A 3-axis mill reaches only the faces it can see from above, so a part with features on several sides needs several set-ups. A 5-axis machine tilts the part or the head and can usually finish it in one. The 5-axis hourly rate is higher, but on complex parts in small and medium batches the set-ups, fixtures and handling it saves can make it the cheaper route per part.
Key points
- 3-axis machines move in X, Y and Z; 5-axis machines add two rotary axes to reach more faces in one clamping.
- 3+2 (positional) machining covers most multi-face parts; full simultaneous 5-axis is for sculpted surfaces.
- 5-axis costs more per hour and per programming hour, but saves set-ups, fixtures and re-clamping.
- Fewer set-ups also means features on different faces line up better.
- For simple parts in long runs, a 3-axis machine with good fixtures is often cheaper.
What the axes mean
3-axis machining moves the cutter left and right (X), forward and back (Y) and up and down (Z). The tool always points straight down. Anything the tool can reach from above can be cut in one set-up; anything on a side face, the underside or at an angle needs the part re-clamped in a new orientation. Each re-clamp is another set-up, often with its own fixture or soft jaws.
5-axis machining adds two rotary axes, either by tilting and rotating the table (trunnion machines) or by swivelling the head. The tool can approach the part from most directions, so five faces and angled features can be cut without re-clamping. Only the face held by the workholding is out of reach.
Between the two sits 4-axis machining (one rotary axis, usually for parts that wrap around an axis) and 3+2 machining.
| 3-axis | 3+2 (positional 5-axis) | Simultaneous 5-axis | |
|---|---|---|---|
| How it cuts | Tool vertical, three linear axes | Tilts the part to a fixed angle, then cuts in 3 axes | All five axes move together during the cut |
| Faces per set-up | One | Up to five | Up to five, plus continuous curved surfaces |
| Angled holes and faces | Need a new set-up or angled fixture | Yes | Yes |
| Sculpted surfaces | Only with small step-overs and limits | Partly | Yes: impellers, blades, moulds |
| Programming | Simplest | Moderate | Most demanding, needs collision checking |
| Typical hourly rate | Lowest | Higher | Highest |
Why 5-axis can be cheaper per part
The hourly rate is only half the picture. A part that needs four set-ups on a 3-axis machine pays for each set-up once per batch, for any fixtures those set-ups need, and for loading and unloading each part four times. Our article on CNC set-up time shows how quickly those fixed blocks add up.
A 5-axis machine also allows shorter tools. When the head can tilt, the cutter can approach a deep feature from the side rather than reaching down into it, and short tools cut faster with less vibration and a better finish. That shortens the toolpath and the cycle.
Accuracy is the other gain. Each re-clamp introduces a small location error. Features cut in one clamping line up with each other as well as the machine can manage; features cut in different set-ups line up only as well as the operator relocated the part.
Why 3-axis is often still the right answer
A 5-axis machine costs considerably more to buy than a 3-axis machine of similar size, and that shows up in its machine hour rate. Programming takes longer, because the programmer has to choose tool orientations and check for collisions. Trunnion tables also limit the size and weight of the part.
For a flat plate, a simple pocketed part or anything machined from one or two sides, the 3-axis machine is cheaper and perfectly capable. In long runs, a 3-axis shop with good multi-part fixtures can match or beat a 5-axis machine per part, because the fixture cost is spread thinly and re-clamping is quick.
Workholding is different on a 5-axis machine too. To let the cutter reach the sides of the part, the part is often held on a small dovetail or a raised fixture, which may need extra stock left on the underside and then removed in a second, short operation. That extra material and the final facing-off belong in the estimate. Large or heavy parts may not fit a trunnion table at all and go to a 3-axis machine with a larger bed, or to a head-type 5-axis machine with a higher rate still.
Worked example: finding the break-even batch size
The figures are illustrative assumptions for one shop, not market rates. The part is an aluminium housing with features on four faces. On the 3-axis machine it needs four set-ups and two custom fixtures; on the 5-axis machine it needs one set-up using a standard vice. Programming is charged at €70 per hour on both.
