You can have a perfectly programmed part and a sharp tool and still end up out of tolerance — because the part moved, distorted, or vibrated in the fixture. These five mistakes account for most of it.
1. Over-tightening thin-walled parts
Clamping or chucking force distorts thin sections elastically. The part measures fine in the vise and goes out of round the moment it's released. Use the minimum clamping force that prevents movement, and consider soft jaws or multiple lighter clamping points instead of one hard squeeze.
2. Skipping deburring before clamping
A single burr or chip under a part lifts it off the true surface by thousandths — enough to throw off a precision feature. Always wipe the vise/fixture and the part before clamping.
3. Insufficient support for long or thin parts
Unsupported overhang deflects under cutting force, especially in turning. Use a center, steady rest, or follower rest for long shafts, and support thin plates under load-bearing areas, not just at the ends.
4. Not checking runout or squareness
A vise that isn't indicated true, or chuck jaws with accumulated wear, silently introduces error into every part you make. Indicate critical fixtures in periodically — it takes two minutes and saves scrapped parts.
5. Clamping too close to the cut
Clamps or vise jaws directly under or very near the cutting zone transmit vibration straight into the part, causing chatter. Where possible, clamp away from the area being machined, or add support specifically near the cut.