Solid Bottom
When does a feature require a true solid bottom instead of a radiused or pointed bottom?
A true flat bottom is required when the feature must seat hardware, support bonded laminates, or mate with a part that references a planar floor. Radiused floors are acceptable only when the design allows corner relief and the tool radius is accounted for.
What tooling is used to create a solid bottom, and when are flat-bottom drills preferred?
Flat end mills and flat-bottom router bits are standard for machining flat floors. Flat-bottom drills are used when a hole or angled hole must start or finish on a plane rather than a point, reducing drill walk and location error.
Why might a pocket measure correct in X-Y yet still not function?
The part may be in tolerance at the perimeter while the floor is out of flatness due to cutter wear, runout, chip packing, or step-over errors. That prevents full seating of inserts or mating components, so floor flatness and depth consistency matter most.
Solid bottom in CNC machining and millwork refers to a flat, closed, unbroken floor surface at the base of a pocket, recess, groove, or drilled feature—not a point, taper, radius, or through-opening. It is normally produced with a flat-bottom end mill, flat-bottom router bit, or flat-bottom drill, and provides a consistent depth reference, clean support surface, and reliable seating for mating parts.
On the shop floor, a solid bottom is the result of a deliberate toolpath choice. In three-axis work, operators run a flat end mill or flat-bottom router bit to full depth with enough step-over to avoid scalloping. The feature usually serves as a seating surface for inserts, laminates, hardware, or mating parts, so floor geometry matters more than nominal width or depth. In millwork, pocket bottoms must be flat for adhesive bond area and laminate support; in metal cutting, they must be flat for load-bearing contact and assembly datum control. Deep pockets demand attention to chip evacuation and rigidity, because vibration and recutting can leave a dished floor. On the setup side, true position and machine leveling affect how consistently that floor is produced across a run. A solid bottom is not a given—it is earned through correct cutter geometry, programmed engagement, and controlled fixturing.
Rounded or tapered floor: A ball nose, V-bit, or pointed drill leaves a radius, taper, or point instead of a true flat floor, causing poor seating, inconsistent adhesive lines, and interference with mating components.
Dished or scalloped floor in deep pockets: Chip recutting, heat, and tool deflection in deep, narrow cavities produce a floor that looks flat in CAM but is actually dished in the part, ruining dimensional accuracy.
Inconsistent floor depth from Z-zero error: A bad work offset, probe, or fixture setup leaves extra stock or cuts through the intended floor, and this becomes critical when that bottom is the reference for the next operation.