What is the tap drill size for a 1/4-20 thread?

#7 (0.201 inch) for 75% thread engagement, the standard choice in aluminum, brass and plastics. In steel, stainless and iron, 50% engagement with a 7/32 (0.2188 inch) drill taps far easier and loses little practical strength.

What is the formula for metric tap drill size?

Tap drill = nominal diameter minus pitch, for roughly 75% thread. M10 x 1.5 gives 10 - 1.5 = 8.5 mm. This one rule covers every standard metric thread.

Is 75% or 50% thread engagement better?

75% is the standard for softer materials. In harder materials 50-65% is often the better call: tapping torque drops sharply while assembled joint strength barely changes, because the bolt usually breaks before the nut threads strip. Deeper engagement mainly increases tap breakage risk.

Are number and letter drills a US system?

Yes. Number drills #80-#1 and letter drills A-Z are the US wire-gauge system, used because standard fractional steps are too coarse for tap drills. Each has an exact decimal-inch equivalent, shown in the chart.

DEVELOPERS · PUBLIC API · NO KEY

Cross-reference API. Free, no key, CORS on.

One HTTP GET turns a carbide grade code into its ISO application class and the grades other manufacturers place at the same position. Built for machine shop software, ERP lookups, CAM plugins and AI agents. No signup, no rate-limit form to fill in, and every row carries the batch it came from so you can judge it yourself.

01 · ENDPOINT

One endpoint, one required parameter

GET https://cuttingtoolsai.eu/api/xref

grade: Required. The grade code as printed on the box, case-insensitive. Example: GC4325, KC5010, AH725.
brand: Optional. Narrows the match when the same code exists at more than one manufacturer. Example: Sandvik.
Auth: None. No key, no token, no signup.
HTTPS: Required. Plain HTTP is not served.
CORS: Enabled, Access-Control-Allow-Origin: *, so it works from the browser.
Cache: public, max-age=3600. Please cache; the data changes rarely.
Format: JSON. Errors return a JSON body with an error field and a 4xx or 5xx status.

02 · TRY IT

Call it right now

Nothing to set up. Paste either of these into a terminal or a browser console.

curl

curl "https://cuttingtoolsai.eu/api/xref?grade=GC4325"

JavaScript, from the browser

const r = await fetch("https://cuttingtoolsai.eu/api/xref?grade=GC4325");
const data = await r.json();
console.log(data.matched, data.count, data.results.slice(0, 5));

Live check

03 · RESPONSE

What comes back

Two blocks. matched is what your query resolved to, which is worth showing the user because a code can exist at more than one manufacturer. results is the actual cross-reference: other brands sitting at the same ISO application class, with the matched brand excluded.

Shape

{
  "query": "GC4325",
  "matched": [ { "brand": "Sandvik", "grade": "GC4325", "iso_app_class": "P25" } ],
  "count": 128,
  "results": [
    {
      "brand": "Kennametal",
      "grade": "KCP25",
      "iso_app_class": "P25",
      "equivalence_type": "comparable_at_iso_position",
      "verified": false,
      "independently_confirmed": true,
      "source_note": null
    }
  ]
}

Fields that decide how much you should trust a row

equivalence_type: comparable_at_iso_position means the two grades occupy the same ISO application class. It does not mean identical substrate, coating or edge behaviour.
verified: true only for rows built from multi-source consensus and spot-checked by a human.
independently_confirmed: true when the ISO placement was confirmed by two or more independent manufacturer charts.
source_note: Present when a row comes from a single manufacturer comparison chart, so you can weight it accordingly.

04 · HONEST LIMITS

What this data is, and what it is not

Read this before you put the output in front of a machinist.

Limits

Position, not equality: Two grades at the same ISO class are comparable starting points. Substrate, coating and edge prep still differ, and so will tool life. Always prove a substitution on a sample part before production.
Source: Published manufacturer comparison charts and cross-reference tables, extracted and normalised. Rows carry their provenance; nothing is generated or guessed.
Coverage: Turning and milling insert grades across the major manufacturers. Coverage is uneven by brand and thin on solid carbide.
Corrections: If you find a wrong row, tell us and it gets fixed. On 2026-08-19 we corrected 245 rows after re-reading a source chart from its own vector geometry, because a column had drifted during extraction.
Not affiliated: CuttingToolsAI is brand-neutral and sells no tooling. Manufacturer names and grade codes are the trademarks of their owners and are used here for identification only.

05 · USE AND ATTRIBUTION

Fair use

Free for commercial and non-commercial use. No key, no quota form. Two requests: cache responses rather than hammering the endpoint, and if you surface the data in a product, credit cuttingtoolsai.eu so a user can check the source. There is also an MCP server if you want an AI agent to call this directly rather than through raw HTTP.

ISO 1832 insert decoder ISO 5608 holder decoder Carbide grades chart Designation reference ISO material groups