Checking in CNC machine Single Block and Dry Run.

  

First part checking in CNC machining is a critical step to ensure the accuracy, safety, and efficiency of the manufacturing process before full production begins. Two indispensable modes used during this phase are Single Block and Dry Run. Both serve to identify and correct potential errors in the CNC program and machine setup, thereby preventing costly damage to the machine, tools, and workpiece.  

Program Checking in Single Block Mode

Necessity:

The primary necessity of single block mode is to execute the CNC program one line (or "block") at a time, giving the operator complete control and the ability to meticulously inspect each movement and command. This is crucial for:

• Collision Prevention: By stepping through the program line by line, the operator can visually confirm that the tool paths are clear of the workpiece, fixtures, and machine components, significantly reducing the risk of crashes, especially during rapid traverses or complex movements.
• Verification of G and M Codes: It allows the operator to verify that the G-codes (preparatory functions, like G00 for rapid traverse, G01 for linear interpolation) and M-codes (miscellaneous functions, like M03 for spindle start, M08 for coolant on) are executing as intended and in the correct sequence.
• Offset and Coordinate System Confirmation: The operator can check if the tool length offsets, diameter compensation, and work coordinate system (G54, G55, etc.) are correctly applied and that the tool is positioned accurately relative to the workpiece datum.
• Tool Path Visualization: It provides an opportunity to visualize the exact tool path in real-time, ensuring that the programmed movements match the intended machining strategy and that no unexpected or incorrect moves are occurring.
• Identification of Logic Errors: Complex programs can have subtle logic errors that might not be apparent in simulation. Single block mode helps uncover these by showing the machine's actual response to each command.
• Safe Tool Changes: Tool changes (M06) can be carefully monitored to ensure the correct tool is loaded, the magazine or turret moves safely, and the new tool's offsets are correctly activated.

Method:

1. Activate Single Block Mode: On the CNC machine's control panel, locate and activate the "Single Block" or "Single Block Execute" button/switch. This is usually a physical button or a soft key on the control screen.
2. Initial Setup and Safety Precautions:
   • Ensure the machine is in "Program Edit" or "Memory" mode (depending on the control) and the program is loaded and at its start.
   • Set the rapid override and feed rate override to a very low percentage (e.g., 5-25%). This provides ample time to react and press the E-stop or feed hold if something goes wrong.
   • Consider raising the Z-axis datum (the "air cut" method) initially so that the tool runs several inches above the workpiece, eliminating any chance of collision during the initial run-through. This is often done by temporarily adjusting the Z-axis work offset.
   • Have one hand ready on the Feed Hold or E-Stop button.
3. Step-by-Step Execution:
   • Press the "Cycle Start" button. The machine will execute only the first block of the program.
   • Observe the machine's movement, the coordinates displayed on the position screen, and the commands being executed.
   • Mentally (or physically, with a printout) check the next block in the program.
   • Once you are satisfied with the executed block, press "Cycle Start" again to execute the next block.  
   • Repeat this process, carefully reviewing each movement and command, especially during rapid traverses, tool changes, and movements near the workpiece.
4. Adjustments and Corrections: If any error or unexpected behavior is observed, immediately press "Feed Hold" or "E-Stop." Identify the problematic block in the program, enter "Edit" mode, and make the necessary corrections. After editing, always re-verify the changes and may need to restart the single block process from a safe point or the beginning.
5. Gradual Increase in Confidence: As the program proves to be correct, the operator can gradually increase the rapid and feed rate overrides, or even switch off single block for safe sections of the program (e.g., long rapid moves between operations), but always remaining vigilant and ready to intervene.

Program Checking in Dry Run Mode

Necessity:

Dry run mode is a machine control feature that allows the program to execute at an accelerated feed rate, but without the tool actually engaging with the workpiece (i.e., "cutting air"). The necessity of dry run mode is primarily for:  

• Rapid Program Flow Verification: It allows for a quick run-through of the entire program or significant sections to confirm the overall sequence of operations, tool changes, and general machine movements without spending excessive time on detailed single-block checks.
• Collision Detection (Macro Level): While single block offers fine-grained control, dry run can help identify larger-scale potential collisions or machine limit overtravels that might occur over the course of the entire program.
• Cycle Time Estimation: Although not for actual cutting, dry run can provide a rough estimate of the total machining time, which can be useful for production planning.
• Confirmation of Tool Change Positions: It verifies that tool changes occur at safe and unobstructed positions, preventing tool crashes with the workpiece or machine components during tool changes.
• Syntax Error Detection (some controls): Some CNC controls may detect syntax errors during a dry run, especially if they simulate parsing the entire program.
• Warm-up of Machine Components: Running the machine in dry run can help warm up the axes and spindle, bringing them to operating temperature before actual machining.  

Method:

1. Activate Dry Run Mode: Locate and activate the "Dry Run" button or switch on the CNC machine's control panel.  
2. Safety Precautions and Setup:
   • Similar to single block, ensure the program is loaded and at the start.
   • Crucially, ensure the Z-axis work offset is adjusted to "cut air." This means setting the part zero in Z significantly above the actual workpiece surface (e.g., 50mm or 2 inches higher). This ensures the tool never makes contact with the material.
   • Set the rapid override and feed rate override to a low or moderate percentage initially.
   • Keep a hand ready on the Feed Hold or E-Stop button.
3. Execute the Program:
   • Press "Cycle Start." The machine will begin executing the program.
   • Observe the machine's movements, paying attention to:
     • Tool paths: Are the tools moving in the expected patterns?
     • Clearances: Is there sufficient clearance between the tool, workpiece, and fixtures at all times?
     • Tool changes: Do they occur smoothly and without collision?
     • Axis limits: Is the machine approaching its axis limits?
     • Spindle and coolant activations: Are they turning on and off at the appropriate times (though actual cutting will not occur)?
4. Intervention and Correction: If any issue is observed (e.g., potential collision, unexpected movement), immediately press "Feed Hold" or "E-Stop." Switch to "Edit" mode, correct the program or setup error, and then re-run the dry run from the beginning or a safe restart point.
5. Progressive Acceleration: As confidence in the program grows, the override percentages can be increased to speed up the dry run, allowing for faster verification.

Important Considerations for Both Modes:

• Operator Vigilance: Both single block and dry run modes require constant attention from the operator. Never leave the machine unattended during these checks.
• Program Printout: Having a printed copy of the CNC program available allows the operator to follow along line-by-line and compare the machine's actions with the programmed commands.
• Machine Graphics/Simulation: Many modern CNC controls have integrated graphics or simulation features that can show the tool path on the screen. While helpful, these should always be supplemented by physical checks using single block and dry run, as simulations may not always account for every real-world machine parameter or setup error.  
• Gradual Approach to the Workpiece: After successful single block and dry run checks with "air cuts," a common practice is to gradually lower the Z-axis offset (or adjust the part zero) in small increments (e.g., 0.1mm or 0.004 inches) and perform a "skim cut" to verify the final Z-depth and offset before full material removal.
• Documentation: Any changes made to the program or setup during first part checking should be documented and saved.

By diligently applying both single block and dry run modes, CNC operators and programmers can effectively "prove out" a new program, identify and rectify errors, and ensure a safe and successful machining process, minimizing scrap, tool breakage, and machine downtime.