Worked example: costs per route (illustrative rates)
| Line | 3-axis at €60/h | 5-axis at €96/h |
|---|---|---|
| Programming | 3 h × €70/h = €210.00 | 4 h × €70/h = €280.00 |
| Set-ups | 4 × 30 min × €1.00/min = €120.00 | 1 × 45 min × €1.60/min = €72.00 |
| Fixtures | 2 × €80.00 = €160.00 | none |
| Fixed cost per order | €490.00 | €352.00 |
| Cutting per part | 32 min | 26 min |
| Loading per part | 4 loads × 3 min = 12 min | 1 load × 3 min = 3 min |
| Machine time per part | 44 min × €1.00/min = €44.00 | 29 min × €1.60/min = €46.40 |
Cost per part at three batch sizes: (fixed ÷ quantity) + per-part cost
| Quantity | 3-axis | 5-axis |
|---|---|---|
| 1 | €490.00 + €44.00 = €534.00 | €352.00 + €46.40 = €398.40 |
| 10 | €49.00 + €44.00 = €93.00 | €35.20 + €46.40 = €81.60 |
| 100 | €4.90 + €44.00 = €48.90 | €3.52 + €46.40 = €49.92 |
Break-even: 490 + 44q = 352 + 46.4q, so q = 138 ÷ 2.4 = 57.5 parts.
With these assumptions, the 5-axis machine is cheaper for orders of up to 57 parts and the 3-axis machine from 58 upwards. The crossover moves with the inputs: more faces, more fixtures or more expensive fixtures push it up; a simpler part or a cheaper 3-axis rate pulls it down. The point is not the number but the method: compare fixed and per-part costs for each route and find where the lines cross.
Which parts need which machine
| Part | Usually best on | Why |
|---|---|---|
| Flat plate with holes and pockets | 3-axis | One or two faces, simple fixturing |
| Bracket machined from two sides | 3-axis | Two set-ups, quick with soft jaws |
| Housing with features on four or five faces | 3+2 | One set-up instead of several, better alignment |
| Part with angled holes or faces | 3+2 | No angled fixtures needed |
| Impeller, turbine blade, mould with deep cavities | Simultaneous 5-axis | Continuous tool tilt to reach curved surfaces |
| Round parts with flats | Lathe or mill-turn | See CNC milling vs turning |
Features a 3-axis cutter cannot reach from any vertical direction, such as an undercut on a side wall, need either a special cutter, an extra set-up or more axes. Designers can often avoid them; the design for CNC machining guide shows how.
How quoting software decides
To price a milled part automatically, software has to work out which directions a cutter can reach each face from, how many set-ups that implies on each machine class the shop owns, and then price each route the way the worked example does. The CNC machining cost guide covers the rest of the estimate. Quotometry does this today: it measures which directions a tool can reach, counts set-ups for 3-axis, 3+2 and 5-axis machines, and prices the part on the cheapest machine that reaches it (see CNC milling).
Questions people ask
What is the difference between 3-axis and 5-axis machining?
A 3-axis mill moves the cutter in X, Y and Z, so it can only reach faces visible from above; other faces need the part re-clamped. A 5-axis machine adds two rotary axes that tilt and turn the part or the head, so the cutter can reach five sides, and angled features, in one set-up.
What is 3+2 machining?
3+2 (positional 5-axis) machining uses the two rotary axes to tilt the part to a fixed angle, then cuts with ordinary 3-axis moves. It reaches most faces in one clamping and is simpler to program than full simultaneous 5-axis, which moves all five axes at once and is needed mainly for sculpted surfaces such as impellers and blades.
Is 5-axis machining always more expensive?
Per hour, usually yes: the machine costs more to buy and program. Per part, not always. Removing set-ups, fixtures and re-clamping can make a 5-axis part cheaper, especially for small batches of parts with features on many faces. For simple prismatic parts in long runs, a 3-axis machine with good fixtures is often cheaper.
Does 5-axis machining improve accuracy?
It often improves the accuracy between features on different faces, because they are cut in one clamping rather than relocated across several set-ups, each of which adds a little error. It does not make a single feature more accurate than a well-run 3-axis machine would. The benefit is in relationships between faces, not in individual dimensions.
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