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ASME B18.6.7M-1999 (2005) — Metric Machine Screws — Flat Countersunk, Oval Countersunk, Pan, Hex and Hex Flange Head Styles
🔩 ASME B18.6.7M-1999 (reaffirmed 2005) is the complete product standard for metric machine screws within the ASME B18 fastener series. Developed by Subcommittee 6 (Slotted and Recessed Head Screws), it supersedes and revises the 1985 edition (reaffirmed 1993). The Standard provides complete general and dimensional data for flat countersunk, oval countersunk, and pan head machine screws with slotted and Type I/IA cross-recessed drives, as well as hex and hex flange head machine screws. Sizes cover M2 through M12, and steel screws are available in property classes 4.8 and 9.8.
💡 The dimensions in this Standard are coordinated to the greatest possible extent with the corresponding ISO standards. Except for recessed flat countersunk heads, the dimensional differences between machine screws made to this Standard and to ISO standards are very few and relatively minor, and none affect functional interchangeability — a convenience for cross-border procurement and export assembly.
1. Scope and General Framework 📌
A machine screw is a threaded fastener of small nominal diameter (up to M12 in this metric series) that is either mated with a nut or threaded into a tapped hole in a metal part. It is used extensively in instruments, electronics, appliances, automotive, and general machinery assembly. B18.6.7M covers the following head styles:
- Flat Countersunk Head: a 90 deg countersunk conical head that sits flush with the part surface, minimizing protrusion and snagging risk.
- Oval Countersunk Head: a countersunk head retaining a raised oval crown for an attractive, decorative appearance.
- Pan Head: a cylindrical, pan-shaped head with a generous bearing surface — the most widely used head style in general assembly.
- Hex Head: wrench-driven for high torque transmission where high tightening loads are required.
- Hex Flange Head: carries an integral flange under the head that acts as a built-in washer, offering good driveability and replacing the hex washer head, which was difficult to manufacture and had less desirable driving capability in some sizes.
Drive provisions include slotted and Type I and Type IA cross-recessed. The inclusion of dimensional data does not imply that all sizes are stock production items — purchasers should consult suppliers on availability. Recommended diameter-length combinations for metric machine screws are given in Table 17 of the Standard.
⚠️ Metric machine screws and metric tapping screws of comparable sizes share consistent head envelopes, but they differ fundamentally in thread form, hardness, and function: machine screws engage pre-formed internal threads, while tapping screws form their own mating threads. Do not mix them in design or procurement.
2. Head Styles, Size Range, and Property Classes 🔧
| Head Style | Drive Provision | Nominal Size Range | Typical Applications |
|---|---|---|---|
| Flat Countersunk | Slotted / Type I, IA recess | M2–M12 | Flush panel and instrument assembly; heat-treated products have a short shoulder |
| Oval Countersunk | Slotted / Type I, IA recess | M2–M12 | Decorative metal and wood fastening |
| Pan | Slotted / Type I, IA recess | M2–M12 | General machinery, electrical, and electronics assembly |
| Hex | Wrench (across-corners drive) | M2–M12 | High tightening torque connections |
| Hex Flange | Wrench, integral flange under head | M2–M12 | Automotive and agricultural vibration service, washerless design |
Steel screws are supplied in property classes 4.8 and 9.8: class 4.8 for general strength requirements and class 9.8 for high-strength, high-reliability joints. Heat-treated flat countersunk head screws feature a short shoulder under the head, with an attendant increase in the minimum grip length, ensuring adequate head-to-shank strength and improved driving performance.
3. Dimensional Tolerances and Gaging Methods 📏
The appendices of the Standard define several dedicated gaging procedures that control head geometry and ensure manufacturing feasibility and assembly compatibility:
- Protrusion Gaging: for flat countersunk head screws; controls the protrusion or depression of the head in its countersink.
- Across-Corners Gaging: for hex head screws; controls the corner-to-corner dimension for wrench fit.
- Penetration Gaging: for recessed head screws; verifies recess depth and effective driver engagement.
- Wobble Gaging: detects eccentricity and wobble of the recess relative to the head, ensuring reliable driver engagement in automated assembly.
The Standard also provides clearance hole recommendations for design drawings of assemblies using these screws.
🚨 Recess geometry (depth, concentricity, and wobble) directly affects the cam-out rate and driver-bit damage on automated assembly lines. For high-volume automated applications, penetration and wobble gaging must be part of the process control plan, or “stripped recess” scrap rates will soar.
4. Engineering Practice Essentials 🛠️
- 🔍 Selection workflow: determine the strength class (4.8 or 9.8) → choose diameter and length from joint thickness and thread engagement → select head style (flush: countersunk; appearance: oval countersunk; general: pan; high torque: hex) → choose drive type (cross-recessed preferred for automated driving).
- 🔧 ISO interchangeability: recessed flat countersunk heads share identical head envelopes with ISO, but one ISO standard incorporates shallower recesses without a shoulder — specify recess depth and shoulder consistent with this Standard when interchangeability matters.
- 📋 Surface treatment: dimensions in the Standard apply before coating; the finished dimensions after zinc, chromium, or Dacromet plating are agreed between supplier and purchaser, and coating thickness affects thread fit.
- 📊 Tightening torque: select torque by property class and friction coefficient. Class 9.8 screws tolerate much higher preload but are more susceptible to hydrogen embrittlement — de-embrittlement (baking) after electroplating is essential.
5. Frequently Asked Questions ❓
❓ Q: What is the difference between B18.6.7M and B18.6.4M?
A: B18.6.7M is the product standard for metric machine screws (flat countersunk, oval countersunk, pan, hex and hex flange heads, M2–M12), while B18.6.4M covers other parts of the metric machine screw series. Check the scope of each document for the exact product coverage.
A: B18.6.7M is the product standard for metric machine screws (flat countersunk, oval countersunk, pan, hex and hex flange heads, M2–M12), while B18.6.4M covers other parts of the metric machine screw series. Check the scope of each document for the exact product coverage.
❓ Q: Why do heat-treated flat countersunk head screws need a short shoulder under the head?
A: Heat treatment raises head strength, and the head-to-shank transition must maintain adequate minimum grip length and bearing area. The short shoulder increases shear and bending capacity, preventing head tear-out under high tightening loads.
A: Heat treatment raises head strength, and the head-to-shank transition must maintain adequate minimum grip length and bearing area. The short shoulder increases shear and bending capacity, preventing head tear-out under high tightening loads.
❓ Q: Why must recess depth be checked with a penetration gage?
A: A shallow recess reduces effective driver engagement and causes cam-out, damaging both screw and bit. The penetration gage simulates the driver tip to measure true engagement depth — the most direct way to control recess quality.
A: A shallow recess reduces effective driver engagement and causes cam-out, damaging both screw and bit. The penetration gage simulates the driver tip to measure true engagement depth — the most direct way to control recess quality.
🔍 Q: Can metric machine screws be driven with inch-series drivers or wrenches?
A: No. Head dimensions and recess parameters of metric and inch series screws are defined independently; mixing tools results in poor engagement, damaged heads, or drive failure. Use tools matched to the metric sizes.
A: No. Head dimensions and recess parameters of metric and inch series screws are defined independently; mixing tools results in poor engagement, damaged heads, or drive failure. Use tools matched to the metric sizes.
