Sample approval is a decision, not a formality
A needle, sinker or cam that looks correct in a photograph can still ruin a production week. The sample approval stage exists to catch that gap before a purchase order commits a year of consumption to the wrong part. A circular knitting spare parts sample approval supplier acceptance checklist turns a subjective judgement into a recorded decision: this sample is acceptable, for this machine, at this gauge, under these conditions, with these records attached.
Factories that skip this stage usually do so because the sample looked fine and the price was good. Three months later the machine runs hotter, breaks needles faster and produces fabric with a stubborn defect, and nobody can prove whether the part failed or the machine did. The approval record is what makes that conversation short.
Define the sample identity before you test it
Start with identity, not measurement. A sample without a documented identity cannot be approved, because approval attaches to a specific part, revision and batch. Record the supplier name, the part description, the part number and revision, the machine model and gauge the part is intended for, and the batch or lot code.
Ask for the material or treatment evidence that the part claims to carry. For a needle or sinker, that may be the steel grade and a heat-treatment record. For a cam, it may be the alloy and surface treatment. For a coated part, it may be the coating type and thickness. The point is not to demand a laboratory file for every screw. It is to make the supplier commit in writing to what the part is, so that a later batch can be compared against the approved reference.
Two suppliers can offer the “same” sinker and deliver different parts. One uses a spring steel with a stated friction coefficient under a defined lubricant, and the other substitutes a cheaper alloy that wears differently. Identity records separate them before the order, not after the claim.
Dimensional and material checks
Measure the sample against the machine’s requirement, not against a feeling. The drawing or the original part defines the dimensions. Record the measured value, the instrument and the tolerance for each critical feature: overall length, hook or blade geometry, groove or slot width, and any locating surface.
The values below come from Aisunny’s published technical-specifications database. They are reference benchmarks for structuring a check, not a claim that every part on the market meets them. Confirm each figure against the original machine manual and the drawing for the specific part.
| Reference benchmark | Published value | Where it applies in a sample check |
|---|---|---|
| Needle and sinker tolerance | ±0.005 mm | Dimensional check on needles and sinkers |
| Needle and sinker material | German/Japanese specialised spring steel | Material verification on the sample certificate |
| Friction coefficient | 0.08–0.12 (with ISO VG 15–22 lubricant) | Confirms the lubricant assumption behind the part |
| Needle and sinker heat resistance | Stable up to 180°C | Reference for high-speed or hot-condition use |
| Cylinder hardness | HRC 58–62 after induction hardening | Hardness spot-check on cam or cylinder-related parts |
| Cylinder trick tolerance | ±0.01 mm | Dimensional reference for cylinder-related samples |
Hardness is the check buyers most often skip, and it is one of the cheapest to add. A portable hardness tester records a value on the spot. If the sample is a cam or a cylinder-related part, the reading is compared against the reference band and the drawing. If the sample is a coated part, the coating type and thickness matter more than the base hardness, and those belong on the supplier certificate.
The sample approval checklist as a buyer-facing table
Use one table for every sample, whatever the part. The last two columns are where approval becomes defensible.
| Check | What to record | Pass condition | Evidence retained |
|---|---|---|---|
| Identity | Supplier, part number, revision, batch | Matches the request | Sample label and request |
| Material | Steel grade, coating, treatment | Matches the stated specification | Supplier certificate |
| Dimensions | Critical dimensions and instrument | Within drawing tolerance | Measurement sheet |
| Hardness | Reading and test method | Within the reference band | Hardness record |
| Visual | Surface, burrs, marking | No defects that affect function | Photographs |
| Friction or feel | Lubricant assumption, movement | Consistent with the reference part | Test note |
| Machine trial | Machine, gauge, yarn, speed | Agreed defected limit not exceeded | Trial run log |
| Fabric result | Structure, defects, sample | Meets the reference sample | Signed fabric sample |
| Corrective action | Open findings and owner | Closed before volume order | Closure record |
| Sign-off | Approver, date, scope | Named and dated | Approval form |
The trial run is where a sample earns approval
A sample that passes dimensional and material checks still has to survive the machine. Run it on the intended machine, at the intended gauge and speed, with the yarn the factory actually uses. Record the number of parts fitted, the run duration, the speed and the yarn lot. Watch for the failure modes that the part is most likely to cause: needle breaks, sinker marks, cam wear, stitch irregularities or heat build-up.
Keep the trial short but honest. A ten-minute run at half speed proves little about a high-speed machine. A several-hour run at production speed, with a defect inspection at the end, gives the approval a foundation. If a full trial is impractical, say so in the record and mark the approval as conditional, with a follow-up plan.
The trial produces two things worth keeping: the run log and a fabric sample. Label the sample with the machine, gauge, yarn lot, speed and date. That sample becomes the reference for the first volume delivery, and it settles the argument if the second batch behaves differently. When parts are consumed in volume, the needle consumption cost calculator turns the trial’s breakage rate into a reorder point, so the approval and the replenishment plan stay connected.
