Fine Calibration.

When I first started 3-D printing I found the calibration process difficult as there were few online resources or information about this critical process. I have now built quite a reasonable experience of 3-D printing after working with my machine for 18 months, and have improved my calibration technique. I will discuss the calibration now to add another online resource for newcomers to 3-D printing.

There are three elements which are critical to the fused deposition modelling (FDM) process, bed levelling, first layer height and plastic flow rate, underperformance in any one of these can cause total failure of the printed model. 

Bed Levelling:

Most 3-D printers on the market currently use a three point bed levelling system, where the adjustment of three nuts changes the height of the bed in relation to the Printer Heads. It is imperative that the printheads move parallel to the bed in both the X and Y axes. So that the space between the extrusion nozzle and the bed is constant. If this distance changes the amount of adhesion between the printed model and the bed varies, causing the model to lift from the bed and/or distort. Some 3-D printers are starting to appear which have automatic bed levelling removing this time consuming process. 

The tried and tested technique is to slide a business card between the nozzle and the bed and then, with the mains power disconnected, manually move the printhead across the X axis. Rather then see the gap between the printhead and the bed you feel it with the business card, if the card becomes tight then the bed needs to be dropped by tightening the nuts on the underside. This process is then repeated in the Y axis direction. It is worthwhile repeating this process many times and devoting a large amount of time as a unlevelled bed can prevent sticking of the first layer and cause unsuccessful printing. 

First Layer Heigth:

For good adhesion of the printed model it is best advised to have a thicker first layer to aid adhesion, but this layer should be compressed to help it bind to the bed, especially important when printing on blue tape which relies on the physical bond between the tape rather then heat. The height of the first layer can be controlled in the slicing software, software which converts 3-D model into layers and generates commands for the printer to follow, both Cura and Slic3r allow variable inputs. A value of between 0.15mm to 0.2mm has good results. To compress the first layer you manually move the position of the Z axes home. This varies from printer to printer but usually involves an adjustable component which breaks a light to gate telling the printer it is at zero for that axis. So the position the printer thinks is zero on the Z axis is actually minus  0.1mm, for example, so when the printer is making a line which is 0.2mm thick, it is actually 0.1mm and is being compressed onto the bed, providing a solid foundation for a long print and reduce the risk of the print model curling and lifting. 

Plastic Flow Rate:

The plastic flow rate effects the wall thickness of the printed models, and is affected by the material feed stepper motors rotational speed, the filament thickness and the nozzle diameter. It is important that the exact diameter of the nozzle is known as this value is used by the slicing software to calculate the number of lines required for a specified wall thickness. 

If the flow rate is too low and then the walls thin and can break causing a failed print. The most common cause of this problem is the filament jamming in the printhead, due to the printhead not being at, or slightly above, the glass point temperature (The temperature at which the material starts to become a liquid and flow, the higher the temperature above this point the less viscous the liquid) of the printing material or that the feed stepper motor is forcing too much material into the head causing it to jam. Both issues I have experienced. 

Temperature is easily fixed when using Printrun software to control the 3-D printer as it can be manually changed. When working with a new batch of material I manually push some filament through the Print Head, varying the temperature and testing the resistance to find a suitable temperature for good quality printing, as each batch of plastic can vary. 

If the material feed stepper motor is forcing too much plastic into the head this is usually due to the diameter of the nozzle diemention being inaccurate, and the Slicing software calculating the flow rate to be greater than it is, so the material enters the head at a greater rate than it can exit, thus causing jamming. This highlights the importance of an accurate measurement of the nozzle diameter. 

The flow rate also affects the precision of the printed models, if too much plastic is being extruded then the wall thicknesses will be greater than designed thus the model's dimensions will be larger than expected and this can affect the assembly of multiple components.