So, you’ve reviewed the drawing with the engineers and designers. You’ve verified the dimensions and double and triple-checked the features. You’ve asked the team if they have any flexibility. You’ve explored every option, but alas that +/-0.0005 tolerance (or tighter sometimes) is correct. Now, you have to make the part. Here are some tips for maintaining tight tolerances.
Machine maintenance is critical for the best performance. While CNC mills are designed to have minimal deflection, things move over time. It is a good idea to contact the manufacturer to service and calibrate machines annually. They can use specialized tools like the interferometers and laser calibration systems to ensure machines are in tip-top shape.
A warmup routine might be standard procedure for any milling operation, but these routines are mostly designed to ensure the lubrication and bearings are warmed up. A high-precision operation should include a more aggressive routine to warm up all the internals. This exercise will minimize any dimensional changes that occur as the machine reaches operating temperature.
Thermal stabilization is different than warm up. While getting to operating temperature is essential, it is also vital for the environmental temperature to be stable. Shop temperatures might fluctuate throughout the day, or a specific machine might be near a vent, window or sunny spot. All these things can cause the workpiece or the machine to change dimensionally. Don’t forget to stabilize the material’s temperature too. If it is stored outside or in a cold room, it should be allowed to stabilize to an ambient temperature near the machine before starting.
The right tooling is a necessity for achieving tight tolerances. Consider using radiused tools for rough cutting to reduce tool wear and allow for faster machining. Then use a sharp, square end tool for finishing. The tool should have the maximum number of flutes allowable. For holes, a reamer has better precision than a bit and leaves a fine polished finish.