---
title: "Which Custom Manufacturer Should You Choose for Metal Inserts in Security Hardware?"
description: "A security hardware quality engineer struggles with plated inserts showing pitting and OD drift. The response explains how to set clear requirements, manage tooling-related cost and lead time, and judge supplier capability before mass production."
url: "https://www.hyneu.com/questions/custom-manufacturer-metal-inserts-security-hardware.html"
language: "en"
type: "Q&A"
category: "Custom Manufacturing Q&A"
datePublished: "2026-10-27"
dateModified: "2026-10-27"
brand: "HYNEU"
manufacturer: "HYNEU"
answerCount: 10
---

# Which Custom Manufacturer Should You Choose for Metal Inserts in Security Hardware?

## Question

 At my company, we manufacture a line of electronic lock chassis, and I am the quality engineer handling incoming metal parts. We purchase a custom zinc alloy insert, part 47-221, which is die-cast, machined on the inner bore, and plated on the visible face. The insert is pressed into a plastic housing and acts as the wear surface around the locking bolt. In the most recent production lot, we saw two separate problems that forced our line to switch from normal sampling to 100% sorting. After plating, around 1.8% of the inserts developed tiny pitting on the counterbore face. In parallel, we found a dimensional shift on the outer diameter, measuring 9.78 mm when the drawing requires 9.85 mm +/-0.02 mm. The six parts per thousand that fall below 9.83 mm mean the press fit no longer secures the insert, causing the lock torque test to fail on the assembly station. My supplier says pitting after plating is a normal appearance issue and claims their yield is still above 99%. Their PPAP was approved a year ago, but they never updated it after they changed their plating subcontractor. I need to know how to evaluate a custom manufacturer for these metal inserts, what evidence I should demand from the supplier in terms of process capability and quality control, and what the realistic cost and lead time impact is if we ask for tooling changes and a re-pilot lot. I also want to avoid overspecifying the part to the point where every incoming lot is difficult to inspect. 

## Answers
                            
### Answer 1 — Best Answer

Start with the requirement set, not with the desire to change suppliers. For a metal insert in security hardware, you need to define which surfaces are functional, which are appearance-related, and which defects are unacceptable regardless of yield. The counterbore face on your part looks like a plating appearance issue, but if the lock torque test fails at assembly, decide whether the pitting creates a path for corrosion or weakens the surface. Write the drawing note so that porosity exposed by machining or plating is a rejection cause. That removes the ambiguity the supplier is using to call it normal.

The outer diameter drift is not an appearance issue. The dimension 9.78 mm against a 9.85 +/-0.02 mm specification is 0.05 mm below the lower limit and means the press fit is gone. Record that as a critical defect. The supplier's "six parts per thousand" claim ignores the point that a security mechanism should not allow functional failures in any batch. Ask for a capability study on the outer diameter, based on at least 300 consecutive production parts, and **require a Cpk of at least 1.33**. Also check the measurement method. Plated parts change their effective OD with plating thickness and temperature. Measure at 20-25 °C after parts stabilize in the metrology room. Ask the supplier for their gauge R&R result to confirm the measuring system can resolve the tolerance band.

The cost and lead time question is tied to where the root cause is. Pitting after plating usually comes from subsurface porosity in the zinc die-casting. Plating chemistry will not cure porosity; the plating bath opens it up and makes it visible. The real correction is in the die-cast tool: gate position, overflow design, cooling, or casting parameters. Modifying an existing die and re-piloting typically takes two to three weeks of tool work plus another two to three weeks for process validation and first article reporting. If the supplier instead plans to machine the insert from solid brass or stainless bar to eliminate porosity, tooling cost may drop but piece price will go up because of material waste and machining time. A re-PPAP after a tooling change can take **four to six weeks** if it includes dimensional, material, and plating validation. Ask the supplier for a calendar, not a general promise.

Supplier judgment starts with their willingness to isolate the source. If they say pitting is normal or reject your tolerance check, treat that as a red flag. A manufacturer that wants your security hardware business should examine porosity, cast skin, machining allowance, pre-plating cleaning, and plating chemistry. Then require a **written corrective action** with a root cause statement and measurable acceptance criteria. Ask for the floor evidence: a tooling inspection record, in-process inspection sheets from the machining step, plating bath records, and a cross-cut adhesion test result after plating. For the pilot lot, inspect first-off and last-off parts from the same batch, measure the OD on the same fixture you plan to use in incoming quality, and cut a microsection of the counterbore face to see whether the porosity is closed or connected to the surface.

