CALIBRATION & SETUPCalibration, collision, workholding and tools

◎Kinematics calibration (measuring the center of rotation)

• Why it's neededOne cause of error in multi-axis machining is the offset between the kinematics held by the NC (a model of the positions and orientations of the rotary axes) and the real machine. When the rotary axes move, this offset becomes error in the part, so the model must be brought closer to the real machine. The rotary axis vectors change with temperature and so on, so periodic checks are needed.^[1][2][3]
• How it's doneMount a rigid reference sphere (calibration sphere) on the table and, with a calibrated touch probe, measure the sphere center with the rotary axes turned to several angles. From the measured values, calculate the center position and orientation of the rotary axes and rewrite the NC's machine configuration parameters (kinematics table).^{[2][1][4][3][5][6]}

Example procedure

1. Calibrate the touch probe (a prerequisite for kinematics measurement)^[1]
2. Fix a rigid reference sphere of accurately known radius on the table (any position, as long as nothing collides when the rotary axes turn)^[1][2]
3. Before measuring, save the current kinematics (so you can restore it if the result is bad or after a power failure)^[1]
4. Measure the sphere at 3 or more angles for each rotary axis. Siemens CYCLE996 measures 3 times per axis, at angles about 120° apart (ideally forming an equilateral triangle)^[2]
5. After measuring all rotary axes, calculate and write the correction values (you can choose measure only or measure and rewrite)^[2][1]
6. Changing the kinematics also changes the presets (workpiece origins). Reset the presets after correcting^[1]

Each maker's calibration functions

• Example prerequisitesConditions for using KinematicsOpt: the touch probe is calibrated, the tool axis is Z, the kinematics description is complete with dimensions entered to about 1 mm accuracy, and the geometry of the whole machine has been measured by the machine maker. Coordinate transformations (datum shift, mirror, rotation, scaling) must be turned off (collision risk).One source / reference^[1]

≡Accuracy test standards and the ballbar

• JIS B 6336-6:2018 (ISO 10791-6:2014) Test conditions for machining centres — Part 6: Accuracy of feeds, speeds and interpolationsAccuracy tests of spindle speed and feed rate, and of interpolated paths made by simultaneous control of two or more linear and rotary axes. Applies to machines with three orthogonal axes plus one or two rotary axes (A, B, C). The annexes specify the motion tests for 5-axis machines for each configuration (two axes on the spindle head side = Annex A, two on the table side = Annex B, one each on the spindle head and table sides = Annex C).^[8][9]
• 5-axis interpolation test with a ballbarWith tool center point control enabled, move the rotary and linear axes simultaneously and use the ballbar to measure whether the distance between the tool tip and the workpiece stays constant. Most ISO 10791-6 tests can be done with a ballbar. Enter the tool length accurately as ‘from the gauge line to the center of the setting ball.’ The first time, run the program without the ballbar attached to check it.One source / reference^[9]
• JIS B 6336-7:2018 (ISO 10791-7:2014) Part 7: Accuracy of a finished test pieceEvaluates machining accuracy by cutting a standard test piece under finishing conditions. Covers 3- to 5-axis machining centers.One source / reference^[10]
• Example test piece at a university (square frustum)In research at Osaka Institute of Technology, a test piece from the then-proposed ISO/CD 10791-7 (a square frustum cut with simultaneous 5-axis linear motion) was machined using tool center point control, tool orientation control and workpiece setup error compensation, and evaluated by straightness, squareness and parallelism of the slopes. They identified abrupt feed changes and ‘a setting error in the C-axis center of rotation coordinates’ as error sources, and the result improved after resetting the center of rotation and lowering the feed.One source / reference^[11]
• Related standards (names only)JIS B 6336 is organized in parts from Part 1 (geometric accuracy of machines with horizontal spindle) to Part 10 (thermal distortion tests). Part 4 covers positioning accuracy of linear and rotary axes. It cites JIS B 6190-1 (corresponding to ISO 230-1:2012), the general rules for machine tool test methods.^[8][10]

!Collision checking and simulation

• 5-axis machines and multitasking machines have complex motion, and there is a risk of the spindle and table colliding not only in automatic operation but also during manual operation. There are two methods: constant checking with a simplified model in the NC (boxes, cylinders, planes), and checking ahead of time by running a 3D model close to the real shape on a PC.One source / reference^[12][13]
• Within the CAM workflow, run toolpath simulation and collision checking before outputting NC code with the post-processor. The post-processor normally outputs the path of the tool tip (TCP).One source / reference^[2]
• There is also a way to verify the program before the real machine with a virtual machine (Siemens VNCK) that runs the control software the same as the real machine.One source / reference^[2]
• Even with 3+2 indexing the tool and workpiece may collide, so there are times you want to index while watching with manual feed (handle).One source / reference^[14]
• Before indexing a tilted plane or rotating, retract the tool to a safe position (Heidenhain: pre-position to a non-colliding position. Haas: retract with G53 before rotating).^[15][16]

For CAM-side checks, see the CAD/CAM page.

⊓Holding the workpiece (5-axis vises)

• In 5-axis machining the tool also goes around the sides and underside of the workpiece, so you must hold the workpiece securely while avoiding collisions with the fixture and vise. Low-profile vises with small jaws are used.^[17][18]
• Grips the bottom of the stock with a minimum gripping depth of 3.5 mm for 5-face machining. The low-profile body gives the tool easy access.One source / reference^[17]
• Stamps the jaw shape into the stock in advance under high pressure (stamping) and holds it by form fit (positive locking). Gripping depth is 3 mm. Standard stamping jaws are for materials up to 35 HRC.One source / reference^[18]
• Kitagawa describes the V75V as a 5-axis centering vise with repeatability of ±0.01 mm that can do roughing and finishing on one vise.One source / reference^[17]

⌀Choosing tools

• Keep the stick-out short: with 5 axes you can tilt the tool to avoid holder–workpiece collisions, so you don't have to stick the tool out as far as on 3 axes. A short stick-out increases rigidity and improves accuracy, feed, and chatter.^[12][19][20]
• When you tilt a ball end mill, use an angle at which the tool center does not take part in cutting. Tilting and cutting with the center still digging in left marks (KISTEC experiment). The boundary angle is θ = arccos(1 − ap/R).One source / reference^[21]
• Barrel tool: a tool whose profile is barrel-like when rotated. In addition to the tip radius, several radius edges run along the periphery and back end, the peripheral edge in particular having a large radius. The large radius edge on the side allows high-efficiency cutting in 5-axis machining, tilting spindles and indexed machining.One source / reference^[22]

For the tilt angle calculation, use5-axis calculations. For ball end mill speed and feed, use theBall end mill calculator.