By Sam Desai, Technical Team, Beta Diamond Products
The rule that organizes everything: hard and napless early, soft and napped late, and no more nap than the step requires. Cloth hardness controls flatness and edge retention. Nap controls final finish and scratch hiding. Use a hard low nap or napless cloth for 9 and 6 micron diamond, a low nap cloth for 3 micron, a low to medium nap for 1 micron, and reserve the soft high nap cloth for the brief final oxide polish, if you use one at all. Get the pairing wrong and the abrasive takes the blame for problems the cloth created.
What nap does, for better and worse
Nap is the pile of fibers standing up from the cloth backing, like very short carpet. During polishing, abrasive particles ride partly buried in that pile, presenting gently to the sample. More nap means a softer touch, fewer visible scratches, a more forgiving polish.
It also means three problems. Napped fibers wrap over sample edges and round them, which destroys coating and case depth measurements. They conform to hard and soft phases differently and generate relief. And they slow real material removal, so deformation from earlier steps can get glossed over rather than removed. A surface polished entirely on soft napped cloths can look perfect and etch terribly, because the nap hid scratches instead of removing the damaged layer beneath them.
Napless and low nap cloths invert the tradeoff. The abrasive sits exposed on a firm surface and cuts aggressively in a single flat plane. Flatness, edge retention, and true removal are excellent. The visible finish at any given micron size is slightly coarser, which is fine, because the next step erases it.
A pairing that works for most metals
Nine and six micron diamond: hard woven cloth, silk type or hard synthetic, or a paper backed pad. This is the workhorse removal stage and you want every particle cutting.
Three micron: low nap woven or pressed cloth. Still cutting, starting to refine.
One micron: low to medium nap synthetic. Finish starts to matter here and a touch of nap helps, but resist the urge to jump to the softest cloth in the drawer.
Final oxide, colloidal silica or 0.05 micron alumina: soft porous synthetic or chemical resistant pad, for sixty to one hundred twenty seconds. Short. Relief and edge rounding grow by the minute on this cloth.
It is worth saying that our final polishing cloth is the one customers compliment most, and the reason is honest rather than flattering: the final step is where every upstream mistake either gets erased or immortalized, so quality differences in the last cloth are the ones you can actually see. But no single cloth is "the best." The right cloth depends entirely on your material and the finish your work requires, which is why we stock the whole range rather than pushing one.
For hard materials like carbides and ceramics, shift everything one notch harder and consider finishing at 1 micron on a napless pad. For coated samples where the edge is the whole point, stay napless as long as humanly possible and keep any final napped step under a minute.
Cloth discipline that saves samples
One cloth, one micron size, forever. Write the size on the cloth backing the day you mount it. A cloth that has seen 9 micron diamond can never again be trusted at 1 micron, no matter how hard you rinse it.
Charge a new cloth before its first samples: a thin film of diamond paste worked into the surface, so it cuts properly from the start instead of starving the first few samples while a dispenser slowly loads it.
Replace cloths on evidence, not schedule. Falling removal rate, scratches that fresh suspension does not cure, or visible glazing all mean the cloth is done. A few dollars of cloth is never worth a morning of rework.
We carry the full range, from hard woven cloths to final polish pads. Tell us your material and your current scratch complaints, and we will tell you which cloth in your sequence is most likely betraying you. Samples free, as with everything we sell.