---
title: "CNC Routing DFM: Designing Flat Parts for a Router, Not a Mill"
description: "Routing is held down, not clamped, and it cuts sheets rather than blocks. How hold-down, tabs, nesting and tool diameter change what you should draw."
canonical: https://openspindle.com/blog/cnc-routing-dfm
author: "Tom"
datePublished: 2026-09-22T00:00:00.000Z
dateModified: 2026-09-22T00:00:00.000Z
category: "Process & DFM"
---

# CNC Routing DFM: Designing for a Router, Not a Mill

A CNC router cuts flat sheet material that is held down by vacuum rather than clamped in a vise, so the design rules differ from milling in three ways. Cost follows cut path length and sheet yield rather than material removed, small parts need tabs or onion skins to stop them moving once they are cut free, and the smallest internal corner radius is set by the cutter, which on a router is usually larger than on a mill. Design in sheet terms: nest-friendly outlines, generous internal radii, and features no smaller than the tool can reach.

## Three differences that change every rule

A router looks like a [mill](/capabilities/cnc-machining) and behaves differently in ways that matter for design.

**The part is held by vacuum, not by a vise.** Routing runs on a flat bed that pulls the sheet down with suction. That grip is strong across a large area and weak on a small one, which is why a small part behaves completely differently from a large one on the same machine. Nothing is holding the part from the sides.

**The stock is a sheet, not a block.** You are buying an area of material, and what you pay for is your share of the sheet including whatever is wasted around your part. Material cost is a nesting question, not a volume question.

**Cost follows the path, not the volume.** A [milled part](/blog/what-is-dfm) is priced largely by how much material comes off. A routed part is priced largely by how far the cutter travels and how many times it plunges. A large simple outline is cheap. A small part with forty internal cutouts is not, even though the second part uses less material.

Everything below follows from those three facts.

## Hold-down is the constraint nobody designs for

The moment a part is cut free of the sheet, the vacuum holding it disappears with the material around it. A loose part on a moving spindle is a scrapped part at best.

Shops solve this in three ways, and each has a design consequence.

**Tabs.** Small bridges of uncut material left around the outline, broken out by hand afterward. They leave a witness mark that has to be sanded. If one edge of your part is cosmetic or is a sealing surface, say so, and the shop will put tabs elsewhere. If you say nothing, tabs go wherever is convenient.

**Onion skin.** Cutting almost through and leaving a thin continuous layer, then separating afterward. Cleaner than tabs and it leaves a light film to remove on the back face.

**Fixturing or sacrificial substrate.** The part is glued, taped or screwed to a spoilboard. Better results, more setup cost, usually reserved for small parts or thin material that vacuum alone cannot hold.

**Design response:** the smaller your parts are relative to the sheet, the more hold-down work the job carries. A part below roughly the size of your palm is in the territory where tabs and fixturing drive the price. If you have many small parts, that is worth knowing before you compare a routing quote to a [laser](/capabilities/co2-laser-cutting) or [waterjet](/capabilities/waterjet-cutting) quote, where hold-down is not the same problem.

## The tool sets your inside corners, and router tools are not small

Like milling, an internal corner cannot be sharper than the cutter that makes it. Unlike milling, routers commonly run larger diameter cutters, because they are removing material fast through thick sheet and a small cutter would deflect or snap.

If a shop is running a 6 mm cutter, every internal corner in your part has a 3 mm radius whether you drew it or not. Drawing a sharp corner does not produce one. It produces a corner with the tool's radius and, if the tolerance implies sharpness, a conversation.

Two practical responses. Draw the radius you expect and make it slightly larger than the likely tool radius, which lets the cutter sweep the corner without dwelling. Or, where a mating part needs to sit into a true corner, design a relief: a small circular cutout at the corner, larger than the cutter, so the mating part has clearance where the radius would otherwise interfere. That relief is standard practice in joinery and panel work and it is far cheaper than asking for a sharp corner.

**Design response:** decide your minimum internal radius early, tell the shop what it is, and keep slots and narrow channels wider than the cutter that has to travel down them.

## Nesting is where the material money is

Since you are buying sheet area, the shape of your outline determines how much sheet gets wasted.

Parts that tile well, meaning rectangles, and outlines whose convex shapes can interlock, use far less sheet than parts with large concave areas that nothing fits into. A long thin diagonal part is the worst case: it can consume a surprising share of a sheet while using very little material.

Three things a designer controls:

**Overall size relative to standard sheet dimensions.** A part 1300 mm long on a 1220 mm wide sheet forces a larger and more expensive sheet, or a different material. Check your dimensions against the standard sheet sizes for the material before finalizing them. Trimming a few millimeters can change which sheet the job runs on.

**Whether parts can share cut lines.** Rectangular parts nested against each other can sometimes be cut with a shared pass, which cuts path length and cost.

**Quantity.** Yield improves substantially with quantity, because leftover areas get filled. This is why routing quotes often drop sharply between one part and ten, more so than machining quotes do.

**Design response:** ask what sheet size the shop runs and design to it. This is one of the few places where a conversation before the design is frozen saves real money.

