Quality Planning at Hydroforce: What Is Decided Before the First Part Is Made

An inspection report says whether one part is good. Whether the next thousand parts will be good is settled much earlier, when the order is planned. This article explains how we plan quality for each order and each technology: the control plan, first article approval, checks during the run and the feedback that keeps the process where it was validated.

Our quality page describes the three stages of inspection we run: raw material verification, in-process monitoring and final acceptance. Customers who audit us usually ask a different question: how do we know in advance that a batch will pass? The honest answer is that the checks were designed before the batch existed, and the batch passes because of that. That design work is quality planning, and it looks different for a hydraulic cylinder, a die-cast housing, a sintered gear and a carbide nozzle.

Why We Plan Instead of Sorting

A supplier who relies on final inspection is really running a sorting operation. Good parts go to the customer, bad parts go to scrap, and the process itself learns nothing. The cost is paid twice: once in scrap and once in schedule, because a rejected batch is discovered at the end, when there is no time left to make another.

Planning moves the effort to the front. Before an order is released to production, a small group of people sits over the drawing: a process engineer, a quality engineer and, for tooled parts, a tooling engineer. They leave the meeting with four documents: a process flow with every operation in sequence, a risk analysis that names what can go wrong at each operation, a control plan that says what will be measured, where, how often, with what and what happens when a reading is out, and an acceptance protocol agreed with the customer. Those four documents are what an ISO 9001:2015 system looks like when it reaches the shop floor.

The Planning Sequence

1. Contract and drawing review

Every order starts with a review of the drawing and the specification against what our processes can hold. Typical findings are a missing tolerance, a reference to a withdrawn standard, a surface finish the chosen process cannot reach, or a material grade with no European equivalent. Each of them is resolved with the customer in writing before anything is cut or cast. The review also classifies characteristics. A few dimensions on any part decide whether it works: a bore diameter, a sealing face, a thread, a hardness range. Those become critical characteristics, and the control plan treats them differently from a chamfer.

2. Process design and measurement design

The route is written operation by operation. For each operation the planner picks the machine, the fixture, the cutting or pressing parameters and the gauge. The gauge is chosen with the same care as the machine. A tolerance of 0.02 mm cannot be controlled with an instrument that resolves 0.01 mm, so the measuring method has to be roughly ten times finer than the tolerance. The CMM program for the part is also written at this stage, well before the first parts arrive at the lab.

3. Risk analysis

For each operation the team asks where the process can drift and what the consequence would be. The answers depend on the part. For a chrome-plated rod the weak spot is coating thickness at the ends, for a die-cast housing it is porosity under a machined sealing face. A sintered gear can vary in density across a thick section, and a carbide insert made from a new powder lot may shrink by a different amount than the last one. Each risk found this way gets its own line in the control plan. For hydraulic cylinders this analysis is formal: a failure mode and effects study per design, fed by our warranty and field records, so that a problem seen once does not appear in the next design.

4. The control plan

The control plan is the single document an operator, an inspector and a customer’s auditor all read. A short extract for a machined cylinder tube shows the format.

Two columns carry the most weight. Frequency fixes the cost of control, and for critical characteristics it is negotiated with the customer. Reaction is what turns a measurement into control: without a reaction rule, the same measurement only produces a report.

5. Tooling validation and first article

For any part that depends on a die, a mould or a press tool, the tool is validated before the plan is signed. The first shots or the first pressings are measured completely, every dimension on the drawing, and compared with the CAD model. Where the tool is corrected, the sample is measured again. Only a tool that has produced an approved first article is released, and the first article report becomes the reference for every later batch. For machined parts the same logic applies to the first piece from a new program or a new fixture.

What Is Planned for Each Technology

The sequence above is the same for every order, while the content of the control plan changes with the technology. The list below shows the planning decisions that carry the most weight in each of our production areas.

Hydraulic cylinders

The plan starts with the material: honed tube and chrome-plated bar arrive with certificates that are checked against the drawing before the lot is released. Welding is done to written procedures, and load-bearing welds get non-destructive testing at the frequency fixed in the plan. Assembly is planned around cleanliness and seal handling, because most early leaks trace back to a damaged seal or a contaminated bore rather than a machining error. Final testing covers every unit. Each cylinder is pressure tested at working pressure and at overpressure, checked for external and internal leakage, and the record is kept with the serial number. Documentation is planned from the start when the customer needs it for classification: ISO, DIN, DNV, ABS or API packages, with strength calculations and test protocols.

