What Makes Good Screw Heading and Rolling?

To judge whether a screw's front-end processing was done well, check three things: head dimensions, thread forming and shank straightness. All three are locked in during cold heading and thread rolling, and heat treatment or plating can't fix them later.

What should you check in cold heading?

• Head height and diameter: First, are they within drawing tolerance? Then, how much do they vary within a batch? Growing variation usually means the dies are starting to wear.
• Recess depth and shape: Too shallow, and the bit cams out. Too deep, and the head gets thin enough to snap under torque.
• Underhead and head surface: There should be no laps or cracks at the underhead fillet. These are common cold forging defects and the usual starting point for head breakage.

How can you tell if thread rolling was done right?

• Blank diameter: This decides whether the thread fills out. Too large, and the crests push out burrs. Too small, and the crests come up short with an undersized major diameter.
• Thread form: Look for full crests, consistent pitch, and no drunken or doubled threads.
• Major and minor diameter: These directly affect driving torque and pull-out strength. For wood screws such as hardwood screws, also check the point and the first thread, since they decide how quickly the screw bites.

Thread rolling cold-forms the thread without cutting through the grain, so rolled threads are usually stronger than cut threads. That only holds true when the blank diameter and the rolling dies are properly matched.

Why do long screws need straightness checks?

The longer the screw, the more likely it is to bend or run out of true during heading and rolling. A bent blank gets locked in during heat treatment, and on site that means screws that drive in crooked or snap. Long screws should be checked for straightness batch by batch, not just spot-checked for dimensions. We process custom lengths from 13mm to 440mm and check long screws on a bending tester.

What should you ask a front-end processing supplier?

• Do they make their own heading and thread rolling dies? In-house tooling means faster adjustments to thread forms and dies. Our heading and rolling teams develop their own tooling, so we can adapt to custom specifications.
• What equipment do they inspect with? Thread form needs optical projection, heads need height measurement, and long screws need straightness checks. We inspect every batch against customer drawings using an automatic optical projection machine, a head height tester and a bending tester.
• How do they leave allowance for downstream processing? Front-end dimensions have to account for heat treatment and plating thickness. If a supplier can't explain this clearly, problems tend to show up later in the process.
• Are quality records traceable? Check for ISO 9001:2015 certification and whether batch inspection records can be pulled when something goes wrong.