Why Grit Size vs. Surface Roughness Matters

You know which diamond powder to use for which die material. You know how to inspect it. You avoid common waste. But one question still comes up: "If I use 3 μm powder, what Ra will I actually get?" This article gives you a practical answer – and shows how to plan your polishing sequence based on the finish you need.

Ra Basics (Short Version)

Ra (average roughness) is the most common surface finish spec for drawing dies. Typical requirements:

• Rough drawing (large wire): Ra 0.05–0.1 μm
• Fine / ultra‑fine wire: Ra 0.01–0.02 μm
• Mirror finish (optical quality): Ra ≤ 0.008 μm

Achieving these numbers isn't magic – it's about matching your diamond powder grit to the target Ra, step by step.

Experience‑Based Grit → Ra Table

The following numbers come from real PCD die polishing (similar for WC and CVD, though material hardness shifts results by ~10–20%). These are typical ranges after proper polishing technique (light pressure, clean environment, correct carrier).

These numbers assume you spend enough time at each grit (usually 2–4 minutes total per die per grit, in short 10‑second intervals). If you rush, Ra will be higher. If you over‑polish, you might get a slightly lower Ra but risk surface burns or orange peel.

What Happens When You Skip Grits? A Real Example

Let's say your target is Ra 0.01 μm. The correct path: 9 μm → 6 μm → 3 μm → 1 μm → 0.5 μm. That gets you there.

Now imagine you jump from 9 μm straight to 1 μm. The 9 μm step leaves scratches about 0.15–0.25 μm deep. 1 μm powder can only improve Ra by roughly one order of magnitude – it will reduce the peaks but cannot remove the deep valleys. The result? A final Ra of maybe 0.04–0.06 μm, not 0.01 μm. You'd have to go back to 3 μm or 6 μm, wasting time and powder.

Bottom line: the Ra you end with is limited by the coarsest grit you used unless you properly step down.

Other Factors That Influence Final Ra

Grit size is the biggest factor, but not the only one. Keep these in mind:

• Polishing pressure: Too heavy → deeper scratches, higher Ra (and burns). Too light → inefficient.
• Time per grit: Insufficient time leaves remnants of coarser scratches. A good rule: polish until the surface looks uniform under a microscope (or until you see no improvement).
• Carrier / lubricant: Oil‑based carriers generally give lower Ra than water‑based because they cool better and reduce micro‑welding.
• Die material hardness: Softer materials (WC) allow slightly lower Ra with the same grit compared to PCD or CVD.
• Diamond crystal shape: Rounded polycrystalline powder typically produces a lower Ra than sharp monocrystalline at the same nominal grit, because the scratches are shallower and more uniform.

How to Measure Ra on a Drawing Die

You can't just stick a standard roughness probe into a small die bore. Here's what works:

• Use a stylus with a small tip radius (2 μm or smaller) and a probe designed for small bores.
• Take measurements at multiple positions around the die circumference (at least 3–4 points).
• Specify the cut‑off length (typically 0.08 mm for very fine finishes, 0.25 mm for rougher). Keep it consistent for comparison.
• If you don't have a profilometer, a 100× microscope comparison with known roughness standards is a cheap alternative.

Practical Planning: From Target Ra to Grit Sequence

Work backwards from your target:

1. Identify required Ra (e.g., 0.01 μm).
2. Look at the table – you'll need to finish with 0.5 μm or 0.25 μm grit.
3. Now choose your starting grit based on how rough the die is initially. If it's a new die after EDM, start at 15 μm. If it's a re‑polish of a used die, start at 9 μm or even 6 μm.
4. Build the full sequence: starting grit → intermediate steps (no more than 2× jump per step) → final grit.

Example for Ra 0.01 μm on a new PCD die: 15 μm → 9 μm → 6 μm → 3 μm → 1 μm → 0.5 μm. That's six steps. For a used die in decent shape: 6 μm → 3 μm → 1 μm → 0.5 μm (four steps).

Common Misconceptions

• "Finer powder always gives finer Ra" – Not if you skip the intermediate steps. The coarsest grit used still dictates the deepest scratches.
• "I can replace 3 μm + 1 μm with just 1 μm for longer" – No. You'll spend way more time and still may not remove the coarse marks.
• "Lower Ra is always better" – For drawing dies, extremely low Ra (<0.005 μm) can actually reduce lubricant retention. Know what your wire actually needs.

Conclusion

Matching diamond powder grit to your target Ra is straightforward: use the table as a starting point, respect the step‑by‑step sequence, and account for material and technique. Measure Ra consistently to confirm.

Once you've chosen the right sequence, refer to the earlier articles in this series for proper polishing procedures, quality inspection, material‑specific tips, and cost control.