Measuring thermal drift of machine tools

Transcription

1 Measuring thermal drift of machine tools dr.ir. G.H.J. Florussen 19 th November 2015, Precisiebeurs, Veldhoven IBS Precision Engineering bv, Esp 201, 5633 AD Eindhoven Tel : + 31 (0) , Fax + 31-(0) ,

2 Outline Introduction Drift measurement setup and equipment Examples of classic drift measurements (ISO230-3) Example of new drift measurements (5-axes milling machine) IPT research Aachen (ir. M. Klatte) Rotary Inspector Closure 2

3 Introduction Relevant heat sources concerning machine tools like milling & grinding machines: Spindle (i.e. 40 KW is not uncommon) Hydraulic pumps, linear motors, gears, electronics Cutting process (i.e. turning, milling, grinding) Environment: air temperature ( day-night and summer-winter cycle) Why do machines drift? α Materials expand with temperature rise (α 0) No separate metrology frame; just 1 machine frame ( 0) Temperature changes limited by conditioning (expensive, energy intensive): Control machine component temperatures Control air temperature 3

4 Introduction 1) Methods to limits thermally induced drift by applying thermal machine models: Analytical models Empirical models Hybrid models These models relate measured sensor temperatures to displacements of the spindle with respect to the table in X, Y and Z-direction. 2) Best results however are obtained by simply controlling machine component temperatures within 1 degree or better. 3) Applying materials with limited/no thermal expansion is uncommon (invar, zerodur). Rarely a separate metrology frame is used (for special machines only) Measure drift of the machine while spindle is running (i.e. imitate machine s normal use) 4

5 Measurement setup and equipment Title ISO 230-3: Determination of thermal effects Measurement Setup: 1. Put precision ball or cilinder in spindle 2. Measure displacement in X, Y and Z (capacitance sensors) 3. To include tilt error, 5 sensors are used Vertical spindle Horizontal spindle Lathe, turning machine 5

6 Measurement equipment 1. Roundness error of target is typically 50 nm 2. Capacitance sensor 3. Probe nest Dual ball is used for tilt measurement Setup 1 (X,Y,Z) Setup2 (X,Y,Z,Rx,Ry) 6

7 Measurement equipment Capacitance sensor: Non contact 1 MHz oscillation excitation Spot size is 130% sensor diameter 15 khz bandwidth Measurement uncertainty < 5 nm Change in C is measured and this is converted into displacement Oscillating electric field One coil for exciting and one for sensing Target must be > 130% sensor diameter THE GAP BETWEEN SENSOR AND TARGET MUST BE CLEAN 7

8 Applied spindle speeds during test Heat up machine by applying spindle speeds: 1. Constant RPM (usually maximum) 2. Spindle spectrum (ISO, DIN): 0,25,50,75 or 100% of maximum spindle speed 3. Apply RPM as used when machining ISO DIN

9 Example 1 of thermal drift measurement no compensation Test: spindle runs at 2500 RPM for 60 min dx= 8.3 m dy = 38.9 m dz = 117 m!! 9

10 Example 2 of thermal drift measurement machine model applied Spindle runs at RPM Spindle stop after 30 min Machine adjusts its setpoint stepwise Most drift in Z (=common) Grounding cable used to reduce electrical noise (ceramic bearings) 10

11 Example 3 of thermal drift measurement machine model applied RPM for 90 minutes 35k stop 35k - stop Error remains < 15 um Most drift in Z (=common) Graph left: machine comes back to initial position as desired 11

12 Example 4 of thermal drift measurement machine is cooled periodically Spindle runs at RPM for 1 hour Error remains < 4 um Spindle temperature is controlled within 1 degree C (wave-period is about 3.5 minute) Machine is used for milling turbine blades (having a large machining time) 12

13 Example 5 of thermal drift measurement machine s spindle is cooled (Röders) Spindle stop. Spindle runs at RPM for 1 hour Error remains < 2 µm Spindle temperature is controlled tightly (0.2 degree C) This jump is caused by target eccentricity and can be ignored 13

14 Example 6 of thermal drift measurement machine s spindle is cooled Spindle runs at 2500 RPM for 30 minutes Drift < 2 µm Test was stopped after 30 minutes: very stable! Machine in factory, no temperature conditioning of environment! 14

15 Example 7 of thermal drift measurement presence of lubrication pulses Thermal compensation can be very nice, but can be distorted seriously too. Lubrication: a pulse is seen each 15 minutes, destroying the machine s stability. 15

16 Basic Concept for TCP Dislocation Measurement (ir. M. Klatte) Procedures for thermal loading Heating solely by rapid movement of linear axis No use of spindle No use of milling Automated measuring procedure for repeatable long-term measurement procedures Relative measurements to reference state Measuring with ball plate artifact for stable positions of balls on table Measuring small number of balls for quick measurements Y Z C X A Reference Bodies + = 3D Probe Geometry Check Thermal Check Dynamic Check Probe Artifact 5 Axis Characterization WZL/Fraunhofer IPT Seite 16

17 Example: Detail Analysis of thermal drift (ir. M. Klatte) Heating of Y-Axis Graph shows traces of reference balls in XY- Plane Slightly more symmetric behavior than X axis heating Left side exhibits more drift Smaller overall thermal drift WZL/Fraunhofer IPT Seite 17

18 Five axes machines: drift of pivot points Other method: R-test movie With R-test the position of a rotary axis is determined 18

19 Five axes machines: drift of pivot points R-test Meas1 & 2 right before milling & meas 3 & 4 right after milling a simple test piece (15 min): A-axis shifts 8 um in Y (=YOA) Vertical displacement A-axis shifts 7 um in Z (=ZOA) Horizontal displacement (=spindle axis) 19

20 Closure Thermal compensation is currently applied in industry for machine tools Measurement setup must be stable and clean (no oil, no water!) Drift in often worst in Z-direction (spindles often grow axially) Large variety in machine s thermally induced drift is seen: Best = Active cooling drift limited to a few µm Machine models drift becomes stable after heat-up period (10 15 min) Advanced machine models drift < 20 µm No compensation > 100 µm Machine shops with air-conditioning do exist and are increasing (but are not common) A trend can be seen in machine thermal stability and its price (typically 100k 1M ) Questions? 20