Self-Threading Screws, Thread Inserts and Hole Design: Cross Recess Types

Cross Recess Types

ANSI B18.6.4 defines four cross recess configurations used with self-tapping screws:

The Pilot Hole — Where the practitioner Went Wrong

This is the section that would have saved the practitioner's contract.

The pilot hole is not an approximation. It is a precision engineering decision that directly determines whether a self-threading screw will:

1. Form or cut clean, strong threads
2. Apply the right preload without splitting the material
3. Achieve adequate thread engagement without stripping

The consequences of getting it wrong are binary and brutal — either the screw strips the material on installation, or the joint fails in service.

The Three Hole Categories

Before you look at a single specification table, understand the three fundamentally different hole types used with self-threading fasteners:

1. Pierced or Extruded Holes Holes formed by punching or extruding, which creates a raised collar of material around the hole perimeter. This collar provides additional thread engagement depth. These holes can be slightly smaller than drilled holes for the same screw because of the added engagement.

2. Drilled or Clean-Punched Holes Standard drilled or cleanly punched holes with no raised collar. These are the most common hole type and require precise sizing for the screw type and material.

3. Clearance Holes Not installation holes — clearance holes allow the screw to pass through a top layer freely and engage only the bottom layer. Three classes exist: Close, Normal (Preferred), and Loose.

Inch-Series Hole Sizes: Type AB Thread Forming Screws

In Steel, Stainless Steel, Monel, and Brass Sheet Metal

Key Insight: Notice how hole size increases as metal thickness increases for the same screw size. Thicker metal provides more thread engagement depth, so a slightly larger hole still achieves adequate grip without requiring excessive forming force.

Thread Cutting Screws in Sheet Metal (Types D, F, G, T)

In Steel and Aluminum Alloy

the practitioner's Lesson: Aluminum alloy consistently requires a slightly smaller hole than steel of the same thickness. This is because aluminum is softer and the cutting edges can remove material more easily. Using the steel-specification hole in aluminum reduces thread engagement depth and is a primary cause of stripped threads.

Thread Cutting Screws in Cast Metals and Plastics (Types BF and BT)

In Die Cast Zinc and Aluminum

In Plastics (Phenol Formaldehyde vs. Acrylic/Styrene Resins)

Penetration depth matters. In through-material applications, depth is self-regulating. In blind holes, you must ensure the screw achieves sufficient thread engagement before bottoming. The "Min Penetration" column tells you the minimum screw-to-material engagement required for acceptable pull-out resistance.

Type U Metallic Drive Screws — Hole Sizes

Type U screws are driven by force (pressing or light hammer blows), not rotation. They create permanent anchors.

Approximate Installation Holes for Type U in Various Materials:

Thread and Point Dimensions — The Full Specification Tables

Types B and BP — Thread Forming (ANSI B18.6.4-1981)

Clearance Holes — The Other Side of the Joint

A clearance hole is drilled in the upper member of a joint so the fastener shank passes through freely. Thread engagement occurs only in the lower member. The clearance fit class determines how freely the screw moves through the top layer and how much adjustment the joint allows.

Inch-Series Clearance Holes for Self-Tapping Screws

Types AB, B, BF, and BT — Clearance Holes

Refer to ANSI B18.6.4-1981 for complete close/normal/loose clearance tables for all screw types. The following metric tables are complete per ANSI/ASME B18.6.5M-1986.

The Metric System — Complete Specifications

Metric Self-Threading Screw Types (ANSI/ASME B18.6.5M-1986)

The metric standard mirrors the inch-series structure but uses millimeter dimensions and slightly different type designations:

Thread Forming (Metric):

• Type AB — Spaced thread, gimlet point; thin metal, resin-impregnated plywood, asbestos compositions
• Type B — Spaced thread, blunt point, same pitches as AB; thin metal, non-ferrous castings, plastics, plywood, asbestos

Thread Cutting (Metric):

• Types BF and BT — Spaced threads, blunt point, cutting edges/chip cavities; for plastics, asbestos, similar materials
• Types D, F, and T — Machine screw diameter-pitch combinations, 60° thread form approximation, blunt point, cutting edges; for aluminum/zinc/lead die castings, steel, cast iron, brass, plastics

Metric Thread Dimensions — Types BF and BT

Metric Clearance Holes — Complete Table

For Types AB, B, BF, and BT (left) and Types D, F, and T (right)

Always select Normal Clearance (Preferred) unless your design specifically demands close alignment precision or maximum adjustment range. The preference notation in the standard is deliberate — it reflects decades of manufacturing experience about what works best in typical assemblies.

