All about Probing
If your machine is equipped, usage of a probe and touch plate greatly simplifies the process for establishing your working origin / home position. There are a couple flavors of touch plates available and this is an important consideration when introducing probing into your CNC workflow.
A typical touch plate is nothing more than a square conducting block. There are also touch plates available that have a portion of their bottom surface cutout which enables you to place it on the corner of a workpiece to also set the starting position for your X/Y axes. The concept remains the same between the two and you must provide your control software the dimensions of the touch plate to accurately calculate your origin.
I've also seen a very simple approach to touch plates involving disposable foil tape and two alligator clips. Taking this approach greatly reduces the amount of free X/Y/Z axes movement you need to have available above and around your workpiece.
- Confirm all Z-Probe actions - Once you are familiar with how multi point and chained x/y/z probing works, you may decide to skip the confirmation steps for acknowledging spindle x/y axes movement. This will speed up the process of traveling from location to location. All actual probing commands require confirmation prior to initiating them. Only confirmation of X/Y movement commands meant to position the spindle, typical moves with minimal risk, can be suppressed.
- Z-Probe Method - Can be Single Point Z-Probe or Multi Point Z-Probe. Further details are provided below.
- Multipoint Calculation - Can be Highest, Lowest, Mean distance, or Average distance. Further details are provided below.
- Touchplate Height - The height of your touch plate measured in millimeters. This value will be automatically included in probing results.
- Z Probe Distance - The distance (in millimeters) the Z axis will travel when attempting to make contact with a touch plate. If the value is 0 your z axis limit will be used.
- Z-Probe End Position - The distance the z axis will travel positively (upward) after a successful probe operation.
- Touchplate Width (X) - The offset / overhang width of your touch plate. Include your tool's radius or diameter to place your spindle over or inside the calculated origin.
- Touchplate Length (Y) - The offset / overhang length of your touch plate. Include your tool's radius or diameter to place your spindle over or inside the calculated origin.
- X/Y Probe Distance - The distance (in millimeters) the X/Y axes will travel when attempting to make contact with a touch plate.
Single Point Z-Probe is just like the name suggests. If Single Point Z-Probe is the selected Z-Probe Method, when you press the Probe button in Grbl Control, you are prompted to confirm your intent to initiate a Z Probe operation at the spindle's current x / y axes position.
Before using this function, ensure you have provided the correct Touch plate height for you touch plate within Plugin Settings. You can also optionally set a maximum travel limit (it defaults to your z-limit) and desired Z End Probe Position.
Upon confirmation, the operation is initiated and, assuming the probe makes contact to complete its circuit, the reference position is returned, the touchplate height offset applied to it, and then stored temporarily as the machine's Z axis working coordinate home position as an offset.
This setting is persistent. It will survive a power cycle and / or reset of the machine.
Multi Point Z-Probe is driven by the values provided within Material Framing. The length / width parameters and the starting location are used to create a multi-point sampling area. With Multi Point Z-Probe set as the Z-Probe Method, Better Grbl Support steps you thru each point and executes a Z-Probe at each.
Once completed, it computes your Z axis working home position using one of the 4 Multipoint Calculation methods provided.
- Highest - The highest point detected. Most often attributed to "carving air" your first few passes.
- Lowest - The lowest point detected. Keep in mind with this setting there will be some portion of your material (the gap) that will be plunged / gouged. If the distance is too great this may create a problem for your spindle / bit / material.
- Mean Distance - Takes the difference of Highest and Lowest, divides them by two, and adds the result to Lowest value. This method is an attempt to minimize the amount of excessive plunge / gouging that may occur on your first couple passes.
- Average Distance - Sums up all points and divides the result by the count of them. This method is an attempt to minimize the amount of excessive plunge / gouging that may occur on your first couple passes.
The result is then set as your Z Axis working home position. Unlike Single Point Z-Probe, the Multi Point Home position is saved in your controller's EEPROM and will survive a power cycle / reset.
With the X, Y, XY, and ALL axis selections in the Axis Selection Bar you can also probe the X/Y origin of your workpiece. X and Y probing relies on the Starting Position you have selected in Material Framing. With this point of reference, Better Grbl Support is able to set the proper direction for movement of the probe.
Before initiating a probe of your X/Y axes, ensure you have the Starting Position in Material Framing set to the same location you placed your touch plate.
If at any time you need to abort the probe operation, hit the Soft Reset button in Grbl Control.
All Multi Point routines are 5 point routines with the exception of CENTER as your starting point. CENTER performs a 9 point check, sampling the edge mid points in addition to each corner.
Pick a Starting Location / datum that you like best and stick with it. The more consistent you can be in your workflows the less likely you'll forget a step along the way and potentially jam / jolt / crater your spindle / tool.
