CNC Tool Offset Adjustment

 

CNC Tool Offset Adjustment for Close Tolerance Dimensions on First Part

When machining parts with close tolerance dimensions on a CNC machine, especially during the first article setup, careful adjustment of tool offsets is crucial to avoid part rejection. A common technique involves intentionally "oversizing" or "undersizing" the initial cut to provide room for fine-tuning.

Understanding the Problem

• Close Tolerances: These leave very little room for error. A slight deviation in the tool's cutting position can result in a part that is outside the specified limits.
• First Part Uncertainty: The first part produced after a setup is always the most uncertain. Factors like tool wear, machine warm-up, and material variations can affect the initial dimensions.
• Cost of Rejection: Rejecting a part, especially a complex or expensive one, leads to wasted material, time, and effort.

Oversizing/Undersizing Strategy

To mitigate the risk of rejection, machinists often employ the following strategy:

• External Dimensions (e.g., diameters, lengths): Intentionally machine the first part slightly oversize. This means leaving a small amount of extra material.
• Internal Dimensions (e.g., holes, slots): Intentionally machine the first part slightly undersize. This means the hole or slot is slightly smaller than the final required dimension.

Why This Works

• Controlled Material Removal: It is easier and more precise to remove a small amount of additional material (for external dimensions) or enlarge a hole slightly (for internal dimensions) than it is to add material back.
• Iterative Adjustment: This approach allows the machinist to make small, iterative adjustments to the tool offsets based on the measurements of the first part.
• Preventing Rejection: By starting on the "safe" side, the machinist can gradually approach the target dimension without the risk of immediately producing a reject.