Consider each step of how the machine functions:
- Carbide Motion sends G-code to the machine over USB — make sure the USB cable is secure at both ends (I like to drape it so that a coil is tensioned so as to push the cable end into the connector)
- Grbl interprets the G-code into electrical impulses which are sent by the stepper drivers to the stepper motors overs the wiring — make sure all wiring is in good condition and all connectors secure
- the stepper motors rotate based on the electrical impulses which they receive turning either the pulleys (make sure that these are secure and that the set screws are in place) or the lead screw
- the turning pulleys move belts (make sure that they are secure at both ends and well-tensioned as noted at: Measuring belt tension, squaring and calibration ) or the turning lead screw moves the Z-axis up/down by rotating in the lead nut (make sure that it is securely in place on the Z-axis spindle carriage plate)
- the moving parts are held in line by the Delrin V-wheels which are adjusted by the eccentric nuts (X- and Y-axes) or the linear rails and blocks (Z-axis — make sure these are lubricated: CNC Machine Maintenance and the video https://youtu.be/SE-36MmjrMQ?si=i3zgKQbyx7VlJMBU )
- all to move the spindle which is turning so as to cut with the tool — make sure that the tool is securely held by the collet (which is clean and in good condition) — use the button only to hand-tighten, then use a pair of good quality wrenches to fully tighten
- at the specified feeds and speeds — make sure that the feeds and speeds are appropriate to the selected tool and the material which you are cutting
- cutting through stock which is securely held in place on the machine by suitable work-holding