How do you machine an inclined face?
There is a face on the part that is not parallel to the table, and you have to drill into it or mill a pocket on it. There are three ways:
- Clamp the part at an angle using an angled fixture. Every angle needs its own fixture and setup takes a long time.
- Work the coordinates out by hand — rotate every point trigonometrically and write it into the program. Tedious and error-prone.
- Rotate the coordinate system — tell the machine “this face is now your XY plane”. After that you program the machining as ordinary X/Y.
The cycle that does the third is CYCLE800. This is usually called 3+2 axis machining: two rotary axes position the part or the head, then stay in that position, and the cutting is done by the three linear axes.
What exactly does CYCLE800 do?
The cycle rotates the coordinate system into the plane to be machined. You can program it in two different ways:
- Program the machine’s rotary axes (A, B, C) directly, or
- Program the rotation about the geometry axes (X, Y, Z) of the workpiece coordinate system — that is, the way it appears on the drawing.
If you choose the second, the control automatically converts the rotation of the workpiece coordinate system into the rotation of the machine’s rotary axes during machining. You enter the angle you read off the drawing and the control does the kinematic maths. This is what makes 3+2 accessible.
Three rules
- The rotary axes are always swiveled so that they are perpendicular to the tool axis of the plane being machined.
- The machining plane stays fixed during machining.
- When the coordinate system is rotated, the zero offset that was already set is automatically rotated with it.
That last point matters: you do not have to re-take G54, the control rotates it too.
Machine kinematics: types T, P and M
Machines fall into three groups depending on what performs the swivel:
| Type | Configuration | What rotates? |
|---|---|---|
| T | Swivel head | The tool carrier can be swiveled; the part is fixed |
| P | Rotary table | The part is swiveled; the tool axis is fixed |
| M | Swivel head + rotary table | Mixed motion — both |
This distinction is not academic: enter the same angle and the machine moves completely differently depending on the type. On a rotary-table machine the part swings and the fixture can end up in the tool’s path; on a swivel-head machine the head swings and there is a risk of hitting the column.
The programming logic: three steps
The source document sums it up in three lines:
- Rotate the coordinate system into the plane to be machined.
- Program the machining normally in X/Y. After the rotation you write as if you were working on a flat face.
- Rotate the coordinate system back to its original position.
Forget the third step and everything that follows runs in the rotated coordinate system — one of the most common mistakes in 3+2.
Setting the cycle up
- Call the swivel function in the program.
- Enter the name of the swivel data record.
- In the swivel prompt choose new or additive.
- Enter the reference point before swiveling (X0, Y0, Z0).
- Choose the swivel mode: axis by axis, projection angle or solid angle.
- Enter the angle to swivel through. In axis-by-axis mode each axis gets its own angle.
- Enter the zero point after swiveling.
The input screen parameters
| Parameter | What it is for |
|---|---|
Swivel data record _TC | Select among the configured swivel data records. Each record has a name; if there is only one record the name need not be given. Entering 0 deselects the record. |
Retraction _FR | Retraction before swiveling on the rotary axis. Options: no retraction · retract Z · retract Z and XY · maximum retraction in the tool direction · incremental retraction in the tool direction (the last two from Cycles SW 6.5 upwards). |
Swivel direction _DIR | Swivel yes → the rotary axes are positioned, or moved into the correct position by hand. Swivel no (calculate only) → the rotary axes do not move; only the calculation is done. Plus the minus / plus direction choice. |
Swivel plane _ST | New → previous frames are deleted and the values entered become an absolute swivel frame. Additive → the swivel frame is programmed additively on top of the previous one (like AROT / ATRANS) and the current rotation in the zero offset is taken into account. |
| Reference point X0, Y0, Z0 | The reference before swiveling. The help screens are based on the G17 machining plane (tool axis Z). |
Swivel mode _MOD | Axis by axis · projection angle · solid angle (below). |
| Rotation about A, B, C | The angle values entered, according to the selected mode. |
| Zero point after swiveling | The zero to work from in the swiveled plane. |
Swivel modes: three ways of describing an angle
Axis by axis
You enter the rotation about the axes of the coordinate system. The axis sequence can be chosen freely — Z first then X, for instance. Each axis gets its own angle.
