What does a CNC machining service actually do?
A CNC machine moves a cutting tool along paths set by a computer program. The tool removes material from a solid piece, a block of aluminium say, until what is left is your part. Because the machine follows a program, it can repeat the same part again and again with little variation.
Compared with 3D printing, machining works from a solid piece, so the part has the strength of the material. It can give flat surfaces, accurate holes and good fits. It is also slower to set up for one-offs, and it cannot make everything. A tool has to reach each surface, so a hollow shape with a closed inside is a job for printing, not machining.
What can it make, and what can it not?
Typical jobs are brackets, plates, housings, fixtures, spacers, shafts and mounting parts. Common materials include aluminium, brass, steel and plastics such as acetal or nylon. Which of those a fabricator can cut depends on their machine. A small desktop mill usually handles plastics and soft metals, within its limits. Harder metals and bigger parts need a stiffer, more powerful machine.
Be careful with these:
- Sharp inside corners. A round cutter leaves a radius in every inside corner, so a perfectly square pocket is not possible. Design for a corner radius, or ask what the shop can do.
- Deep, narrow pockets. A long thin tool bends and chatters, so the shop may refuse or charge more.
- Thin walls. They flex while being cut and can end up uneven.
- Very small features. The tool has a size. If a slot is narrower than the smallest cutter, it cannot be cut.
- Parts that need several set-ups. Machining the back of a part means flipping it, and each flip adds time and a chance of misalignment.
If you are not sure whether a shape can be machined, send it and ask. A fabricator will tell you, often with a suggestion that costs less.
What do I need to send for a quote?
- A drawing or a CAD file. A STEP file is the most useful format because the shop can open it in any software. A PDF drawing with dimensions helps for anything with tolerances. If you have only a sketch and measurements, say so.
- The material, or what the part has to do. If you give the job, the fabricator can suggest the material.
- Quantity. One part and fifty parts are different jobs. Setup time is spread across the batch.
- Tolerances on the dimensions that matter. Mark which ones are critical and say the rest can be general.
- Surface finish and treatment: as machined, deburred, anodised, painted, plated. Some of these are done by another shop, which affects time.
- The deadline, and pickup or shipping.
Why do tight tolerances cost more?
Tolerance is how much a dimension is allowed to vary from what you asked for. A tight tolerance asks the shop to cut slower, take lighter passes, measure the part during the job and sometimes cut it twice. Parts can be rejected and remade. All of it takes machine time and care, and that is what the price is built on.
The practical advice is simple. Apply tight tolerances only where the part really needs them, such as a bearing seat or a hole that takes a pin. Leave the rest loose. A drawing where every dimension is marked tight costs more than one that marks the important ones. What a shop can hold depends on the machine, the material and the part, so ask what they can reliably achieve.
What should I ask a fabricator?
- Can you make this on your machine, in this material, at this size?
- What tolerance can you hold on the critical dimensions, and how will you check them?
- What is included: material, programming, finishing, deburring, packing?
- What happens if a part is out of tolerance?
- How long, counting from the day I pay?
- Do you have examples of similar parts?
The fabricator directory lets you search for CNC. Look at profiles for examples of work, and message two or three with the same brief. Keep the agreement and the order on PrintCommons; how it works explains payment and delivery. If you are working out which process fits, custom parts made near me compares printing, machining and cutting.
Where do I start?
Write the brief, find a CNC fabricator and send it. If you would first like to see what is available as finished designs, try the marketplace. For a first handover in person, read how to prepare your first local pickup.