Finally, balance the value of the existing relationship against the cost of building a new one. Re-qualifying another manufacturer is expensive and time-consuming, and it will not fix a defect that actually started in the tooling and process. Use this bad batch as a test of the supplier's engineering discipline. Give them a deadline for the corrective action, define the acceptance criteria before they start, and require them to update the PPAP before any future production. If they cannot meet that level of control, that is the only reason to move your project to a different factory.

**status:** accepted
**Author:** Leo Shen
**Date:** 2026-10-27

### Answer 2

The pitting after plating at the counterbore face is usually a casting distribution problem rather than a plating problem. In die casting, the gate location determines how the molten zinc fills the cavity and where the last material to solidify stays.

If the counterbore region fills last or sits behind a local wall-thickness change, micro-porosity can appear after machining and plating. Ask the supplier to share the die layout and gate position.

If they cannot show the tooling layout or a flow simulation result, their correction plan is guesswork. A supplier that understands tooling will discuss overflow wells, gate geometry, or cooling changes instead of arguing that the defect is normal.

Also confirm the machining allowance after casting. A shallow facing pass on the counterbore can break through the dense cast skin and expose subsurface bubbles.

That is one of the most common reasons for plated zinc defects. During the pilot run, request microsection slices across the counterbore face and compare the defect to an un-machined cast surface.

You need to know whether the porosity is closed under the plating or connected to the surface. Closed porosity can remain hidden and still cause adhesion failure later when the lock is exposed to humidity or thermal cycling.

**status:** suggested
**Author:** Marcus He
**Date:** 2026-10-27

### Answer 3

The supplier's yield figure of six parts per thousand is not a justification; it is a symptom. For a security part, any defective insert that reaches the line is a process failure, not an acceptable output. When the OD can drift to 9.78 mm, ask what the in-process check frequency is and how quickly the operator reacts when a reading nears the control limit.

A capable line uses a gauging fixture, records readings on a control chart, and changes the insert before the tool shifts parts outside the tolerance band. If the supplier only measures at final inspection, the damage is already done.

Plating defects can also hide housekeeping issues. Cutting fluid residue, chips, or fingerprints left on zinc inserts before plating will react in the plating bath and appear as small pits. Ask whether the parts are washed and rinsed after machining and whether the wash solution is monitored or replaced on a schedule.

For the pilot lot, request time-stamped in-process data from one complete shift. Confirm that the pilot uses the same feed, speed, cleaning, and packing method as the mass production run. A supplier that builds a perfect pilot with extra operator attention is not giving you a true picture of their production capability.

**status:** suggested
**Author:** Irene Tang
**Date:** 2026-10-27

### Answer 4

Inserts that fail on outer diameter are often linked to tooling wear, not to the operator. In a zinc die-casting die, core pins and moving slides wear in predictable directions, and the part then starts to track around the worn segment of the cavity. Ask the supplier for the die maintenance record and the recent cavity measurement.

If the tool has not been traced, the dimensional consistency claim has no evidence. Also ask whether the OD is final-machined or as-cast. If as-cast, the die condition is the only thing controlling the dimension.

The die steel and its heat treatment determine how long the spacing and shut-off face hold. A tool built for a small pilot run may not be production-hardened. For security hardware, the manufacturer should record cavity dimensions before and after each batch and follow a maintenance schedule.

During the re-pilot, ask them to measure the tool cavity with a CMM and submit the data with the part inspection results. Comparing the cavity size against the part will show whether the dimension drift comes from tool wear or from the molding process. If they resist this, they do not understand their own process control.

**status:** suggested
**Author:** Victor Luo
**Date:** 2026-10-27

### Answer 5

Because these inserts go into a security hardware assembly, the appearance and dimensional defects are also a documentation problem. Require a salt spray test report that corresponds to your finish spec and to the substrate alloy actually used in the batch.

The report should mention the plating cycle, coating thickness, and result after the agreed number of hours. If the supplier cannot match their certificate to the batch number, you cannot rely on their plating quality.

Traceability matters for future liability. Ask for the material certificate with the lot number, the casting date or melt batch, and the plating job number.