Projection angle
The angle of the swiveled face is projected onto the first two axes of the coordinate system. If the drawing gives the angles across two views, this mode makes life easier.
Solid angle
The rotation is given first about Z, then about Y. The order is fixed.
New or additive? And the two-solution problem
New (absolute) and additive swiveling
With new, the previous frames are deleted and the values you enter become an absolute swivel frame. With additive, the new swivel is added on top of the previous one and the current rotation in the zero offset is taken into account.
Several swivel operations can be programmed one after another, and a later one can build on the previous one.
The NCU may find two solutions
This is the most confusing part of 3+2. Depending on the angular range of the machine’s rotary axes, the NCU can calculate two possible solutions — the same plane can be reached with two different axis combinations. Usually one of them is technologically suitable.
- Which rotary axis (1st or 2nd) is used as the reference for the two solutions is selected in the CYCLE800 start menu.
- The choice of “minus” or “plus” direction determines which of the two solutions is applied.
What changes after the swivel?
- The zero offset rotates by itself. A zero offset that was already set is automatically rotated with the coordinate system; you do not re-take G54.
- You program as if on a flat face. After swiveling, machining is programmed normally in X/Y.
- The machining plane stays fixed. The rotary axes do not move while cutting.
- Tool length becomes critical. When the head swings, the volume a long tool sweeps is large — and the whole point of 3+2 is reaching the inclined face with a short tool.
The block structure above is there to show the logic; the CYCLE800 parameters depend on the machine’s kinematics, the active swivel data record and the mode selected, and must be filled in from the input screen on the panel. Do not guess the parameter order.
Common mistakes
Not retracting before the swivel
The most expensive mistake in 3+2. If the swivel starts with the tool in or near the part, a crash is unavoidable.
Forgetting to swivel the coordinate system back
Everything that follows runs in the rotated plane. The third step is not optional.
Entering the angle in the wrong swivel mode
Axis-by-axis, projection angle and solid angle want different numbers for the same face.
Choosing “additive” instead of “new” (or the other way round)
With additive the swivel stacks on the previous one and the angles add up.
Picking the wrong one of the two solutions
The minus/plus direction and the reference rotary axis decide which route the machine takes.
Swiveling with a long tool
When the head swings, the volume a long tool sweeps grows enormously. Use the shortest tool you can.
Running another machine’s program without knowing the kinematic type
Type T, P and M machines move completely differently for the same angle.
Frequently asked questions
Do I need a 5-axis machine for 3+2?
You need two rotary axes — a swivel head, a rotary table or a combination. The difference is that the rotary axes do not move while cutting; you do not need simultaneous 5-axis dynamics.
Do I have to re-take G54 after swiveling?
No. When the coordinate system is rotated, the zero offset that was already set is automatically rotated with it.
Can I have it calculate without moving the rotary axes?
Yes. The “swivel no (calculate only)” option in _DIR is exactly for that; use it when you want an auxiliary swivel plane based on the drawing.
If there are several inclined faces, do I write a separate cycle for each?
Yes, and several swivel operations can be programmed one after another. If a later swivel is to build on the previous one, choose additive.
Can swiveling be done in JOG as well?
Yes, swiveling can be done in both JOG and AUTOMATIC. The input screens come with help graphics.
Why 3+2 rather than simultaneous 5-axis?
Most die and mold features can be machined in 3+2. It is simpler to program, easier to control, lets you keep the tool short and usually gives a more consistent surface. Simultaneous 5-axis is only necessary where the surface normal changes continuously.
Sources
- Siemens AG — SINUMERIK Advanced CNC Operation and Programming, 05/2010: Section 3.3 Swiveling — CYCLE800 (function, the T/P/M kinematic types, the programming procedure, the input screen parameters _TC, _FR, _DIR, _ST, _MOD, the two solutions and the minus/plus direction), 3.4 Programming example — swiveling, 4.4 and 4.5 Milling with 3 axes or with 3+2 axes
- Siemens AG — SINUMERIK 840D sl / 828D Job Planning, Programming Manual, 01/2015, document no. 6FC5398-2BP40-5BA2: coordinate transformations (frames) section — the relationship between the swivel frame and the zero offset