A drilled hole can come out oversize or undersize even when the drill nominal diameter is correct. The real hole size is affected by runout, tool wear, point geometry, material springback, coolant, alignment, and how the tool enters the workpiece.
Common reasons for oversize holes
- Spindle or holder runout makes the drill cut wider than its nominal diameter.
- The drill is not guided well at entry and starts off center.
- The cutting edges are not balanced because of wear or damage.
- The setup is not rigid enough, so the drill vibrates or walks.
- Coolant or chip evacuation is poor, causing heat and rubbing.
Why holes can come out undersize
Undersize holes are common in materials that spring back, smear, or close slightly after the drill passes. Copper, some stainless steels, and other ductile materials may not behave like free-cutting steel. A worn drill can also rub rather than cut cleanly, leaving a poor surface that measures differently along the hole depth.
That said, do not assume the material is the only reason. Check actual drill diameter, runout, holder condition, and whether the drill point is damaged.
Use a measurement sequence, not guesswork
- Measure the drill itself and inspect the cutting edges.
- Indicate runout at the tool tip.
- Check entry condition and whether a spotting or pilot operation is needed.
- Measure the hole at the mouth and deeper in the hole.
- Inspect chip shape and coolant delivery.
If the hole size must support tapping, reaming, or press-fit assembly, see HEYI’s broader guide to hole machining tool selection by material. For carbide drilling review, the through-coolant carbide drill page is a practical starting point.
When requesting help, include the nominal drill size, measured hole size range, material, hole depth, machine type, holder, coolant method, and whether the tool is new or used. Submit those details through the RFQ form.