Defect log and the corrective action loop
Every sample trial produces findings, even a good one. Write them down. A defect log should list what was observed, on which part, under what condition, and with what severity. Keep defects that affect function separate from cosmetic marks that do not.
If a sample fails, the supplier should respond with a corrective action, not a discount. A corrective action names the cause, the change, and the evidence that the change worked. Ask for a revised sample and repeat the affected checks. A supplier who cannot describe the cause is likely to send the same part again with a different batch number.
When the part is a substitute for an obsolete or unavailable original, the review deepens. The substitute must be approved against the machine, not only against the drawing. The counterfeit detection and supplier verification checklist covers the packaging, serial and supplier checks that support a substitute decision, while the needle types guide helps confirm that the substitute is the correct type for the fabric being produced.
Substitute approval and traceability records
Approval scope matters. Approving a sample for one machine and gauge does not approve it for a fine-gauge machine that stresses the same part differently. Write the approved scope into the record: which machine, which gauge, which speed range, which yarn types.
Traceability makes the approval usable later. Link the approved sample to the supplier, the part number, the revision, the batch and the approval date. When the next delivery arrives, the incoming check compares it against the approved reference. A part that matches the number but not the revision is not the approved part, and the record should say so.
The European Commission notes that textile-specific Digital Product Passport obligations are still to be defined through a future delegated act, and that possible information areas include identification, origin, repair and maintenance data. A supplier-readiness record that already captures part number, revision, material and approval status puts a factory ahead of that framework without claiming a rule that is not yet in force. Keep the caveat: the delegated act is not final, so no supplier should be told that a textile DPP obligation is already mandatory.
When to reject a sample
Reject when the identity is unclear, when the material or hardness evidence is missing or inconsistent with the claim, when a critical dimension is out of tolerance, or when the trial produces defects beyond the agreed limit. Also reject when the supplier treats a corrective action as a discount instead of a fix. A cheap sample that fails on the machine is the most expensive part in the store.
A rejection is not the end of the conversation. State the finding, the standard it failed against, and the evidence. A supplier who receives a clear rejection with data usually returns a better sample. A supplier who receives “it doesn’t work” returns the same part.
For parts that sit alongside the machine’s main wear items, the circular knitting machine spare parts guide provides the category context that helps place a sample in the wider parts list, and the circular knitting spare parts RFQ template gives a structured way to request the next sample with the right data attached.
Frequently Asked Questions
Q: How many samples should we test before approving a supplier?
A: One approved sample is enough to open a trial relationship, but approve it for a defined scope and a defined batch. For a new supplier, test a second batch before a volume order to confirm that the approved quality repeats.
Q: Do we need a certificate for every spare part sample?
A: Not every screw, but every part whose function depends on material, hardness or coating should carry evidence. For needles, sinkers and cams, the material and treatment record is the difference between a spec and a hope.
Q: How long should a sample trial run?
A: Long enough to reach production speed and inspect a meaningful length of fabric. Several hours at operating speed is more informative than a full day at half speed. Write the duration into the approval record.
Q: What if the sample passes but the volume delivery differs?
A: This is what the approval record and the retained reference sample are for. Compare the delivery against the approved batch, part number and revision. A mismatch on revision or material evidence is a rejection, not a negotiating point.
Q: Can we approve a cheaper substitute part?
A: Yes, if it passes identity, material, dimensional, hardness and machine-trial checks for the defined machine and gauge. Approve the scope in writing, because a substitute approved for one machine may not suit another.
Q: Does a sample approval replace incoming inspection?
A: No. Approval defines the reference. Incoming inspection confirms that each delivery matches that reference. Approval without incoming inspection lets a good decision drift into a bad delivery.
Q: What records should be kept after approval?
A: The sample label, supplier certificate, measurement sheet, hardness record, trial log, fabric sample and signed approval form. Keep them together so the next batch can be checked against a single reference.
Conclusion
Sample approval is the cheapest quality control a knitting factory can run. It costs a few measurements and a trial run, and it prevents a year of consumption from being committed to a part that only looked right. Define the sample identity, check material and dimensions against the drawing, run the part on the machine, log the findings, and approve a defined scope with a retained reference. Reject on missing evidence, not on price.
Done well, the approval record and the reference sample do the arguing for you. Done badly, every delivery becomes a debate the factory cannot win.
References
Lists possible textile DPP data areas, including identification, origin, repair and maintenance, under a future delegated act.
Provides the policy context for repairability and traceability that supplier-readiness records anticipate.
Defines AQL-indexed sampling schemes referenced when setting an incoming-inspection acceptance basis.
Shows supplier listings organised around OEM/ODM capability, MOQ and sourcing requests for knitting components.
Illustrates component categories and supplier positioning for circular-knitting spare parts.
Manufacturer reference for industrial knitting needles and precision parts used to frame sample material expectations.