If a field failure happens, these links allow you to isolate the affected parts. Also verify that the finish and material meet current RoHS and REACH restrictions for nickel release and chromium categories.

The presence of pitting after plating should make you ask whether the plating subcontractor is still approved. If the supplier changed their plating vendor without updating your PPAP, that is a process change and should require a re-qualification at your incoming audit.

**status:** suggested
**Author:** Evan Qian
**Date:** 2026-10-27

### Answer 6

The insert in your lock chassis must be judged as an assembly, not as an isolated dimension. An outer diameter of 9.78 mm will fail more than the dimensional print; it will change the press-fit force and the torque path through the plastic housing.

When you run a pilot lot, include assembly-level tests after the parts have stabilized for at least 24 hours. Test push-in force, pull-out force, and torque at low and high temperatures if your lock sees seasonal or outdoor use.

Part geometry also controls retention. A small chamfer on the leading edge helps the plastic deform around the insert in a repeatable way. A knurl or through-groove on the OD can compensate for a wider tolerance band by creating a mechanical lock.

If the supplier focuses only on the 9.85 mm and does not review the assembly condition, they are missing the design function. The best answer from a manufacturer will connect the insert tolerance to the housing bore tolerance and to the expected plastic shrink after molding.

**status:** suggested
**Author:** Owen Jiang
**Date:** 2026-10-27

### Answer 7

Set the inspection plan so it distinguishes functional defects from appearance defects. The outer diameter fall-off is functional; pitting is functional only if it is on a contact area or if it exposes base metal.

Write that distinction into the incoming quality checklist. Do not use one AQL category for both. Typical security hardware practice is to keep critical dimensions on a zero-defect or AQL 0.65 sampling plan and allow a broader AQL for minor appearance flaws.

The measurement environment is not a side issue. Plated parts should sit in the metrology room at 20-25 °C before measurement.

The plating thickness can affect the effective OD, and the heat from the plating process can alter the part temperature. Ask the supplier what their measurement step is and confirm their gauge repeatability.

Also ask for an internal quality check at the machining station: first-piece inspection at setup, in-process checks every 30 to 60 minutes, and final OQC with a plating adhesion test. If the supplier cannot produce these checkpoints, a tightened incoming inspection will only catch defects after the part reaches your door.

**status:** suggested
**Author:** Claire Ren
**Date:** 2026-10-27

### Answer 8

When metal inserts sit inside a plastic lock chassis, the consistency of the insert matters long before assembly. If the OD is at the low end, plastic shrinkage can still grip it, but the residual stress distribution changes.

The failure might not show at the initial torque check; it may appear after temperature cycling. A pilot lot should therefore include molded samples and assembled torque tests, not only measurement of the insert on the gauge.

Insert design details affect the injection molding process. Sharp corners create stress concentration in the plastic and can nucleate cracks near the bore.

An unclean knurl or sharp flash can trap air and cause poor plastic filling. Ask the supplier to review the insert print and recommend a chamfer, an undercut, or a specific surface roughness on the bonding area.

They should also propose a process window for the overmolding step: preheat, melt temperature, injection speed, and hold pressure. If their process settings depend on a single operator trial, the next batch can behave differently.

**status:** suggested
**Author:** Henry Cao
**Date:** 2026-10-27

### Answer 9

Set the approval roadmap before the next order, otherwise you are only reacting to each defective lot. The current PPAP is no longer valid because the plating subcontractor changed without notification and because the OD process is not stable.

Require a re-PPAP submission with a written corrective action, a capability study on the critical OD, and a visual and microsection comparison of the counterbore face before and after tooling adjustment. The submission should be checked against your drawing and against the assembly test that failed.

Lead time prediction should be part of the approval plan. Tooling modification, a pilot run, and a plating re-validation take time, and the supplier should give you a milestone chart rather than a promise. After the pilot lot passes, define the change control rules.

If the supplier replaces the die, changes the plating vendor, or adjusts the material grade without notifying your team, the same failure can return. This is not only a quality issue; it is a change management issue that belongs in the supplier agreement.

**status:** suggested
**Author:** Aaron Fang
**Date:** 2026-10-27

### Answer 10

The defect pattern is also a material selection question. If the insert is zinc die-casting, asking which alloy is graded into the part matters. Zamak 3 offers good dimensional stability and is easy to cast, but Zamak 5 has higher strength and hardness.

