Troubleshooting
Part Dimensional Inaccuracy: Beating Tool Deflection
Parts drifting out of tolerance? It's almost always deflection. Shorten the tool, lighten the cut, and hold size — here's how RobbJack engineers diagnose it.
When a feature comes in oversize or tapered and the program is right, the tool is bending. End mill deflection is elastic — the tool pushes away from the cut under load and springs back, so the wall leans and the size wanders. Every fix below comes down to the same thing: reduce the load on the tool or make the tool stiffer.
Dimensional inaccuracy — causes & fixes
Deflection scales with the cube of length
A tool's stiffness falls off dramatically as it gets longer — bending deflection grows with the cube of the unsupported length. That's why trimming even a little stick-out, or stepping to a stub tool, does more for accuracy than any feed change. Use the shortest tool that clears the part, and let RobbJack's necked geometry reach into deep features while keeping the body short and rigid.
Take the load off, then take a spring pass
When you can't shorten the tool, lighten the cut: reduce radial width and axial depth so the tool sees less side force. For finishing critical walls, leave a thin, even allowance and take a spring pass — a final light pass at the same Z that lets a deflected tool clean up to true size. RobbJack has run long aluminum tools with multiple successful spring passes at full cutting length.
Shorten the tool, cut lighter, and finish with a spring pass — control deflection and dimensions fall into line.
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