A die print says G6. Most buyers cannot translate that into what actually arrives in the box, and most suppliers will not explain it, because the honest answer is that the grade number covers only part of what determines whether the part comes out concentric. This page explains what cold heading die precision means in practice, what the G grades cover, and where extra precision is worth paying for.
When we talk about a precision die we mean four separate measurements. A supplier quoting only one of them is telling you a quarter of the story.
Two dies can both be G6 and still behave completely differently, because the grade typically constrains bore tolerance while concentricity and runout are held by the grinding setup. That is the part worth asking about.
The G scale is a tolerance band expressed in micrometres, applied to the nominal bore. The tighter the grade, the narrower the band, and the more grinding passes and inspection time it takes.
| Grade | Bore band | Typical use |
|---|---|---|
| G9 | Widest | General fasteners, short run, generous print |
| G7 | Standard | Commercial screws and bolts |
| G6 | Tight | Our default for automotive and load-bearing fasteners |
| G5 | Tighter | Precision screws, tight thread-blank control |
| G4 | Tightest practical | High-voltage and electrical contact screws, instrument fasteners |
The numbers matter less than the consequence. At G6 with a 6 mm nominal bore, the band is narrow enough that a well-set header will hold the thread-blank diameter without adjustment across a full shift. Move two grades looser and the operator starts chasing size at the machine.
G6 is the point where precision stops being free. Going from G7 to G6 costs a modest amount of grinding time. Going from G6 to G5 roughly doubles inspection effort, because every piece has to be gauged rather than sampled, and the reject rate on the grinding line rises.
For most automotive and load-bearing fasteners G6 delivers everything the application needs. We go tighter only when the print demands it or when the failure mode is measurable, for example a contact screw where a few micrometres of eccentricity shows up as resistance drift.
A certificate is a claim. Measurement is proof. Every precision die we ship goes through the same sequence:
The records travel with the die. If a customer later sees a size problem on the machine, the first question is whether the die left our shop inside the band, and the answer should be a document, not an opinion. 100% inspection, not sampling, is our standard on precision orders.
A G6 die does not remain a G6 die for a million hits. Four things move it:
This is why we treat nib and casing as one matched assembly. A precision nib inserted into a relaxed casing loses the precision you paid for on the first day of production.
Tighter is not automatically better. Pay for G5 or G4 when one of these is true:
Skip it when the part is a standard fastener in a clearance hole. Nobody will measure it, and the money is better spent on a better cut-off or a better coating. Material choice interacts with all of this as well; our die material selection guide explains when a harder grade buys more than a tighter one.
Send the drawing with the tolerance callouts and the annual volume. We will confirm the achievable grade, flag any callout that will drive cost without improving the part, and quote including inspection records. The full product range is on the cold heading dies page, and the deformation side of the problem is covered under extrusion ratio limits.