Sheet 3 · Core holes · ISO 261 · ISO 965-1 · ASME B1.1 · ISO 228

One thread. Three holes.

The same thread is made three different ways, and each way wants a different hole. Pick the standard and the size; the section below is redrawn to it and every dimension on this sheet is computed from the standard profile, not read off a chart.

Standard

Size

M6 × 1 Form-tap target %
Section through the tapped hole, drawn to the selected thread, with the three core-hole diameters laid over the finished profile.
Section A–A · M6 × 1 · 60° · ISO 68-1 · enlarged, major Ø fitted to the view

D · Entry chamfer

Included angle 90° To Ø 6.10 Depth from face 0.55

Taken clear of the major diameter so the first thread comes out complete and burr-free. The depth above is measured against the core hole of the selected process: countersink after drilling, before tapping. A formed thread also lifts material at the mouth, so take that one deeper or deburr afterwards.

Basic dimensions and 6H limits — M6 × 1
Dimension Sym Basic Min 6H Max 6H

General notes

  1. Two thread forms are drawn here, both to their real geometry. The 60° form of ISO 68-1 carries metric coarse, metric fine, UNC and UNF; the 55° Whitworth form of BS 84 carries BSPP and BSW, with its rounded crest and root at the true 0.1373 P radius.
  2. Thread engagement is quoted in the workshop convention: 100 % is a full flank height. On the 60° form that is 0.6495 P per side, so a hole at the basic minor diameter D1 reads 83.3 % and a hole at D − P lands at 77 %, which is exactly why that became the classic core drill. On the Whitworth form the full depth is 0.6403 P and the basic minor reads 100 %, so the two families' percentages are not directly comparable.
  3. For metric the hole window is the real class 6H minor-diameter band from ISO 965-1. For UNC, UNF, BSPP and BSW it is a practical engagement window: 60–83 % on the inch 60° threads, 75–100 % on Whitworth. Those are not the 2B or ISO 228 gauge limits, which this sheet does not reproduce. Every standard core drill listed falls inside its window.
  4. A formed thread finishes in the upper half of the minor band and shows the split crest that identifies it. Gauge it rather than assume it. Form tapping needs material that will flow, roughly A5 above 5 %; grey cast iron, free-cutting brass and hardened steel are cut, not formed. It is not offered on this sheet for the Whitworth families, where the relation behind card B is not calibrated.
  5. Two metric coarse sizes are deliberately not D − P: M8 takes 6.8 rather than 6.75 and M12 takes 10.2 rather than 10.25, because those are the stock drills ISO 2306 lists. Both sit well inside the 6H band, and the theoretical value is shown beside them.
  6. Every headline diameter is a drill you can actually chuck. Where a standard already names a metric drill (ISO 2306 for metric, ISO 228 for BSPP) that size is used exactly as listed, hundredths included: M2.5 really is 2.05 and M10×1.25 really is 8.75. Converted and calculated figures are snapped to a standard metric rack instead (0.05 steps below 3 mm, 0.1 to 20 mm, then 0.25 and 0.5), and never outside the hole window, so UNC 1/4-20 reads Ø 5.10 rather than the 5.105 that #7 converts to. The exact figure sits beside it on the card, and the engagement quoted is always the one the listed drill gives.
  7. There is no ISO standard that fixes the entry chamfer of a tapped hole. ISO 6410-1 covers how it is drawn, DIN 76-1 the thread run-out. The values here follow tap-maker practice: 90° included, taken to Ø D + 0.1 P. Where depth is short or the wall is thin, 120° is used instead.
  8. Tapered threads (NPT, NPTF, BSPT and Rc) are not on this sheet. Their hole is a taper, not a parallel bore with a minor-diameter band, so nothing above would describe them honestly. They need a sheet of their own.
  9. Everything here is computed live from the standard, not transcribed from a table. It is still a starting point that has to meet a real machine: verify against your tap, your material and your fixture.

Need the same rigour on the rest of the process?

This sheet exists because the question comes up on every job. The day job is the harder version of it: CAM programming, machining simulation, probing and the workflow around them.

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