CNC machining and gears

Precision parts are planned around IT6 to IT7 tolerance grades and the general tolerance standards on the drawing: ISO 2768, ISO 286 for fits, ISO 1101 for geometry. The CMM program is written and proven on the first piece. During the run, critical dimensions are measured at fixed intervals, with a scheduled check every 45 machine hours and a control chart for the dimensions where tool wear is the known drift. For gears the plan includes profile, lead and pitch measurement to ISO 1328 on a gear measuring machine, and hardness and case depth checks after heat treatment. Large components up to 16 metres are planned differently: a laser tracker or a 3D scanner replaces the CMM, and the measurement setup is designed together with the fixture before the part is loaded.

High-pressure die casting

Here quality is planned into the tool. Mould design, gating and cooling are reviewed for the risk of porosity in the zones that will later be machined or sealed, and those zones are marked as critical in the plan. Tool trials produce the first article. In series, the process parameters of the machine are monitored in real time and charted, because a die-casting cell that holds its parameters holds its dimensions. Dimensional tolerances of ±0.1 to 0.3 mm and surface roughness Ra 1.6 to 6.3 µm are verified on the CMM and by surface measurement. Where the part is pressure tight or structural, X-ray inspection is added to the plan for the critical zones at an agreed frequency.

Powder metallurgy and MIM

Sintered parts are controlled by material and process rather than by machining. The plan begins with the powder lot: composition and flow are checked on receipt. Compaction is planned for green density above 95 percent, and the press is monitored for pressure and fill. Sintering furnaces run programmed cycles with temperature held to ±5 °C in a controlled atmosphere, and the cycle record is part of the batch file. Each lot is checked for density, hardness and dimensions, with metallographic examination at the frequency fixed in the plan. For MIM parts, weight control of green parts is the early warning: a change in weight shows a feedstock or moulding drift before the dimensions move. Tolerances of ±0.05 mm after sintering and ±0.02 mm for MIM are the values the plan is written against.

Cemented carbide

Carbide adds a planning step that no other technology needs: the shrinkage trial. A part shrinks 15 to 20 percent linearly during sintering, and the exact figure depends on the grade and the powder lot. Before a die is finalised, a trial sintering fixes the coefficient. In production the plan covers the grade itself, selected from the ISO 513 range, and the properties that prove it: density, hardness and, for critical grades, transverse rupture strength on bars sintered in the same cycle. Sintering tolerances of ±0.15 mm are brought to ±0.01 mm by diamond grinding, and the ground parts are verified on the CMM with surface roughness Ra 0.1 to 0.4 µm. Where the application demands full density, hot isostatic pressing is added to the route and confirmed by density measurement.

Hydraulic motors and pumps

Motors and pumps are planned as tested units. Every unit passes a functional test, a pressure test and a performance check against the catalogue values, and leaves with its documentation package.

Managing Quality While the Batch Runs

A plan only works if someone keeps it alive during the run. Four routines do that.

• When a reading goes out of tolerance, the operation stops. Parts made since the last good reading are set aside and measured again, and the cause is found before the operation restarts. None of those parts travel on to the next stage.
• Every gauge, from a torque wrench to the CMM, is on a calibration schedule. An instrument past its calibration date is taken out of service. Measurement records name the instrument used, so a calibration finding can be traced back to the parts it measured.
• A defect found in the shop, at a supplier or by a customer is recorded, contained and analysed to its root cause. The corrective action changes the plan, the tooling or the procedure, and the change is verified on the next batch.
• Material and subcontracted operations fall under the same plan. Suppliers are audited, incoming certificates are checked against the specification, and a supplier lot that fails incoming inspection is held before it reaches a machine.

Operators take part in all of this. They are trained in the measurement techniques their control plan calls for, and the first-piece and interval checks are theirs to perform and to sign.

What the Customer Can Ask For

Planning also gives the customer a view before production starts. On request we share the control plan for the part, agree the critical characteristics and the check frequency, and add witnessed tests or third-party inspection where the application calls for it. With the shipment, or on request, we provide material certificates, the first article report, measurement records for the characteristics in the plan, test protocols for cylinders, motors and pumps, and a certificate of conformity for the batch. The Acceptance Conditions document on our quality page describes the acceptance procedure in detail.

In Short

Inspection catches a defect after it has been made. Planning works on the reason for it. At Hydroforce the plan is written before the order is released, it depends on the technology the part is made with, and during the run it is kept alive by calibration, reaction rules and corrective action. This is why we can commit to the tolerances on our technology pages, and why the tenth batch comes out like the first.

Send us your drawing and requirements through the contact form. We will reply with a manufacturing proposal and the outline of the control plan for your part.