CNC Bits 101: Which Router Bit Should You Use?
Choosing a router bit can feel confusing when you are getting started with a hobby CNC. Several bits may look similar, yet leave very different results in the same material.
The best place to start is with two questions: What material are you cutting, and what do you want the bit to do? Cutting out a plywood shape, carving a wooden sign, and machining an acrylic panel each call for a different approach.
Whether you use a Shark CNC or another hobby CNC router, understanding a few common bit types will help you choose with more confidence.
| Project or task | Bit to consider | What to know |
| Flat pockets and general wood cutouts | Flat end mill or suitable straight bit | Produces a flat bottom; choose flute direction for the surface and chip removal |
| Wood pockets and deeper grooves | Upcut spiral end mill | Pulls chips out, but can lift fibers on the top edge |
| Plywood with a clean visible top | Downcut spiral end mill | Protects the top surface; chips need room to escape |
| Plywood with both faces visible | Compression bit | Both cutting sections must engage correctly |
| Lettering and decorative grooves | V-bit | Angle and cutting depth determine the shape of the groove |
| Rounded 3D surfaces | Ball nose bit | Stepover affects finish and machining time |
| Detailed 3D reliefs | Tapered ball nose bit | Small tip reaches detail; taper adds support |
| Acrylic and many other plastics | Material-specific O-flute bit | Helps evacuate chips and manage heat |
| Flattening a spoilboard | CNC surfacing bit | Match cutter size and speed to your equipment |
These are starting points. Always check the bit manufacturer’s material recommendations and your machine’s capabilities.
A flat-bottom cutter is one of the most useful tools in a hobby CNC shop. Use it for pockets, slots, and cutting shapes from sheet material. Think of a recessed area in a sign, a tray pocket, or the outline of a wooden ornament.
“Straight bit” and “flat end mill” do not mean exactly the same thing. A straight-flute bit has cutting edges that run along the tool without a spiral. A spiral end mill can also have a flat cutting end. Both can make flat-bottom cuts, but their flute designs handle chips differently.
Choose a bit designed for your material and the way it will enter the cut. Do not assume every straight router bit can plunge vertically. Check its plunge-cutting capability and use the manufacturer’s recommended entry method.
For general wood projects, a 1/4-inch flat end mill is a practical starting size when your collet supports it. A 1/8-inch cutter can reach smaller details, but needs cutting settings appropriate for its smaller size.
These terms describe cutting action. Choosing between them often comes down to which surface needs the cleanest edge and how the chips will leave the cut.
An upcut spiral pulls chips upward. It is useful for pockets and grooves where clearing chips matters. The tradeoff is that it can lift the workpiece or tear fibers along the top edge.
A downcut spiral pushes chips downward and helps preserve a clean top surface. It can be useful for plywood and veneered material, but chips can pack into a closed groove. A clean top edge does not guarantee a clean bottom edge.
A compression bit combines an upcut section at the tip with a downcut section above it. It can leave clean edges on both faces of plywood or laminated sheet goods. However, the cutting depth must position those sections correctly: if the first pass is shallower than the upcut tip length, the top edge may still tear out. Choose an appropriate upcut length and follow the manufacturer’s pass-depth guidance.
V-bits cut angled grooves and are a popular choice for names, decorative borders, and carved signs. With a V-carving toolpath, the software varies cutting depth to fit the design.
Two common choices are 60-degree and 90-degree bits. For the same groove width, a 60-degree bit cuts deeper than a 90-degree bit. Narrower angles can be useful for fine lettering; wider angles can produce broader grooves without cutting as deeply.
The angle alone does not guarantee crisp lettering. Tip condition, material, surface flatness, and an accurate Z-zero also affect the result. Enter the correct included angle in your software and preview the toolpath before cutting. A 60-degree bit and a 90-degree bit are not interchangeable in the same programmed toolpath.
A ball nose bit has a rounded cutting end. It is commonly used to finish curved 3D surfaces, such as carved landscapes and decorative reliefs.
