These is a discrepancy between the measurements of your design and the piece cut with the Emblaser.
You may want to cut a box that is 100mm x 100mm and you end up with 99.8mm x 98mm, for instance.
Sometimes slight differences between stepper motors lead to a very small difference in the amount a motor turns. These small differences can accumulate and cause your Emblaser to not cut to exact measurements.
You can perform a fine calibration by making adjustments to the GRBL parameters controlling the steps going to the motor.
The GRBL parameters that control the number of steps going to the motors are $100 & $101. By default they are both set to 200.
1: Cut a square of known dimensions and measure its actual dimensions.
For instance you may create a square 100mm x 100mm.
Your cut piece measures 99.8mm x 101mm, for instance.
2: Make a small change to the GRBL $100 & $101 parameters and cut the same square again. You can change the parameters by entering commands like this in the Ultimate GCode Sender command window:
In this case we are increasing the one value by 0.1, and decreasing the other by 0.1.
These values to not relate directly to a distance. Use only small increments and check the effect.
Adding a small amount to the value will increase the size of your part and subtracting a small amount will make it smaller.
3: Once again cut your material and measure the actual dimensions. Hopefully they are closer to what they should be.
4: Repeat the process making small adjustments to the GRBL values until your actual dimensions are what they should be.
With some trial and error, you will be able to perfectly tune out any discrepancy.