If the insert sees repeated torque from the latching system, a switched alloy could change the mechanical behavior. Ask for the material certificate and compare the declared grade against your approved drawing.

For outdoor or humid applications, a sealed zinc casting or a stainless steel insert may be a better engineering choice than fighting plating defects on a zinc part. Pitting after plating often points to porosity, and porosity must be solved in the casting or by changing the material route.

The cost difference between zinc die-casting and stainless machining is significant, but it can be justified by a lower rejection risk over the product life. Base the decision on the lock environment and the required load, not on the supplier's standard price sheet.

**status:** suggested
**Author:** Grace Sun
**Date:** 2026-10-27

## Related Resources

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            "text": "Set the inspection plan so it distinguishes functional defects from appearance defects. The outer diameter fall-off is functional; pitting is functional only if it is on a contact area or if it exposes base metal. Write that distinction into the incoming quality checklist. Do not use one AQL category for both. Typical security hardware practice is to keep critical dimensions on a zero-defect or AQL 0.65 sampling plan and allow a broader AQL for minor appearance flaws. The measurement environment is not a side issue. Plated parts should sit in the metrology room at 20-25 °C before measurement. The plating thickness can affect the effective OD, and the heat from the plating process can alter the part temperature. Ask the supplier what their measurement step is and confirm their gauge repeatability. Also ask for an internal quality check at the machining station: first-piece inspection at setup, in-process checks every 30 to 60 minutes, and final OQC with a plating adhesion test. If the supplier cannot produce these checkpoints, a tightened incoming inspection will only catch defects after the part reaches your door.",
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            "text": "When metal inserts sit inside a plastic lock chassis, the consistency of the insert matters long before assembly. If the OD is at the low end, plastic shrinkage can still grip it, but the residual stress distribution changes. The failure might not show at the initial torque check; it may appear after temperature cycling. A pilot lot should therefore include molded samples and assembled torque tests, not only measurement of the insert on the gauge. Insert design details affect the injection molding process. Sharp corners create stress concentration in the plastic and can nucleate cracks near the bore. An unclean knurl or sharp flash can trap air and cause poor plastic filling. Ask the supplier to review the insert print and recommend a chamfer, an undercut, or a specific surface roughness on the bonding area. They should also propose a process window for the overmolding step: preheat, melt temperature, injection speed, and hold pressure. If their process settings depend on a single operator trial, the next batch can behave differently.",
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            "text": "Set the approval roadmap before the next order, otherwise you are only reacting to each defective lot. The current PPAP is no longer valid because the plating subcontractor changed without notification and because the OD process is not stable. Require a re-PPAP submission with a written corrective action, a capability study on the critical OD, and a visual and microsection comparison of the counterbore face before and after tooling adjustment. The submission should be checked against your drawing and against the assembly test that failed. Lead time prediction should be part of the approval plan. Tooling modification, a pilot run, and a plating re-validation take time, and the supplier should give you a milestone chart rather than a promise. After the pilot lot passes, define the change control rules. If the supplier replaces the die, changes the plating vendor, or adjusts the material grade without notifying your team, the same failure can return. This is not only a quality issue; it is a change management issue that belongs in the supplier agreement.",
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          ,          {
            "@type": "Answer",
            "text": "The defect pattern is also a material selection question. If the insert is zinc die-casting, asking which alloy is graded into the part matters. Zamak 3 offers good dimensional stability and is easy to cast, but Zamak 5 has higher strength and hardness. If the insert sees repeated torque from the latching system, a switched alloy could change the mechanical behavior. Ask for the material certificate and compare the declared grade against your approved drawing. For outdoor or humid applications, a sealed zinc casting or a stainless steel insert may be a better engineering choice than fighting plating defects on a zinc part. Pitting after plating often points to porosity, and porosity must be solved in the casting or by changing the material route. The cost difference between zinc die-casting and stainless machining is significant, but it can be justified by a lower rejection risk over the product life. Base the decision on the lock environment and the required load, not on the supplier's standard price sheet.",
            "upvoteCount": 0,
            "url": "https://www.hyneu.com/questions/custom-manufacturer-metal-inserts-security-hardware.html#suggestedAnswer-10",
            "datePublished": "2026-10-27T21:53:49Z",
            "author": {"@type": "Person","name": "Grace Sun","url": "https://www.hyneu.com/team/grace-sun.html"}          }
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