Hole design looks simple in CAD, but a poorly specified hole can create unnecessary machining cost or assembly problems. This CNC hole design guide explains how mechanical designers should think about drilled, reamed and tapped holes.
Start With the Function
First determine whether the hole is for clearance, fastening, locating, bearing support, fluid flow or another function. The required accuracy should come from the function rather than habit.
Drilled Holes
Drilling is one of the most common CNC operations. Standard drill sizes can reduce tool changes and simplify manufacturing. Avoid specifying unnecessarily tight tolerances for ordinary drilled holes.
Reamed Holes
When a hole requires better size and surface control, a reaming operation may be appropriate after drilling. The drawing should clearly communicate the required final diameter and tolerance.
Tapped Holes
For threaded holes, specify the thread standard, nominal size, pitch and depth. Consider the available tool and the required full thread length. Blind holes also require space at the bottom for the tool tip.
Hole Depth
Differentiate between total hole depth, thread depth and required effective depth. This prevents manufacturing confusion and helps the machinist select an appropriate tool.
Counterbores and Countersinks
Use a counterbore when a fastener head needs a flat-bottom recess. Use a countersink for a conical head. Define the diameter and depth or angle clearly.
Tolerances
Do not make every hole a precision hole. Tight tolerances can increase machining time and inspection requirements. Apply tighter tolerances only where the assembly or function needs them.
DFM Tip
Standard tools, accessible hole locations and sensible tolerances generally make CNC manufacturing easier. Review the design with manufacturing before releasing the drawing.
Quick Checklist
- Is the hole function defined?
- Can a standard drill or cutter be used?
- Is the tolerance justified?
- Is the depth manufacturable?
- Are threads clearly specified?
- Can the tool reach the feature?