A tap hole size chart is essential for clean, accurate threads. In most shops, machinists refer to ANSI/ASME B94.9 for inch sizes and ISO standards for metric.
Always adjust slightly depending on material for softer metals, a slightly smaller hole ensures tighter threads; for harder alloys, follow the standard exactly.
Many suppliers offer laminated charts for quick reference. When working with aluminum, use cutting fluid to reduce friction and prolong tap life. Consistency in pilot hole sizing will improve both thread quality and tool longevity.