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Thermal compensation, calibrated screws, linear encoders

In order to clarify our efforts "to build the most accurate PCB prototyping mill", we are showing below the typical calibration graphics of a high quality lead screw (similar lead screws are used by all manufacturers/competitors in this business).
The graphics shows the absolute positioning error along the length of the screw.
It is visible that the error consist of two components:

• error from the screw by it self, related to the production of the screw and nonlinearity of the driving motor (we will call this - screw error)
• screw thermal expansion error

Most of the screws specifications are given at 68 °F (20 °C), but actually they never work at this temperature. In fact different axes has different screw temperatures etc. all related to the particular machine design.

Absolute positioning errors at 36°C(97°F) & 20°C (68°F)

As it is seen on the graphics both error components affect the accuracy differently:

• The screw error affect mostly small distance movements and may reach 25 μm @ 2.54 mm (1 mil @ 100 mil). It is quite logical that it has repeat equal to one screw turn.
• The thermal expansion error is quite linear and affect the accuracy in the long distance movements. Here it reaches 110 μm @ 381 mm (4.3 mil @ 15 inch)

The material under machining also has its own thermal expansion that has to be compensated in order to achieve maximum accuracy at desired temperature.

As it is seen from the diagrams rotational encoder or servo control using one is not a solution.
We resolve the problem two ways, designing two different positioning systems that are using linear encoders.

The first one has the encoders permanently mounted and works using servo feedback from them.  The encoders have 1μm resolution and better than 10 μm @ 1000 mm @ 68°F (20°C) accuracy.  This positioning system shows less than 5μm positioning error @ 254 mm (10 inch).  The measuring reference is quartz glass ruler with NIST certificate and 0.2μm accuracy @ 68°F (20°C).

The second one is based on the same encoders, but they are mounted temporary during the production cycle of the system. A calibration procedure is applied and it records the difference (the error) of the screw compare to the linear encoder. The process that we call "screw calibration" maps the differences every 0.01 inch and stores them in the flash memory of our PhSTdrive™. It is an intelligent stepper motor drive (DSP microcontroller based) that allows using servo feedback and/or dynamic real time corrections based on the screw calibration map.

This type of positioning system also shows incredibly high accuracy of less than 8 μm positioning error @ 254 mm (10 inch). It has the advantage that the cost of relatively expensive linear encoders is eliminated and the accuracy is still close to the first one.

We offer models with both positioning systems.
Despite of the type of the positioning system (servo or calibrated screws) we are taking in account all thermal expansion factors, such as screws temperature, encoders temperature and material temperature. All factors related to the material under machining are settable by the operator.
After mapping the screw errors or using servo feedback (depend of the model selected), we have eliminated the main sources of positioning errors and the thermal expansion appear to be the main factor to the final accuracy of the machined product.
Here are some requirements that will help to achieve maximum absolute accuracy:

• select the right MTCLE (Material Thermal Coefficient of Linear Expansion). The default value is for FR4 PCB material.
• select the temperature that you need to have the best dimensional accuracy. Can be any, but normally is 68°F (20°C).
• keep the machine on for 30 minutes or more before usage
• mount the material under machining 30 minutes before usage. This will equalize its temperature with the machine table.

Notes:

In some sources MTCLE is called CTE (Coefficient of Thermal Expansion).