Metric Clean-Punched and Drilled Hole Sizes — Type AB Thread Forming in Sheet Metal

In Steel, Stainless Steel, Monel, and Brass

In Aluminum Alloy Sheet Metal

Pierced or Extruded Hole Sizes for Metric Types AB and B in Steel/Monel/Brass

Pierced/extruded holes are consistently larger than drilled holes for the same screw and material. This is because the raised collar around the hole provides additional threaded material depth. If you drill a hole to pierced-hole specs without the collar, you will have inadequate thread engagement.

Metric Hole Sizes for Types BF and BT in Cast Metals

In Die Cast Zinc and Aluminum

Nominal Screw Lengths — How to Specify Without Ambiguity

Nominal length is always measured from the underside of the head to the tip of the screw, with one critical exception: countersunk (flat or oval) heads are measured from the top surface of the head, since the head itself is intended to sit flush with or below the surface.

Minimum Practical Screw Lengths (Inch Series — Types AB)

Why does countersunk need a longer minimum length? Because part of the screw's nominal length is consumed by the tapered head sitting in the countersunk recess. The effective shank-in-material engagement is shorter than the nominal length. This is one of the most common specification errors on shop floors worldwide.

Minimum Practical Screw Lengths — Metric Types BF, BT, D, F, T

The Method of Designation — Specifying Screws Without Ambiguity

An incorrectly specified fastener causes the same downstream damage as a wrong fastener. ANSI standards define designation sequences precisely.

Type U Metallic Drive Screw Designation

Sequence: Nominal Size × Nominal Length — Product Name (Head Type) — Material — Finish

Examples:

10 × 5/16 Round Head Metallic Drive Screw, Steel

0.312 × 0.50 Round Head Metallic Drive Screw, Steel, Zinc Plated

Material and Heat Treatment — What the Screw Is Made Of Matters as Much as Its Type

the practitioner had a third problem he hadn't yet discovered: some of his replacement screws were standard carbon steel. The new aluminum housing was anodized to a harder finish. On close examination, the screw tips were deforming before they could cut threads.

The material of the screw must be harder than the material it's threading into. This sounds obvious until it isn't.

Standard Fabrication Materials

Per ANSI/ASME B18.6.5M, tapping screws are normally fabricated from carbon steel and suitably processed to meet performance and test requirements. However, they may also be made from:

Important: When using screws in materials other than standard carbon steel, the material properties, performance characteristics, and torque requirements should be mutually agreed upon between manufacturer and purchaser — they are not automatically covered by standard tables.

Torsional Strength Requirements (ANSI B18.6.4-1981)

The screw must survive the installation torque without shearing. Torsional strength is the minimum torque a screw can withstand without permanent deformation at the shank-head junction.

Thread-cutting screws (D, F, G, T fine thread) require higher torsional strength than their thread-forming counterparts in many sizes. This makes mechanical sense: cutting through harder material requires higher torque, which demands a more robust screw cross-section.

Thread Inserts — The Professional's Secret Weapon

This is where the practitioner's story takes its final turn.

After resolving his hole-size errors and material mismatches, the practitioner still faced a problem: some of the aluminum enclosures were already in the field, with threads that had been stripped by the incorrect installation. He couldn't scrap them. He needed to repair them — without increasing the hole size significantly.

The solution existed all along. He'd simply never been taught about it.

Self-Tapping Thread Inserts

Self-tapping thread inserts are essentially hard bushings with both internal and external threads. The internal thread conforms to standard Unified/American classes 2B or 3B. The external thread has cutting edges on the end that provide the self-tapping feature.