It is not the best choice when you want a flat pocket bottom. Its strength is following contours that a flat-bottom cutter cannot reproduce as smoothly.
The spacing between neighboring tool passes is called stepover. A smaller stepover generally leaves smaller ridges and a smoother surface, but increases cutting time. Preview the finish and estimated machining time to find a useful balance for your project.
A tapered ball nose combines a small rounded tip with a wider tapered body. That shape provides more support than a straight tool with the same small tip diameter, making it useful for detailed relief carvings, wildlife scenes, and textured designs.
Read the specifications carefully. Shank diameter, tip diameter, tip radius, and taper angle describe different parts of the tool. A 1/16-inch tip diameter has a 1/32-inch tip radius. Mixing up those measurements changes how the software models the cutter.
The taper also limits access near steep walls. Enter the actual tool geometry and inspect the preview rather than treating the cutter as a tiny straight ball nose. A separate roughing pass with a larger tool can remove bulk material before the small finishing tool handles the detail.
Acrylic is a plastic, but it deserves special attention because edge appearance and heat control matter so much. For small hobby CNC cutters, a single-flute spiral O-flute bit made for acrylic is a useful starting point. Onsrud identifies this geometry as particularly effective for acrylic edge finish at diameters of 1/4 inch and smaller.
The open flute provides room for chips to leave the cut. An upcut design can help lift them out, provided the sheet is held securely.
Check whether your material is cast or extruded acrylic. They machine differently, so a successful setup for one may need adjustment for the other. Also, a freshly routed edge is not automatically a polished, glass-clear edge; additional finishing may be needed for the appearance you want.
If plastic begins sticking to the cutter or welding back into the slot, stop and investigate. Possible causes include unsuitable tooling, poor chip evacuation, and an incorrect balance of feed rate and RPM. Simply slowing the machine’s travel can make the problem worse by reducing the size of each chip while the bit keeps spinning.
Do not assume every plastic will cut like acrylic. HDPE, PVC, ABS, and polycarbonate have different machining characteristics, and even products in the same material family can behave differently.
A plastic-cutting O-flute is a useful option for many softer plastics. Choose a cutter whose manufacturer specifically lists your material. Harder plastics may call for a different geometry or a finishing tool.
For a hobby project such as an HDPE jig or a plastic sign component, make a short test cut first. Check chip evacuation, edge finish, and whether the part stays firmly supported. Adjust one variable at a time so you can identify what improved the result.
A surfacing bit sweeps a wider area to flatten a spoilboard or suitable project blank. A flat spoilboard helps support consistent cutting depth, especially for shallow engraving and V-carving.
Select a cutter diameter, shank, and operating speed that your CNC and router or spindle support. A larger cutter is not automatically a better choice for a hobby machine. Check clearance around clamps and dust collection, and follow the cutter’s speed limit and cutting recommendations.
Feed rate is how fast the bit travels through the material. RPM is how fast it rotates. Chip load describes the material removed by each cutting edge and connects feed rate, RPM, and flute count:
Chip load = Feed rate ÷ (RPM × Number of flutes)
Use consistent units: inches per minute produces inches per tooth. More flutes are not automatically better; at the same feed rate and RPM, adding flutes reduces chip load.
Start with the manufacturer’s data for the exact cutter and material, then account for your machine, workholding, pass depth, and toolpath. If an industrial recommendation exceeds your machine’s capacity, seek a suitable combination of tool size, flute count, RPM, and cutting depth. Avoid copying a feed rate by itself.
You do not need every bit style to begin. For someone starting with wood signs and simple cutouts, our suggested first purchases are a flat end mill and a V-bit. Add a downcut or compression cutter when plywood edge quality becomes a priority, a tapered ball nose for detailed 3D work, and a material-specific O-flute when you begin cutting plastics.
Before each project, confirm that the cutter fits your collet, the tool definition matches the bit, the material is secure, and the toolpath clears your clamps. Follow your equipment’s operating instructions and test unfamiliar combinations on scrap.
Choose the bit for the cut you want to make. As your projects expand, your bit collection can grow with them.