## The material matters more than in milling

A router runs across a much wider range of materials than a mill typically does, and each behaves differently.

| Material | How it cuts | Design consequence |
| --- | --- | --- |
| Plywood and MDF | Fast, forgiving. MDF cuts cleanly, plywood can splinter on the top face. | Specify which face is cosmetic. Climb versus conventional cutting changes which face tears out. |
| Acrylic and polycarbonate | Melts if the chip does not clear. Acrylic chips and cracks at sharp corners. | Generous radii, no sharp internal corners, and expect a flame or vapor polished edge to be a separate operation. |
| HDPE and UHMW | Cuts easily, moves a lot. Internal stresses release when cut. | Expect some warp in large thin parts. Symmetric cutouts help. |
| Aluminum sheet and plate | Routable with the right tooling and feeds, slower than wood or plastic. | Thinner sheet may chatter without good hold-down. Deburring is a real line item. |
| Carbon fiber and G10 laminates | Abrasive. Destroys tooling, generates hazardous dust. | Expect tooling cost in the quote, larger radii, and edge sealing considerations. See the composites guide. |
| Foam and soft tooling board | Very fast, very deep passes possible. | Nearly no constraint on geometry. Cost is almost entirely path length. |

*The right cutter, feed and spindle speed for each of these is the shop's decision, not yours. What you control is radii, part size, and how much of the sheet you use.*

## Tolerances and what to send

**Be realistic about tolerance.** A router working on a large sheet holds position well, but the material moves. Wood breathes with humidity, plastics relax when cut, and large thin parts are never as flat as your model. Ask for tight tolerance on the features that mate with something, not across the whole part, and expect looser tolerance across a long dimension than across a short one. The arithmetic for deciding which dimensions earn a tight tolerance is in [tolerance stack-up](/blog/tolerance-stack-up).

**Send flat geometry in a flat format.** A DXF is the natural format for routed parts, with two conventions worth respecting. Closed vectors: an outline that does not close is ambiguous about what is being cut. One entity per feature: overlapping duplicate lines cause double cutting, and they are the single most common problem in customer-supplied DXF files.

**Separate cut types by layer.** Through cuts, pockets, engraving and drill points on their own layers with clear names. A file where everything is on layer 0 needs interpretation, and interpretation is where errors enter.

**State the thickness you mean.** Nominal sheet thickness and actual thickness differ, particularly in plywood and in composite laminates. If the design depends on the thickness, such as a slot that another panel fits into, say whether you designed to nominal or to measured, since that decides whether the joint is loose or tight.

**Say which face is up.** The top face and the bottom face have different edge quality, and the bottom face is against the spoilboard. If one side is visible in the finished product, that is a requirement, not a preference.

## Related reading

- [CNC routing](https://openspindle.com/capabilities/cnc-routing.md)
- [Waterjet cutting](https://openspindle.com/capabilities/waterjet-cutting.md)
- [CO2 laser cutting](https://openspindle.com/capabilities/co2-laser-cutting.md)
- [CNC machining](https://openspindle.com/capabilities/cnc-machining.md)
- [Carbon fiber cutting methods](https://openspindle.com/carbon-fiber-cutting.md)
- [Which manufacturing process at what volume](https://openspindle.com/blog/choosing-a-manufacturing-process.md)
- [What is DFM?](https://openspindle.com/blog/what-is-dfm.md)

## Frequently asked questions

### What is the difference between CNC routing and CNC milling?

A router is built to move a spindle quickly over a large flat area, holding sheet material down with vacuum. A mill is built for rigidity and holds a block in a vise. Routing suits large flat parts in sheet materials, milling suits smaller parts with deep features and tight tolerances, and the design rules differ mainly in hold-down, corner radii and how cost is driven.

### Why do routed parts have tabs?

Because the vacuum holding the sheet stops holding a part the moment it is fully cut free. Tabs are small uncut bridges that keep the part attached until the job finishes, then get broken out by hand. They leave a small witness mark, so if one edge must stay clean, tell the shop which one before the job is programmed.

### What is the smallest internal radius a router can cut?

Half the diameter of the cutter, and router cutters are typically larger than mill cutters because they are moving fast through thick material. Ask the shop what diameter it plans to run and design your corners slightly larger than that radius. A sharp corner on the drawing does not produce a sharp corner on the part.

### How do I get a true sharp internal corner in a routed joint?

Add a corner relief: a small circular cutout at the corner, slightly larger than the cutter, so the mating part has somewhere to sit. This is standard in panel and joinery work, and it is much cheaper than asking for a corner the tool physically cannot make.

### How should I prepare a DXF for routing?

Closed vectors, one entity per feature and no duplicate overlapping lines, with different operations on clearly named layers. Duplicated geometry causing double cuts is the most common problem in customer-supplied files, and it is easy to miss in CAD because the duplicates sit exactly on top of each other.

### Does the quantity change the price per part much in routing?

More than it does in machining, because of nesting. One part occupies a sheet inefficiently, while ten parts fill the gaps around each other. If you are near a quantity break, ask for the price at your quantity and at two or three times it, since the yield improvement can be larger than you expect.