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SAE J58-2015: Engineering Guide to Flanged 12-Point Screws
The SAE J58-2015 standard defines the detailed dimensional and performance specifications for flanged 12-point screws. These fasteners are widely used in automotive, ground vehicles, and industrial equipment where high-strength, positive drive engagement is required. The 2015 revision brought several clarifications, updated gaging methods, and aligned the standard with modern industry practices.
1. Overview and Scope
Included in SAE J58-2015 are specifications covering head dimensions, threads, material options, and workmanship requirements for flanged 12-point screws. The standard applies to general-purpose screws in inch sizes from 1/4 in. to 1-1/2 in. nominal diameter. Key updates in this revision include:
- Clarification that dimensions apply to unplated/uncoated product.
- Adoption of the two-ring gage method for head acceptability, similar to metric 12-point flange screw standards.
- Revised underhead fillet dimensioning for easier interpretation without changing physical part requirements.
- Updated thread class defaults and inclusion of constant-pitch series as a purchaser option.
- Improved straightness inspection references using ASME B18.2.9.
- Added caution regarding hydrogen embrittlement for screws exceeding HRC 39 hardness.
2. Key Dimensional and Thread Specifications
All dimensions in the standard are in inches and apply to unplated products unless otherwise noted. The table below summarizes critical dimensions for selected sizes. For complete tables, refer to the full standard.
| Nominal Size | Body Dia Min | Flange Dia Max | Head Height Max | Wrenching Height Min | Flange Thickness Min |
|---|---|---|---|---|---|
| 1/4 | 0.2435 | 0.375 | 0.260 | 0.15 | 0.058 |
| 3/8 | 0.3678 | 0.562 | 0.375 | 0.21 | 0.095 |
| 1/2 | 0.4919 | 0.750 | 0.500 | 0.29 | 0.129 |
| 3/4 | 0.7406 | 1.125 | 0.750 | 0.44 | 0.200 |
| 1 | 0.9886 | 1.500 | 1.000 | 0.60 | 0.268 |
Thread Requirements 🛠️
Threads are preferably rolled and must meet ASME B1.1. Unless otherwise specified by the purchaser, the thread form shall be UNR. The thread series can be UNRC, UNRF, or a constant-pitch series per purchaser. For uncoated threads, the default class is 2A. Coated threads are accepted using class 3A GO and 2A NOGO gages.
3. Design Insights and Best Practices
Design Insight: Two-Ring Gage Method for Head Acceptability 🛠️
The 2015 revision introduced a consistent method using two gaging rings (B and T). To verify head corner fill and wrenching height, first place Ring B on the screw head, then place Ring T over it. The wrenching height is acceptable if Ring T does not contact Ring B after both rings are in place. This method aligns with metric practices and ensures reliable wrenching engagement.
Underhead Fillet: The fillet must be smooth and continuous. For full-length threaded screws, the minimum fillet transition diameter (Ea min) can be reduced by the difference between the basic screw diameter and the specified minimum pitch diameter. This change in dimensioning method in the 2015 revision improves clarity without altering physical geometry.
⚠️ Caution: Hydrogen Embrittlement Risk
SAE J58-2015 warns that screws heat-treated to a hardness exceeding HRC 39 are susceptible to hydrogen embrittlement. Proper baking after plating and careful material selection are essential. Additional material options are referenced from SAE J429, ASTM F468, ASTM F593, and ASTM A574.
Common Mistakes and Considerations
- Coating adjustments: Dimensions are for unplated product. Coating thickness must be accounted for in thread gaging and final dimensions.
- Two-ring gage procedure: Place Ring B first, then Ring T. Misordering can lead to incorrect acceptance.
- Thread series not specified: If the purchaser does not specify UNRC, UNRF, or constant pitch, the default is UNR thread form and default series per manufacturer discretion.
- Runout measurement: Bearing surface runout must be measured as close to the periphery as possible with the screw held at a distance of one diameter from the head.
Frequently Asked Questions
Q1: What default thread class applies to uncoated flanged 12-point screws? 🔍
A: Unless otherwise specified, uncoated threads are class 2A per SAE J58-2015.
Q2: How is head acceptability verified?
A: Using the two-ring gage method. Place Ring B on the head, then Ring T. If Ring T does not contact Ring B, the wrenching height is acceptable.
Q3: What materials are permitted?
A: The standard references SAE J429 for mechanical requirements, ASTM F468 for nonferrous, ASTM F593 for stainless steel, and ASTM A574 for alloy steel. Material selection should account for strength and hardness limits to avoid hydrogen embrittlement.
Q4: What is the straightness requirement for longer screws?
A: The 2015 revision adopted straightness gaging per ASME B18.2.9, with graduated tolerances for longer screws, replacing the old Appendix A.
This concludes our overview of SAE J58-2015. For full details, consult the official standard document.
