A new operator burned 12 percent of the zinc alloy zipper pulls in the first week, and every rejected piece showed the same blue-black edge. Wheel grade, wheel speed and pressure cause most of these burns, not the part or the bath.
This guide is the junior standard for hand polishing bag hardware: wheel types, compound grades, surface speed in meters per second, and the pressure and timing checks that keep edges cool and clean.
Common Mistakes and How to Avoid Them
| Mistake | Why It Happens | Practical Fix |
|---|---|---|
| 1. Wheel speed too high | Friction melts edge metal | Set surface speed to 25-35 m/s |
| 2. Heavy hand pressure | Edge burns and metal smear | Use 1-3 kg, let the wheel cut |
| 3. Emery compound on finish step | Deep scratches and heat | Match compound to wheel stage |
| 4. Wrong wheel grade | Too aggressive on thin edges | Use sisal only for the cut stage |
| 5. Wheel not dressed | Glazed surface skids and heats | Dress wheel every 20-30 minutes |
| 6. Parts held too long | Local heat builds at the edge | Keep contact 3-5 seconds per part |
| 7. Same wheel for all parts | Thin edges burn first | Separate wheels by part geometry |
| 8. No edge check during run | Burns found at batch end | Check edges every 10 pieces |
| 9. Operator untrained on speed | Speed drifts with pressure | Verify speed with a tachometer |
Best Practices
- Set wheel surface speed with a tachometer, not by feel
- Use the cut-finish wheel sequence: sisal, stitched muslin, loose cotton
- Keep contact time under 5 seconds and let the wheel do the work
- Check part edges every 10 pieces during the run
- Dress wheels on a fixed schedule and log it
- Cool parts in small batches between polishing and plating
Implementation Roadmap
| Step | Frequency |
|---|---|
| Set wheel speed | Each shift start |
| Select wheel and compound | Per finish requirement |
| Dress the wheel | Every 20-30 minutes |
| Edge check | Every 10 pieces |
| Log wheel and result | Per batch |
Edge burn prevention flow
- Set speed - 25-35 m/s surface speed. (High speed melts thin edges.)
- Select wheel - Sisal cut, cotton finish. (Wrong grade over-cuts.)
- Polish - 1-3 kg pressure, 3-5 s. (Heavy pressure burns.)
- Check edge - Every 10 pieces. (Late checks waste the batch.)
Surface speed = pi x wheel diameter x rpm divided by 60. | A cool edge is the first sign of correct pressure.
Wheel and pressure data
Reference values for hand polishing zinc alloy bag hardware.
| Parameter | Reference | Why It Matters |
|---|---|---|
| Surface speed | 25-35 m/s | Below 20 m/s cuts poorly; above 38 m/s burns edges |
| Speed formula | rpm = 60 x v / (3.14 x D) | D is the wheel diameter in meters |
| Cut stage | Sisal wheel + emery, 120-240 grit | Removes die flash and gates |
| Finish stage | Loose cotton + green rouge | Builds color without heat |
| Pressure | 1-3 kg on small parts | More pressure adds heat, not cut |
| Contact time | 3-5 seconds per part | Long contact raises edge temperature |
| Material removal | 0.03 mm max per pass | More removal weakens thin walls |
Wheel and compound selection
| Finish required | Wheel | Compound | Speed |
|---|---|---|---|
| Heavy cut / flash | Sisal | Emery 120-240 | 25-30 m/s |
| Satin finish | Stitched muslin | Tripoli | 28-32 m/s |
| Mirror color | Loose cotton | Green rouge | 30-35 m/s |
Edge burn check card
| Check | Target | Action |
|---|---|---|
| Edge color | No blue or black | Reduce speed or pressure |
| Edge shape | Radius unchanged | Stop if metal is smeared |
| Part temperature | Cool to touch | Reduce contact time |
| Wheel face | Dressed, no glaze | Dress before next part |
Case 1 - twelve percent edge burns in a new operator's first week
Scenario. A bag hardware plant in Guangdong put a new operator on zipper pull polishing; within the first week 12 percent of parts failed with blue-black burned edges and were rejected before plating.
Action. A tachometer showed the wheel running at 42 m/s, and the operator pressed hard with emery compound on the finish step. Speed was cut to 30 m/s, pressure limited to 2 kg, and the wheel sequence was fixed at sisal, muslin, cotton.
Result. Edge burns dropped to 0.5 percent in the next week; the speed check and edge check became part of the shift start.
Case 2 - shift-to-shift burns on die-cast buckles in Vietnam
Scenario. A finishing shop in Ho Chi Minh City rejected buckles with melted edges every afternoon shift while the morning shift ran clean; both shifts used the same parts and bath.
Action. The afternoon operator changed to a fresh sisal wheel but kept the old glazed wheel speed; wheel dressing was added every 25 minutes and the edge check every 10 pieces on both shifts.
Result. Afternoon rejects matched the morning shift within three days, and the wheel log showed the dressing gap.
Frequently Asked Questions
Why do edges burn during polishing?
Thin edges lose heat faster than flat faces, so friction raises edge temperature quickly; high speed and heavy pressure push it past the melt point.
What wheel speed should a beginner use?
Set the surface speed at 25-35 m/s with a tachometer; below 20 m/s the wheel stops cutting and above 38 m/s thin edges overheat.
How do I choose the wheel grade?
Match the wheel to the stage: sisal for heavy cut, stitched muslin for satin, loose cotton for final color.
Which compound goes with which wheel?
Emery with the cut wheel, tripoli with muslin, green rouge with cotton; mixing stages leaves deep scratches.
How much pressure is correct?
1-3 kg for small zinc alloy parts; extra pressure adds friction heat, not cutting speed.
Why does the edge turn blue or black?
Blue-black color means the metal reached high temperature; reduce speed, pressure or contact time immediately.
How often should the wheel be dressed?
Every 20-30 minutes of running, or whenever the face looks glazed; a glazed wheel skids and heats parts.
How long can I hold a part on the wheel?
Keep contact at 3-5 seconds per part; longer contact builds heat at the edge before the cut improves.
What is the first check after polishing?
Look at the edges of the first 10 pieces; a cool, clean edge means speed and pressure are correct.
What Would You Like to Solve?
Describe your wheel type, wheel speed, compound and the burn pattern on your parts; the sequence here can be matched to your line's exact geometry.