutting length, diameter, maximum speed or flute count does not match the setup.
Seven dimension checks
- Shank vs. collet: use the exact matching size. Do not treat 6 mm and 1/4 in as interchangeable.
- Cutting length: it must cover the programmed cutting depth, including any intentional spoilboard allowance.
- Reach and stick-out: use only the reach needed; unnecessary projection reduces rigidity.
- Internal radius: a round cutter cannot create a sharp inside corner; the radius cannot be smaller than half the cutter diameter.
- Compression transition: place the upcut/downcut transition correctly within the panel.
- Maximum RPM: commanded spindle speed must not exceed the exact cutter’s published limit.
- Flute count: the machine must feed fast enough to achieve the manufacturer’s chip-load range without rubbing.
Use chip load as the relationship—not a guessed feed
Chip load = Feed rate ÷ (RPM × number of flutes)
Start with the cutter manufacturer’s data for the exact series, diameter and material. Calculate a feed, compare it with the machine’s real capability, then test progressively on scrap. A value copied from another diameter or geometry is not equivalent.
For plastics, chips must leave the cut and carry heat away. Fine dust, melting, re-welding or a hot tool are rejection signals—not reasons to keep running the same parameters.
Worked example
18 mm plywood, both faces visible
This example shows the reasoning path without inventing a universal RPM or feed rate.
- Machine
- Rigid CNC router
- Material
- 18 mm veneered plywood
- Operation
- Through profile cut
- Finish
- Clean top and bottom
- Units
- Metric
Reasoned recommendation
A compression spiral is the first geometry to investigate because the two visible faces require opposing cutting directions. A nominal 6 mm cutter is acceptable only if the CNC has a true 6 mm collet and the exact bit is manufacturer-rated for plywood.
The cutting length must exceed the full programmed depth. More importantly, the first cutting pass must be deep enough to place the compression transition correctly; a shallow first pass that remains in the upcut section can damage the top veneer. If the machine cannot safely make that entry, the process plan—not just the feed—must change.
Output shown to the user: geometry, exact shank requirement, minimum usable cutting length, transition warning, manufacturer RPM/feed source, hold-down and extraction checks, then a scrap-test instruction. No precise cutting parameter is shown until the exact cutter and machine limits are known.
Refusal logic
When MatchMyTools must not recommend
A trustworthy selector sometimes says “not enough information.” Refusing false precision is more useful than returning a plausible-looking number.
Stop the recommendation when any critical condition is unresolved
Unknown or mismatched shank and collet size
Exact cutter maximum RPM is unavailable
Cutting length is shorter than the programmed depth
Machine type is unknown or the cutter is listed as CNC-only
Material is described only as “wood,” “plastic” or “aluminum”
Compression transition cannot be placed correctly
Workholding cannot resist cutting forces
Acrylic is melting, re-welding or producing only dust
Spindle cannot reach a usable manufacturer-backed speed/feed combination
Guarding, extraction or safe clearance is missing
First diagnostics
Read the cut before changing numbers
Symptoms can have several causes. Change one variable at a time and never compensate for a mechanical defect with software settings.
Top-face tear-out
Check helix direction, compression transition, entry strategy, tool sharpness, runout and surface support.
Bottom-face tear-out
Check spoilboard support, tool exit, compression engagement, workholding and whether the bottom layer is already damaged.
Burning in wood
Check dull or dirty edges, feed that is too low for RPM, excessive rubbing, recutting chips and long dwell at corners.
Melting acrylic
Stop. Check chip formation, cutter geometry, feed/RPM relationship, dwell, ramp entry and chip evacuation.
Chatter or rough walls
Check workholding, collet condition, runout, stick-out, machine rigidity, cutter deflection and toolpath direction.
Unexpected breakage
Stop. Verify collision, flute packing, excessive reach or depth, wrong material, damaged collet, runout and manufacturer limits.
Safety & scope
A starting point, never a machine override
The machine manual and exact tool manufacturer remain controlling sources. This page explains selection logic; it cannot inspect the machine, collet, workholding or tool condition.
Before every test cut
- Confirm the tool, collet and spindle are clean, undamaged and correctly tightened.
- Verify maximum tool RPM and every machine limit before starting the spindle.
- Secure the workpiece and keep guards, enclosures and dust extraction in service.
- Use suitable eye, hearing and respiratory protection for the process and material.
- Run a controlled scrap test and stop for abnormal noise, heat, vibration, movement or chip behavior.
- Keep hands and tools away from the rotating cutter; isolate power before inspection or adjustment.
What this guide deliberately does not do
It does not publish one universal speeds-and-feeds table. Those values depend on the exact tool series and diameter, material composition, spindle power, machine rigidity, runout, hold-down, depth of cut and evacuation.
It also does not claim that every CNC router bit is safe in a hand-held router or router table. Some manufacturer catalogs explicitly restrict certain cutters to CNC use.
Editorial status: reviewed technical pillar page for the United States market. Exact product data must be rechecked whenever a manufacturer revises its specifications.
Evidence
Manufacturer and safety sources
Sources support the selection rules; they do not imply endorsement of a seller or product.
- LMT Onsrud — Fixturing and Routing of Plastics with CNC: chip load, heat, chip re-welding, ramp entry and workholding.
- LMT Onsrud — Cutting Data Recommendations: manufacturer data by tool family and material.
- Amana Tool — Down-cut spiral cutter specification: top-surface cutting direction and chip-clearance behavior.
- Amana Tool — CNC compression spiral router bits: intended use on double-sided melamine, laminates and MDF.
- Amana Tool — Router Bit Technical Information: CNC-only warnings and general cutter maintenance.
- Amana Tool — Maximum RPM Guide: published maximum speed for the exact tool.
- Amana Tool — ACM V-groove cutter set: flat-bottom V-grooving and folding use.
- OSHA — Woodworking routers: guarding and rotating-tool hazards.
- OSHA — Wood dust solutions: local exhaust ventilation and protective measures.
Last technical review: August 9, 2026. Recheck source pages and exact product specifications before purchase, publication of numeric parameters or machining.
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