CNC Bits Explained: Which Endmill for Which Job

The machine came with two bits. One of them has been in the collet since the day you set it up, and it has cut every project since, because it works.

Then you carve a detailed sign and the fine parts come out mushy. Or you cut a plywood part and the top face splinters along every edge. Or you try aluminum and the bit welds itself to the workpiece.

The bit was never wrong. It was just answering a question nobody asked it.

The Vocabulary

Five numbers describe any bit, and the listings assume you know all five.

Shank diameter is the part that goes in the collet, usually 1/4 inch or 1/8 inch on hobby machines. Your collet decides what you can use, and adapters exist but reduce rigidity.

Cutting diameter is the width of the cut itself. It is often the same as the shank and frequently is not.

Flute count is how many cutting edges spiral around the bit.

Flute length is how deep it can cut. A bit whose flutes are shorter than your material cannot cut through it in one pass no matter how you set the depth.

Overall length matters for rigidity. Long bits deflect, and deflection shows up as tapered walls and inaccurate parts.

Flute Count Changes Everything

More flutes means more cutting edges touching the work, which sounds better and is not automatically.

The tradeoff is chip clearance. Every flute is also a channel for waste to get out of the cut. Fewer flutes means bigger channels, which is exactly what soft gummy materials need to avoid packing and melting. More flutes means a smoother finish, as long as the material makes small chips.

Plastics in particular want a single flute. Running acrylic with a four-flute bit is a reliable way to produce a welded mess.

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Upcut, Downcut, Compression

This is the distinction that fixes the splintered-edge problem, and it is about which way the spiral throws the chips.

Upcut spirals waste upward, out of the cut. Excellent chip evacuation, which means less heat and less burning, and it leaves a clean bottom edge. The cost is that it lifts as it cuts: the top surface can fuzz or splinter, and thin or poorly held parts can be pulled up off the bed.

Downcut spirals waste downward. It presses the material against the bed, which holds small parts in place and leaves a beautifully clean top edge. The cost is that chips have nowhere to go but into the cut, packing in and building heat, which means burning on deep cuts.

Compression does both: upcut geometry at the tip, downcut above it. Both faces come out clean, which is exactly what veneered plywood needs. The catch is that it only works properly when the cut is deep enough for both sections to engage, so it is a full-depth-cut tool rather than a shallow-pass one.

For most hobby work in solid wood and MDF, an upcut two-flute is the right default. Switch to downcut when the top face is what shows and the part is well held. Reach for compression when you are cutting plywood with a face veneer you care about.

Shaped Bits for Shaped Jobs

V-bits come to a point at a set angle, usually 60 or 90 degrees. As the machine plunges deeper, the cut gets wider, which is what lets a V-carve produce lines that vary in width. A narrower angle gives finer detail at the same depth; a wider angle covers more width for less plunge. Our V-carving explainer covers how the toolpath actually works.

Ball nose bits have a rounded tip, which is what you want for 3D contours and relief carving. They leave a scalloped surface, and how visible those scallops are depends on your stepover: closer passes give a smoother result and take much longer. This is the bit for turning a depth map into a carved relief, which is what ReliefMaker generates.

Tapered ball nose bits give you a small tip for fine detail on a thicker, more rigid body. They are the standard choice for detailed 3D work where a straight small-diameter bit would deflect or snap.

Straight flute bits do not spiral at all. Less aggressive than either spiral direction, they sit in the middle on both edge quality and chip clearance.

Surfacing bits are wide and flat, made for flattening a spoilboard or a slab rather than cutting parts.

Bit Diameter Is a Design Constraint

This is the one that catches people who design first and cut later.

A round bit cannot cut a sharp internal corner. The tightest inside corner you can produce has a radius equal to half your bit diameter. Design a slot with square internal corners and a 1/4 inch bit, and you get corners with a 1/8 inch radius whether you wanted them or not.

Two consequences worth knowing:

Joinery needs relief. If a tab has to seat fully into a slot, the slot needs relief cuts at its corners, usually dogbone or T-bone shaped, so the leftover radius does not block the tab. Laser cuts do not have this problem, which is why a file that cuts perfectly on a laser can fail on a router.

Fine detail needs a small bit, and small bits are fragile. A 1/16 inch bit will reach detail a 1/4 inch bit cannot, and it will also snap if you feed it like the bigger one. That is a feeds and speeds conversation, and our feeds and speeds guide covers it.

Picking by Job

Making Them Last

Bits are consumable, and most die early for avoidable reasons.

A dull bit burns rather than cuts. If cuts that used to be clean are now brown and smell scorched, suspect the bit before the settings.

Resin buildup acts like dullness. Pitch and glue residue coats the flutes and stops them cutting. Cleaning it off restores a bit that seemed finished.

Never let bits touch each other. Loose in a drawer, carbide edges chip against each other. Keep them in the tubes or a block.

Full-depth cuts kill bits. Multiple shallower passes take longer and cost less in tooling, and they produce better edges.

Seat the shank properly in the collet, and not so deep the flutes are buried. A poorly seated bit can pull out mid-cut, which ruins the part and often the bit.

Our common CNC mistakes post covers the rest of the early lessons, and workholding for beginners covers keeping the material where it should be while all this is happening.

Frequently Asked Questions

What is the difference between an upcut and a downcut bit?

Upcut spirals chips up and out, giving good chip clearance and a clean bottom edge, but it can lift the workpiece and fuzz the top. Downcut pushes chips down, holding the material and leaving a clean top edge, at the cost of packing chips into the cut.

When should I use a compression bit?

On plywood and other veneered sheet goods where both faces need to be clean. It needs enough cut depth for both the up and down sections to engage, so it suits full-depth cutting rather than shallow passes.

How many flutes do I need for wood?

Two is the general-purpose answer for wood, MDF, and plywood. Use one flute for plastics and aluminum where chip clearance matters most.

What bit do I use for carving letters?

A V-bit, typically 60 or 90 degrees. The narrower angle gives finer detail, the wider angle covers more width for the same depth.

Why do my inside corners come out rounded?

Because a round bit cannot cut a sharp inside corner. The smallest radius you can get is half your bit diameter, which is why joinery needs relief cuts at the corners.

Why did my bit break?

Usually feeding too fast for its diameter, cutting too deep in one pass, or a small bit run at settings meant for a bigger one. Deflection and heat kill bits well before wear does.

Buy Three, Not Twelve

If you are starting out, you do not need a set of thirty. A 1/4 inch two-flute upcut, a 60 or 90 degree V-bit, and a small ball nose will cover parts, lettering, and 3D work between them.

Add a compression bit when you start cutting nice plywood, and a single flute when you first cut acrylic. Everything else can wait until a specific job asks for it.

When you are ready to make something for those bits to cut, Vector Studio generates machine-ready SVG designs, and ReliefMaker turns a photo or a prompt into the depth map a ball nose carves. If you are unsure which bit a particular job wants, Craft Chat is free with your account and happy to be asked.