How they work:

BEFORE:                        AFTER INSTALLATION:

  Damaged or soft               ┌──────────────────┐
  base material                 │  Base Material    │
       ↓                        │  ┌────────────┐  │
  ┌─────────┐                   │  │  Insert    │  │
  │         │                   │  │ (hard      │  │
  │  ████   │ ← stripped        │  │  bushing)  │  │
  │  ████   │   threads         │  │            │  │
  │         │                   │  │ Internal   │  │
  └─────────┘                   │  │ thread:    │  │
                                │  │ 2B or 3B   │  │
                                │  └────────────┘  │
                                └──────────────────┘
                                         ↑
                                Machine screw goes here
                                (standard thread spec)

Compatible base materials:

• Magnesium
• Aluminum
• Cast iron
• Zinc
• Plastics
• Wood (brass inserts specifically)

Available insert materials:

• Case-hardened carbon steel — standard general use
• Stainless steel — corrosion-resistant environments
• Brass — specifically for wood installation

Screw Thread Inserts (Heli-Coil Type)

Screw thread inserts are helically formed coils of diamond-shaped wire — typically stainless steel or phosphor bronze — that screw into a specially tapped hole to form a high-quality internal mating thread.

These inserts serve two primary purposes:

1. Thread Repair When threads in a housing are stripped, a Heli-Coil insert restores full thread integrity without enlarging the external dimensions of the part.

2. Thread Reinforcement In soft materials where direct tapping produces threads too weak for the application, a coil insert delivers dramatically higher pull-out resistance by converting the soft material's threads into high-strength stainless or bronze.

According to the fastener supplier., conventional design practice for boss diameters and edge distances can generally still be applied — because the major diameter of a hole tapped to receive a thread insert is not much larger than the major diameter of thread the insert provides.

Available thread sizes for Heli-Coil inserts:

Wire material options:

• Stainless steel — most common; corrosion resistant; high strength
• Phosphor bronze — for electrical conductivity requirements or non-magnetic applications

When to Choose Each Insert Type

Hole Preparation Mastery — The Decision Framework

the practitioner built a laminated decision card for his shop floor after his ordeal. Here is the engineering logic behind it.

Step 1: Identify the Base Material

The base material determines the screw type (forming vs. cutting) and the specific hole size column to use.

Is the material ductile (aluminum, mild steel, brass, plastic)?
     YES → Thread Forming may be appropriate (Types AB, B)
     NO  → Thread Cutting is required (Types D, F, G, T, BF, BT)

Is the material brittle (cast iron, hard cast zinc, hard plastics)?
     YES → Thread Cutting required (chip removal prevents cracking)

Step 2: Determine Hole Type

Is the hole made by drilling or clean punching?
     → Use Drilled/Clean-Punched hole tables

Is the hole made by piercing or extruding?
     → Use Pierced/Extruded hole tables (larger values permitted)

Is the screw passing through the top layer only?
     → Use Clearance Hole table (not installation table)

Step 3: Confirm Material Thickness

Hole size is thickness-dependent. Never select a hole size without also recording the material thickness. Using the wrong thickness row is equivalent to selecting the wrong hole size entirely.

Step 4: Verify Screw Length

Ensure the selected nominal length achieves:

• Minimum practical length for the head style
• Sufficient engagement depth in the threaded member
• Penetration within specified range for blind holes

$$\text{Effective Engagement Depth} = L_{\text{nominal}} - L_{\text{clearance member thickness}}$$

For countersunk heads: $$\text{Effective Engagement Depth} = L_{\text{nominal}} - L_{\text{clearance member thickness}} - h_{\text{head}}$$

Where $h_{\text{head}}$ is the embedded depth of the countersunk head.

Step 5: Check Torsional Strength Against Installation Torque

Your installation torque must not exceed the screw's minimum torsional strength. If your installation process consistently requires higher torque (due to hard material, poor lubrication, or heavy finish), you must:

• Increase screw size, or
• Switch to a thread-cutting type (lower driving torque for the same material), or
• Verify the pilot hole size is not undersized (a common cause of excessive driving torque)